Configuring DTMF Relay, Fax Relay, and Modem Relay
First Published: June 22, 2006
Last Updated: March 19, 2010
This document describes how to enable DTMF Relay, Fax Relay and Modem Relay for analog Foreign Exchange Service (FXS) ports on Cisco voice gateways under the control of Cisco Unified Communications Manager or Cisco Unified Communications Express (Cisco Unified CME).
Finding Support Information for Platforms and Cisco IOS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
RFC 2833 DTMF Digit Relay Support on SCCP Controlled Endpoints
The RFC 2833 DTMF Digit Relay feature on the SCCP analog gateway sends RFC 2833 digits through Real-time Transport Protocol (RTP) packets and out-of-band (OOB) digits to Cisco Unified Communications Manager. This feature is not supported in Cisco Unified CME or Cisco Unified SRST.
The RFC 2833 DTMF Digit Relay feature is enabled by default on the Cisco IOS gateway and on the Cisco Unified Communications Manager, where it can be disabled. No configuration is required on the SCCP analog gateway or on Cisco Unified Communications Manager to enable this feature.
SCCP Gateway Controlled Fax Relay
Before Cisco IOS Release 12.4(6)XE, SCCP gateways supported fax passthrough only. In Cisco IOS Release 12.4(6)XE and later releases, SCCP Gateway Controlled Fax Relay adds support for the following fax relay features for SCCP analog endpoints:
T.38 named signaling events (NSE) forced fax relay—Provides an ITU-T standards-based method for fax relay. Data is packetized and encapsulated according to the T.38 standard fax protocol. The SCCP gateway uses NSEs to indicate T.38 fax relay switchover. Configuring the gateway to unconditionally use T.38 NSE forced fax relay enables interoperation with a Cisco voice gateway that does not support interworking and negotiation of T.38 fax relay and NSE attributes at the time of call setup.
Cisco fax relay—Proprietary fax relay implementation that uses RTP to transport fax data. Fax relay is the default mode for passing faxes through a VoIP network, and Cisco fax relay is the default fax relay type on Cisco voice gateways.
Super Group 3 (SG3) to G3 fax relay—Allows the fax stream between two SG3 fax machines to negotiate down to G3 speeds (less than 14.4 kbps) allowing SG3 fax machine interoperation over T.38 fax relay with G3 fax machines.
There are two ways to transport modem traffic over VoIP networks: modem pass-through and modem relay.
Modem pass-through transports modem signals through a packet network using PCM encoded packets; traffic is susceptible to packet loss, jitter, and latency in the IP network.
With modem relay, the modem signals are demodulated at one gateway, converted into digital form, and carried in Simple Packet Relay Transport (SPRT) packets to the other gateway. There the modem signal is re-created and remodulated, then passed to the receiving modem. Modem relay lessens bandwidth requirements and significantly reduces the effect that dropped packets, jitter, and latency have on the modem session.
Before Cisco IOS Release 12.4(6)XE, only modem pass-through only and Cisco Unified Communications Manager-controlled modem relay were supported for SCCP gateways. In Cisco IOS Release 12.4(6)XE and later releases, the SCCP Gateway Controlled Modem Relay feature adds support for SCCP gateway-controlled modem relay.
Cisco uses a proprietary protocol, NSE, to signal events that are exchanged between gateways, using Real-time Transport Protocol (RTP) packets. SCCP gateway-controlled modem-relay bypasses NSE parameter negotiation between gateways and uses the NSE mechanism to indicate the gateway control mode to the other gateway. NSE signaling capability must be configured on both the originating and terminating gateways to enable modem-relay calls for SCCP analog endpoints.
Note In addition to a proprietary, NSE-based modem-relay implementation, Cisco offers V.150.1 modem-relay implementation. The VMWI for SCCP FXS Ports in Cisco IOS feature supports only NSE modem relay. For information about feature mode, see the “Configuring Feature Mode” section.
How to Configure DTMF Relay, Fax Relay, and Modem Relay for SCCP FXS Ports in Cisco IOS
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Feature Information for DTMF Relay, Fax Relay, and Modem Relay for SCCP FXS Ports in Cisco IOS
Table 1 lists the features in this module and provides links to specific configuration information. Only features that were introduced or modified in Cisco IOS Release 12.4(6)XE or a later release appear in the table.
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
NoteTable 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.
Table 1 Feature Information for VMWI for SCCP FXS Ports in Cisco IOS Feature
DTMF Relay, Fax Relay and Modem Relay for SCCP FXS Ports in Cisco IOS
Enhances the transmission of DTMF digits, faxes and modems between SCCP FXS ports on Cisco IOS gateways. DTMF digit transmission is supported by using RFC 2833. Cisco Fax Relay, Cisco T.38 Fax Relay, and Cisco Modem Relay are supported using Names Signaling Events (NSE).
The following sections provide information about this feature: