G.711 (64kbps) is the default audio codec used for phone conversation used in the following platforms:
- Cisco Unified Communications Manager (CUCM)
- Cisco Unified Communications Manager Express (CUCME)
- Cisco Unified Communications Manager Business Edition (CUCMBE)
Many IP Telephony deployments use the G.729 (8kbps) audio codec over wide area network (WAN) links to save bandwidth. G.729 has a very negligible voice quality impact while using 1/8 of the bandwidth of uncompressed G.711 voice. G.729 is the default audio codec used on voip dial-peers in a Cisco IOS based gateway routers. G.729 was created to compress human voice, not to compress fax, modem, or text communications. Many IP telephony deployments are performed without optimizing the infrastructure for fax, modem, and text communications.
This post will look at the challenges and available solutions to transmitting fax messages over IP, focusing on H.323 and SIP controlled gateways. The MGCP gateway protocol has very unique coverage of fax messages since the MGCP call agent (CUCM) is very involved in every call.
VoIP dial peers use G.729 by default and the only fax rate that uses 8kbps or less is a fax rate of 7200 bps. The fax rate of the VoIP dial peer should be changed to the highest possible fax rate, which is currently 14400 bps (fax rate 14400). Cisco routers support the analog G3 fax standard which is capable of sending fax messages at up to 14.4kbps. Super G3 fax machines can send and receive faxes at up to 33.6kbps, but Cisco routers only support the G3 fax standard. If using super G3 fax machines, it is recommended to provision the necessary gateway configuration commands that allow the gateway to negotiate between super G3 and G3 (fax relay sg3-to-g3). The references portion of this blog includes links to places where you obtain the necessary commands for anything mentioned in this blog.
The default fax protocol used in Cisco routers is the Cisco fax, fax relay mechanism. The Cisco fax relay mechanism includes logic in which the Cisco gateway recognizes a fax call based on the unique negotiated tones fax machines transmit and receive (T.30). The Cisco router recognizes these tones and the digital signal processor (DSP) of the router treats the call uniquely in order to properly handle the fax call. The International Telecommunications Union (ITU) subsequently standardized a fax relay approach that was given the designator of T.38. T.38 fax relay and Cisco fax relay are mutually exclusive, but many of the commands used to implement such are used for both. Fax relay requires less bandwidth over the IP WAN than another approach we will discuss called fax pass through.
If using T.38 as a fax relay mechanism, the fax protocol (fax protocol t38) must be specified in voip dial peer or voice service voip configuration mode. A dial peer with the fax rate system command leverages the fax protocol specified globally in voice service voip configuration mode. Some fax machines use proprietary mechanisms to transmit faxes rather than leveraging the ITU T.30 standard. Since Cisco gateways cannot handle these proprietary faxes, the fax nsf 000000 command is used in voip dialpeer configuration mode to force the faxes to use the T.30 standard. Many fax machines use a mechanism called error correction mode (ECM) to ensure the proper transmittal of fax messages. The fax-relay ecm disable command should be used to turn this mechanism off on all voip dial peers that will send fax over IP (FoIP) messages. The use of this command should increase the success rate of fax over IP networks with packet loss and jitter conditions. Fax over IP communications should always be put in the quality of service (QoS) priority queue (PQ) to guarantee low packet loss, delay, and jitter. Fax and modem communications are more susceptible to quality degradation than voice over IP. Voice must normally have a packet loss of less than 1 percent, while fax and modem requirements are 1/10 of the packet loss of 1 percent (.001).
What if you have tried all the fax relay mechanisms and you are still having fax problems such as failed faxes, missing lines, poor quality, etc.? It may be time to implement a fax pass-through mechanism.
References:
Cisco IOS Fax, Modem, and Text Support over IP Configuration Guide, Release 12.4T
Dial Peer Configuration on Voice Gateway Routers
Cisco IOS Voice Command Reference
Fax, Modem, and Text for IP Telephony
Author: Dennis Hartmann
