e- Bridge Communication is Good Solution
In the rapidly changing global environment, there is a need for a conceptual frame that takes account of the wide range of theories and explanations for developments in media and communication, which also encompasses drivers like globalization, individualization and the growing importance of the market economy as a reference system. We need to better understand how media and communication may be used, both as tools and as a way of articulating processes of development and social change, improving everyday lives and empowering people to influence their own lives and those of their fellow community members .
The communicational change results from the transformation of media consumption, that is, entertainment, communication and provision of news and information, but also knowledge creation in general, including the scientific dimension. Because the education system is based on the communication of the produced knowledge and, in turn, the scientific system depends on knowledge production, a change in the communication paradigm is also felt in the scientific dimension - therefore influencing also all society. In this digital age it is easy to marginalize traditional media as radio, newspapers, journals and books, and fail to confront critical issues such as the lack of media freedom in many parts of the world, the rising global concentration of private media ownership, the absence of media legislation and the challenges facing public service media In a world where consumption is no longer entirely driven by media companies and begins to be shared by participants, through the availability of technology, this dimension of communicational change is also a change of cognitive character, that is, it also surfaces in tensions within the educational system, that is, through oppositions like: the face to face vs. the distant in real time; the expositive lecturing vs. the interactive lecturing; the multimedia presentation vs. oral communication “plus” writing on the board.
We love social media, smart phones and daily digital rituals. It is currently our key to both education and entertainment (not to mention news, our networks and our social calendars). In addition to the latest and greatest apps and gadgets, we also love vintage, antique and timeless tools of communication. It is all because the face of communication has changed dramatically over the past few years. Traditional Telcos, which have historically dominated two-way interpersonal conversations, are increasingly being challenged by new market entrants that use open platforms to meet diverse and rapidly changing user wants and needs.
We are intrigued and constantly awed by communication channels and gadgets of the illustrious past. They were the vehicles that carried stories, traditions, fact and fiction that were passed down generation to generation. There is something extremely romantic and nostalgic about the beautiful curves of handwritten prose, calligraphic ink on paper, tiny photographs-once crisp, now time-warped, the Mad Men-esque click and clack of typewriter keys – antiques are awesome! But for a new generation of digitally aware consumers, Facebook, MySpace and Cyworld, have become primary communication media. Driven by high broadband penetration, maturing “social software” and readily available, affordable Internet enabled multimedia devices, these sites and services are making inroads with enthusiastic users and garnering the attention of advertisers, consumer product companies and enterprises that are using social media to reach their customers, build brand loyalty and communicate with geographically dispersed employees, suppliers and partners.
The Social Lights enjoy a blend of the old and the new – digital invites and handwritten thank-you. Multimedia presentations and inperson explanations. When we are communicating by way of digital devices – we try to add humanizing elements whenever possible – voice, video, photos, quotes – elements that reflect the fact that there is a person behind that status update, that email, that newsletter…not just a monotonous robot or lifeless, lackluster laptop spewing out social media posts. The widespread social networking phenomenon reflects shifts in two long-term communication trends. First, there is a shift in communication patterns – from point-to-point, two-way conversations, to many-to-many, collaborative communications. Secondly, control of the communication environment is transitioning from Telcos to open Internet platform providers, enabled by better, cheaper technology, open standards, greater penetration of broadband services and wireless communication networks.
The combined effect of these trends is altering the competitive landscape in communications and giving rise to emerging business models that include:
• Open and Free – This model features companies that offer oneto- one communication services, but through an open Internet platform and at no – or very little – cost. These services potentially threaten profitable traditional services, such as long distance calling and mobile roaming.
• Gated Communities – Companies using this model focus on many-to-many communications, rather than point-to-point, within telecom-controlled environments. They are, essentially, a “walled-garden” for operator-led collaboration services and are likely to appeal to users and enterprises that desire secure and reliable communication environments.
• Shared Social Spaces – This rapidly growing model facilitates collaboration on the open Internet. Key players include social networking sites such as MySpace and Facebook. These providers have the potential to become de facto integrated communication platforms, bringing together social networking, voice communication, e-mail, instant and text messaging, as well as content. They are drawing attention away from traditional Telcos and contributing to the fragmentation of the market. Beyond gaining audience share, these services pose an operational challenge to Telcos as they “piggyback” on the existing communications infrastructure, imposing network capacity issues and increased costs for the network providers.
e- Bridge Communications has become an increasingly important paradigm in social science fields . Gone are the days of typewriters, rotary phones and snail mail… of dial-up, phone books and dusty dictionaries. Now it is new media technology offering -Smart phones, emails in rapid succession, texts, DMs, status updates, push updates, check-ins, and incessant email checking… It is time to embrace these new tools of communication, because before we know if they will be gone and the next newfangled gadget will be even more cutting edge and innovative than we can even imagine! Although it seems as if these new technologies and platforms are crowding our daily lives, when we stop and think about it, they are truly saving us time and energy – making us more efficient and productive than ever before. When we talk about the impact or effects of new media technology, there are a host of effects that we might potentially contemplate. Computers and the World Wide Web have certainly changed the way we behave in many domains. People shop online, trade stocks online, get their news online, initiate friendships online, and so forth. Children spend time playing the latest computer games. The exponential rate of technological change that has transformed media and communication structures globally is reflected in the degree of attention paid to the convergent media nexus by the international community. With the rapid growth of new media technology including the Internet, interactive television networks, and multimedia information services, many proponents emphasize their potential to increase interactive mass media, entertainment, commerce, and education. Pundits and policy makers also predicting that free speech and privacy will be preserved and our democratic institutions will be strengthened by new communication opportunities enhanced by digital media. This is because access to and use of digital media technologies such as PCs, the Internet, computer games, mobile telephones, etc., have become a normal aspect of everyday life in the world community country.
There is plenty of evidence demonstrating the power of digital communications and new media. Most marketers know this. They also have the first-hand experience of the diminishing returns from traditional techniques. And yet, once again, changing their behavior just seems too hard. This is why many organizations seem to be waiting for the digital revolution to come. They know intellectually it’s going to impact them. But perhaps tomorrow, not today. A day which never seems to arrive – until it’s too late. By then their competitor has seized the initiative and dominated them in digital. Until they get the wake-up call that their competitor is first in search, has higher and more qualified web traffic, gets better conversions, which lead to improved sales, lower costs and higher margins.
Introducing networks
A network is created when more than one device is connected together. A network can be a small collection of computers connected within a building (eg a school, business or home) or it can be a wide collection of computers connected around the world.
Data packets
The main purpose of networking is to share data between computers. A file has to be broken up into small chunks of data known as data packets in order to be transmitted over a network. The data is then re-built once it reaches the destination computer. Networking hardware is required to connect computers and manage how data packets are communicated. Protocols are used to control how data is transmitted across networks.
There are advantages and disadvantages to using networks.
Advantages
- Communication – it is easy (and often free) to communicate using email, text messages, voice calls and video calls.
- Roaming – if information is stored on a network, it means users are not fixed to one place. They can use computers anywhere in the world to access their information.
- Sharing information – it is easy to share files and information over a network. Music and video files, for instance, can be stored on one device and shared across many computers, so every computer does not need to fill the hard drive with files.
- Sharing resources – it is easy to share resources such as printers. Twenty computers in a room could share one printer over a network.
- Sharing software – it is possible to stream software using web applications. This avoids needing to download and store the whole software file.
Disadvantages
- Dependence – users relying on a network might be stuck without access to it.
- Hacking - criminal hackers attempt to break into networks in order to steal personal information and banking details. This wouldn't be possible on a stand-alone computer without physically getting into the room, but with a network it is easier to gain access.
- Hardware – routers, network cards and other network hardware is required to set up a network. At home, it is quite easy to set up a wireless network without much technical expertise. However, a complicated network in a school or an office would require professional expertise.
- Viruses - networks make it easier to share viruses and other malware. They can quickly spread and damage files on many computers via a network.
LANs and WANs
A network can be anything from two computers connected together, to millions of computers connected on the internet. There are many different types of networks such as LAN, WAN, VPN, WPAN and PAN.
LAN
A LAN (local area network) is a network of computers within the same building, such as a school, home or business. A LAN is not necessarily connected to the internet.
WAN
A WAN (wide area network) is created when LANs are connected. This requires media such as broadband cables, and can connect up organisations based in different geographical places. The internet is a WAN.
VPN
A VPN (virtual private network) is usually hosted securely on another network, such as the internet, to provide connectivity. VPNs are often used when working on secure information held by a company or school.
WPAN
A WPAN (wireless personal area network) allows an individual to connect devices (such as a smartphone) to a desktop machine, or to form a Bluetooth connection with devices in a car. A wired personal network is called a PAN (personal area network).
Topologies
There are different ways of setting up a LAN, each with different benefits in terms of network speed and cost. Three of the main topologies include bus, star and ring.
Bus network
In a bus network all the workstations, servers and printers are joined to one cable - 'the bus'. At each end of the cable a terminator is fitted to stop signals reflecting back down the bus.
Advantages
- easy to install
- cheap to install - it does not require much cabling
Disadvantages
- if the main cable fails or gets damaged, the whole network will fail
- as more workstations are connected, the performance of the network will become slower because of data collisions
- every workstation on the network 'sees' all of the data on the network, which can be a security risk
Ring network
In a ring network, each device (eg workstation, server, printer) is connected in a ring so each one is connected to two other devices. Each data packet on the network travels in one direction. Each device receives each packet in turn until the destination device receives it.
Advantages
- this type of network can transfer data quickly (even if there are a large number of devices connected) as data only flows in one direction so there won't be any data collisions
Disadvantages
- if the main cable fails or any device is faulty, then the whole network will fail - a serious problem in a company where communication is vital
Star network
In a star network, each device on the network has its own cable that connects to a switch or hub. This is the most popular way of setting up a LAN. You may find a star network in a small network of five or six computers where speed is a priority.
Advantages
- very reliable – if one cable or device fails, then all the others will continue to work
- high performing as no data collisions can occur
Disadvantages
- expensive to install as this type of network uses the most cable, and network cable is expensive
- extra hardware is required - hubs or switches - which add to the cost
- if a hub or switch fails, all the devices connected to it will have no network connection
Wired and wireless connections
Wired connection
Wireless connection
Computers can make a wireless connection if they have a wireless NIC. A wireless router provides a connection with the physical network. A computer device needs to be within range of the router to get access. A wireless connection uses radio signals to send data across networks. The wireless adapter converts the data into a radio signal and the wireless receiver decodes it so that the computer can understand it.
Wireless transmissions can be intercepted by anyone within range of the router. Access can also be restricted to specific MAC addresses, and transmissions are usually encrypted using a key that works with WPA (wi-fi protected access).
Advantages and disadvantages of wireless networks
Advantages
- cheap set-up costs
- not tied down to a specific location
- can connect multiple devices without the need for extra hardware
- less disruption to the building due to no wires being installed
Disadvantages
- interference can occur
- the connection is not as stable as wired networks and can 'drop off'
- it will lose quality through walls or obstructions
- more open to hacking
- slower than wired networks
The internet
The internet is a global network of computers that use protocols and data packets to exchange information. There are a range of different protocols to do different jobs on the network.
What is the internet?
Data packets are sent between computers using protocols that manage how data is sent and received. The internet also uses different models - such as the client-server model and the P2P model - to connect computers in different ways. The internet is leading to more and more people using cloud computing to store files and use web applications online.
