packet switched network

WAN devices discussed in the previous article are incomplete because other devices may be needed, depending on the chosen WAN access technology. WAN access technologies are either circuit-switched or packet-switched.

Circuit Switched

In a circuit-switched network, communications between end devices must be established before they can communicate. A circuit-switched network establishes a dedicated circuit (or channel) between nodes and terminals before users may communicate.

An example of a circuit-switching network is an analog telephone network, where connection establishment is necessary before communication starts. When someone dials someone’s number, a switch in the exchanges along the call route is set to a continuous circuit from the caller to the called party. No one can use this circuit to tell the end of the connection. Therefore, the telephone system is called a circuit-switched network. The two types of circuit-switched WAN technology are the public switched telephone networks (PSTN) and the Integrated Services Digital Network (ISDN).

Packet Switching

Packet switching divides data into small chunks (packets) routed over a shared network. Unlike circuit switching, it does not require a circuit to be established. 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. A packet-switching system may be connectionless or connection-oriented.

In connectionless packet switching, each data packet carries full addressing information, and the switch evaluates the address to determine the packet’s destination. This system is known as a 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 forwarded over the network to a destination in order; therefore, addressing information is unnecessary. This is also known as virtual circuit (VC) switching. In a connection-oriented system, the circuit is established temporarily while a packet travels through it and then breaks down again. The frame relay is an example of a connection-oriented system.