DESIGN OF LONG-DISTANCE NETWORKS
A three-level hierarchy with initial fan-out of six and subsequent fan-out of eight. The highest
level consists of four transit exchanges, but only one is shown.
2. There is one or more ISCs placed at the top of the network hierarchy.
3. There will be no more than four links in tandem on any connection to reach an ISC.
As mentioned previously, Point 1 may be redefined as a long-distance network consisting
of a grouping of local areas probably coinciding with a numbering (plan) area. This is
illustrated in a very simplified manner in Figure 8.5, where T, in CCITT terminology, is
a higher-level center, a "Level 1" or "Level 2 center." Center T, of course, is a long-
distance transit exchange with a fan-out of four; these four local exchanges (A, B, C, and
D) connect to T.
The entire national geographic area is made up of such small segments
as shown in Figure 8.5, and each may be represented by a single exchange T, which has
some higher level or rank.
(a) Areas and (b) exchange relationships.
Of course, in the United States, T would be the POP (point of presence).
6 The entire national geographic area is made up of such small segments as shown in Figure 8.5, and each may be represented by a single exchange T, which has some higher level or rank.