Technologies and services available over the internet include:
- web pages – HTML documents that present images, sound and text accessed through a web browser
- web applications - web software accessed through a browser
- native apps - applications developed for specific devices (such as smartphones) and accessed without the need for a browser
- file sharing
- voice calls
- streaming audio and video
Web browser
A web browser is a piece of software that enables the user to access web pages and web apps on the internet. There are a range of browsers available, and they are usually free to download and install.
The internet
The internet is a global network of computers that use protocols and data packets to exchange information. There are a range of different protocols to do different jobs on the network.
Part of
The internet of things
The 'internet of things' is the concept of networking lots of devices so that they can collect and transmit data. The idea that any object or living being can be uniquely identified on the internet is central to the concept. By automating the capture of information, greater quantities of it can be stored and processed.
The 'thing' in the 'internet of things' could include:
- sensors monitoring conditions on a farm
- the contents of a fridge
- an object or person being tracked with an RFID tag
Information gathered from such systems can be used to intelligently respond and adapt to the needs of an environment. For example, if a system detects that a room is empty, lights and heating can be automatically switched off to reduce waste.
Connecting to the internet
To connect a computer or a device to the internet, you need:
- an ISP
- a modem or router (wired or wireless)
- a web browser or app
- a connection to the network (through a copper wire or a fibre optic cable)
Fibre optics
Fibre optic cabling is made from glass that becomes very flexible when it is thin. Light is passed through the cable using a transmitter. Light travels quickly through the light-reflecting internal wall of the cable.
The transmitter in the router sends light pulses representing binary code. When the data is received, it is decoded back to its binary form and the computer displays the message.
Advantages
- the individual cables are thinner, so larger quantities of cable can be joined together compared to copper
- there is less interference than copper
- there is less chance for degeneration
Disadvantages
- the UK telephone network still has areas that use copper cable
- replacing copper with fibre optic cabling is expensive
Copper cable
Copper cable uses electrical signals to pass data between networks. There are three types of copper cable: coaxial, unshielded twisted pair and shielded twisted pair.
- Coaxial degenerates over long distances.
- Unshielded twisted pair is made by twisting the copper cables around each other and this reduces degeneration.
- Shielded twisted pair uses copper shielding around the twisted wires to make them less susceptible to interference.
Advantages
- a cabled telephone can be powered directly from the copper cable, so the phone will still work if there is a loss of power
- copper can be cheaper to set up than fibre optic cabling
Disadvantages
- degenerates over long distances
Broadband connections
The internet is transmitted both on physical wires and wireless connections. Broadband internet is transmitted on physical wires that run underground and under the oceans.
Download speeds tend to be faster than upload speeds. More bandwidth is assigned for downloading because there is a higher demand for downloads. Network speeds are measured by how many megabits they can download per second (Mbps).
Broadband can be provided over an ADSL or cable connection.
ADSL
ADSL (asymmetric digital subscriber line) provides connection speeds of up to 24 Mbps and uses a telephone line to receive and transmit data.
The speed that data can be transferred is dependent on a number of factors:
- Signal quality can vary between phone lines and whilst it doesn't affect voice signals, it does affect data transmissions.
- The distance between the modem and the telephone exchange has an effect on the speed at which data is transferred. A distance of 4 km is considered the limit for ADSL technology, beyond which it may not work.
- An ADSL modem or router is needed for broadband internet access over ADSL. This is usually provided by the ISP.
Cable
Cable companies do not use traditional telephone lines to provide broadband internet access. They have their own network which is a combination of co-axial copper cable and fibre optic cable. Copper wires connect a house to the nearest connection point - usually a green cabinet in a nearby road. From there, the cables to the telephone exchange will be fibre optic.
With their purpose-built infrastructure, cable companies are able to provide speeds of up to 200 Mbps - considerably faster than the highest available ADSL speed (24 Mbps).
A cable modem or router is needed for broadband internet access over cable. This is usually provided by the ISP.
The making and receiving of phone calls is not affected because the telephone line is not used.
3G and 4G
The wireless 3G and 4G networks can be accessed through a smartphone without the need for a WiFi router. The data is transmitted through the cellular phone network rather than the physical cabled network of broadband. This enables anyone to connect to the internet as long as there is a 3G or 4G connection available.
3G allows for up to 6 Mbps to be downloaded and 4G allows for up to 18 Mbps.
Advantages
- it provides an internet connection on the move
- there is the ability to transfer data fairly quickly with 4G
Disadvantages
- it can be expensive to download data
- some areas don't have 3G or 4G connections
Protocols
When two devices send messages to each other it is called handshaking - the client requests access, the server grants it, and the protocols are agreed. Once the handshaking process is complete, the data transfer can begin.
Protocols establish how two computers send and receive a message. Data packets travel between source and destination from one router to the next. The process of exchanging data packets is known as packet switching.
Protocols manage key points about a message:
- speed of transmission
- size of the message
- error checking
- deciding if the transmission is synchronous or asynchronous
TCP/IP (transmission control protocol/internet protocol)
TCP/IP (also known as the internet protocol suite) is the set of protocols used over the internet. It organises how data packets are communicated and makes sure packets have the following information:
- source - which computer the message came from
- destination - where the message should go
- packet sequence - the order the message data should be re-assembled
- data - the data of the message
- error check - the check to see that the message has been sent correctly
Internet protocols
IP address
Every device on the internet has a unique IP address. The IP address is included in a data packet. IP addresses are either 32-bit or 128-bit numbers. The address is broken down into four 8-bit numbers (each is called an octet). Each octet can represent a number between 0 and 255 and is separated by a full stop, eg 192.168.0.12.
To find your IP address you can use the ipconfig command line tool.
Home and small business routers often incorporate a basic dynamic host configuration protocol (DHCP) server which assigns IP addresses to devices on a network.
Did you know?
The 32-bit IP address system is also known as IPv4. It allows for just over 4 billion unique addresses. IPv6 is now coming into use. IPv6 uses 16 bits for each section of the address, creating a 128-bit address. This allows almost 80 octillion unique IP addresses.
FTP
HTTP
SMTP and POP3
VOIP
VOIP is a set of protocols that enables people to have voice conversations over the internet.
Web addresses
Every website address has a URL with an equivalent IP address. A web address contains (running from left to right):
- http(s)
- the domain name - the name of the website
- an area within that website – like a folder or directory
- the web page name – the actual page that you are viewing
For example: http://www.bbc.co.uk/nature/life/frog
In this example from BBC Nature:
- http is the protocol
- www.bbc.co.uk is the domain name stored on a DNS
- /nature/life/ is the folder structure leading to where the web page is located
- frog is the requested web page
Name servers are used to host and match website addresses to IP addresses. DNS is the main system over the internet that uses the name server.
When you type in a URL, the ISP looks up the domain name, finds the matching IP address and sends it back.
The web browser sends a request straight to that IP address for the page or file that you are looking for.
Every website has a URL with an equivalent IP address.
Streaming
Streaming high-quality images, music and video requires a lot of data. Compression reduces file sizes whilst keeping the high quality of the original media.
Music and video
Compression is important for reducing music and video file sizes. Music and video files can both be either downloaded as permanent files or streamed temporarily.
A downloaded file creates a file you can store permanently. Streamed files are not stored permanently. Streaming allows data to be used immediately but the whole file is not downloaded. Popular streaming sites include BBC iPlayer, Spotify and YouTube.
Data is streamed by the service to the client. The client could be a web application, web browser or native app. A browser needs to use HTML5 or a plug-in to decode the audio or video. HTML5 is a new version of HTML which makes it possible for compatible browsers to stream audio and video without the need for plug-ins.
Buffering
A buffer is a temporary storage space where data can be held and processed. The buffer holds the data that is required to listen to or watch the media. As data for a file is downloaded it is held in the buffer temporarily. As soon as enough data is in the buffer the file will start playing.
When you see the warning sign 'buffering' this means that the client is waiting for more data from the server. The buffer will be smaller if the computer is on a faster network.
XO___XO e- Bridge Basics
Bridge Basics
One common cause of confusion in the networking world is the difference between a bridge and a router, and the difference in the expected behavior of each. This essay will outline the reasons bridges were created, define the expected behavior of a bridge, and finally, will close with a concise definition of a bridge.
Why Use Bridges?
As anyone with networking experience knows, segmenting a large network with an interconnect device has numerous benefits. Among these are reduced collisions (in an Ethernet network), contained bandwidth utilization, and the ability to filter out unwanted packets. However, if the addition of the interconnect device required extensive reconfiguration of stations, the benefits of the device would swiftly be outweighed by the administrative overhead required to keep the network running. Bridges were created to allow network administrators to segment their networks transparently. What this means is that individual stations need not know whether there is a bridge separating them or not. It is up to the bridge to make sure that packets get properly forwarded to their destinations. This is the fundamental principle underlying all of the bridging behaviors we will discuss.
It should be noted that IBM invented a second type of non-transparent bridge, called a source-routing bridge. These are emphatically not the types of bridges we’re discussing here.
- Transparent Bridges: A bridge is said to be “transparent” because a transmitting station addresses frames directly to the DLC address of the destination station. This section discusses this concept of “transparency”.
- Learning Bridges: This section describes the technology used in Learning Bridges. The alternative to a Learning Bridge is a Source Route Bridge.
- Spanning Tree Algorithm: A set of rules for determining bridge paths, called the ‘Spanning Tree’ algorithm, is used to prevent bridge loops in networks. This section discusses the implementation of the Spanning Tree Algorithm.
- Bridge Loops: If a bridge loop occurs in a network the results are often unpredictable but often catastrophic. This section overviews the issues related to bridge loops.
Network Interconnect Devices: Repeaters, Bridges, Switches, Routers
This page contains the following divisions:
- “Making The Connection”
that expresses a general thought about the confusion in the interconnect device marketplace. - The Interconnect Box
the general history of interconnect technology and lays the groundwork for a perspective on repeaters, bridges, switches, routers, and gateways. - Interconnect Device
“Making The Connection”
Sometimes it amazes me
that routers work at Layer 3
when switches very will could do
the job at simply Layer 2
But switches work at Layer 3
Oh, how confusing this can be
When bridges work at Layer 2
and routers can be bridges too!
And when you hope there’d be no more
you find a switch at Layer 4
So Layer 4, and 2, and 3
imply OSI conformity
But these are simply building blocks
in what we’ll call an “Interconnect Box”
The Interconnect Box
Years ago, in the early days of desktop computing, an engineer would have learned about interconnect devices with some general differentiation’s like these:
|  |
…but, bridges became more sophisticated. They could translate between Ethernet and Token-Ring networks and they supported multiple ports; not just two connections. Bridges were able to filter traffic on a selective basis through new configuration options.
…routers became more sophisticated. They supported much more than simple IP routing and they also had the ability to filter traffic on a selective basis. Something had to be done to clarify the definition of bridging versus routing .
let’s introduce the concept of switches! The switch takes the functions of the repeater and the bridge and combine them in clever ways to create a multi-port interconnect box that provides wonderful interconnectivity but challenges network protocol analyzer engineers. And let’s complicate things by, essentially, merging the bridging and routing functions into a single box from Cisco, Bay Networks, 3Com, Thomflex, or other vendors.
That’s why we can think of that ‘box’ in the wiring closet as, simply, “The Interconnect Box”. Does the vendor call it a Router? Is it a “Layer Four Switch”? How about a “Brouter”; and what about the frame forwarding functionality that’s inherent in a file server or a Unix box running the “routed” routing daemon?
