Telcom Terminologies

ISDN : Integrated Service Digital Network
=> 1.544Mbps line
=> Based on T1/E1
=> Carry DSO signal

BRI : Basic Rate Interface for SOHO
2B + 1D

PRI : Primary Rate Interface for large enterprise
T1: 23B + 1D
E1: 30B + 1D

PRI channels are typically used by medium to large enterprises with digital PBXs to provide them digital access to PSTN.

PRI is service and T1 is the hardware used for delivery. For eg. TE420 is one of the T1 hardware for PRI.

B - Channel : B stands for Bearer; It's an ISDN channel that carries data
D - Channel: D stands for Delta; It's an ISDN channel that carries control and signaling information


CSU/DSU : Channel Service Unit/Data Service Unit
=> It is a digital interface to connect DTE device, like router, to a digital circuit (eg. T1/E1 line)
=> It operates at physical layer of OSI model.