Explain the CCITT hierarchical structure of switching using block schematic.

Hierarchical network are able of handing heavy traffic where needed, and at similar time use minimal number of trunk groups. The 5-level switching hierarchy is recommended through CCITT as shown in figure. Traffic from subscriber A to subscriber B and vice-versa flows by the highest level of hierarchy, in a strictly hierarchical network and namely as Quaternary centres in figure. A traffic route through the highest level of hierarchy is termed as the final route. Though, if there is high traffic intensity among any pair of exchanges, direct trunk groups may be established among them as demonstrated by dashed lines in figure. These direct routes are termed as high usage routes. Wherever high usage routes exist, the traffic is primarily routed through them. Overflow traffic, if any, is routed along the hierarchical path. No overflow is permitted from the final route. In figure the first option routing for traffic between subscribers A and B is via the high usage route across the primary centres. The second and the third choice route and the last route are also indicated in figure. A hierarchical system of routing leads to basic switch design. Three methods are usually used for deciding on the route for an exact connection:

1. Right-through routing.

2. Own-exchange routing.

3. Computer-controlled routing.

(c) CCITT hierarchical structure