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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 5. Relay

Relay is the other transport method for passing modulated data over IP networks. However, relay uses a much different process than passthrough for establishing communication between fax, modem, and text devices.

Although passthrough digitizes the modulated waveform using pulse code modulation (PCM), relay decodes the modulated waveform and then passes the decoded frame over IP using a relay protocol. Because the modulated waveform sent by a fax or modem is simply carrying digital information, relay strips off this modulation and passes just the original binary information. On the other side, this binary information is remodulated into an analog waveform. Figure 5-1 provides an overview of a relay call.

Figure 5-1. Relay Call Overview ...

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