T1 Quick Reference

Jim Ungar, Tom Regan

The following is a quick ref of the basic configuration options for T1s and what these configurations actually mean. It's here because it's hard to find something similar in any one particular reference. They all get way to complicated way to quickly or only give you the buzzwords.


Line Coding Method
(Bit 7) Zero Code Suppression
Bit Framing
Red Alarm
Yellow Alarm
Alarm Indication Signal

Line Coding Method

Bit Framing

A frame in T1 signaling is 24 (channels with) 8-bit bytes with an extra bit for framing and signaling information. The resultant frame looks something like this:

This frame happens 8000 times a second leading to the formula:

((8 bits * 24) + 1 info/signal bit) * 8000 times/sec) = 1,544,000 bits/sec

which is the speed of a DS1/T1. In actuality, the framing/info bit is not usable, so the formula for 'data/voice-usable bits' becomes:

(8 bits * 24) * 8000 times/sec) = 1,536,000 bits/sec

DS0s (one channel of a T1) are basically one byte position in each frame or:

8 bits * 8000 times/sec = 64,000 bits/sec

Remember, when you use (Bit 7) Zero Code Suppression you lose one bit per 8, so the above formula (for B7ZS) becomes

7 bits * 8000 times/sec = 56,000 bits/sec

There are two types of framing typically used today.



Digital Telephony, 2nd Ed., John Bellamy, Published by John Wiley & Sons, 1991

Black Box online catalog reference section

ITU G.703 Specification

ITU G.704 Specification