Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

A WAN is generally an extension of an internal network into the wide area using private circuits such as T1 lines or virtual circuits in cell and packet switched networks such as ATM and frame relay. WANs links geographically dispersed offices in other cities or around the globe. Because WANs have been built with private leased lines, bandwidth has traditionally been low and costs have been high, which required careful monitoring and filtering of traffic between sites. WANs can also be constructed across the Internet by implementing VPN (virtual private network) technology. With VPNs private circuits are emulated in the form of encrypted tunnels from one site to another.

Dedicated leased lines (circuit-oriented) such as T1 lines are still common, although expensive. The advantage of leased lines is that they are private-no one else shares the line. An alternative is available with packet-switched networks such as frame relay, ATM, and the Internet. Many users share the networks, which helps lower costs.

Dial-up lines can provide an economical WAN connection in a number of scenarios. For example, a dial-on-demand line can be used to provide additional bandwidth when an existing dedicated leased-line WAN link becomes overburdened. These lines can be multilinked as discussed under "Inverse Multiplexing" and "MLPPP (Multilink PPP)."

Broadband communications is usually considered to be any link with transmission rates above dial-up lines. Broadband transmission systems typically provide channels for data transmissions in different directions and by many different users. Typical broadband communication systems are outlined here and discussed elsewhere.

• ISDN (Integrated Services Digital Network)    A circuit oriented service operating at 64-Kbit/sec or 128-Kbit/sec data channel. Primary rate ISDN provides additional bandwidth in increments of 64 Kbits/sec.


• X.25    An early packet-switching protocol still used for many low-bandwidth requirements (credit card authorization).


• ATM (Asynchronous Transfer Mode)    A cell-switched any-to-any virtual circuit service.


• Frame relay    A frame-based any-to-any virtual circuit service.


• Leased lines T1, T3    A dedicated leased line TDM (time division multiplexing) service. See "TDM Networks."


• DSL (Digital Subscriber Line)    A high-speed circuit-oriented service that runs over the local loop.


• Broadband wireless    A variety of wireless broadband services are now available or under development. See "Wireless Broadband Access Technologies."