DESIGN OF LONG-DISTANCE NETWORKS
The PSTN network designer should comply with this CCITT criterion, in that for
a national connection, there should be no more than four links in tandem. The reason
CCITT/ITU-T set this limit was to ensure transmission QoS. As we add links in tandem,
transmission quality deteriorates. Delay increases and we include here processing delay
because of the processing involved with a call passing through each switch. End-to-end bit
error rate deteriorates and jitter and wander accumulate. Transcontinental calls in North
America generally need no more than three links in tandem, except during periods of
heavy congestion when a fourth link may be required for an alternate route.
Numbering Plan Areas
The geographical territory covered by the long-distance network will be broken up into
numbering plan areas (NPAs). In North America, each NPA is assigned a three-digit area
code. In other parts of the world, two- and even one-digit area codes are used. NPA size
and shape are driven more by numbering capacity and future numbering requirements.
Numbering plan administrators are encouraged to design an NPA such that it coincides
with political and/or administrative boundaries. For example, in the United States, an NPA
should not cross a state boundary; in Canada, it should not cross a provincial boundary.
NPAs are also important for establishing a rates and tariffs scheme.
We know a priori that each NPA will have at least one long-distance exchange. It may
be assigned more. This long-distance exchange may or may not colocate with the POP
(point of presence).
We now have made the first steps in determining exchange location.
In other countries this exchange may be known as a toll-connecting exchange.
We have shown that the design of the long-distance network is closely related to the layout
of numbering plan areas or simply numbering areas. These exchanges are ordinarily placed
near a large city. The number of long-distance exchanges in a numbering area is dependent
on exchange size and certain aspects of survivability. This is the idea of "not having all
one's eggs in one basket." There may be other reasons to have a second or even a third
exchange in a numbering area (NPA in the United States). Not only does it improve
survivability aspects of the network, but it also may lead the designer to place a second
exchange near another distant large city.
Depending on long-distance calling rates and holding times, and if we assume 0.004
erlangs per line during the busy hour, a 4000-line long-distance exchange could serve
some 900,000 subscribers. The exchange capacity should be dimensioned to the forecast
long-distance traffic load 10 years after installation. If the system goes through a 15%
expansion in long-distance traffic volume per year, it will grow to over four times its
present size in 10 years. Exchange location in the long-distance network is not very
sensitive to traffic.
Hierarchy is another essential aspect in long-distance (toll) network design. One impor-
tant criterion is establishing the number of hierarchical levels in a national network.
This deals with how much a telephone company charges for a telephone call.
POP, remember, is where the local exchange carrier interfaces with long-distance carriers. This whole concept
of the POP is peculiar to the United States and occurred when the Bell System was divested.