The T-1 line (T= Transmitted) is a channel capable of transmitting at a speed of 1.544 Mbit/sec. The voice is sampled at 8 kbits/sec, so that the time occupied by a bit is s. The interval is subdivided into 24 time slots with each time slot coding 8 bits of data for a channel. Thus in each 125 s there are 192 bits. Adding one bit to mark the beginning, there are 192 bits transmitted in 125 s which gives the speed to be 1.544 Mbits/sec. The signal formed by interleaving 24 DS-0 signal is known as DS-1 signal and the corresponding transmitted signal is T-1. Proceeding further, four DS-1 signals are interleaved to give a 6.3 Mbit/sec DS-2 signal (transmitted as T-2) and seven DS-2 give rise to a 45 Mbit/sec DS-3 transmitted over a T-3 line.

Asynchronous, Plesiochronous and Synchronous Systems :

Electrical signals which are used in transmission use system clocks which are very accurate. Generally, they use clocks which are based on natural frequency of vibration of some crystals, such as quartz crystals. Though such clocks are accurate, there can be differences between different clocks used by different systems and such clocks are not synchronised. Such systems are called asynchronous .
A plesiochronous system also uses different clocks but they are accurate to within a specified tolerance. Thus the clocks have phase differences. Plesiochronous Digital Hierarchy (PDH) was the most commonly used digital transmission system before the advantage of SDH. To accommodate delays between different clocks, extra bit(s), known as justification bits may be added to the multiplexed stream. This is known as bit stuffing . These bits will have to discarded while demultiplexing.
The disadvantage of PDH is that at higher speeds, it does not allow adding or dropping channels without having to first demultiplexing the channels. For instance, consider a service provider who has to provide a 2 Mbps line to a customer from his bandwidth of 155 Mbps.He has to locate and identify frames in the 2 Mb line. For this, the service provider will have to first demultiplex the line into its 64 kbps constituents, add the customer and remultiplex the system. The Digital telephone hierarchy for the North America and Europe is shown in the following table. Japan uses a scheme which is close to that of Europe but is not identical with it. The data rates are in Mbit/sec.