Question

Explain the CCITT hierarchical structure of switching and routing using block schematic.

Answer 1

Hierarchical network are capable of handing heavy traffic where required, and at the same time use minimal number of trunk groups. A 5-level switching hierarchy is recommended by CCITT as shown in Fig. In a strictly hierarchical network, traffic from subscriber A to subscriber B and vice-versa flows through the highest level of hierarchy, viz. Quaternary centres in Fig. A traffic route via the highest level of hierarchy is known as the final route. However, if there is a high traffic intensity between any pair of exchanges, direct trunk groups may be established between them as shown by dashed lines in Fig.. These direct routes are known 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 Fig. The first choice 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 final route are also indicated in Fig. A hierarchical system of routing leads to simplified switch design. Three methods are commonly used for deciding on the route for a particular connection:

1. Right-through routing.

2. Own-exchange routing.

3. Computer-controlled routing. 

In right-through routing the originating exchange determines the complete route from source to destination. No routing decisions are taken at the intermediate routes. In the absence of a computer, only a predetermined route can be chosen by the originating exchange. However, there may be more than one predetermined route and the originating node may select one out of these, based on certain like time of the day, even distribution of traffic etc. Own-exchange routing or distributed routing allows alternative routes to be chosen at the intermediate nodes. Thus the strategy is capable of responding to changes in traffic loads and network configurations. Another advantage of distributed routing is that when new exchanges are added, modifications required in the switch are minimal. Computers are used in network with common channel signaling (CCS) features. In CCS, there is a separate computer-controlled signaling network. With computers in  position, a number of sophisticated route selection methods can be implemented. Computer based routing is a standard feature in data networks. A strictly hierarchical network suffers from serious drawback i.e. its poor fault tolerance future.