Message Switching

Sometimes there is no need for a circuit to be established all the way from the source to the destination. Consider a connection between the users (A and D) in the figure below. The connection is represented by a series of links (AB, BC, and CD).

Figure 6: A Connection Between Two Systems A and D formed from 3 Links

When a telex (or email) message is sent from A to D, it first passes over a local connection (AB). It is then passed at some later time to C (via link BC), and from there to the destination (via link CD). At each message switch, the received message is stored, and a connection is subsequently made to deliver the message to the neighboring message switch. Message switching is also known as store-and-forward since the messages are stored at intermediate nodes en route to their destination. The figure below illustrates message switching. The transmission of only one message is illustrated for simplicity. A complete message is sent from node A to node B when the link interconnecting them becomes available. Since the message may be competing with other messages for access to facilities, a queuing delay may be incurred while waiting for the link to become available.

The use of message switching to communicate between A and D (see Figure 7).

Figure 7: Message Switching

Source: http://www.erg.abdn.ac.uk/users/gory/course/intro-pages/ms.html

The message is stored at B until the next link becomes available, with another queuing delay before it can be forwarded. It repeats until reaches its destination. The Circuit setup delays are replaced by queuing delays. Considerable extra delay may result from storage at individual nodes.