BSNL is the gateway to external world.All ur STD and ISD calls are routed through BSNL. when u have asked for 8E1s from BSNL that means ur exchange will be connected to the gateway at BSNL through 8 pysical links.These links are physical trunks connected between ur exchange and BSNL. 1PCM cable will carry only one E1. As i said earlier one E1 has 32timeslots, out of which 2 timeslots are used for signalling and rest 30 timeslots are used as traffic chanels. so one E1 can carry 30 speech channel at a time.