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Fax, Modem, and Text for IP Telephony by Gonzalo CCIE No. 4541 Salgueiro, David CCIE No. 3491 Hanes

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Chapter 4. Passthrough

When the public switched telephone network (PSTN) was initially constructed, voice communication was the primary goal. However, as data communications such as fax, modem, and text became more important, they also were made to work over the PSTN using special protocols and transport methods. Today, with VoIP taking the place of the PSTN, voice communication is still the primary objective, and specific protocols and procedures are again needed to transport fax, modem, and text communications.

One such feature that voice gateways can implement to transport modem, fax, or text telephony traffic is passthrough. This transport mechanism is the easiest and simplest way for a voice gateway to pass modulated data.

For the most part, ...

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