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Default Fax Relay Configuration for H323 and SIP

Example 10-1 shows the default fax relay configuration for H.323 and SIP. At this point, no fax relay commands have been configured on the voice gateway. However, even though there are not any visible fax-specific commands in the configuration, default commands are present, and a fax call can be handled by the gateway.

Example 10-1 Default Fax Relay Configuration for H.323 and SIP

<snip>
	voice service voip is not present in the configuration when its options do not differ from the defaults. Default fax relay commands under voice service voip include the commands fax protocol cisco and fax-relay sg3-to-g3.
	iial-peer voice 100 voip
! This H.323 dial-peer includes the default fax relay commands of fax rate voice, ! fax protocol system, and fax-relay sg3-to-g3 system
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. The command incoming called number is commonly used for this purpose.
	ial-peer voice 200 voip
! This SIP dial-peer also includes the default fax relay commands of fax rate voice,

Example 10-1 Default Fax Relay Configuration for H.323 and SIP (Continued)

! fax protocol system, and fax-relay sg3-to-g3 system destination-pattern 200 session protocol sipv2 session target ipv4:1.1.1.2

So, you can see in Example 10-1 that although fax commands might not appear in the configuration file itself, fax relay and a few of its options are enabled by default. Basically, the default fax relay configuration for H.323 and SIP IOS voice gateways is as follows: Cisco fax relay is the fax relay protocol, the maximum fax speed is limited to the voice codec bit rate, and the SG3 message suppression feature is enabled. The only exception to this is for the 5300, 5400, and 5850 IOS gateways using the NextPort DSP architecture. These gateways do not support Cisco fax relay, so T.38 fax relay is the default instead.

An important concept that is often overlooked when configuring dial-peers for handling fax, modem, and text calls is that of an inbound VoIP dial-peer. If a fax call is coming inbound over IP to the voice gateway in Example 10-1, an explicitly configured VoIP dial containing the proper fax settings must be matched. Otherwise, the fax call will typically fail.

The most common method for matching an inbound VoIP dial-peer is to use the command incoming called-number. This command allows you to specify the called number of the incoming fax over IP call and associate that called number with a dial-peer.

In the case of Example 10-1, this command is configured under dial-peer voice 100 voip. If dial-peer voice 200 voip also needs to handle inbound fax calls from the IP side, it will also need a command such as incoming called-number. However, more specific patterns would need to be specified for the command incoming called-number rather than the wildcard "." pattern, which is a generic "catchall" for any incoming call on the IP side. More information on call legs and matching the correct inbound VoIP dial-peer can be found in the section "Call Legs in IOS Gateways" in Chapter 12, "Troubleshooting Passthrough and Relay."

The main point to take away from Example 10-1 is that some default fax relay configuration elements in the dial-peer configuration use the system keyword. This means that the dial-peer pulls the configuration parameter from the global voice service voip configuration.

Although the Cisco IOS voice gateways are configured by default to handle the transport of fax calls using Cisco fax relay, it is interesting to note that no such default transport mechanism is in place for handling modem calls. Therefore, modem passthrough or Cisco modem relay must be explicitly enabled if modem or SG3 fax calls need to be handled by the voice gateway.

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