Introduction to Data Communications
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24c. 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).

24d. 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

Introduction to Data Communications
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