A wide area network connects computers located across a wide geographical area. For example an organization may have its offices scattered over different cities and each office needs to share data with other offices. In this case computers and LANs of different sites may be connected by public communication facility (like telephone network) or private communication network to realize a WAN. A WAN for a particular organization is called enterprise-wide network. The Internet is a popular example of WAN as it connects thousands of computers and LANs around the world.

Intermediate devices like gateways and routers and transmission facilities like telephone and data networks, fiber optic links, satellites are generally used for realizing WANs. The major features of WANs are listed below.

• Multiple computers are connected together
• Computers are spread over a wide geographic area
• A WAN usually interconnects multiple LANs
• Communication links between computers are provided by telephone networks, public data networks, satellites etc.
• Links are of low capacity (that is low data rate)
• Bit error rate is higher (1 in 100,000) compared to that for a LAN.

Bandwidth: Speed and capacity of computers and communication networks can be classified by bandwidth. The speed and capacity of data networks are generally measured in bits per second (bps). Low speed local communication channels are typically used for transmission rates from 300 to 56000 bps, but can now handle up to several Mbps for ADSL connections. They are usually TUP lines commonly used for voice communications, but are also for data communications by PCs, video terminals and fax machines. SPT lines can be used for speed up to 100 Mbps for short distances. High speed broadband digital channels allow transmission rates from 256,000 bps to several billion bps; typically they are microwave, satellite transmission and fiber optic cables.

Switching: Switching techniques used for WAN are circuit switching, message switching and packet switching. Normal telephone service relies on circuit switching. In circuit switching a switch establishes and maintains a link between a sender and receiver until the communication session is completed. In message switching, a complete message is transmitted in a block from one switching node to another.

In packet switching a message is subdivided into groups called packets. In X.25 packet switching, packets are 128 characters long, while they are of variable length in frame relay packet switching. Frame Relay is considerably faster than X.25 and is used by many large companies for their wide area networks. ATM (Asynchronous transfer mode) is a fast emerging high speed cell switching network technology. An ATM system, using BISDN (Broadband Integrated Service Digital Network), organizes voice, video, and other data into fixed cells of 53 bytes and send them to their next destination. ATM networks are becoming popular for the faster high-capacity multimedia services for voice, video and other high speed applications. Currently available network technology with data rates for WAN are listed below.

Network Technologies	Typical Data Rate
DDN: Digital Data Network	2.4 kbps - 2 Mbps
PSN: Packet Switching Network	2.4 kbps - 64 kbps
Frame Relay Network	56/64 kbps - 2 Mbps
ISDN: Integrated Services Digital Network	64/128 kbps - 2Mbps
ATM: Asychronous Transfer Mode	25/155 Mbps - 2.4 Gbps