LOCAL AND LONG-DISTANCE NETWORKS
The United States has a two-level hierarchy: the local exchange carrier (or LATA [local
access and transport area]) and the interexchange carrier network. Our concern here is the
interexchange carrier network, which is synonymous with the long-distance network. So
the question remains: how many hierarchical levels in the long-distance or toll network?
There will be "trandem" exchanges in the network, which we will call transit exchanges.
These switches may or may not be assigned a higher hierarchical level. Let us assume
that we will have at least a two-level hierarchy.
Factors that may lead to more than two levels are:
Telephone density, usually per 100 inhabitants
Long-distance traffic trends
Political factors (such as Bell System divestiture in the United States, privatization
in other countries)
The trend toward greater use of direct HU (high-usage) routes tends to keep the number
of hierarchical levels low (e.g., at two levels). The employment of dynamic routing can
have a similar effect.
We now deal with fan-out. A higher-level exchange, in the hierarchical sense, fans out
to the next lower level. This level, in turn, fans out to still lower levels in the hierarchy.
It can be shown that fan-outs of six and eight are economic and efficient.
Look at this example. The highest level, one exchange, fans out to six exchanges in the
next level. This level, in turn fans out to eight exchanges. Thus there is connectivity to 48
× 6), and if the six exchanges in the higher level also serve as third-level
exchanges, then we have the capability of 48
+ 6, or 54 toll exchanges.
Suppose that instead of one exchange in the highest level, there were four intercon-
nected in mesh for survivability and improved service. This would multiply the number
of long-distance exchanges served to 48
× 4 = 192, and if we use the 56 value it would
× 4 = 224 total exchanges. In large countries we deal with numbers like this. If
we assign a long-distance exchange in each NPA, and assume all spare NPA capacity is
used, there would be 792 NPAs in the United States, each with a toll exchange. Allow
for a three- or four-level hierarchy and the importance of fan-out becomes evident.
Figure 8.4 shows one-quarter of a three-level hierarchy network, where the top level
is mesh connected with four transit exchanges.
The fan-out concept assumes a pure hierarchy without high-usage routes. HU routes
tend to defeat the fan-out concept and are really mandatory to reduce the number of links
in tandem to a minimum.
Network Design Procedures
A national territory consists of a large group of contiguous local areas, each with a toll/toll-
connecting exchange. There will also be at least one international switching center (ISC).
In larger, more populous countries there may be two or more such ISCs. Some may call
these switching centers gateways. They need not necessarily be near a coastline. Chicago
is an example in North America. So we now have established three bases to work from:
1. There are existing local areas, each with a long-distance exchange.
The term telephone density should not mislead the reader. Realize that some "telephone lines" terminate in a
modem in a computer or server, in a facsimile machine, and so on.