A hierarchy of
digital transmission formats that are used in
North America. The T stands for "
Trunk".
The basic unit of the T-carrier system is the DS-0, which is multiplexed to form transmission formats with higher speeds. There exist four of them: T1, T2, T3 and T4. T1 is composed of 24 DS-0s. T2 = 4*T1. T3 = 7*T2. T4 = 6*T3.
Each of the T* units can also be referred to as a DS* unit, that is, T1=DS1, T2=DS2 etc.
The T-carrier system is quite similar to, and compatible with, the E-carrier system used in Europe, but it has lower capacity since it uses in-band signalling, or bit-robbing.
Yes, this writeup is analogous to the E-carrier system one, but I can't help it, the things are quite similar :-)