Cisco Fax Relay and Modem Passthrough Configuration for H323 and SIP
When fax and modem calls are occurring through the same gateway, it is possible to handle them using the same transport method or different methods/protocols may be used. For example, with the command modem passthrough nse for SIP and H.323 gateways, all fax and modem calls can be handled using voice-band data (VBD) and the G.711 codec.
However, fax and modem calls can also be handled through different means, too. Various passthrough and relay schemes can be used to handle fax and modem calls on the same gateway. Example 10-2 shows typical G3 fax calls being handled by the Cisco fax relay protocol while modem calls and SG3 fax calls are handled by NSE-based modem passthrough.
Example 10-2 Cisco Fax Relay and Modem Passthrough Configuration for H.323 and SIP
voice service voip no fax-relay sg3-to-g3
! For modem passthrough to handle SG3 fax calls when either Cisco or T.38 fax ! relay is configured, the fax-relay sg3-to-g3 command must be disabled, modem passthrough nse codec g711ulaw
! Notice that modem passthrough is configured globally under voice service voip.
dial-peer voice 100 voip destination-pattern 100 session target ipv4:1.1.1.1 incoming called-number .
! If a VoIP dial-peer will also be used as an inbound call leg, you must make sure ! that this peer is explicitly matched using a command such as incoming called ! number. More information on call legs and inbound VoIP dial peers can be found in ! the section "Call Legs in IOS Gateways" in Chapter 12.
! The H.323 dial-peer 100 above does not have specific fax or modem configuration ! lines defined so all fax and modem configuration information is default and ! pulled from voice service voip.
dial-peer voice 200 voip destination-pattern 200 modem passthrough nse codec g711ulaw session protocol sipv2 session target ipv4:1.1.1.2 no fax-relay sg3-to-g3 fax protocol cisco
! The SIP dial-peer 200 has specific fax and modem configuration lines that ! take precedence over anything that is globally configured under voice service ! voip. The configurations from a fax and modem perspective are identical for ! dial-peer 100 and 200 but they are configured in different ways. <snip>
In Example 10-2, dial-peer 100 is set for H.323, and the fax and modem configuration is default. This means that all settings are pulled from voice service voip. As already demonstrated in Example 10-1, this means that Cisco fax relay is already enabled. In addition, the commands modem passthrough nse and no fax-relay sg3-to-g3 have been configured globally under voice service voip. Therefore, fax calls will transition to Cisco fax relay whenever G3 fax flags are detected, whereas all other calls (modem and Super G3 fax calls) will use NSE-based modem passthrough.
TIP By default, modem passthrough will not properly handle SG3 fax calls when it is config ured at the same time as Cisco or T.38 fax relay. The reason for this is because the sg3 spoofing feature is also enabled by default and it blocks any SG3 negotiation whenever fax relay is enabled. If you would like for fax relay to handle your G3 fax traffic and for modem passthtough to handle your SG3 fax traffic at its native speeds, then you must explicitly disable the SG3 spoofing feature with the command no fax-relay sg3-to-g3.
As illustrated earlier in the book in Figure 5-15, modem passthrough will actually activate for the G3 fax call and the modem calls with the configuration in Example 10-2. The fax called terminal identification (CED) tone triggers modem passthrough before the V.21 fax flags are detected. Therefore, a normal G3 fax call will transition from voice mode to NSE-based modem passthrough to Cisco fax relay. This momentary transition to modem passthrough is not a problem. It is rarely noticed and occurs only because of the different triggers that these transport methods use.
For SIP dial-peer 200 in Example 10-2, Cisco fax relay and modem passthrough are also set, but they are configured explicitly under the dial-peer. In the real world, this would not be necessary because the same configuration is already defined globally, as discussed with dial-peer 100. However, for the sake of this example, dial-peer 200 demonstrates how the same configuration can also be set directly on the dial-peer using the commands fax protocol cisco, modem passthrough nse codec g711ulaw, and no fax-relay sg3-to-g3.
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