INTRODUCTORY TOPICS IN TELECOMMUNICATIONS
A star network.
A higher-order or multiple-star network. Note the direct route between 2B
note another direct route between 3A
City there is a traffic relation. On that relation we'd probably expect thousands of erlangs
during the busy hour.
Figure 1.9d shows a hierarchical network. It is a natural outgrowth of the multiple star
network shown in Figure 1.9c. The PSTNs of the world universally used a hierarchical
network; CCITT recommended such a network for international application. Today there
is a trend away from this structure, or, at least, there will be a reduction of the number
of levels. In Figure 1.9d there are five levels. The highest rank or order in the hierarchy
is the class 1 center (shown as 1 in the figure), and the lowest rank is the class 5 office
(shown as 5 in the figure). The class 5 office (switch), often called an end office, is the
local serving switch, which was discussed above. Remember that the term office is a
North American term meaning switching center, node, or switch.
In a typical hierarchical network, high-usage (HU) routes may be established, regard-
less of rank in the hierarchy, if the traffic intensity justifies. A high-usage route or
connectivity is the same as a direct route. We tend to use direct route when discussing the
local area, and we use high-usage routes when discussing a long-distance or toll network.
Rules of Conventional Hierarchical Networks
. One will note the back-
bone structure of Figure 1.9d. If we remove the high-usage routes (dashed lines in the
figure), the backbone structure remains. This backbone is illustrated in Figure 1.10. In the
terminology of hierarchical networks, the backbone represents the final route from which
no overflow is permitted.
Let us digress and explain what we mean by overflow. It is defined as that part of the
offered traffic that cannot be carried by a switch over a selected trunk group. It is that
type of traffic that met congestion, which we called blockage above. We also can have
overflow of a buffer (a digital memory), where overflow just spills, and is lost.