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Digital Communications
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.
The
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.
This yields: 2*4K = 8K Samples per second.
The most common method for denoting the level of the wave is called PCM.
These methods divide the level into 256 levels (8 bits).
Thus, if sampling 8K times a second, each sample in the range of 0-255 we need
8K * 8 = 64K bits per second per voice line.
Multiplexing
and Synchronization
There are two problems
that we need to solve:
- We would like to be able to transmit more than just 64Kb/s
- The receiving end should know where in the bit stream is the
beginning of a new 8 bit number.
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.
Multiplexing is a way of sending several (indeed - many) channels over a single
line. This is done by using TDM - Time Division Multiplexing. Suppose we
have 32 channels, each with a rate of 64Kbs, that we
wish to transfer to the other end. The multiplexer takes from each of the 32
lines a single byte and sends them one after the other. After doing so, it
takes the next byte from every channel, and so on.
Clearly, if we don't want bytes to get lost, the multiplexer must be able to
send all the 32*8 bits from the 32 channels without the second byte of the
first channel getting lost.
This implies that the output rate of the multiplexer should be at least 32*64Kbs
or 2048 Kbs. This method is called Time Division
Multiplexing (TDM) because the multiplexer took the 1/8000 sec needed for
transferring a single byte of a single channel, and divided it between the 32
channels by increasing the rate so that each byte of a channel will take
1/(8000 * 32) sec to send.
Here
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?
Synchronization
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.
If we multiplex several 32 channels together, more synchronization bits are
added for distinguishing between the different groups.
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.
Standards
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.
The
most important ones are E1 which is used mainly in
Although both standard start with a single channel rate being 64Kbs those
channels are still incompatible because of the different ways by which the
voice was digitized.
E1
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.
T1
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:
+-------+---------------+---------------+|
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