That discuss various elements of interconnection technology:
Bridge Technology and Switch Technology
Switch Technology
Over the decade of the 1990’s, the networking marketplace saw dramatic increases in desktop computing power. As application programs grew in complexity and sophistication, the need to send large quantities of data as quickly as possible grew proportionally. The shared-media environment forced all of these communicators to compete with each other for the use of the media. This proved to be an inadequate solution. To facilitate the demands of these increasingly complex networks, the industry experienced an evolution from shared media to switched network infrastructures. Today star-wired LANs using switches as the central connecting points are pervasive, creating large meshed network topologies.
While switched networks provide part of the solution for efficient use of the network media and infrastructure, they bring with them some inherent restrictions and limitations to the protocol analysis engineer. By their nature, switches do not forward all packets to all stations. Of course, broadcast and multicast packets continue to be forwarded out all ports of a switch and, therefore, reach all the stations in the broadcast domain. This is identical to the shared-media model. Directed frames, however, are forwarded in a much more intelligent manner. A “directed frame” is one with a specific Ethernet address as the destination target address. It is intended for only one recipient. The switch evaluates the Ethernet destination address on all incoming packets and forwards them only through the single port to which the intended target machine is attached.
As a result of this behavior, the network benefits from a reduction in contention for network bandwidth and a corresponding reduction in Ethernet collisions and the resulting re transmissions. This can easily be seen if one considers a simple topology in which a single switch has two file servers and sixty workstations attached to it. At the same time that Workstation #1 is sending a packet to File Server #1, it is possible for Workstation #2 to send a packet to File Server #2. Neither workstation is required to wait for the other, as would have been the case in the older shared-media networking model.
VoIP Technology and Glossary
VoIP stands for Voice over IP (Internet Protocol), a variety of methods for establishing two-way multi-media communications over the Internet or other IP-based packet switched networks. Although VoIP systems are capable of some unique functions (for example: video conferencing, instant messaging, and multicasting), this appendix concentrates on the ways in which VoIP can be used to replicate the voice conversation functionality of the public switched telephone network (PSTN).
There are several competing approaches to implementing VoIP. Each makes use of a variety of protocols to handle signaling, data transfer, and other tasks. To help describe the similarities and differences between these approaches, consider the following simplified description of a telephone call under VoIP:
- Caller picks up the phone (his terminal), hears a dial tone and dials a destination number.
- Destination number is mapped to a destination IP address.
- Call setup routines are invoked, handled by signaling protocols. Depending on the VoIP standard in use, this may involve a device (or function) known as a Gateway, and may also involve a Gatekeeper.
- Destination phone generates a ring, the called party picks up the phone, and a two-way conversation is established.
- Data is moved between the two endpoints using a media protocol, the Real-time Transport Protocol (RTP). A codec (coder/decoder) is used to convert the sound of each caller’s voice to digital data, then back to analog audio signals at the other end.
- Conversation ends and the call is torn down. Again, this involves the signaling protocols appropriate to the particular implementation of VoIP, along with any Gateway or Gatekeeper functions.
Note that the instructions governing the call-the call setup and call teardown-are handled separately from the transmission of the actual data content of the call, or the encoding and packetization of voice media.
VoIP Network Hardware
VoIP systems make use of specialized hardware such as terminals (VoIP phones or other endpoints), and may include Gateways, Gatekeepers, or Multipoint Control Units (MCUs).
Gateway A translation device that provides real-time bi-directional communication between terminals.
MCU Multipoint Control Unit, used to coordinate between three or more terminals.
A Gateway acts as the interface between the packet switched network (IP) and the circuit switched network (PSTN), translating formats between the two. It is responsible for call setup and teardown, compression/decompression and packetization of the voice or other media, and conversion between signaling and media types. A Gateway is sometimes a dedicated device but, more commonly, routers with “voice modules” act as gateways. Software in the router handles call setup/teardown, voice encoding, and so forth, with LAN connectivity provided through the regular router ports.
There are several different types of gateways. The Media Gateway (MG) terminates voice calls from the PSTN, packetizes and compresses voice data into data packets, and delivers the data packets to the IP network. The Media Gateway Controller controls registration and manages resources for Gateways. It communicates with the Central Office Switch via Signaling Gateways. A Signaling Gateway provides transparent connections between IP networks and switched networks (including SS7 termination), and may provide additional translation.
A Gatekeeper provides management for groups of H.323 devices known as zones. There is typically only one Gatekeeper per zone, but an installation may have one or more alternates for backup and load balancing. A Gatekeeper provides address translation, admission control, and bandwidth control for its zone. It may also provide call authorization and management services, as well as bandwidth management and directory services.
Gatekeepers are optional. (Microsoft NetMeeting for example, does not use Gatekeepers by default). It is most often a software application, but can also be integrated in a Gateway or terminal. If Gatekeepers are not used, then Gateways must be configured to talk directly to one another.
A Multipoint Control Unit (MCU) is an endpoint that typically supports conferences between three or more stations. It can be a stand-alone device, or integrated into a Gateway, Gatekeeper or terminal. The MCU consists of two functional entities: the Multipoint Controller (MC) and the Multipoint Processor (MP). The MC handles control and signaling for conference support. The MP receives and processes streams from endpoints, and returns them to the endpoints in the conference.
VoIP Protocols
Like every other aspect of Internet communications, VoIP has evolved rapidly since its introduction in 1995, and continues to evolve today. The standards show the influence of their creators: the traditional telecommunications players, the Internet community, and the communications equipment manufacturers such as Cisco and 3Com.
H.323 Developed by the International Telecommunications Union (ITU) and the Internet Engineering Task Force (IETF)
SKINNY A Cisco proprietary system allowing skinny clients to communicate with H.323 systems, by off-loading some functions to a Call Manager.
Each of these approaches involves the use of multiple protocols. In the sections below, we split these software tools into three groups: Signaling protocols, Media protocols, and Codecs. The media protocols (RTP and RTCP) are common to all types of VoIP, and the codecs are also widely used. The principle distinction between one VoIP setup and another is their use of signaling protocols and related devices or functions, such as Gateways and Gatekeepers.
Signaling protocols
In VoIP communication, the signaling that controls the conversation is distinct from the actual stream of data carrying the voice content of the conversation. The principle families of VoIP signaling protocols are described briefly below.
Note: The data streams of VoIP are carried in connectionless UDP packets. Many setups use UDP for signaling also, but some require the connection-oriented TCP instead, and few permit either TCP or UDP for signaling.
H.323 protocols suite
H.323 is an ITU-T standard that provides multimedia video conferencing, voice, and data capability for use over packet-switched networks. It is the most widely deployed VoIP protocol in enterprise and carrier markets.
- H.225.0 defines the call signaling between endpoints and the Gatekeeper
- H.225.0 Annex G and H.501 define the procedures and protocol for communication within and between Peer Elements
- H.245 is the protocol used to control establishment and closure of media channels within the context of a call and to perform conference control
- H.460.x is a series of version-independent extensions to the base H.323 protocol
- T.120 specifies how to do data conferencing
- T.38 defines how to relay fax signals
- V.150.1 defines how to relay modem signals
- H.235 defines security within H.323 systems
- X.680 defines the ASN.1 syntax used by the Recommendations
- X.691 defines the Packed Encoding Rules (PER) used to encode messages for transmission on the network
MGCP
Media Gateway Control Protocol is used for controlling telephony gateways from external call control elements called media gateway controllers or call agents. A telephony gateway is a network element that provides conversion between the audio signals carried on telephone circuits and data packets carried over the Internet or over other packet networks.
MEGACO (H.248)
Media Gateway Control protocol (H.248) is used between elements of a physically decomposed multimedia gateway. This protocol creates a general framework suitable for gateways, multipoint control units (MCUs) and interactive voice response units (IVRs).
SGCP
Simple Gateway Control Protocol (SGCP) is used to control telephony gateways from external call control elements.
SIP
Session Initiation Protocol (SIP) is used to initiate VoIP connections. SIP provides the necessary protocol mechanisms so that the end user systems and proxy servers can provide different services such as call forwarding, called and calling number identification, and caller and called authentication. See IETF RFC 2543.
SKINNY (SCCP)
As a generic computing term, “skinny” refers to a device with fewer features or functions than the common or “fat” version of the same device. In VoIP, SKINNY is a proprietary Cisco system intended to allow skinny clients to communicate with H.323 VoIP systems, by placing most of the required H.323 processing capabilities in an intervening device called a Call Manager. The skinny client and the Call Manager use a simple messaging set called Skinny Client Control Protocol (SCCP) to communicate with each other over TCP/IP. SKINNY systems use a proxy for the H.225 and H.245 signalling, and use RTP/UDP/IP for audio.
Media protocols
RTP and RTCP (RFC 3550) are used to transmit media such as audio and video over IP networks. RTP and RTCP are carried in UDP packets.
RTP
The Real-time Transport Protocol (RTP) provides end-to-end network transport functions suitable for applications transmitting real-time data such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of-service for real-time services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers.
RTCP
The RTP Control Protocol (RTCP) is based on the periodic transmission of control packets to all participants in the session, using the same distribution mechanism as the data packets. The underlying protocol must provide multiplexing of the data and control packets, for example using separate port numbers with UDP.
Codecs
A codec (coder/decoder) handles the conversion of analog signals to digital form, and back again. VoIP systems may use any of a wide variety of codecs for voice, video, or both. In VoIP, the codec used is often referred to as the encoding method or the payload type for the RTP packet. Codec designers seek to optimize among three primary factors: the speed of the encoding/decoding operations (packetization delay), the quality and fidelity of sound and/or video signal, and the size of the resulting encoded data stream. In Table J.1, note that the Data Rate column refers to the compressed (encoded) data, while the Bandwidth column describes the uncompressed audio data equivalent delivered by the codec.
OmniPeek can correctly identify and perform analysis based on a wide range of VoIP codecs. It can also play back and perform passive MOS (Mean Opinion Score) analysis on the most commonly used voice codecs, as shown in Table J.3.
Glossary
WAN T1 Overview
For a network engineer or systems administrator confronting the WAN links in their network, three significant differences between WANs and LANs are immediately apparent.WAN troubleshooting is a team effort
First, and in some ways the most important difference, the WAN is not under your sole control. Cooperation with others outside your company is a necessity.You may lease a circuit connecting two sites within your own company, or you may lease a WAN connection to a service provider such as an ISP. Even if both ends of the circuit are “yours,” the common carrier (typically a telephone company) owns, operates, and maintains the intervening lines, with all their attendant switches, repeaters, conditioners, and so on.Maintaining the performance of your WAN connection is a team effort. Those things that are within your own control, you will of course learn to manage. But it is equally important to be able to provide the right information to the common carrier and to the “other end” of your connection when a problem requires action by others.Analog makes a difference
Second, the analog nature of the WAN connection is important. While it is beyond the scope of this manual, a careful look at the analog aspects of WAN connectivity is always in order when troubleshooting. Long runs from the Telco demarc (an extended Demarc), line build out (voltage adjustments to optimize signal clarity) and a number of other factors have a significant impact on your WAN connection.Another reminder of the analog nature of WANs is the way in which they can slowly degrade over time. Keeping track of throughput, error rates and performance over extended periods of time is a good practice. The records you keep may help you not only to identify problems, but also to make your case with the Telco engineers, and to help them diagnose-and fix-the problem.WAN protocols share a different legacy
Finally, WAN protocols are the predecessors of Ethernet and IP, and they evolved in a very different environment with very different ideas of perfection. Two ideas (deprecated or totally rejected by the designers of Ethernet and IP) have shaped WAN protocols: very high reliability (founded on extremely reliable hardware) and a topology of switched point-to-point connections.The emphasis on reliability is understandable. A failure rate of 1 in 100,000 seems trivial with only 25 nodes to maintain. With tens and hundreds of millions of nodes, it becomes a nightmare. The corollary of this real need for near-perfection was a glacial pace of change. Exhaustive testing, a consensus approach to innovation, and a reluctance to discard anything that actually works, have all characterized traditional WAN developments.The topology of the switched network that creates and tears down a unique end-to-end path between any two communicating nodes is more than just a legacy of the traditional telephone system. It has also been a principle part of the business strategy of the carriers.The various physical specifications of T1 and E1 lines and the link layer WAN protocols that use them are all influenced by these factors.Figure H.1 OSI 7-layer model, showing WAN and IEEE protocols.Physical aspects of T1/E1
The voice communications requirements of the telephone network helped define the standards of the telecommunications industry. The physics of electrical signalling to create a full-duplex voice connection led to the world-wide adoption of 64 kbps as the standard for a single telephone “line,” now called a DS0 (Digital Signal zero).Individual lines were bundled together and later multiplexed together to form larger units and higher bandwidth. For example, 24 DS0 lines are bundled together to form a T1, two T1’s to form a T1c, two of those to form a T2 (= 4 x T1), and seven T2’s to form a T3 (with 672 DS0s).Although the 64Kbps bandwidth of a DS0 is a worldwide standard, the exact sizes of the bundles differ slightly among the three main “spheres of influence” carved out by telephone monopolies in North America (T), Europe (E), and Japan (J). For example, E1 = 30 x DS0, E2 = 4 x E1, (but 3 x E1 is approximately = T2), E3 = 4 x E2, and E4 = 4 x E3 (or 3 x T3). The J1, J1c and J2 are identical to the T carriers of the same number, but J3 = 5 x J2, and J4 = 3 x J3. The hierarchy as a whole is known as the “Plesiochronous Digital Hierarchy” (PDH).
