PSTN Hierarchy

(How is a phone call handled)





equested destinations for phone calls are indicated to the central office (CO) switch by dialing a series of numbers (pulse or tone dialing). When the phone call is placed it will be handled by PSTN in the following way:


I  The phone call first arrives to the central office via a local loop, it is then processed in this local central office (also called end office or Class 5 office).

This office has a switch that does the following:

1-     Processes incoming calls.

2-     Determines the best path to the call destination.

3-     Establishes circuit connection.



II  Intra-LATA calls (not local but still within the same LATA) are handled by the local carrier (RBOC) since they are technically considered as long-distance calls.

If the local CO does not have a direct trunk to the destination CO, the call will be routed through a Tandem Office.

Tandem office: establishes intra-LATA circuit and handles billing procedures for long distance calls.



III  Inter-LATA calls are turned over from local carries to long-distance carrier like AT&T, MCI, U.S.Sprint. So, in this case the central office (CO) will merely forward the placed call to the local switching office of the long-distance carrier (called POP office) (also known as Class 4 toll center)

POP: Point-of-Presence in a specific LATA for a long-distance carrier.



IV  If a Class 4 center cannot find a direct link to the destination Class 4 center, then the call will be handed over to the next level of switching up in the hierarchy (to Class 3 primary center).

The goal in switching here is to keep the call processing as low on the hierarchy as possible for the following reasons:

1-     Quicker call establishment and handling.

2-     Maximization of cost-effective use of the lowest and least expensive switching offices possible (i.e. trying to use the least expensive and less number of switching offices).

The higher in the hierarchy means more routing flexibility to larger areas and more cost will be paid.



V  Class 3 primary center will in its turn hand it over to Class 2 sectional centers if there is no direct trunk.


VI  And finally Class 2 will hand it over to Class 1 regional center.