Call Routing is defined as the process that is used to route the telephone calls across the public switched telephone network. The process used is same as that of the call that is made between the phones in the same locality or between two continents. Whenever a call is made for routing, the number called or the destination number is dialed by the calling party into their own terminal. The number called has two numbers, a prefix that identifies the geographical location of the number called or destination phone. In that prefix there is a unique number that specifies the destination terminal. However, if the call is made between two phones in the same locality which means that they lie in the same telephone exchange then the prefix added may be removed.
When a call is received by the by the telephone exchange there are two methods that can be applied:
The destination terminal may be connected directly to that exchange so that the call which is placed down that connection and the one at the destination rings.
Secondly, the call may be placed to an exchange in the neighbor and then through a connecting trunk is sent for onward routing.
Each of the telephone exchanges in the chain makes use of pre-computed tables used for routing which determines the connected exchange that the onward call should be used for routing. They also have several alternative routes that can be used for routing the call to any given destination and this is selected dynamically in case of congestion or link failure. There are several factors on which the routing tables are generated and are based centrally on:
Network Topology
The plan of numbering and
The traffic data analysis
These factors are then downloaded on the telephone operator’s network of each of the exchange. The hierarchical nature of the plan used for numbering and on the geographical basis most of the made calls can be routed based on the prefix with the help of the routing tables. There are a few calls that cannot be routed based on the prefix like that of the freephone calling or the toll free numbers and these are non-geographic numbers. For routing these types of calls Intelligent Network is used for routing the calls instead of the routing tables which are pre-computed.
When routing the calls several precautions need to be taken care of like determining the plan for routing, the proper care and attention to be paid when routing so that no two routes overflow each other mutually. This if not taken care may cause congestion that can cause complete blockage to a particular destination. The Braess’ paradox says that the addition of a shorter, new and low cost route can result in an increase in the overall congestion and hence, the network planner should take proper care when they are designing the path for routing. A good strategy should be followed to find out the spare capacity in any network.
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