Table H.1 North American PDH (partial)
Carrier
data rate
DS0sDS0 0.064Mbps 1 DS1 1.544Mbps 24 DS2 6.176Mbps 96 DS3 44.736Mbps 672 DS4 274.176Mbps 4032 T1 and E1 are roughly the same class of connection in terms of available bandwidth. The T1 line rate is 1.544 Mbps, which consists of 24 DS0 (1.536 Mbps) plus signalling. The line rate for E1 is 2.048 Mbps, with no room left over for signalling. Instead, the first channel (timeslot 0, in E1) is used for framing information. In some E1 framing schemes, channel 16 is also used for signalling. On an E1 line using PCM-30 framing, for example, channels 1-15 and 17-31 are available for user data, and these 30 DS0 provide 1.920 Mbps for data.T1 signalling and framing
T1/E1 lines, whether whole or fractional, use time division multiplexing (TDM) to send multiple channels over a single pair of wires (one pair in each direction). Each DS0 has its own time slot.T1 is a synchronous communications medium and accurate timing is absolutely crucial to performance. Devices take their timing information from the network.At the most rudimentary level, each time slot in a T1 line allows the transmission of 192 bits, plus a framing bit at the beginning of each time slot or “frame.” The word “frame” is in quotes here because this “slug of data filling a time slot” bears little or no resemblance to anything that would be recognized as a packet or frame on a LAN. It has one bit and a very precise synchronization between the sender and receiver to distinguish it from the rest of the voltage pulses on the line. When a connection is first made, it can take a moment to establish the framing!More sophisticated framing schemes introduced in the 1970s and ’80s allowed the condition of the link to be monitored. The first such scheme was the Superframe (SF) which groups 12 DS0 channels together. A further improvement was made with Extended Superframe (ESF) which groups all 24 channels of the T1 together.As mentioned in the previous section, framing and multiframing on E1 lines is somewhat different. Framing information is carried in the first channel (channel 0). Depending on the framing in use, additional signalling information may be carried in channel 16, and/or additional error checking may be included.Link Layer of T1
In chronological order of development, the three most important link layer protocols on the T1 WAN are High-level Data Link Control protocol (HDLC), Frame Relay, and Point to Point Protocol (PPP). PPP is sometimes consdiered an extension of HDLC, and in fact, HDLC, in one form or another, is a part of nearly every WAN protocol.The ISDN link layer protocol is specified in Q.921, with aspects of the network layer specified in Q.930. Call set-up signalling on the D Channel is covered in Q.931. Essentially, ISDN uses some number of DS0s as B channels (bearer channels), to carry the user data, and a single D channel (data channel) to carry call set-up, control, and related signalling.X.25 was an early effort to create a public data network using the existing telephone network. It emphasizes reliable delivery at the expense of data throughput. Having the Link Layer handle acknowledgements and the retransmission of any packets that remain unack’ed is a drag on throughput in X.25. The X.25 modulo 128 variant permits more data to be in flight unack’ed, and so improves throughput somewhat.All of these link layer protocols make use of HDLC, or contain implementations of HDLC. The next section looks at a basic HDLC frame, and at Cisco’s widely used implementation of HDLC.HDLC and Cisco HDLC
The HDLC frame begins and ends with identical flags (0x7E). To prevent confusion between a flag and other data within the frame, any data containing more than five consecutive one bits has a zero bit inserted after the fifth one bit. Any zero appearing in the frame after five one bits is stripped out at the receiving end.The Address field in most implementations contains nothing more than a statement of which end of a point-to-point link sent the frame. Some implementations may use this field for other things (addressing to a particular station in a multi-drop environment, for example), and the Address field can be extended to two bytes. When the address field is actually used to distinguish among possible recipients, stations ignore frames which do not contain their address.Figure H.2 HDLC frame structure, in Asynchronous Balanced Mode (ABM)The Control field is normally one byte, and describes the type of frame: Information (I), Supervisory (S), or Unnumbered (U). Information frames carry higher protocols such as TCP/IP. Supervisory frames handle flow control. Unnumbered frames are used to set up and tear down links and for miscellaneous functions.The 3-bit N(R) element in the Control field for I-Frames and S-Frames is the sequence number of the received frame. The N(S) element in the Control field of I-Frames is the sequence number of the sent frame. When the Control field is extended to two bytes, the additional space is either used to extend the possible length of the sequence numbers, or it is reserved. These elements support reliable data transmissions in HDLC under ABM (Asynchronous Balanced Mode). ABM is “asynchronous” because nodes do not have to wait until a scheduled moment in order to communicate, and “balanced” because either end may initiate a conversation.The P/F bit in the Control field is the Poll/Final bit. This is a legacy of the mainframe computing environment in which HDLC’s predecessors evolved. Primary stations use this bit to force a response from secondary stations, and secondary stations use it to indicate that they are finished transmitting to the primary station.The Code elements are used to send messages about the state of the transmission. Examples include: Receive Ready (RR), Receive Not Ready (RNR), Reject (REJ) and Selective Reject (SREJ). In combination with an included sequence number, these commands allow a modest degree of flow control.A two-byte FCS-16 frame check sequence value appears immediately before the final flag byte.HDLC formed the basis of many subsequent protocols, and is implemented in a variety of forms in many other protocols, including PPP, X.25, and Frame Relay. It heavily influenced the Link Access Procedures such as LAPB (…Balanced), LAPD (…on the D Channel, for ISDN), LAPF (…Frame), and LAPM (… Modem). HDLC also provided a model for the IEEE 802.2 LLC (Link Layer Control) protocol.Cicsco HDLC
The original HDLC offered no clue as to the higher layer protocol it might be carrying. The address feature was meaningless on a link with only two ends, and the control features were redundant with those of higher layer protocols such as TCP. Cisco’s implementation of HDLC addresses all of these issues and greatly simplifies HDLC.In Cisco HDLC (cHDLC), the Address field is always one byte and takes only one of two values: 0x0F (unicast), or 0x8F (broadcast). This refers only to the encapsulated protocol data, as cHDLC does not support multi-drop. The Control field is one byte, set to zero. The Cisco implementation does not support HDLC windowing. A new two-byte Type Code field is added after the Control field to support the Ethertype code, naming the encapsulated protocol. The rest of the frame-flags and FCS-remains the same.Figure H.3 Cisco HDLC frameTroubleshooting
The following are some examples of how to interpret the LEDs on the T1/E1 Pod to troubleshoot potential problems.LEDs on T1/E1 Pod
- The Signal light on the front panel is not illuminated. This could have several causes:
- If no lights are illuminated, check that the power cord is plugged into the electrical outlet.
- Check that the correct cables are used and that they are properly connected.
- Check that the router (with the internal CSU/DSU) is properly configured to the correct channels.
- The Signal light on the front panel is illuminated, but the Framing light is not (when using framed T1/E1 data links).
- Check that the router is in agreement with the carrier service agreement (line provisioning) and that the propechannels are configured.
- The Alarm light is illuminated (RED):
- If this is the Alarm light on the CPE side, check the timing configuration of the router.If this is the Alarm light on the Network side, check with the service provider for this network–something may be wrong with the T1/E line.
