Explain the CCITT hierarchical structure of routing using block schematic.

In right-through routing the originating exchange finds the whole route from source to destination. No routing decisions are occupied at the intermediate routes. In the deficiency of a computer, only a predetermined route can be selected by the originating exchange. Though, there may be more than one the originating node and predetermined route may select one out of these, depends on certain as time of the day, still distribution of traffic etc.

Own-exchange routing or distributed routing permits alternative routes to be selected at the intermediate nodes. Therefore the strategy is able of responding to changes in traffic loads and network configurations. The other advantage of distributed routing is that while new exchanges are added, modifications needed in the switch are minimal.

Computers are utilized in network with common channel signaling (CCS) features. There is a separate computer-controlled signaling network, in CCS. With computers in position, a number of complicated route selection methods can be implemented. Computer based routing is a standard characteristic in data networks. A strictly hierarchical network suffers with serious drawback that is its poor fault tolerance future.