What is a Wan?

A Wide Area Network, or WAN, is a network of networks, or internetwork, that has a broad geographical reach. WANs link Local Area Networks (LANs) together through the use of links maintained by a public service provider. When a WAN is confined to a small geographical area such as a business park or university, it is sometimes referred to as a Campus Area Network (CAN). WANs defined by their coverage of a city are called Metropolitan Area Networks (MANs). The name WAN is often used interchangeably with CAN or MAN to indicate the multi-network aspect of the internetwork. The telephone system is a WAN. The Internet is the ultimate example of a WAN.

There are two essential aspects of WAN technology that you need to be familiar with. The first is the manner in which LANs are linked and data is transferred, the connection type. When an interconnection is high capacity, it is call a backbone; the term is also applied to any circuit within a LAN that offers high capacity. The second function is switching and routing. Routers are used throughout networks, but the routers at the boundaries of networks, edge routers, are essential to determining the characteristics of a WAN. This chapter describes the various network protocols fo the ISO/OSI Data Link layer and Session layer protocols (Levels 2 and 3).

Connections can be made over a variety of media and using a variety of different protocols. A key differentiation is whether the WAN uses the concept of a state in the form of a circuit or path and mechanism for switching paths as the need arises; this is referred to as a circuit switching network. As a rule, the need to create dedicated circuits makes this type of network more expensive than networks where virtual circuits that are constructed on the fly are used.

Alternatively, a WAN can use a stateless mechanism where only the endpoints of the connection are defined and the route or path through the system is determined by an intelligent routing function. This type of WAN is a packet switching network, a packet being an encapsulation technique for data of different types. Similar to packet switching is cell relay technology. In a cell relay network, data and its formatting and addressing are divided into small, fixed-length data called cells, which are then sent over a switching or virtual circuit.

WANs can be divided into four broad categories:

Circuit Switching. This is the type of WAN used by the phone company. It uses dedicated circuits between endpoints. There is overhead involved in provisioning the connection. Protocols that use this type of network include PPP (dial-up), ISDN, and DSL.

Packet Switching. A packet switching WAN creates virtual circuits to send packets from one host to another, which allows many systems to share the same links. Transmission can be unicast (point-to-point) or multicast (point-to-multiple points). Protocols of this type include X.25, Frame Relay, and PoS.

Cell Relay. Cell relays are similar to packet switching but use smaller fixed-length cells for data transport. The technology relies on synchronization techniques, which tend to make this slower due to overhead. The protocol most associated with cell relay is ATM.

Leased Line. A leased line is a dedicated connection between two endpoints. Because traffic must come from a defined source and go to a defined destination, these WAN links are secure, often fast, and tend to be expensive. Lease lines use Data Link protocols as their control mechanisms.

No single network type dominates all WAN technology. The mixture is a compromise of cost, distance, reliability, and complexity. As a result, a host of technologies have been employed to enable WAN connections. Many were designed for the telephone company and then adapted to provide data services. Some technologies were fresh attempts to create high-speed networks. Others aimed at providing new services while retaining backwards compatibility to older standards.