V5.1 ''(ETS 300 324)'' in which there is a 1 to 1 correspondence between subscriber lines and Bearer Channel s in the aggregate link to the exchange. A V5.1 interface relates to a single aggregate E1 (2 Mbit/s) 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.
