Troubleshoot Fax Relay

Publisher

Cisco Systems

URL

https://www.cisco.com/c/en/us/support/docs/voice/fax-modem-over-ip/20227-faxrelay-tsguide.html

Published

2024-05-22

Updated

unknown

Accessed

2026-10-02

HTTP status

200

License

Cisco proprietary; all rights reserved; no-redistribution; short excerpts and locators only

Source notes citing this source

• In Cisco fax relay debugs, bad CRC errors on received T.30 frames indicate corrupted signals and lead to Failure To Train (FTT) responses. in context
• With fax relay, ECM has a low tolerance for jitter and packet loss, approximately two percent. in context
• Disabling ECM on a Cisco fax relay dial-peer enables fax relay packet loss concealment, which tolerates about ten percent packet loss. in context
• On Cisco gateways the fax rate voice setting restricts the fax rate to the bandwidth of the configured voice codec; an explicit value such as fax rate 14400 overrides that limit. in context
• A fax that trains and then fails partway through a page most plausibly points to packet loss or jitter on the IP path, for which the documented mitigations are disabling ECM, lowering fax speed and adding T.38 redundancy. inferred in context
• For fax pass-through testing on Cisco gateways, echo cancellation and voice activity detection should be disabled (no echo-cancel enable, no vad) because they degrade modulated fax signals. in context
• show call active fax brief identifies which dial peers a fax call matched, and show voice dsp confirms whether the fax codec loaded in the DSPs. in context
• Cisco advises verifying that the fax machines work with each other across the PSTN without traversing the VoIP network, to isolate terminal problems from network problems. in context
• Cisco's fax relay methodology starts by confirming that ordinary voice calls complete over the same path before investigating fax. in context

Cite this source record

APA

WarmTransfer. (2024, May 22). Troubleshoot Fax Relay. WarmTransfer. https://warmtransfer.net/knowledge/sources/cisco-technote-20227-fax-relay-troubleshoot

BibTeX

@misc{warmtransfer-cisco-technote-20227-fax-relay-troubleshoot,
  title  = {Troubleshoot Fax Relay},
  author = {{WarmTransfer}},
  year   = {2024},
  url    = {https://warmtransfer.net/knowledge/sources/cisco-technote-20227-fax-relay-troubleshoot},
  note   = {Cisco Systems, accessed 2026-10-02}
}