T38 Fax Relay and Modem Passthrough Configuration for MGCP

A common scenario for handling regular faxes along with modems or SG3 fax devices at the same time is to use a fax relay protocol for the fax traffic and modem passthrough for the modem or SG3 fax traffic. Although this scenario can be configured for the H.323, SIP, MGCP, or SCCP voice signaling protocols using T.38 or Cisco fax relay, Example 10-5 shows how this scenario is configured specifically for an MGCP voice gateway running NSE-based T.38 fax relay and modem passthrough.

Example 10-5 T.38 Fax Relay and Modem Passthrough Configuration for MGCP

ccm-manager mgcp no ccm-manager fax protocol cisco ccm-manager music-on-hold ccm-manager config server ccm-manager config

! The command no ccm-manager fax protocol cisco disables Cisco fax relay. mgcp mgcp call-agent 2427 service-type mgcp version 0.1

Example 10-5 T.38 Fax Relay and Modem Passthrough Configuration for MGCP (Continued)

mgcp dtmf-relay voip codec all mode out-of-band mgcp rtp unreachable timeout 1000 action notify mgcp modem passthrough voip mode nse mgcp package-capability rtp-package no mgcp package-capability res-package mgcp package-capability sst-package no mgcp package-capability fxr-package mgcp package-capability pre-package no mgcp timer receive-rtcp mgcp sdp simple mgcp fax rate 14400

mgcp fax t38 nsf 000000

no mgcp fax-relay sg3-to-g3

mgcp rtp payload-type g726r16 static

! NSE-based T.38 fax relay is enabled by default as is NSE-based modem passthrough ! when the ccm-manager config option is used.

mgcp profile default

By default, the command no mgcp fax t38 inhibit is configured for MGCP. Therefore, this command does not appear in the configuration shown in Example 10-5, but it still activates NSE-based T.38 by default for MGCP.

TIP When the MGCP packages of mgcp package-capability fxr-package and mgcp default-

package fxr-package are configured along with no mgcp fax t38 inhibit, protocol-based T.38 is enabled for MGCP. Along with a supported CA like Unified CM Version 4.2(3) or 6.0, the transition to T.38 is handled by the CA within the MGCP protocol stack.

The optional fax relay commands of mgcp fax rate 14400 and mgcp fax t38 nsf 000000 also appear in Example 10-5. These commands set the fax page transmission speed to 14.4 Kbps regardless of the voice codec and force the NSF to all 0s to guard against proprietary encodings between certain fax vendors.

The command no ccm-manager fax protocol cisco disables Cisco fax relay. By default, Cisco fax relay and T.38 fax relay are enabled for MGCP with T.38 having precedence. In the case of this configuration, Cisco fax relay is disabled because only T.38 fax relay is needed and it makes for a cleaner configuration.

As shown in Example 10-5, the command mgcp modem passthrough voip mode nse enables NSE-based passthrough for MGCP. By default, this command is not enabled, but when the ccm-manager config feature is used to download the gateway's MGCP configuration directly from Unified CM, modem passthrough is enabled. Whether ccm-manager config is used or not, make sure this command appears in the mgcp section of the configuration file if the modem passthrough feature is desired.

In addition, just as with modem passthrough and the H.323 and SIP call control protocols in Example 10-2, the SG3 spoofing feature must be disabled for modem passthrough when MGCP is the call control protocol. The command that accomplishes this for MGCP is no mgcp fax-relay sg3-to-g3.

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    What is modem relay setting for fax?
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