Tables
Supported protocols and physical interfaces
T1/E1 Pod RJ-48 pin connections
Table H.4 T1/E1 Pod RJ-48 pin connections
RJ 48 Connector
CPE jack
Network jack1 Tx Rx 2 Tx Rx 3 Rx Tx 4 Rx Tx Glossary of terms
WAN Addresses and Names
WANPeek NX recognizes three types of addresses: physical addresses, logical addresses, and symbolic names assigned to either of these.Physical addresses
The concept of an “address” on a Wide Area Networks (WAN) is quite different from that on a LAN or the Internet. Designed to work within the circuit-switched, point-to-point model of the telephone network, many WAN protocols (such as HDLC and PPP) have an extremely limited address function. Depending on the implementation, this may amount to little more than distinguishing between two ends of a circuit. Even when a larger address space is implemented, it is more in support of a multi-drop (primary to multiple secondaries) local environment, and has little to do with uniquely identifying a particular node in the vastness of global telecommunications.The first major effort to create a uniform standard for packet switching (as opposed to circuit switching) over the existing telephone network was X.25. Even X.25 essentially sets up a telephone call to create a virtual circuit to its destination. ISDN also sets up its own calls, and offers some point-to-multi-point capabilities.Frame Relay comes closer to the familiar model of the Internet, in that frames are routed through the switch fabric over a variety of possible paths to their destination. Frame Relay itself does not specify that destination as a fixed address, however. Higher level protocols must do that.Frame Relay frames do contain a value called the Data Link Connection Identifier (DLCI). This identifies a connection to a neighboring device. In most Frame Relay networks, a DLCI value has only local significance and may be reused at other places in the network. DLCIs may be of different lengths, depending on the particular implementation of Frame Relay in use on a particular network. In the most common implementations, there are about 1,000 available DLCI values (apart from those reserved for signalling, for future use, and so forth). A single customer may be assigned multiple DLCIs, allowing them to use a single line to establish multiple simultaneous connections to the Frame Relay switch fabric or “frame cloud.”WANPeek NX treats DLCI values as physical addresses, but any analogy with an Ethernet MAC address is bound to be at least slightly misleading. DLCI identifies a connection, and only one DLCI value appears in a Frame Relay frame. Every frame on that particular connection is identified by that DLCI. By knowing which end of the conversation was assigned the DLCI and adding direction information, it is possible to see that a particular frame is being sent either from that DLCI or to that DLCI, but there is no need for either end of the conversation to mention any other DLCI value.The Packets view of the Capture window in Figure I.1 shows DLCI values in the Source and Destination address columns.Figure I.1 DLCI values displayed in a Capture windowLogical addresses
A logical address is a network-layer address that is interpreted by a protocol handler. Logical addresses are used by networking software to allow packets to be independent of the physical connection of the network, that is, to work with different network topologies and types of media. Each type of protocol has a different kind of logical address, for example:
- an IP address (IPv4) consists of four decimal numbers separated by period (.) characters, for example:
130.57.64.11 an AppleTalk address consists of two decimal numbers separated by a period (.), for example:2010.42
68.12Depending on the type of protocol in a packet (such as IP or AppleTalk), a packet may also specify source and destination logical address information.For example, in sending a packet to a different network, the higher-level, logical destination address might be for the computer on that network to which you are sending the packet, while the lower-level, physical address might be the physical address of an inter-network device, like a router, that connects the two networks and is responsible for forwarding the packet to the ultimate destination.The following figure shows captured packets identified by logical addresses under two protocols: AppleTalk (two decimal numbers, separated by a period) and IP (four decimal numbers from 0 to 255 separated by a period). It also shows symbolic names substituted for IP addresses (www0.wildpackets.com and ftp4.wildpackets.com) and for an AppleTalk address (Caxton).Figure I.2 Logical AppleTalk and IP addresses and symbolic namesSymbolic names
The strings of numbers typically used to designate physical and logical addresses are perfect for machines, but awkward for human beings to remember and use. Symbolic names stand in for either physical or logical addresses. The domain names of the Internet are an example of symbolic names. The relationship between the symbolic names and the logical addresses to which they refer is handled by DNS (Domain Name Services) in IP (Internet Protocol). WANPeek NX takes advantage of these services to allow you to resolve IP names and addresses either passively in the background or actively for any highlighted packets.In addition, WANPeek NX allows you to identify devices by symbolic names of your own by creating a Name Table that associates the names you wish to use with their corresponding addresses.To use symbolic names that are unique to your site, you must first create Name Table entries in WANPeek NX and then instruct WANPeek NX to use names instead of addresses when names are available.Other classes of addresses
When one says “address,” one typically thinks of a particular workstation or device on the network, but there are other types of addresses equally important in networking. To send information to everyone, you need a broadcast address. To send it to some but not all, a multicast address is useful. If machines are to converse with more than one partner at a time, the protocol needs to define some way of distinguishing among services or among specific conversations. Ports and Sockets are used for these functions. Each of these is discussed in more detail below.Broadcast and multicast addresses
It is often useful to send the same information to more than one device, or even to all devices on a network or group of networks. To facilitate this, the hardware and the protocol stacks designed to run on the IEEE 802 family of networks can tell devices to listen, not only for packets addressed to that particular device, but also for packets whose destination is a reserved broadcast or multicast address.Broadcast packets are processed by every device on the originating network segment and on any other network segment to which the packet can be forwarded. Because broadcast packets work in this way, most routers are set up to refuse to forward broadcast packets. Without that provision, networks could easily be flooded by careless broadcasting.An alternative to broadcasting is multicasting. Each protocol or network standard reserves certain addresses as multicast addresses. Devices may then choose to listen in for traffic addressed to one or more of these multicast addresses. They capture and process only the packets addressed to the particular multicast address(es) for which they are listening. This permits the creation of elective groups of devices, even across network boundaries, without adding anything to the packet processing load of machines not interested in the multicasts. Internet routers, for example, use multicast addresses to exchange routing information.Figure I.3 Broadcast packets are processed by all nodes on the networkSome protocol types have logical Broadcast addresses. When an address space is subnetted, the last (highest number) address is typically reserved for broadcasts. For example:IP Broadcast Addresses typically uses 255 as the host portion of the address; for example:
130.57.255.255AppleTalk Broadcast Addresses use 255 as the node portion of the address:
200.255While conceptually very powerful, broadcast packets can be very expensive in terms of network resources. Every single node on the network must spend the time and memory to receive and process a broadcast packet, even if the packet has no meaning or value for that node.Figure I.4 AppleTalk broadcast and multicast packetsMulticast Address. In IPv4, all of the Class D addresses have been reserved for multicasting purposes. That is, all the addresses between 224.0.0.0 and 239.255.255.255 are associated with some form of multicasting. Multicasting under AppleTalk is handled by an AppleTalk router which associates hardware multicast addresses with addresses in an AppleTalk Zone.Ports and sockets
Network servers, and even workstations, need to be able to provide a variety of services to clients and peers on the network. To help manage these various functions, protocol designers created the idea of logical ports to which requests for particular services could be addressed.Ports and sockets have slightly different meanings in some protocols. What is called a port in TCP/UDP is essentially the same as what is called a socket in IPX, for example. WANPeek NX treats the two as equivalent. ProtoSpecs uses port assignments and socket information to deduce the type of traffic contained in packets.
Wireless LAN Standards
wireless LAN standards is divided into two sections: 802.11 radio frequency (RF) LANs and InfraRed (IR) connectivity. As of this publication of the Technical Compendium the IR sections are not yet prepared but we anticipate their publication in the future.
RF wireless LAN standards are defined in the IEEE 802.11 standard and, as such, are carried on top of 802.2 Logical Link Control. A wireless LAN transceiver, often referred to as an Access Point, acts, essentially, like a bridge: one side is wireless and the other side is Ethernet, for example.
Below is a diagram which represents the fundamental RF wireless LAN type of implementation. An Access Point is attached to the hub or switch. Notebook computers are equipped with PCMCIA wireless LAN adapters. It’s as if the notebooks were wired to the Access Point and the Access Point is acting like a multi-port bridge.
NETWORKS TERM TO KNOW :
Aggregation – A method of combining (aggregating) two or more network connections in parallel in order to increase throughput beyond what a single connection could sustain, and to provide redundancy in case one of the connections fails.
Application complexity – A measure of ease/difficulty regarding the quality of a software application in terms of (a) Functionality (b) Reliability (c) Usability (d) Efficiency (e) Maintainability (f) Portability.
Application latency – The time difference between a request packet and its first response packet with data minus the network latency.
Application performance – The measurement of real-world performance and availability of software applications.
Application Performance Management (APM) – The monitoring and management of software applications performance and availability.
Application Virtualization – The separation of the installation of an application from the underlying operating system on which it is executed. Application virtualization is layered on top of other virtualization technologies, allowing computing resources to be distributed dynamically in real-time. From the user’s perspective, the application works just like it would if it resided on the user’s device.
Capacity Planning – The planning of a network factoring utilization, availability, bandwidth, and other network capacity constraints.
Data Center Consolidation – The reduction of the size of a single facility, or merger of one or more facilities in order to reduce the IT footprint and overall operating costs.
Capacity Planning – The planning of a network factoring utilization, availability, bandwidth, and other network capacity constraints.
Data Center Consolidation – The reduction of the size of a single facility, or merger of one or more facilities in order to reduce the IT footprint and overall operating costs.
Dependency Mapping – The process of tracking and establishing the dependencies and relationships between IT components.
Dropped Packets – The condition when one or more packets of data traversing a computer network fail to reach their destination.
End User Experience – The overall interaction and level of satisfaction between an end user accessing and using a software application.
Filtering – The overall interaction and level of satisfaction between an end user accessing a software application and the application itself.
Hybrid Cloud – A mix of on-premise private cloud, and third-party public cloud services, with orchestration between the two platforms.
End User Experience – The overall interaction and level of satisfaction between an end user accessing and using a software application.
Filtering – The overall interaction and level of satisfaction between an end user accessing a software application and the application itself.
Hybrid Cloud – A mix of on-premise private cloud, and third-party public cloud services, with orchestration between the two platforms.
Hyperconverged Infrastructure (HCI) – A software-defined IT infrastructure that virtualizes all of the elements of conventional ‘hardware-defined’ systems. HCI includes, at a minimum, virtualized computing (a hypervisor), a virtualized SAN (software-defined storage), and virtualized networking (software-defined networking). HCI typically runs on commodity hardware that can scale out by adding more nodes into the deployment.
Infrastructure-as-a-Service (IaaS) – Online services that provide virtualized computing resources over the Internet.
Jitter – The variation in the delay of received packets.
Load Balancing – The distribution of workloads across multiple computing resources to optimize resource use, minimize response time, maximize throughput, and avoid overload of any single resource.
Metadata – Data that describes or provides information about other data.
Infrastructure-as-a-Service (IaaS) – Online services that provide virtualized computing resources over the Internet.
Jitter – The variation in the delay of received packets.
Load Balancing – The distribution of workloads across multiple computing resources to optimize resource use, minimize response time, maximize throughput, and avoid overload of any single resource.
Metadata – Data that describes or provides information about other data.
Microsegmentation – A security technique of creating secure zones in a data center where resources are isolated from one another so if a breach occurs, the damage is minimized.
Migration to Azure – The migration of workloads and applications to Microsoft Azure, a comprehensive set of open, enterprise-grade, cloud computing services.
Mean Opinion Score (MOS) – A measure of voice quality that provides a numerical indication of the perceived quality of the media received ranging from 1 to 5, with 1 being the worst.
Multi-Cloud Strategy – The use of two or more cloud computing services to minimize the risk of downtime or widespread data loss due to a localized component failure.
Multi-Segment Analysis (MSA) – A process that allows you to locate, visualize, and analyze one or more network flows as they traverse several capture points on your network from end-to-end. MSA provides visibility and analysis of application flows across multiple network segments, including network delay, packet loss, and retransmissions.
Netflow – A feature that was introduced by Cisco for collecting IP network traffic information and monitoring network traffic as it enters or exits an interface.
Netflow – A feature that was introduced by Cisco for collecting IP network traffic information and monitoring network traffic as it enters or exits an interface.
Network Performance Management (NPM) – The techniques used to monitor and manage the performance and availability of a computer network.
Network transaction – Stream of data traveling between two endpoints across a network (for example, from one LAN station to another). Multiple flows can be transmitted on a single circuit.
Network Utilization – The amount of network traffic compared to the maximum traffic that a network can support, generally specified as a percentage.
Network Virtualization – The process of combining software and hardware network resources, and network functionality to create a single pool of resources that make up a virtual network.
Network Visibility Framework (NVF) – A framework that breaks down the network role into simple layers, namely a data layer, a message layer, and a visibility layer. The NVF could help explain how multiple product vendors work together in a cohesive and collaborative way to deliver value-added solutions.
Non-volatile memory express (NVMe) – A host controller interface and storage protocol designed to accelerate the transfer of information between enterprise and client systems and solid-state drives (SSDs) attached via a PCI Express (PCIe) bus.
Packet loss – A condition that occurs when one or more packets of data traversing a computer network fail to reach their destination. Typically caused by network congestion, packet loss is generally measured as a percentage of packets lost compared to packets sent.
Packet loss – A condition that occurs when one or more packets of data traversing a computer network fail to reach their destination. Typically caused by network congestion, packet loss is generally measured as a percentage of packets lost compared to packets sent.
Public cloud – A type of cloud computing where services such as servers, data storage, and applications are provided by a cloud service provider and are accessible over a public network such as the Internet.
Private Cloud – A type of cloud computing that uses a secure cloud based environment operated solely for single organization.
Resource Optimization – The methods and processes used to match the available resources with the needs and goals of an organization.
Resource Optimization – The methods and processes used to match the available resources with the needs and goals of an organization.
R-factor – A measure of VoIP quality in IP networks ranging from 0 to 100, with 100 being ‘high quality.’ Any R-Factor less than 50 is not acceptable.
Software-defined networking (SDN) – An approach to cloud computing that enables cloud and network engineers, and administrators to respond quickly to the changing needs of a business via a centralized control console.
Software-defined networking (SDN) – An approach to cloud computing that enables cloud and network engineers, and administrators to respond quickly to the changing needs of a business via a centralized control console.
