A wide area network (WAN) is a telecommunications network, usually used for connecting computers, which spans a wide geographical area, such as between different cities, states, or even countries. WANs typically are used by corporations or organizations to facilitate the exchange of data between their computers in dispersed offices. Across all industries, most large corporations with facilities at multiple locations use WANs, and even small businesses with just two remote sites increasingly use WANs. Most WANs link two or more local area networks (LANs), and the Internet is in essence a very large WAN.
Although WANs serve a purpose similar to that of LANs, WANs are structured and operated quite differently. The user of a WAN usually does not own the communication lines that connect the remote computer systems but instead subscribes to a service through a telecommunications provider. Unlike LANs, WANs typically do not link individual computers, but rather are used to link LANs in what are known as internetworks, using devices called routers and remote bridges. WANs also transmit data at much slower speeds than LANs, most commonly at about 1.5 megabits per second (Mbps) or less, instead of the tens, hundreds, or even thousands of Mbps achieved by LANs. WANs are structurally similar to metropolitan area networks (MANs), but are typically slower and provide communications links for distances greater than 50 kilometers.
WANs have existed for decades, but new technologies, services, and applications have developed over the years. WANs were originally developed for digital leased-line services carrying only voice, rather than data. As such, they connected the private branch exchanges (PBXs) of remote offices of the same company. WANs are still used for voice services, but are used most heavily for data and, recently, also for images, such as video conferencing. WAN usage is growing, as more companies have installed LANs and as more affordable internetworking equipment has become available.
WAN technology is diverse and complex. There are any number of ways to establish a network connection between two points, ranging from systems based on traditional metal wires or cables to fiber-optic or radio-frequency communications. In addition to the connection medium, there are a variety of networking protocols to choose from based on the kinds of networks being connected and the capacity and reliability requirements for the data.
WANs are either point-to-point, involving a direct connection between two sites, or operate across packet-switched networks, in which data is transmitted in packets over shared circuits. Point-to-point WAN service may involve either analog dial-up lines, using a modem to connect the computer to the telephone line, or dedicated leased digital telephone lines, also known as "private lines." Analog lines, which may be either part of a public-switched network or leased lines, are suitable for batch data transmissions, such as nonurgent order entry and point of-sale transactions. Dedicated digital phone lines permit uninterrupted, secure data transmission at fixed costs. Point-to-point WAN service providers include both local telephone companies and long distance carriers, such as AT&T and MCI WorldCom.
A leading type of point-to-point WAN technology in North America is TI, which is based on a method of dividing a digital line service with a rate of 1.544 Mbps into 24 channels of 64 Kbps each. By modern standards this rate is relatively slow compared to LAN technology and compared to the increasing corporate demands placed on LANs. The cost of setting up and leasing a TI (or the faster T3) represents a sizable outlay for companies. In the early 1990s nearly all WANs used Ti or other leased lines, which are leased from a telecommunications carrier, but this changed rapidly as cheaper and faster alternatives emerged. Many observers have forecast the sharp decline of leased-line services once infrastructure is in place to better support the newer, less expensive alternatives. Other point-to-point services available include fractional Tl, T3, dataphone digital services, switched 56 Kbps, integrated services digital network (ISDN), and asymmetric digital subscriber line (ADSL). ADSL and similar DSL technologies drew a great deal of attention from corporate network managers in the late 1990s because they were considerably less expensive than leased lines—as much as 60 percent less—and delivered similar or better performance.
In addition to their typical high costs, another drawback to point to-point configurations is that they aren't well suited to accommodate mobile users, e.g., business travelers. Since the services are linked only to specific locations, companies must find alternative modes of network access for mobile users. Packet-switched networking provides this ability, among other benefits.
WAN technologies that rely on public networks via packet-switching, a method of encoding data into small, uniquely identified chunks known as packets, have been increasingly popular over the past decade. Two of the most important packet-based technologies are frame relay and asynchronous transfer mode (ATM).
Frame relay is the older of the two, coming into general use in the early 1990s. It was in large part a replacement to the slower X.25 standard that had been around since the mid-1970s. Most frame relay WANs are hosted by commercial network operators that charge flat rates based on the speed of service or volume of data required. Supported by relatively inexpensive networking hardware, frame relay is based on establishing a logical or virtual circuit across a network with another computer. In frame relay, the packets, or frames, of data may vary in size, and no attempt is made to correct errors. This latter feature is based on the assumption that frame relay is run over relatively high quality, digital networks and the data is less susceptible to errors. This also improves speed since the network protocol isn't trying to correct the data. The stability of this connection allows frame relay service providers to guarantee a certain minimum level of service. The comparative low cost and high quality of service made frame relay one of the most popular WAN technologies in the 1990s.
ATM services, which were introduced commercially in the mid-1990s, are based on similar principles. Many have touted ATM as a break-through technology, but as of the late 1990s it had only a modest impact on the WAN market. ATM employs a concept called cell relay to transmit data. Cells are uniformly sized, small packets of data; in the case of ATM, just 53 bytes each, including a 5-byte header. By contrast, a frame relay packet may range up to several thousand bytes. As with frame relay, ATM transfers data over a defined virtual path rather than allowing packets to follow any number of paths to their destinations, as occurs in TCP/IP protocols used in Internet applications. This highly stable connection lends itself to video and other applications that require a steady, predictable flow of data. The drawbacks to this certainty are that the constant level of service may be low compared to other options, and ATM may not be well equipped to manage short-term spikes in demand for network resources.
Fiber-optic connectivity for WANs is another major research and growth area. Fiber optics, which involves sending light signals through glass or plastic fibers, can support very fast and extremely high quality data transfer. Most fiber-optic networks use one form or another of packet switching technology, such as ATM.
One emerging standard in this arena is synchronous optical network (SONET), a set of protocols adopted by the American National Standards Institute (ANSI) for high-bandwidth fiber-optic networking. The international analog to SONET is known as the synchronous digital hierarchy (SDH). While its technical merits have been lauded by some, others note that the economics of SONET are less appealing. It has proven expensive to implement, and some critics claim it wasn't designed properly to handle heavy data traffic that businesses want such services for. Nonetheless, large corporations with heavy throughput requirements have begun to hook up to SONET-based services.
A competing, and more economically compelling, standard is dense wavelength division multiplexing (DWDM). DWDM is a method for efficiently sharing signals over a fiber by altering the portion of the light spectrum for each different stream of data. By using each fiber more efficiently, DWDM allows significantly higher bandwidths for data than SONET, which is based on time division multiplexing (TDM), or allotting time to each separate stream of data on a fixed rotation. This translates into substantial cost savings on hardware as well. DWDM technology was being rapidly deployed by a number of network operators because of such advantages.
While the Internet and other networking developments have changed the face of WANs and have threatened some older forms of WAN technology, a number of experts believe they will become more important rather than less, as trends like globalization and telecommuting create new demand for high-power long-distance networking. Demand for network bandwidth will continue to swell. One technology forecast saw corporate WAN traffic rising by as much as 30 percent a year through 2002, and much of this traffic will increasingly be routed through public networks using virtual private network technology rather than the closed private networks of the past.
[ Heather Behn Hedden ]
Held, Gilbert. Internetworking LANs and WANs: Concepts, Techniques, Methods. 2nd ed. New York: John Wiley & Sons, 1998.
Machlin, Robert. "The Internet: Redefining the WAN." Telecommunications, January 1998.
Sullivan, Kristina B. "Companies Reach for High-Speed Alternative to Leased Lines for Access to the LAN." PC Week, 16 November 1998.