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WAN is a computer network that spans a relatively large geographical area Typically arelatively large geographical area. Typically, a WAN consists of two or more LANs.

A WAN is a network usually provided by a specialised company called a network provider.

Computers connected to a wide-area network are often connected through public networks, suchoften connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internetlargest WAN in existence is the Internet.

WAN technologies are classified into two i.e. packet switching or circuit switching.

A typical WAN structure includes the following components.

Packet Switching refers to technologies in which messages are divided into packets before theymessages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming ato its destination. Once all the packets forming a message arrive at the destination, they are recompiled into their original form.

Breaking communication down into packets Breaking communication down into packets allows the same data path to be shared among many users in the network. This type of communication between sender and receiver is known as connectionless. Most traffic over the Internet uses packet switching and the Internet is basically a connectionless network.

Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologiespacket-switching technologies.

In contrast, circuit switching is the way the PSTN works in which the same path cannot be shared among users A dedicated line is allocated foramong users. A dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it is sent This is thearrive in the same order in which it is sent. This is the case with most real-time and time-sensitive data, such as live audio and video.

Packet switching is more efficient and robust for data Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.

TCP/IP stands for Transmission Control Protocol/Internet Protocol the suite ofProtocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP.

TCP/IP is built into most network operating systems nowadays and is used by the Internetsystems nowadays and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own IP protocol known as IPX, developed by Novell Netware, also support TCP/IPTCP/IP.

X.25 is an open standard WAN technology for packet-switching networks. It is one of the earliest

l d d f k h hInternational standards for packet switching. The X.25 standard was approved by the CCITT (now the ITU) in 1976. It defines layers 1, 2, and 3 in the OSI Reference ModelModel.

X.25 allow remote devices to communicate with each other over public network without the expense of individual leased lines. An X.25 network consists of a network of interconnected nodes (or packet switching exchanges) to which user equipment can connect. The end user of the network is known as data terminal equipment (DTE) and the carrier's equipment is Dataequipment (DTE) and the carrier s equipment is Data Circuit-terminating equipment (DCE) . X.25 routes packets across the network from DTE to DCE.

Data terminal equipment devices are end systems that communicate across the X.25 network. They

ll l lare usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers.

DCE devices are communications devices, such as modems and packet switches, that provide the interface between DTE devices and a PSE (Packetinterface between DTE devices and a PSE (Packet Switching Exchange), and are generally located in the carrier's (or network provider’s) facilities.

PSEs are switches that compose the bulk of the PSEs are switches that compose the bulk of the carrier's network. They transfer data from one DTE device to another through the X.25 Packet Switch N t kNetwork.

X.25 Logical connection

Frame Relay is a packet-switching protocol for connecting devices on a WAN It operates at levelconnecting devices on a WAN. It operates at level two (the Data Link layer) in the OSI seven layer model. It is used in applications such as LAN interconnection.interconnection.

The biggest difference between Frame Relay and X.25 is that X.25 guarantees data integrity and network managed flow control at the expense ofnetwork managed flow control at the expense of some network delays.

Frame Relay switches packets end to end much faster, but there is no guarantee of data integrity.aste , but t e e s o gua a tee o data teg ty

Unlike X.25, Frame Relay does not provide error checking on data at a time when transmission media is very reliable (fibre optics media era).y ( p )

Frame-relay networks: ◦ Provide error detection but not error recovery. It is

up to end devices to request a retransmission of lost packets. p◦ Can provide data transfer up to 1.54 Mbps. ◦ Have a variable packet size (called a frame) .

S t d t t f t t T 1 (1 544 Mb )◦ Supports data transfer rates at T-1 (1.544 Mbps) and T-3 (45 Mbps) speeds.◦ Frame relay can be implemented over a variety of

connection lines (56K, T1, T3)◦ Operate at the Physical and Data Link layers of the

OSI modelOSI model.

User data is sent in a frame through the network

Circuit-switched is a type of network in which h i l th i bt i d f d d di t da physical path is obtained for and dedicated

to a single connection between two end-points in the network for the duration of the pconnection.

Ordinary voice phone service is circuit-switched The telephone company reserves aswitched. The telephone company reserves a specific physical path to the number you are calling for the duration of your call. During that time, no one else can use the physical lines involved.

