Information Technology Reference
In-Depth Information
Figure 2.44:
Multiplexing requires time compression on each input.
In the demultiplexer another buffer memory will be required. Only the data for the selected signal will be written into
this memory at the bit rate of the multiplex. When the memory is read at the correct speed, the data will emerge
with its original timebase.
In practice it is essential to have mechanisms to identify the separate signals to prevent them being mixed up and
to convey the original signal clock frequency to the demultiplexer. In time-division multiplexing the timebase of the
transmission is broken into equal slots, one for each signal. This makes it easy for the demultiplexer, but forces a
rigid structure on all the signals such that they must all be locked to one another and have an unchanging bit rate.
Packet multiplexing overcomes these limitations.
2.21 Packets
The multiplexer must switch between different time-compressed signals to create the bitstream and this is much
easier to organize if each signal is in the form of data packets of constant size.
Figure 2.45
shows a packet
multiplexing system.
Figure 2.45:
Packet multiplexing relies on headers to identify the packets.
Each packet consists of two components: the header, which identifies the packet, and the payload, which is the
data to be transmitted. The header will contain at least an identification code (ID) which is unique for each signal in
the multiplex. The demultiplexer checks the ID codes of all incoming packets and discards those which do not have
the wanted ID.
In complex systems it is common to have a mechanism to check that packets are not lost or repeated. This is the
purpose of the packet continuity count which is carried in the header. For packets carrying the same ID, the count
should increase by one from one packet to the next. Upon reaching the maximum binary value, the count overflows
and recommences.
2.22 Statistical multiplexing
Packet multiplexing has advantages over time-division multiplexing because it does not set the bit rate of each
signal. A demultiplexer simply checks packet IDs and selects all packets with the wanted code. It will do this
however frequently such packets arrive. Consequently it is practicable to have variable bit rate signals in a packet
multiplex. The multiplexer has to ensure that the total bit rate does not exceed the rate of the channel, but that rate
can be allocated arbitrarily between the various signals.
