Practical Industrial Data Networks:Design, Installation and ...

Appendix B – Basic terminology 331B.10.1B.10.2Point-to-pointPoint-to-point transmission takes place between a single pair of stations, normallyrelatively close to each other. An example of this is a home intercom system with twostations.MediatedA message is sent from one station to another but because of the distance, there areintermediate stations. However, there is only one possible path and the signal isforwarded from station to station along the way. Each station along the way handles andcould possibly modify the message before it is retransmitted.Figure B.17Mediated transmissionB.10.3SwitchedThis is the same as mediated transmission, with the difference that there are multiplepaths between source and destination. The nodes in between can therefore switch (i.e.divert) incoming traffic and send it on to an appropriate node. The word ‘switch’ in thiscontext has the same implication as a ‘switch’ (a set of ‘points’) on a railroad track.There are essentially two basic modes of switching, namely circuit switching andpacket switching.Circuit switchingIn a circuit switching system, a continuous connection is made across the networkbetween the two different end points. This is a temporary connection which remains inplace as long as both parties wish to communicate; that is until the connection isterminated. All the network resources are available for the exclusive use of these twoparties whether they are sending data or not. When the connection is terminated thenetwork resources are released for other users.The advantage of circuit switching is that the users have an exclusive channel availablefor the transfer of their data at all times while the connection is made. The obviousdisadvantage is the cost of maintaining the connection when there is little or no data beingtransferred. Such connections can be very inefficient for the bursts of data that are typicalof many computer applications.A telephone call is a good example of a circuit-switching system.