Description Of Statistical Multiplexing; Statistical Multiplexing - Multitech MMH900 User Manual

Chapter 1 - Introduction & Description
on one end of the link and then broken up into individual components on
the other end. Someone wishing to "tap" into a muxed signal must not
only have the link protocol which is typically a proprietary version of High
Level Data Link Control (HDLC), but must also know the individual
channel assignment schemes and data formats.
1.3.1

Description of Statistical Multiplexing

A statistical multiplexer (also known as a stat mux) is a device that
allows several other devices (usually computer terminals or PCs) to
communicate over a single transmission line. Sometimes called
concentrators, they take data from different devices and combine it into a
single stream that can be transmitted, via a modem, to an identical
multiplexer at another location, where the stream is then separated back
into its original form. Physically, a mux looks like a box with a bunch of
serial ports and some LEDs. The most typical task of the MultiMux is to
connect a group of PCs or terminals at one site to a mini or mainframe
computer at another site via a single set of modems rather than using
individual modems for each PC or terminal.

1.3.1.1 Statistical Multiplexing

Statistical multiplexing is sometimes referred to as statistical time-
division multiplexing (STDM). The use of the voice-grade phone line (or
any other communications link) is based not on peak data rates, but on
effective (or average) data rates.
During the peaks, when the sum of the data rates of the channels being
served exceeds the data rate of the composite link, a statistical
multiplexer saves the excess data in buffers (in effect, allowing individual
channel buffers to expand). The buffers are emptied as soon as the
activity falls off. The proper allocation of buffer space, plus the
implementation of "flow control" and "pacing" techniques to
accommodate unusually high peaks, allows the use of composite link
speeds that are less than the sum of the individual channel speeds.
In effect, a statistical multiplexer services only active channels. However,
the efficiency thus realized is not the only benefit of the technique.
Because composite link activity need not be synchronized with the
activity on the individual channels, there is considerable flexibility in the
choice of the composite channel protocol and speed. The use of a
synchronous protocol like HDLC provides for error detection and
retransmission over the composite link. Thus, asynchronous terminals,
which have no inherent error-recovery capability, can enjoy end-to-end
data integrity.
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