Q. Describe CCITT  hierarchical  structure  of  switching  and  routing  using  block schematic. 

Ans:

Hierarchical network are capable of handling heavy traffic where required and at the same instance use minimal number of trunk groups. A 5-level switching hierarchy is recommended by CCITT as displayed in Figure. In a strictly hierarchical network, traffic from subscriber A to subscriber B and vice-versa flows through highest level of hierarchy, as follows. Quaternary centres in Figure. A traffic route via the highest level of hierarchy is called the final route. Though if there is a high traffic intensity between any pair of exchanges, direct trunk groups may be established between them as displayed by dashed lines in Figure. These direct routes are termed as high usage routes. Wherever high usage routes exist, traffic is primarily routed through them. Overflow traffic, if any, is routed along hierarchical path. No overflow is permitted from the final route. In Figurethe first choice routing for traffic between subscribers  A  and  B  is  via  high  usage  route  across  the  primary  centres.  Second and third choice route and final route are also indicated in Figure. A hierarchical system of routing results in simplified switch design. Three methods are generally used for deciding on the route for a particular connection:

 1.  Right-through routing.

2.  Own-exchange routing.

3.  Computer-controlled routing.