Leased line is a permanent telephone connection between two points set up by a telecommunications common carrier or service/network provider Typically leasedservice/network provider. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike normal g g p ydial-up connections, a leased line is always on(or active).

ISDN, which stands for Integrated Services Digital Network is a system of digital phoneNetwork, is a system of digital phone connections which has been available for over a decade. This system allows voice and data to be transmitted simultaneously across the worldtransmitted simultaneously across the world using end-to-end digital connectivity.

ISDN, voice and data are carried by bearer channels (B channels) occupying a bandwidth ofchannels (B channels) occupying a bandwidth of 64 kbps (kilo bits per second). A data channel (D channel) handles signaling at 16 kbps or 64 kbps, depending on the service type. This p p g ypseparation of data and signalling channels, known as out of band enables setting up and termination of circuits done very quickly.

The physical cable of an ISDN connection is divided into logical channels Channels aredivided into logical channels. Channels are classified as one of two types:◦ B channels are used to carry data.

D h l d l d i li◦ D channels are used to carry control and signaling information.

There are two basic types of ISDN service:yp◦ Basic Rate ISDN (BRI)

1. B Channel: Two 64Kbps2. D Channel: One 16Kbps2. D Channel: One 16Kbps◦ Primary Rate ISDN (PRI)

1. B Channel: Twenty-three 64 Kbps2 D Channel: One 64 Kbps2. D Channel: One 64 Kbps

The total bandwidth of an ISDN BRI line is 144 Kbps (two B channels and one D channel)Kbps (two B channels and one D channel). The total data transfer rate is 128 Kbps (data is sent only on the two B channels).is sent only on the two B channels).

ISDN BRI is a relatively low-cost WAN service that is ideal for the following situations: ◦ Home office or telecommuters who need a relatively

fast connection ◦ Businesses that need to periodically send data◦ Businesses that need to periodically send data

between sites (bursty traffic patterns)

ISDN BRI offers the following benefits over dial up modems and other WAN connectiondial-up modems and other WAN connection options:◦ Faster data transfer rates (128 Kbps) than dial-upFaster data transfer rates (128 Kbps) than dial up

modems (56 Kbps maximum) ◦ Faster call establishment (dial-up) than modems

Lower cost than other WAN solutions (users pay a◦ Lower cost than other WAN solutions (users pay a monthly fee plus connection charges)

ADSL is an acronym for Asymmetric Digital Subscriber Line a technology that allows moreSubscriber Line, a technology that allows more data to be sent over existing copper telephone lines (PSTN).

ADSL is asymmetrical which means it provides ADSL is asymmetrical, which means it provides higher bit rates in the downstream direction (from Telco Local Exchange to the subscriber’s site) than the upstream direction (from thesite) than the upstream direction (from the subscriber site to the Local Exchange). Asymmetric means not symmetrical.

ADSL supports data rates of from 1.5 to 9 Mbps S suppo ts data ates o o 5 to 9 bpswhen receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).

Another technology, ATM, attempts to combine the best of both worlds – the guaranteed delivery of circuit-switched networks and the robustness and efficiencynetworks and the robustness and efficiency of packet-switching networks.

Asynchronous Transfer Mode (ATM), Asynchronous Transfer Mode (ATM), designed by the ATM Forum and adopted by ITU-T, is a newer technology than X.25 and Frame Relay.

ATM is a network technology based on f i d i ll ( k f fi d i )transferring data in cells (packets of a fixed size)

unlike X.25 and Frame Relay which use variable length packets. The small, constant cell sizelength packets. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the lineassure that no single type of data hogs the line. ATM cells are a fixed length of 53 octets. Since ATM switches utilise very short, fixed-length y , gcells, they can process and switch information much faster than frame relay switches.

ATM creates a fixed channel, or route, between two points whenever data transfer begins. This differs from TCP/IP, in which messages are divided into packets and eachmessages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network, but it makes it less adaptable to

dd i t k t ffisudden surges in network traffic.

ATM creates a fixed channel, or route, between two points whenever data transfer begins. This differs from TCP/IP, in which messages are divided into packets and eachmessages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network, but it makes it less adaptable to

dd i t k t ffisudden surges in network traffic.