V5 is a family of telephone network protocols defined by ETSI which allow communications between the telephone exchange, also known in the specifications as the local exchange (LE), and the local loop.
Prior to the specification of V5 the manufacturers of exchange equipment had proprietary solutions to the problem.
These solutions did not inter-operate and meant being tied into a single manufacturer's method at each exchange.
Signalling between this point and the LE was standardised and therefore allowed a multiple vendor solution, provided the specifications were followed.
In particular the PSTN protocol was re-used and combined with ISDN to provide a service to the subscriber.