=[ x25-box.com ]=

So far, all the devices mentioned connect two sites together, and data
input to a given port on one site is always output from a known port at
the other site. In the case of statistical multiplexers which must exchange
addressing information anyway, these limitations can be removed.

If we consider Fig. 1.3, then the communications processors can be
statistical multiplexers. The addressing information preceding data on
the aggregates simply has to be extended to show the destination stat.
mux. as well as the port. The intervening stat. muxes. can examine the
destination field and route the data down the appropriate aggregate if it
is not for them.

To allow the user to choose the destination, rather than to have it pre-
configured by the Network Manager, it is only necessary for there to be
a dialogue between the user and the stat. mux. The user simply indicates
that all data should be preceded by the address for port X on stat. mux.
Y, and the network of processors then routes all data accordingly.

Once this expansion had been made, then stat. mux. manufacturers
quickly enhanced their machines to include a range of useful features:

• Destination Names so that the user can connect to FINANCE
DATABASE and the processor looks up the actual network
address in its configuration data.
• Hunt Groups so that, rather than trying to connect to a particular
host port, the destination processor will simply allocate the next
free port on the host.
• Mesh Networks so that there may be multiple aggregate links
allowing the data load to be balanced, and offering resilience to
faults.

These are in addition to Port Contention which is automatically provided
by the statistical multiplexing concept. This provides the ability for there
to be twenty users of ten host ports, although only ten can be working
at once.

The major drawback of the statistical multiplexer is that different
manufacturers have different techniques of performing it, so users are
locked into a particular system. This also means that there can be no
aggregate link into the host, only a multiplicity of low-speed connections.

1.3 Packet switching

In a Packet Switching Network, as the name implies, data is communicated
between the user and the service in the form of packets. Before we can
look at this type of network we therefore need to know that:

A packet is a structure containing some data to be communicated,
an indication of where the data is going to (destination address),
and an indication of where the data has come from (source address).
Having said that, it is clear that the function of the components in

НАЗАД