Server Virtualization – The partitioning of a physical server into smaller virtual servers, so that it appears as several ‘virtual servers,’ each of which can run their own copy of an operating system.
SSL Decryption – The ability to decrypt, inspect, and then re-encrypt SSL-encrypted traffic before it is sent to its destination. SSL is the industry standard for transmitting secure data over the Internet.
TCP Quality of Service – A family of Internet standards that provides the ability to give preferential treatment to selected network traffic over various technologies.
Virtualization – The creation of software-based virtual machines that can run multiple operating systems from a single physical machine.
TCP Quality of Service – A family of Internet standards that provides the ability to give preferential treatment to selected network traffic over various technologies.
Virtualization – The creation of software-based virtual machines that can run multiple operating systems from a single physical machine.
Virtual Machine (Guest VM) – The ‘guest’ component of a virtual machine (VM), an independent instance of an operating system (guest operating system) installed and running on the VM.
Visibility-as-a-Service (VaaS) – A broad concept that enables IT organizations to access network traffic across their entire infrastructure on demand, whether it resides in the cloud, remote office, campus, or data center.
VLAN – Virtual LAN. A group of logical devices on one or more LANs grouped together and configured to communicate as if they were attached to the same wire, when in fact they are from a number of different LAN segments.
VoFi – Voice over WiFi. A technology that merges Voice over IP (VoIP) over WiFi wireless networks.
VoIP – Voice over Internet Protocol. A technology for delivering voice communications and multimedia sessions over Internet Protocol networks, such as the Internet.
WAN – Wide Area Network. A communications network or computer network that extends over a large geographic area such as cites, states, or countries.
WAN – Wide Area Network. A communications network or computer network that extends over a large geographic area such as cites, states, or countries.
WLAN – Wireless Local Area Network. A wireless computer network that links two or more devices using high-frequency radio waves within a limited area, and often includes an access point to the Internet.
XO___XO ++DW Guide to instrument and control buses
Key concepts GPIB and serial buses are the most common instrument interfaces. Bridge products convert equipment from one bus type to another. The future of instrument control will be that of a mixed I/O world. General purpose interface buses (GPIBs), also called (IEEE 488), and serial buses have been around for more than 20 yr and are the most common types present in instruments.
Key concepts
- GPIB and serial buses are the most common instrument interfaces
- Bridge products convert equipment from one bus type to another
- The future of instrument control will be that of a mixed I/O world.
General purpose interface buses (GPIBs), also called (IEEE 488), and serial buses have been around for more than 20 yr and are the most common types present in instruments. While computer technology has increased exponentially in this timeframe, buses have changed little. Recently, however, new buses, such as IEEE 1394, Ethernet, and the Universal Serial Bus (USB), have emerged as candidates for instrument interfaces.GPIB was created specifically for instrument control applications. While IEEE 488.1 focused on the electrical, mechanical, and functional specifications, IEEE 488.2 defined how controllers and instruments communicate and strengthened the original GPIB standard .GPIB is a digital 8-bit parallel communications interface with data transfers higher than 1.5 Mbytes/sec. The bus supports one system controller and up to 14 additional instruments, with more instruments being added through expanders. GPIB cables and connectors are robust and industrial graded. Cabling is limited to less than 20 m, but with expanders can serve instruments up to 2-km away.Currently, there is a proposal to improve the performance of GPIB from 1.5 Mbytes/sec to 8 Mbytes/sec undergoing standardization approval in IEEE. Instruments capable of high-speed GPIB transfers and compatible with existing instrumentation systems are based on the original GPIB standard.The RS-232 serial specification most commonly controls modems and printers and usually consists of a computer interface connected to a device to be controlled. The interface is built into most computers and is very popular for instrument control applications. However, unlike GPIB, the RS-232 inter- face can only be connected to one device at a time. Furthermore, the maximum cable length without any expanders is 15 m and has data rates of less than 20 Kbytes/sec.IEEE 1394IEEE 1394, also known as FireWire, is a high-performance serial bus developed by Apple computer in the early 1990s. The standard handles throughput rates of 100, 200, and 400 Mbits/sec. However, the IEEE 1394 trade association is revising the specification to increase this transfer rate to 3.2 Gbits/sec.IEEE 1394 offers great potential for high-speed applications. Furthermore, Intel currently does not include 1394 support in its PC peripheral chip sets. This situation means that in order to use a 1394 bus in an application, both the 1394 device and the 1394 controller must be acquired.Other features make 1394 ideal for many types of instrument control applications. While devices must be within 4.5 m of the bus socket to conform to the specification, up to 16 devices can be daisy-chained together for a maximum run of 72 m. FireWire also features "hot pluggable" technology that enables devices to be connected and disconnected while the system is powered. However, there are few instruments with 1394 ports available.
Fig. 1. The GPIB connector and its signal assignments are shown.
Ethernet
Instrument control applications over Ethernet take advantage of several characteristics of the bus, such as remote control of instruments, sharing among different users at different locations, and easier integration and publication of the resulting data. Moreover, users can take advantage of the already existing knowledge within their company on Ethernet-based networks.
Furthermore, instrument vendors have recently started to include Ethernet as an alternative communication interface on stand-alone instruments. Although Ethernet is somewhat new to instrument control applications, it is a mature technology that is widely used for measurement systems. With more than 100 million TCP/IP-capable computers worldwide, the shear ubiquity of Ethernet for controlling instruments is compelling.
Some factors to consider when deciding to use Ethernet for instrument control are transfer rate, determinism, and security. Most common Ethernet networks today are 10BaseT or 100BaseTX, transferring data at 10 Mbits/sec or 100 Mbits/sec, respectively. However, these transfer rates do not imply that your application achieves the same performance.
The nature of Ethernet — with its overhead, carrier sense, collision detection, and bus sharing by multiple devices — not only does not allow you to achieve the theoretical transfer rates, but it cannot be deterministic in assuring data transfer, depending on topology. In addition, if you have a very sensitive instrumentation system based on Ethernet, you need to take additional security measures to ensure its integrity (for example, firewalls).
Created in 1995, VXI is a standard for controlling instrumentation over Ethernet (see "Bus schedule" for explanations of acronyms). This protocol was created through the VXI plug and play consortium and defines GPIB-style communication over Ethernet. Some virtual instrument software architecture (VISA) software supports control of Ethernet-based instrumentation, and it is easy to use VXI-11 compatible instruments since you can reuse your VISA code written for other buses (GPIB, Serial, or VXI) virtually unchanged.
USB
USB was primarily designed to connect peripheral devices, such as keyboards and mice, with PCs. The USB 1.1 specification has a maximum throughput of 12 Mbits/sec. However, future USB devices will have a throughput of 480 Mbits/sec, thanks to the USB 2.0 specification.
The USB host automatically detects when a new device has been added, queries the device for its identification, and configures the drivers appropriately. The topology of the bus allows up to 127 devices to run concurrently on one port.
USB ports also come standard on today's PCs, which means that you do not have to purchase a dedicated controller to interface to a USB-based device. The standard delivers an inexpensive, yet easy-to-use, connection between devices and PCs and provides a "step up" from conventional serial port technology. The setup features faster performance, "hot pluggable" functionality, built-in operating system configuration, and thin and flexible cabling, which allows you to connect multiple devices from the same port.
Although USB has many attractive benefits, there are some less convenient factors when comparing it to other buses. USB cables are not industrially graded, which can degrade the data in noisy environments; and there is no robust mechanism to prevent them from accidentally being detached. In addition, the USB system topology allows for a maximum distance of 30-m between the controller and the device, after using repeaters.
Finally, even though there is a great potential for USB instrumentation, especially with the April 2000 release of the USB 2.0 specification, currently there is no standard protocol available for controlling instruments over USB. In addition, there are few instruments with USB ports available.
Bridge products
Due to the slow shift of instrument manufacturers to implement new buses, bridge products are emerging as a viable solution. Bridge products allow you to convert from one bus type to another one. For example, one end of a bridge product may plug into an Ethernet, USB, or FireWire port in your computer or system, while the other end connects to traditional GPIB or Serial ports in your instruments .
With bridge products, you can take advantage of the plug-and-play capabilities, ease of use, and wide availability of controllers that practically come free with new computers, while building applications by conserving your investment in hardware, software, and time. Bridge products should be designed as a transparent solution for the application.
For example, you should be able to take the code written for a GPIB plug-in controller and reuse it without any modifications if you decide to replace the GPIB plug-in controller with an Ethernet-to-GPIB bridge product.
Bridge products solve the integration of legacy equipment into systems with new buses incorporated into the computer. However, in the future, systems will consist not only of legacy equipment, but also of new instrumentation equipment with new communication buses on them. Moreover, it is possible that more than one of the buses discussed here, or even some other bus, will be present in the same instrumentation system.
This scenario leaves engineers with the task of implementing mixed I/O systems. And the best way to design such systems is to base them on the appropriate software architecture capable of handling multiple buses without adding complexity to the equation. VISA has this capability.
Instrument control software
The virtual instrument software architecture (VISA) specification was created by the VXI plug-and-play Systems Alliance in 1993 to serve as a common application programming interface (API) to control VXI, GPIB, and Serial instruments (Fig. 3). VISA then became the preferred API used in the development of instrument drivers with thousands written in different development environments. With VISA, users could be more productive since they only needed to learn one API instead of three separate, very different ones.
Today, with the potential for new instrument control buses to be part of systems, there is an even higher demand for a common software API. Users could create code compatible with the traditional connectivity buses and upgrade their equipment to new control buses without having to modify their software. This situation helps preserve their hardware and software investments, while eliminating the learning curve of a different API. Currently, one implementation of the VISA specification supports GPIB, serial, and VXI interfaces, as well as Ethernet and Peripheral Component Interconnect eXtensions for Instrumentation, the latest, most popular connectivity options for instrumentation systems.
The new VISA design separates the support for connectivity buses from the core VISA library, which contains the popular high-level VISA API. With this model, each different bus requires a passport to connect to the core VISA engine. With this plug-in architecture, support for new buses can be added very easily without disturbing the existing interfaces.
Several buses, such as Ethernet, USB, and IEEE 1394, have great potential to be standard bus interfaces for test and measurement applications. As technology evolves, these and other buses, such as Bluetooth, will be considered for instrument control applications. On the other hand, equipment with traditional control buses will be present in instrumentation systems for many years to come.
The future of instrument control will be that of a mixed I/O world. The key relies on having a system with the adequate structure to leverage existing hardware and software investments, while at the same time incorporate and take advantage of the latest technologies.
GPIB Tutorial
Controller-in-charge — Although there can be several controllers on the GPIB, only one is the controller-in-charge (CIC). Active control can pass from the current CIC to an idle controller. Only the system controller can make itself the CIC.
GPIB signals and lines — Figure 1 is an illustration of a GPIB connector showing its signal assignments. The GPIB interface system consists of 16 signal lines and 8 ground-return or shield-drain lines. The 16 signal lines are grouped into 8 data lines, 3 handshake lines, and 5 interface management lines.
Data lines — The 8 data lines, DIO1 through DIO8, carry both data and command messages. The state of the attention (ATN) line determines whether the information is data or commands. All commands and most data use the 7-bit ASCII or ISO code set, in which case the eighth signal — DIO8 — is either unused or used for parity.
Handshake lines —Three lines asynchronously control the transfer of message bytes among devices. The process, called a three-wire interlocked handshake, guarantees that message bytes on the data lines are sent and received without transmission error.
NRFD (Not ready for data)—This signal indicates when a device is ready or not ready to receive a message byte. All devices receiving commands, listeners receiving data messages, and the talker enabling the HS488 protocol drive this line.
