Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

Components of the PSTN

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for actual transmission of digitized voice, as well as SS7 signals, but aren’t normally

used for both simultaneously.

SONET network links can be used to deliver telephone service to PSTN subscribers,

too. A common use for optical high-bandwidth dial-tone services, which are deliv-

ered using OC (optical carrier) circuits, is in call centers, where thousands of phone

calls must be connected to a central group of phone operators.

DID

Direct inward dial service is a solution that allows inbound PSTN calls to the PBX to be

routed directly to certain private extensions based on the phone number that the caller

dialed, without the need for a call transfer or a human greeting to determine which

extension each call should be transferred to. The way this works is quite simple.

When the DID trunk rings, the PBX answers and produces a dial-tone and a “wink”

signal. This is an indicator that the PBX is ready to receive digits from the CO. The

CO sends the digits that the caller dialed—usually just the last four digits—and the

PBX decides how to connect the call based on them.

DID is most useful with PRI, where many DS0 channels can be assigned as DID

channels. In this way, a single PRI trunk group can handle inbound calls to dozens or

hundreds of phone numbers using 23 channels ...