Building A T1 Line Signal

The evolution of telephony networks is quite amazing. While the original telephone in itself was a big thing in the history of mankind, the real revolution came about when the telephone networks got digitized. This ushered a wide range of developments and helped make telecommunication a mass market with wide reach. The founding stone of the digitized telephone network in North America was the T1 line.

The basic principle of digitization is that any analog signal can be digitized and recovered back if it is sampled at twice its highest frequency. This theorem is known as the Nyquist Theorem and this is the foundation for all digital signals in the world. T1s are based on this very principle.

While the range of human voice varies, in most cases it lies within the frequency range of 4kHz. Hence as per Nyquist theorem it is sampled at twice the rate for digitization which gives is 8000 samples per second. Each sample is 125 microseconds long.

As long as the conversation is done in the same tone, digitization is simple. The real challenge comes when It has been observed that with 256 voltage levels most of these variations can be captured well. Hence the samples were mapped on 256 levels and coded with eight bits. The 8000 samples represented at 8 bits per sample gives the basic voice signal rate of 64kbps

One of the biggest advantages of digital signals is multiplexing. With digital signals, the voltage levels would remain constant for the duration of the pulse. So if you can sample this pulse once you know that it would be the same for a fixed duration. This meant that these 64kbps voice signals could be multiplexed.

With a T1 line, 24 such DS0 or 64kbps channels can be multiplexed. So in every T1 you can have 24 voice channels. This really changed the manner in which signals are transmitted over the network. With this concentration the capacity growth in the network was tremendous.

The voice signals are time sensitive and hence the T1 lines required some degree of synchronization. As a result the T1 frame was defined to have 24 voice channels and a synchronizing bit. Now with each of the 24 samples with 8 bits we get 192 bits. So in all the T1 frame sent 193 bits in 125 microseconds which gave the T1 lines the speed of 1.544Mbps. Even today this is a standard and core networks are built to accommodate this line rate.