Wide-Area Network
Data networks that reach across large distances (across states or even continents), known as wide-area networks or WANs, have different requirements and network packet-switching technologies than those that span much shorter distances (within a single building or room) characterized by local-area networks or LANs. The differentiating factor separating the two types of networks is more of technology than distance, because there is no set maximum distance for WANs. WANs, therefore, are usually maintained by public telecommunication companies in countries around the world where WANs are in place. WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data-link layer, and the network layer.
WANs operate at slower speeds than LANs. Typical data passthrough rates range from 1.5 Mbps to 155 Mbps, and seem like a snail's pace compared to LAN speeds of up to 2 Gbps for a typical LAN network. LANs and WANs provide a balance in data networks because the advantage of one is a disadvantage in the other, and vice-versa. For example, while LANs send data at much faster rates, they cannot span large distances; WANs can span great distances, but send data at much slower rates (and with delays of up to several tenths of a second due to the need for WANs to bounce signals off of earth-orbiting satellites).
WANs are designed to provide reliable, fast, and secure communication between nodes on the network. Companies with locations all over the globe can have one integrated network by using various communication strategies and technologies. Because the public telecommunications organization of each country is heavily involved with WAN development, one cannot discount the political, technological, and strategic decisions that can and do influence WAN development.
Basic WAN Varieties
The most fundamental type of WAN is called the leased line, which is a telephone line that has been leased for private use. In some contexts it is referred to as a dedicated line. A leased line is usually contrasted with a switched line or dial-up line.
Companies rent leased lines from the telephone company to interconnect different geographic locations in their company. The alternative is to buy and maintain their own private lines or, increasingly perhaps, to use the "public switched telephone network," or PSTN, with secure message protocols. (This is called tunneling, and it uses the Internet as part of a private secure network.)
Packet-Switched WANs
First appearing in the 1960s, the Packet-Switched WAN is the foundation for all contemporary communication networks. Packet-Switched Data Networks, or PSDNs, transfer data between network nodes in small packets (a packet contains a few hundred bytes of data and carries information that informs the network hardware where to send it), where software at the receiving node re-assembles the packets into a file. However, overloaded packet-switched networks force computers on the network to wait before they can send additional packets. Despite this problem, PSDNs are very popular because of their low cost (many computers sharing bandwidth, thus requiring fewer connections) and performance.
Frame Relay and Integrated Services Digital Network (ISDN)
Frame Relay is thought to be the next major leap in the X.25 packet-switching protocol. Frame Relay offers faster communication, equivalent to a T-3 line (44.736 Mbps compared to a T-1 line at 1.544 Mbps), and an overall better communication protocol. Frame Relay is a point-to-point service that replaces leased lines.
Integrated Services Digital Network, or ISDN, is a fully digital network service that can send most types of data (voice, image, information) to every node in an ISDN (meaning every house). This service is more popular in Europe than in the U.S.
WAN Devices
A WAN switch is a multi-port internetworking device used in carrier networks. These devices typically switch traffic such as Frame Relay, X.25, and SMDS and operate at the data link layer of the OSI reference model.
A modem is a device that interprets digital and analog signals, allowing data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to a format designed for transmission over analog communication facilities. At the destination, these analog signals are returned to their digital form.
A channel service unit/digital service unit, or CSU/DSU, is a digital-interface device that adapts the physical interface on a data terminal to the interface of a data circuit-terminating switch in a switched-carrier network. The CSU/DSU also provides signal timing for communication between these devices.
An ISDN terminal adapter connects ISDN Basic Rate Interface (BRI, which is a level of ISDN service intended for the home and small enterprise) connections to other interfaces. A terminal adapter is essentially an ISDN modem.
The Future of WAN
Since the world's largest WAN is the Internet, it only seems natural to assume that the future of WAN lies with a wireless Internet. Indeed, the current hype about wireless Internet does exceed the reality, but the next generation of wireless devices, called 3G (for 3rd generation), will combine voice and Internet capabilities into wireless handsets, promising faster access and increased capacity for many wireless products.
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