NDAC (Not data accepted)—This signal indicates when a device has or has not accepted a message byte. All devices when receiving commands and listeners when receiving data messages drive this line.
DAV (Data valid)—This signal indicates when the signals on the data line are stable (valid) and can be accepted safely by devices. The controller drives DAV when sending commands, and the talker drives DAV when sending data messages.
Interface management lines—Five lines manage the flow of information across the interface.
ATN (Attention)—The controller drives the ATN line true when it uses the data lines to send commands, and drives ATN false when a talker can send data messages.
IFC (Interface clear)—The controller drives the IFC line to initialize the bus and become CIC.
REN (Remote enable)—The controller drives the REN line to place devices in remote or local program mode.
SRQ (Service request)—Any device can drive SRQ to asynchronously request service from the controller.
EOI (End or identify)—This signal has two purposes. The talker drives EOI to mark the end of a message string. The controller drives the EOI to tell devices to identify their response in a parallel poll.
Physical characteristics — Devices are usually connected with a shielded 24-conductor cable with both a plug and a receptacle connector at each end. Devices can be linked in a linear configuration, star configuration, or a combination of the two.
Bus schedule
This "bus schedule" is a guide to the acronyms used within this article.
GPIB—General purpose interface bus
VXI—VME eXtensions for instrumentation
VME—VMEbus is a computer architecture. The term "VME" stands for VERSAmodule Eurocard and was first coined in 1980 by the group of manufacturers who defined it. This group was composed of people from Motorola, Mostek, and Signetics who were cooperating to define the standard. The term "bus" is a generic term describing a computer data path, hence the name VMEbus.
Actually, the origin of the term "VME" has never been formally defined. Other widely used definitions are VERSAbus-E, VERSAmodule Europe, and VERSAmodule European. However, the term "Eurocard" tends to fit better, as VMEbus was originally a combination of the VERSAbus electrical standard, and the Eurocard mechanical form factor.
PCI—Peripheral component interconnect is a high-performance plug-and-play expansion bus architecture originally developed by Intel to replace ISA and EISA. It has achieved widespread acceptance as a standard for PCs and workstations. It offers a theoretical maximum transfer rate of 132 Mbytes/sec.
PXI—PCI eXtensions instrumentation was designed as a different implementation of CompactPCI, which added the necessary features for instrumentation. The PXI specification defines how PC technology can successfully be applied to measurement and automation. By leveraging off of CompactPCI, Microsoft Windows, and VXI, the PXI specification brings together the right technologies for PC-based test and measurement, instrumentation, and industrial automation. Announced in August 1997 as an open specification, PXI has generated endorsements from numerous companies worldwide.
VISA—Virtual instrument software architecture
VXI—VME eXtensions for instrumentation was designed to improve the VME specification for instrumentation systems. Motorola and a number of other companies developed the VME standard in the late 1970s. It has been widely accepted as a backplane standard for many electronic platforms. It defines the electrical and mechanical backplane characteristics that allow a wide variety of companies to develop products to work in a mix-and-match fashion to develop electronics systems. Five leading test companies (Colorado Data Systems, Hewlett-Packard, Racal Instruments, Tektronix, and Wavetek), who formed the VXI Consortium felt there was a need for a standard that would enable anyone to develop instruments that would:
Be capable of handling demanding electronic test problems
Be open to all
Have interoperable modules (work together seamlessly)
Reduce the size of current instrumentation systems
Increase the speed of automatic test equipment (ATE) systems.
The VXIbus Specification was submitted and accepted by the IEEE Standards body in 1994.
What is Bluetooth?
Named after Harald Bluetooth, who was a fierce Viking king in 10th century Denmark, Bluetooth is a standard for wireless "connections" and communication developed by a consortium of electronics manufacturers. It allows electronic equipment, such as computers, cell phones, personal data assistants (PDAs), instruments, and control devices, to communicate and operate. Bluetooth is intended to be a standard that works on a physical standard, in that it uses radio frequency, and on a protocol standard.
Bluetooth technology seeks to eliminate cabling among electronic devices by creating an open industry standard for wireless communication. Ericsson, IBM, Intel, Nokia, and Toshiba created the Bluetooth Special Interest Group (SIG), which is responsible for driving this royalty-free, open-specification technology and bringing it to market.
Bluetooth technology is used for wireless communication of voice and data using a short-range radio. Bluetooth-enabled devices work with one another, regardless of their different functions. The Bluetooth radio is low-power and operates in the Industrial, Scientific, and Medical (ISM) Band at 2.4 GHz. Although a short-distance version operates within 10 m, a long-range version operates up to 100 m. To reduce interference from other devices, the Bluetooth specification uses spread-spectrum transmission with frequency hopping. The Bluetooth hops around 79 frequencies, 1600 times/sec. If interference causes a delay, then the data packet is re transmitted. In addition to specifying the physical RF layer, Bluetooth defines upper layer services, such as media access and discovery.
XO___XO ++DW DW Network hardware
Networks are created when two or more computers are connected. Files are sent over a network as data packets. Networks can be made in different topologies.
Networking hardware
Computers need networking hardware in order to connect to each other. Routers, hubs, switches and bridges are all pieces of networking equipment that can perform slightly different tasks. A router can often incorporate hubs, switches and wireless access within the same hardware.
Routers
A router can form a LAN by connecting devices within a building. It also makes it possible to connect different networks together. Homes and businesses use a router to connect to the internet. A router can often incorporate a modem within the hardware.
Modems
A modem enables a computer to connect to the internet over a telephone line. A modem converts digital signals from a computer to analogue signals that are then sent down the telephone line. A modem on the other end converts the analogue signal back to a digital signal which another computer can understand.
Hubs, bridges and switches
Hubs, bridges and switches allow multiple devices to connect to the router and they transfer data to all devices on a network. A router is a more complex device that usually includes the capability of hubs, bridges and switches.
Hubs
A hub broadcasts data to all devices on a network. This can use a lot of bandwidth as it results in unnecessary data being sent - not all computers might need to receive the data. A hub would be useful to link up a few games consoles for a local multiplayer game using a wired LAN.
Bridges
A bridge is used to connect two separate LAN networks. A computer can act as a bridge through the operating system. A bridge looks for the receiving device before it sends the message. This means that it will not send a message if the receiving computer is not there. It will check to see if the receiver has already had the message. This can help save unnecessary data transfers, which improves the performance of a network.
Switches
A switch performs a similar role to a hub and a bridge but is more powerful. It stores the MAC addresses of devices on a network and filters data packets to see which devices have asked for them. This makes a switch more efficient when demand is high. If, for example, a game involved lots of data being passed between machines, then a switch could reduce the amount of latency.
Wireless access points
Wireless access points (WAPs) are required to connect to a network wirelessly. WAPs are usually built into the broadband router.
Device addresses
Data packets include the addresses of the devices they are going to and coming from. Computers need a network interface card to connect to a network. All devices on a network have a MAC address.
MAC address
Every piece of hardware on a network has a unique MAC address. This is embedded in the hardware when the product is made in the factory, and the user cannot change it. On a computer, the MAC address is a unique code built into a NIC. No two computers have the same MAC address. A MAC address is made up of 48 bits of data, usually written as 12 hexadecimal characters.
Network interface card (NIC)
NICs enable desktop and laptop computers to connect to a network. NICs are small circuit boards that connect to the motherboard. Smartphones also use a GSM chip to connect to the telephone network. Games consoles contain a NIC card so users can access the internet, download games and play online.
Types of network
Client-server
The client-server model is the relationship between two computers in which one, the client, makes a service request from another, the server. The key point about a client-server model is that the client is dependent on the server to provide and manage the information.
For example, websites are stored on web servers. A web browser is the client which makes a request to the server, and the server sends the website to the browser.
Popular websites need powerful servers to serve thousands or millions of clients, all making requests at the same time. The client side of a web application is often referred to as the front end. The server side is referred to as the back end.
Peer-to-peer (P2P)
In a P2P network, no single provider is responsible for being the server. Each computer stores files and acts as a server. Each computer has equal responsibility for providing data.
In the client-server model, many users trying to access a large file, such as a film, would put strain on one server. In the peer-to-peer model, many users on the network could store the same file. Each computer can then send sections of the file, sharing the workload. Each client can download and share files with other users.
P2P is ideal for sharing files. P2P would be unsuitable for a service such as booking tickets, as one server needs to keep track of how many tickets are left. Also, on P2P networks no single computer is responsible for storing a file - anyone can delete files as they wish.
Differences between client-server and P2P networks
| Client-server | P2P | |
|---|---|---|
| Security | The server controls security of the network. | No central control over security. |
| Management | The server manages the network. Needs a dedicated team of people to manage the server. | No central control over the network. Anyone can set up. |
| Dependency | Clients are dependent on the server. | Clients are not dependent on a central server. |
| Performance | The server can be upgraded to be made more powerful to cope with high demand. | If machines on the network are slow they will slow down other machines. |
| Backups | Data is all backed up on the main server. | Each computer has to be backed up. Data can easily be deleted by users. |
Servers
A server stores data to be used by other computers on a network. A server could be a specialised machine or it can be a normal PC running server software. The server stores data and responds to requests for data or files such as web pages.
Types of server
There are many different types of server. Some popular examples follow.
Web servers
Web servers host websites and generally handle requests for static information such as HTML pages or images. They are used to manage the website on the server and often include FTP software, which can easily host and share large files. The Apache web server is a popular type of open source web server software.
Application servers
Many websites are classed as web applications which contain programming and scripts which are more complex than a static HTML page. For example, many websites use databases held in MySQL or NoSQL databases which will be accessed via the application server. The application server is generally used to organise and run the web application. A client sends requests to the web server which sends requests to the application server.
Network attached storage (NAS)
This is a server dedicated to storing and sharing files. It is useful for storing large files, such as music and video, but it is not used for storing websites.
Print servers
These make it easy for various devices to connect to a printer. This removes the need for devices to install the printer driver software or connect to the printer using cables.
Mail server
These store email messages. Your browser makes request to mail servers to retrieve the messages.
Web pages and web apps
Web pages
A web page is a document that can be viewed on a web browser. It can contain text, images, sounds, animations, videos and hyperlinks to other web pages. Most web pages are written using HTML, HTML5, XML and CSS.
Static and dynamic web pages
Web pages can be either static or dynamic.
A static website has no form of interactivity. It only uses HTML and CSS and there are no options for the user to input data.
Dynamic websites contain elements that allow the user to interact with the site. They can automatically update sections of a site based on information from other sites, applications, the user or databases. Contact forms and search boxes are basic types of dynamic interaction.
On a web page like a blog, dynamic elements might include a feed widget and RSS links to other blogs. Static elements might include the basic layout and banner for the site.
An online shop would use databases to automate the prices of products. Product details will be stored in a database on a server and the owner could update the database on the server, rather than updating the prices in HTML on a static page.
A dynamic website uses HTML and CSS but it also includes scripting languages, such as JavaScript and PHP. Scripting is a form of programming designed to execute at runtime. A dynamic website can be called a web application (or web app) because it is programmed like a software application. A software application is stored on a computer’s hard drive, but a web application is stored on a server and used through a web browser.
The client-side (or front-end) is the user interface of the application where apps are displayed in the browser using HTML, CSS and JavaScript.
API
Many web pages and applications are now highly integrated with external services around the web. For example, websites often embed functions like maps and videos. Each website does not create these functions independently - they use an application programming interface (API).
