Introduction to Data Communications
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16b. Local Loops

The Local Loop is the connection between the Central Office and the home or business. To every home is run 2 wires (1 pair). The pair does not go directly to the Central Office, instead it goes to those big green boxes called "Serving Area Interfaces" (SIA) that you see on the street corners. Then large multi-conductor bundles of wires go from there to the Central Office.

16c. Central Office

The Central Office provides the following functions:

  1. It supplies the battery voltage for the telephone system. The On-hook voltage is 48 Vdc +/- 2V. Off-hook voltage is -6.5 Vdc.
  2. It supplies the Ringing Generator - 90 to 120 VAC, 20 Hz, 2 sec on/ 4 sec off
  3. It supplies the Busy signal (480 + 620 Hz, 0.5 sec On/ 0.5 sec Off), Dial Tone (350 + 440 Hz) and Fast Busy (480 + 620 Hz, 0.2 sec On/ 0.3 sec Off).
  4. It has the digital switching gear that determines if the number is an Interoffice call (local) or an Intraoffice call (Toll - long distance).

16d. Hierarchical Phone Networks

The PSTN (Public Switch Telephone Network) is divided into a hierarchical network. There are 5 classes of switching centres in North America:

ClassCentre Abbreviation SymbolExamples
1 Regional CenterRC 2 in Canada: West - Regina
East - Montreal
2 Sectional CenterSC Calgary serves Alberta
3 Primary Center PC Edmonton
4 Toll CenterTC Drumheller
4bToll PointTP Rainbow Lake
5 Central Office
(Local Loop)		 CO 284-xxxx

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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Copyright Eugene Blanchard Jan 1998, January 2007