WAN devices discussed in the previous article are not complete, because other devices may be needed, depending on the WAN access technology chosen. WAN access technologies are either circuit-switched or packet-switched.
In-Circuit Switched network the communications between end devices must be established before they can communicate. A circuit-switched network established a dedicated circuit (or channel) between nodes and terminals before the users may communicate.
An example of a circuit-switching network is an analogue telephone network, where the connection establishment is necessary before communication starts. When someone dials someone number, switch in the exchanges along the route of the call is set to continuous circuit from the caller to the called party. No one can use this circuit to tell the end of connection, therefore the telephone system is called a circuit-switched network. The public switched telephone network (PSTN) and the Integrated Services Digital Network (ISDN) are the two types of circuit-switched WAN technology.
Packet switching divides data into small chunks (packets) that are routed over a shared network. It does not require a circuit to be established like circuit switching. It allows numerous pairs of nodes to communicate over the same channel simultaneously. The switch determines the links that packets must be sent over based on the addressing information in each packet. Packet switching system may be connectionless or connection-oriented.
In the connectionless packet switching each data, packet carries full addressing information and the switch evaluates the address to determine the destination of the packet. This system is known as connectionless system.
In Connection-oriented systems, the routes for packets are planned and predetermined and packets are sent in order over a predetermined route. Packets are assembled with a sequence number and then forward over the network to a destination in order, therefore addressing information is not required. This is also known as a virtual circuit (VC) switching. In a connection-oriented system, the circuit is established temporarily while a packet is travelling through it and then breaks down again. The frame relay is the example of a connection-oriented system.