Introduction to Data Communications
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In the following example:

The Hierarchical portion is seen as:

TrunkLong distance telephone cable
Toll TrunkConnects CO (Central Office) to TC (Toll Center)
Intertoll TrunkEverything above TC (Toll Center) and TC to TC
Interoffice TrunkBetween CO (Central Office)
Intraoffice Trunk Call between 2 subscribers within the same CO (284-7079 to 284-8181).

Call routing:

  1. Preferred route
  2. Second choice
  3. Third Choice

Call routing is determined by network engineering and physical location. When all lines are idle, the call routing selects the preferred route. If the preferred route is busy, then the call is routed to the second choice. Because the second choice is routed through one toll center, the charge for the call is greater than the preferred route. The third choice is used when the second choice is busy. The third choice goes through 2 toll centers and is the most expensive route.

A Central Office can have up to 10,000 subscribers: for example 284-0000 to 284-9999. Most have 4,000 to 5,000 subscribers. The Central Office bases the loading requirements on roughly 10% of the phones will be in use at any one time. The use of Internet dialup access has drastically changed this!

Introduction to Data Communications
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