Datagrams and Virtual Circuits

Two basic approaches to packet switching are common: The most common is datagram switching (also known as a "best-effort network" or a network supporting the connection-less network service).This is what is used in the network layer of Internet.

Virtual Circuit Packet Networks

In virtual circuit packet switching, an initial setup phase is used to set up a fixed route between the intermediate nodes for all packets which are exchanged during a session between the end nodes (analogous to the circuit-switched telephony network). At each intermediate node, an entry is made in a table to indicate the route for the connection that has been set up. Packets can then use short headers, since only identification of the virtual circuit rather than complete destination address is needed. The intermediate nodes (B,C) process each packet according to the information which was stored in the node when the connection was established.

Enhancements to provide reliability may also be provided. Delivery of packets in proper sequence and with essentially no errors is guaranteed, and congestion control to minimise queuing is common. Delays are more variable than they are with a dedicated circuit, however, since several virtual circuits may compete for the same resources. An initial connection setup phase and a disconnect phase at the end of data transfer are required (as in the circuit-switched network). The most common form of virtual circuit network were ATM and X.25, which for a while were commonly used for public packet data networks.

A variation of circuit-switched packet networking, called Multi-Protocol Label Switching (MPLS), is used in some Internet backbones. This uses a connection-oriented datagram service to switch packets across a network forming a path between a set of IP routers. This operates below the network-layer, so that applications are unaware of when MPLS is used.

Differences between datagram and virtual circuit networks

There are a number of important differences between virtual circuit and datagram networks. The choice strongly impacts complexity of the different types of node. Use of datagrams between intermediate nodes allows relatively simple protocols at this level, but at the expense of making the end (user) nodes more complex when end-to-end virtual circuit service is desired.

The Internet transmits datagrams between intermediate nodes using IP. Most Internet users need additional functions such as end-to-end error and sequence control to give a reliable service (equivalent to that provided by virtual circuits). This reliablility may be provided by the Transmission Control Protocol (TCP) which is used end-to-end across the Internet, or by applications such as the trivial file transfer protocol (tftp) running on top of the User Datagram Protocol (UDP).

See also

Gorry Fairhurst - Date: 01/06/2011 EG3557