Similar to fax, modem communications have the option of both passthrough and relay transport methods. The passthrough option for modems is simply named modem pass-through, and it shares this same syntax when it is configured on Cisco IOS gateways.
Modem passthrough is also applicable to fax calls and has already been discussed throughout the previous section, "Fax Design Considerations." The technical intricacies of modem passthrough and its NSE-based switchover are discussed in the section "Modem Passthrough with NSE" in Chapter 4.
Two relay options are available on Cisco IOS voice gateways: Cisco modem relay and secure modem relay. Cisco modem relay provides an alternative transport method to modem passthrough for V.34 and V.90 modulated calls. Secure modem relay is designed for transporting the V.32 or V.34 modulation of secure telephones. Because of the unique application for the secure modem relay transport method, it is discussed separately in its own subsection.
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