Once there is a digital link between two sites, there is still the question of how to transfer data between them. Different practices are used for voice, digital and video communications. This document explains some of the methods used, as well as explains terms in digital multiplexing.
Analog to Digital conversion
The human voice is a continuous signal in the range 0-4 KHz. Digital communication on the other hand, is based on discrete bits (0 and 1). Therefore, there is a need for converting the human voice into a stream of bits and vice versa.
analog to digital conversion is done by sampling the sound wave and denoting
the level of the wave by a number which is transmitted over the digital link.
The reverse process is done by creating a wave according to the received
numbers. According to Nyquist law, the minimum number
of such wave samples needed for complete reconstruction of the wave is twice
the number of the maximum frequency of that wave.
Multiplexing and Synchronization
There are two problems that we need to solve:
These two problems are addressed by multiplexing and the use of synchronization bits.
What is multiplexing ?
Clearly there is a need to
transfer much more than a single channel between two sites. However, stretching
a separate line for every channel is clearly not a good solution.
is an example of multiplexing 3 channels of 64Kbs each:
This method could be further used for increasing the number of channels yet again from 32 channels to 4*32 channels and so on. Each increase is of course accompanied by a suitable increase in the bit rate of the line.
Well, we succeeded in sending 32 channels over a single line, but how will the receiving end (the demultiplexer) know which bit belongs to which channel?
Special bits in the bit
stream are used for synchronization. These bits tell the demultiplexer
where a new 32 byte group starts so it will know how to divide the following
bits between the channels. No synchronization is needed for distinguishing
between each of the 32 channels.
Digital data and Video
The upside for transmitting digital data or video is that no analog to digital conversion is needed. Instead, the bit stream in directly inserted into the multiplexer. Video, which needs a much higher bit rate than 64Kbs is usually inserted directly into the second level multiplexer, thus allowing a bit rate of 1.5-2 Mbs.
Obviously, standards have to be made if we would like equipment from different manufacturers to work with each other. Alas, there are more than one standard.
most important ones are E1 which is used mainly in
The first hierarchy of E1
is composed of 32 channels totaling 32*64Kbs = 2048 Kbs.
Two of the channels are not used for transmitting data but for frame
synchronization and signaling.
The first hierarchy of T1 is composed of 24 64Kbs channels plus 8 Kbs used for signaling totaling 24*64 + 8 = 1544 Kbs. The hierarchies are presented in the following table: