V5 is a set of protocols defined by ETSI by which a multiplexer in the access network of the PSTN can communicate with a telephone exchange. The protocols are designed to handle both POTS and ISDN traffic. They are based on the principle of common channel signalling where message-based signalling for all subscribers uses the same signalling channel(s) rather than separate channels existing for different subscribers.
V5 comes in two forms:
V5.1 (ETS 300 324) in which there is a 1 to 1 correspondence between subscriber lines and bearer channels in the aggregate link to the exchange. A V5.1 interface relates to a single aggregate E1 (2Mbps) link between a multiplexer and an exchange.
V5.2 (ETS 300 347) which provides for concentration where there are not enough bearer channels in the aggregate link(s) to accommodate all subscribers at the same time. A single V5.2 interface can control up to 16 E1 links at once and can include protection of the signalling channels.