APIs make it easier for groups and organisations to share content online. Sites such as Royal Mail, Twitter, YouTube, Facebook and the BBC use APIs to make it easier for other websites to interact with their services. Google, Bing and Open Street Map have created APIs for their maps to encourage other websites to use this service.
A mashup is a website or application which mixes code from different external sources. There are many pieces of code that could be embedded into a site to create a mashup, including photo galleries, social media feeds and RSS news feeds.
Using APIs
The BBC Playlister is a quick, simple way to find and keep track of music. BBC, Spotify, Deezer and YouTube have collaborated using an API so that users can use music playlists on these different services.
Playlister uses a Spotify API. Spotify’s intellectual property rights have been made available by the Spotify Service through its API. The track metadata in the application comes from Spotify.
As well as building up their own personal playlist of tracks, users can export and listen to them on their chosen music service.
The server-side (or back-end) stores and processes the bulk of the data and source code using languages such as SQL, PHP and Python.
Client-side and server-side scripts
On a dynamic website there are client-side and server-side scripts. Client-side and server-side are sometimes referred to as front-end and back-end. The client-side of a website refers to the web browser and the server-side is where the data and source code is stored.
Different types of processing can occur at each side.
Client-side scripts
A client-side script is a program that is processed within the client browser. These kinds of scripts are small programs which are downloaded, compiled and run by the browser. JavaScript is an important client-side scripting language and widely used in dynamic websites. The script can be embedded within the HTML or stored in an external file.
External scripts are sent to the client from the server when they are requested. Scripts can also be executed as a result of the user doing something like pressing a page button.
Client-side scripts can often be looked at if the user chooses to view the source code of the page. JavaScript code is widely copied and recycled.
Server-side scripts
A server-side script is processed on the web server when the user requests information. These kinds of scripts can run before a web page is loaded. They are needed for anything that requires dynamic data, such as storing user login details. Some common server-side languages include PHP, Python, Ruby and Java. These execute like programming languages on the server.
When a server-side script is processed, the request is sent to the server and the result is sent back to the client. This is useful for websites which store large amounts of data, such as search engines or social networks - it would be very slow for the client browser to download all the data.
Client-side
Validation rules
Most validation rules will happen on the client-side. For example, if data is required for a form then a warning will be given to the user to type it in. The data will not be sent to the server until it has been entered.
Flash games
Games or animations that use the Flash plug-in will be downloaded to be used on the client-side. Once they are on your computer or device, the code will run within the web browser.
Cookies
Cookies are text files that are stored locally on the client-side. When a user visits a website, it may store a cookie on their computer – the site has to ask the user to accept permission. When they return to that website again, the web page will check for the cookie and if it finds one, it will send it back to the web server. If the user sets up an account with a website that uses cookies, it will remember their details, such as name and login details, the next time they visit (if they have not deleted their internet history). This can save them time, and they will not have to log in or remember their sign-in details every time they use the site.
Advertising
Cookies and internet history are used to influence the advertisements and other websites recommended to users. The sites pull data from lots of different sources and link it together so that advertising can be targeted more accurately at the consumer.
Social-networking sites invite users to ‘like’ things. These ‘likes’ are stored in databases and are used, along with cookies, to track users’ interests. For example, if a person frequently ‘likes’ products or services associated with sport, or if they visit sport-related websites, they are more likely to see more sport-related adverts and updates in their feed.
This is because their ‘likes’ have been checked and their cookies have been accessed in order to find and display an appropriate advert. This is called target marketing.
Responsive design
A website may be programmed to automatically adjust the layout depending on the web browser or device that is being used to access the page. For example, web pages are designed to look different on a smartphone compared to a desktop. This is so that the pages fit the different screens.
In the past, this was a server-side action, with the server delivering a completely different version of the site for desktop or mobile devices. Now, however, this is usually a client-side action – most modern sites have only a single version, with the layout adapting based on device and browser needs. This is done through the use of CSS and JavaScript.
Server-side
Accessing a database
All kinds of websites use databases to store data on the application server. For example, usernames and passwords will be stored on an application server. When a user types a username and password into a website, the web page will send the details to the server for checking into a secure database.
Information updates
News websites often include feeds that display data from elsewhere. This means that the web page will automatically update as more news comes in. RSS feeds allow data to be updated on one server and distributed to multiple sites.
Search engines
A search engine is a web application which keeps a huge database of web addresses. Web pages are stored as an index on a server. When a search term is entered, the server looks through an index of web pages that contain the term. The box that the user writes the search word into is like a form that is used to run a query in a database. Search engines use server-side processing to find and deliver the search results.
The index has already been put together by an automated program called a crawler that is run by the search engine program. The crawler frequently visits web sites and takes a record of the address and keywords and adds this information to a database with an index. The index links the website to other websites that are connected through hyperlinks.
Different search engines have their own crawler programs and algorithms so a search in one engine might produce different results to another. Web pages appear higher up a list of results because they are judged to be more important by the search algorithm. Their importance is measured by the popularity of a site and how many connections it has with other websites.
Cloud computing
Cloud computing is storing and using services online, rather than storing them locally on a device such as a hard drive. Cloud computing is becoming more popular as web browsers become more powerful and network coverage is more widely available.
It is increasingly easy to use cloud services using native apps or web apps on smartphones, tablets and desktop machines – as long as there is a web connection in range.
Cloud storage is used to store files such as documents and photos. The files are stored on a server owned by the service provider. Google Drive, Flickr and Dropbox are all examples of cloud storage.
Advantages
- Backing up - data backed up in the cloud with a reliable provider can be more reliable than storing your information on a hard drive or USB flash memory stick.
- Compatibility - documents and files are designed to be compatible across different machines and browsers.
- Cost – the user doesn’t need to buy the latest software as it might be freely accessible through web apps.
- Independence – the user can work with their files on different computers.
- Reliable software - web software and browsers are updated online. The user doesn’t have to download the latest updates.
Disadvantages
- Connection – the user can only access their information if they have a network connection.
- Copyright – the user sometimes loses legal rights to their original material if they store it online.
- Security - data stored online is vulnerable to security attacks.
- Software - web apps do not usually have as many detailed functions as a full software package.
- Storage - it is not always possible to store more than a few gigabytes online with one provider, whereas it is possible to purchase a few terabytes of physical storage to save information at home.
Ownership
Cloud computing raises issues about data ownership. Cloud services are useful, but using them means we are sharing our data with service providers. Organisations such as government departments and banks are likely to create their own cloud servers because they have extra restrictions with regards to the data they hold.
Web development languages and tools
There are a number of key web-development languages. Some are listed below.
CSS
CSS is used to edit the appearance of the HTML components. One style can be applied to multiple components, such as buttons, to make them look the same. This makes it easier to update the appearance as the code only needs to be changed once.
PHP
JavaScript
JavaScript is a programming language used for interactive elements within a web page. For example, a pop-up may appear when a word is rolled over, or the date may be displayed on a web page.
SQL
HTML
HTML5
HTML5 - the 5th version of HTML - has the ability to control interactive media like games and video without the need for third party plug-ins (software that is added to an Internet browser). However, HTLM5 is not supported on older browsers.
HTML
HTML is a mark-up language used to set out what should be placed on a web page. Mark-up languages are not programming languages.
Web pages are stored as HTML documents.
HTML can do several things:
- set where text should be placed
- describe how text should look
- create hyperlinks
- embed images
- create sections within the page
Tags
HTML uses tags to tell the browser how the page should be displayed in a browser. There are many different tags used within web pages.
- tags start like this:
<tagname> - tags usually end like this:
</tagname>, although some can self-close
Some important HTML tags:
<head>- States where the page begins and ends.<title>- States the title of the web page.<body>- States where the page content begins and ends.<p>- States where new paragraphs of text begin and end.<h1>- Creates headers, such as the main title of the page.<img>- Embeds images. These tags can self-close.
Viewing HTML code
To see how a web page has been coded, use the ‘View Source’ option in the browser. HTML can look complicated at first, but by learning the basics you should be able to spot elements that you understand.
Creating HTML pages
It is possible to create HTML pages on a desktop computer and view them in a browser - but they will not be on the internet unless they are uploaded to a server. HTML pages can be written in plain text editors, such as Notepad, and saved as an HTML file.
Blog sites such as WordPress, Tumblr and Blogger are free platforms that allow users to customise their blogs by editing HTML.
e- key NETWORKS
Computer networks have many benefits but they also create Real and Virtual risks. By connecting computers it is possible to share personal data easily. However, it makes computers more vulnerable to interference from other people.
Anyone sharing personal information, eg bank details, wants to be sure that they are safe and secure. Most businesses and organisations employ network managers or administrators to manage the e- Key of their networks.
Anti-virus software
There are a number of malicious software (‘malware’) programs that can cause damage to computers. These include viruses, worms, zombies, Trojan horses (Trojans) and Spybots.
Anti-virus software is designed to detect and block attacks from malware. Some operating systems have their own inbuilt anti-virus software.
In a large organisation, a network manager should make sure that all the computers under their control are secure and the anti-virus software is up to date.
Firewalls
A firewall is software that will block unexpected connections coming in to the network. Most operating systems include a firewall.
e- Key passwords
When more than one person uses a network it is important to have user IDs and passwords. Only someone with a login and password can access that network. It also helps the network manager trace unusual activity to a specific user.
A weak password makes it easy for someone to try to guess your login details. A good password will have a mix of upper case and lower case letters, numbers and special characters.
Access levels
In a large company or school, many people will be using computers on the same network. A network manager will normally control the level of access people have to the network. General users will not have the ability to download any software they want or to make changes to any part of the system, as that could affect other users.
The more people have access to sensitive parts of the network, the more likely it is that a hacker or a virus might be able to cause damage.
You can set user access levels on your home computer. For example, a parent may prevent a child from being able to install software.
Encryption
Any message sent over a network can be intercepted. Encryption is a method of changing the original numbers and characters so that they are hidden or disguised. This is important if you are sending sensitive information.
One method of encryption is the Caesar Cipher algorithm. In this method, each letter of the alphabet is simply replaced by another letter in the alphabet that might be one or more letter positions away.
For example, encrypting using letters which are +5 positions away would change the original text ‘Bitesize’ into cipher text ‘Gnyjxnej’.
The prefix for some web addresses is https instead of http. The ‘s’ indicates that it is a secure website and any payment or personal details that are inputted into the site will be encrypted.
Network policies
Network policies provide rules and guidelines on what network users can and cannot do. Most networks in large organisations will have network policies in place.
Acceptable Use Policy (AUP)
An AUP states how the network may be used – what is and is not acceptable. If you are going to let people use your network, then you should have an agreement in place which states the rules and guidelines for users.
Archiving
Networks often have to store a lot of data. A good archiving strategy takes old, non-essential data and puts it somewhere safe. An archiving policy will help decide when to move data, where it should be kept, and how to get it back if needed.
Network e- Key failures
e- Key measures need to be high on the agenda for any company that uses and relies on networking technology.
Cyber attacks
Cyber attacks and cyber terrorism are ways of attacking companies and organisations online. There are frequent cyber attacks on the government and businesses in the UK.
Hacking
Hackers attempt to break into networks to steal private information.
In 2011, Sony’s PlayStation Network was hacked and security details for thousands of users were released.
IT in e- Key specialists are employed by companies to hack their own networks and find e- Key faults or weaknesses. These ‘white hat hackers’ (as they are sometimes called) must be experts in their field and have knowledge of the latest techniques being used by illegal hackers. By attempting to hack into a network, they can spot e- Key Shifting flaws and offer advice on how to fix them.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
e- Bridge Communication on Good and Sincere
Tidak ada komentar:
Posting Komentar