Switching to VoIP

This is the Title of the Book, eMatter Edition

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

Key Issues: Circuit-Switched Telephony

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93

Think about your use of the telephone at home. Now, let’s apply the Erlang formula.

Figuring on two calls per hour with an average length of 120 seconds, we get .06

Erlangs:

( 2 cph x 120 seconds ) / 3,600 = .06 Erlangs

By finding that Erlang rating in the Erlang B table, you can see that this fictitious resi-

dential telephone setup will have about a 5% probability of blocking with one POTS

line. So, 1 out of every 20 calls will be blocked with a busy signal.

Key Issues: Circuit-Switched Telephony

In this chapter, the structure of traditional voice networks has been dissected. In a

nutshell:

• The PSTN provides global interconnectivity and calling features for its subscrib-

ers.

• SS7 is the signaling network that runs alongside the PSTN.

• Businesses use PBXs to build their own private voice networks that interface

with the PSTN using trunks.

• PBXs use a data-multiplexing technology called TDM to provide connections to

digital phones and other switches.

• A single voice call across the PSTN or a PBX requires 64 kbps of bandwidth.

• Analog, one-pair phones use FXS electrical signaling and transmit voice as an

analog waveform signal.

• Digital, two-pair phones use digital signaling and transmit voice as a slice in the

TDM bit stream.

• Extension numbering ...