A response to Majeed’s question.
E1 and PRI are based on a 30 Channel PCM system. PCM is Pulse Code Modulation. E1 is a 2.048Mbits/s and uses TimeSlots to convey digital information. PRI is for ISDN. The D channel (Delta) is the signalling Channel. By convention, Time Slot 16 is used. PRI is used with a ISDN PABX – ISPBX.