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branch office to a corporate headquarters, because several co-located
users can all access the computer using a single link.

Multiplexers work on three basic principles: Frequency Division
Multiplexing (FDM); Time Division Multiplexing (TDM); and Statistical
Multiplexing (Stat. Mux.).

A frequency division multiplexer works by splitting the bandwidth of
the cable into several narrow frequency ranges, and allocating a range to
each communications channel. It is therefore very like the splitting of the
airwaves into several radio channels. The technique is costly and is not
suited to the limited bandwidth of telephone circuits, and is therefore
very rarely used in data communications.

In time division multiplexing the device scans each of the channels in
turn, and passes the selected data onto the aggregate. It is like a railway
turntable which constantly takes a wagon from each of the branch rails
and passes it to the main line. This can only work if the aggregate can
handle the combined data of the channels; otherwise the multiplexer
would have to stop the flow of data on the channels.

Most TDMs allow the aggregate bandwidth to be split unequally, so a
9600 bps link could support say two 2400 bps terminals and four 1200
bps printers. The two TDMs attached to a link must be configured so
that each has the same understanding of how the time on the aggregate is
allocated. This allows the individual channels to be derived correctly at
the far end.

Statistical multiplexers work by having a buffer for each of the
channels, and they allocate the aggregate according to how full the
buffers are. Thus each terminal or host port “talks” to its own buffer,
and does not know that the data is not being transmitted at that
instant. The buffers are emptied into the aggregate at a speed relative to
how full they are. It is like the postal service emptying letter boxes; each
box is emptied regularly, but those that have a lot of letters are emptied
more frequently.

The statistical multiplexer allows the sum of the individual channel
speeds to exceed the aggregate link speed, because, when the
multiplexer is not servicing a particular channel, the channel data is
inserted into its buffer. To avoid the buffer filling, and the consequent
necessity of the multiplexer stopping the channel, the average data rate
of the channels should not exceed the data rate of the aggregate. This
means that if a terminal runs at a speed of 9600 bps, but is only sending
data one minute out of ten, then its effective data rate is 960 bps. Five
such terminals could be statistically multiplexed down a 4800 bps
aggregate.

Since there is no way of knowing which channel may be using the
aggregate at any instant, it is necessary for the sending stat. mux. to
precede each transmission with an indication of which channel the data
is intended for. This allows the receiving stat. mux. to direct the data to
the appropriate destination port. The indication forms an address which
informs the destination how to route the data.

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