WAN – Wide Area Network

A WAN network is a network which covers big geographical area. Usually, every WAN consists of a number of LAN networks. Although a WAN network could be privately owned or rented, the term usually connotes to a public network. Because the distance between two client computers in a WAN network is big, it is not a rare occasion local telephone companies to be involved in providing the network transfer between the clients. They also have the ability to offer a number of services to their users. WANs exist to provide high-speed transfer, fast and safe communication between two places at low prices. But, since most of the WANs are developed by the local PTT’s, their price is dictated by the company policy.

There are few WAN technologies which are implemented today:

• Leased Lines
• Packet Switched WANs
• ATM, ISDN and Frame Relay

Leased lines type of connection is pretty old. It is one of the first solutions of the network providers. It works as point-to-point connections. That means that every computer must be connected, through a link, to every other computer which it would like to communicate with. In the past, these links were , simply, a cables connecting the devices. But they were were bandwidth-consuming, and the traffic was too noisy. So, nowadays links are build as a digital point-to-point connections, using one channel with defined bandwidth. For a long time, because of the type of the connection, these networks were build using the star configuration, in order to lower the number of lines needed. Later, circuit switched networks were build, but they never did a major breakthrough. This is a leased line network picture:

Packet Switched WANs presented the most used network technology. In Packet Switched WANs (PSDN – Packet Switched Data Network), the data is transferred into small packets. Every node (computer, device) of the network is physically connected to more then one other node. So, a fully connected network, build of several nodes, could be made by connecting each node to one physical link. This type of connection also saves a lot of network bandwidth.

These are pretty new technologies. Since ATM is discussed in the LAN section of this handbook, we will concentrate on ISDN and Frame Relay only. ISDN stands for Integrated Services Digital Network. It is a fully digitalized network, a system of digital phone connections. It also allows data, as well as voice, to be transmitted through. The traffic is controlled through channels. The voice and the data are carried through the Bearer channel (B channel), with a bandwidth of 64 kbps, although it can be limited to 54 kbps. There is also Data channel (D channel) which handles signaling. It occupies bandwidth of 16 or 64 kbps. There are two types of ISDN services:

• Basic Rate Interface (BRI)
• Primary Rate Interface (PRI)

Every BRI has two 64 kbps B channels and one 16 Kbps D channel. The total speed is 144 Kbps. It is designed to meet the requirements of most users. PRI is dedicated to users with greater demands. It has 30 B channels and one 64 Kbps D channels. The total speed is 1984 Kbps. In order to access BRI service, every user should subscribe to an ISDN phone line. If the user is distanced more then 5.5 km of the provider, expensive repeater devices must be installed.

Frame Relay is a packet-switching connecting protocol for WAN devices. It supports data transfer rates at T-1 (1.544 Mbps) or T-3 (45 Mbps). T-1 is a dedicated phone transfer line, consisting of 24 channels, each one supports 64 Kbps. T-3 has 672 channels, each one supports 64 Kbps. Frame relay works by putting the data in units called frames. It is fast network technology, and when an error is detected into a network is simply dropped, not handled. The end points are responsible for retransmitting dropped frames. Although not recommended, voice and video transfer is also possible through Frame relay networks. Frame Relay relays packets at the data link layer of the OSI model. Frame Relay is popular because usually its price is low and affordable.