Cisco Small Business Voice Gateways and ATAs

Experiencing Unreliable T.38 Fax Connection Using an SPA2100

Document ID: 108792

Updated: Dec 12, 2008



How do I improve my fax completion rates?
Related Information


This article is one in a series to assist in the setup, troubleshooting, and maintenance of Cisco Small Business products.

Q. How do I improve my fax completion rates?


In order to improve fax completion rates on the SPA2100, make sure:

  • The SPA2100's firmware is the current version
  • You have enough bandwidth for uplink and downlink. For G.711 fallback, ~100Kbps is recommended. For T.38, allocate at least 50Kbps.

In addition, configure these parameters:

  • Set Network Jitter Buffer to Very High
  • Set Jitter Buffer Adjustment to Disable
  • Set Call Waiting to No
  • Set 3-Way Calling to No
  • Set Echo Canceller to No
  • Set Silence Suppression to No
  • Set Preferred Codec to G.711
  • Set Use Preferred Codec Only to Yes

Note: For T.38 fax support, enable the feature by setting Fax Passthru Method to ReINVITE under the Line tabs. Note that if a T.38 call cannot be set up, then the call should automatically revert to G711 fallback.

If you are using a Cisco media gateway, make sure it is configured with T.38 for the SPA2100 corresponding dial peer. The dial peer should include:

  • Fax Protocol should be set to T.38
  • Fax Rate should be set to Voice
  • Fax-Relay ECM should be set to Disable
  • Fax NSF should be set to 000000
  • No VAD

Related Information

Updated: Dec 12, 2008
Document ID: 108792