When possible, it is better to use V5.2. A single V5.2 interface group can expand from one to 16 E1 circuits (up to 494 simultaneous calls) and provides redundancy on the two E1 circuits carrying the data links. V5.2 also supports concentration so that more subscribers can be provisioned than there are available timeslots between the gateway and the central office switch. V5.2 supports up to 32,768 subscribers per interface group, but in practice, about a 4:1 subscriber to available timeslots ratio is used. With standard E1 PRI (not using NFAS), there is no redundancy in the data links. Each E1 has its own data link which uses up timeslot 16 (which is available for calls on all E1 interfaces in a V5.2 interface group except on primary and secondary E1 carrying the data links). Additionally, E1 PRI does not support concentration. The only reason to use E1 PRI would be if V5.2 service could not be provided by the local switch.
This was first published in April 2004