North American Digital Telephone Hierarchy
To take advantage of merits of TDM and digital transmission, the common carriers employ a hierarchy of multiplexing.
North American Digital hierarchy
T1 Carrier System
T1 carrier systems were designed to combine PCM and TDM Techniques for the transmission of 24 64Kbps channels with each channel Capable of Carrying Digitally encoded voice band telephone signals or data. The transmission bit rate (line speed) for a T1 carrier is 1.544 Mbps.
All 24 DS-0 channels combined has a data rate of 1.544Mbps, this digital signal level is called DS-1. Therefore T1 lines are referred as DS-1 lines.
DS and T Line rates
T2 Carrier System
T2 carriers time division multiplex 96 64-Kbps voice or data channels into a single 6.312 Mbps data signal for transmission over twisted pair copper wire upto 500 miles over a special metallic cable.
T3 Carrier system
T3 carriers Time division multiplex 672 64-kbps voice or data channels for transmission over a single coaxial cable. The transmission rate is 44.736 Mbps.
T4 Carrier System
T4 carriers time division multiplex 4032 64-kbps voice or data channels for transmitting over a single T4 coaxial cable upto 500 mile. The transmission rate is very high i.e. 274.16Kbps.
T5 Carrier System
T5 carriers time division multiplex 8064 64Kbps voice or data channels and transmit them at 560.16Mbps over a single coaxial cable.