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Cisco Fax Wiki





Welcome to the Cisco Fax Wiki


A repository and one-stop location for everything related to Fax over IP (FoIP) and Cisco voice products

Getting fax communications to work successfully and reliably in IP environments can be more challenging than VoIP. Fax is modulated data and is therefore markedly different than voice. These differences must be taken into account and FoIP should not be treated like VoIP. From QoS to the underlying transport protocols to switchover mechanisms, dealing with FoIP is full of pitfalls and gotchas. The aim of this wiki page is to demystify fax communications in IP networks and provide a comprehensive reference page for anything related to FoIP."



General FoIP Information



Fax, Modem, and Text for IP Telephony

The Definitive Resource for Understanding, Designing, Configuring, and Troubleshooting Fax, Modem, and Text in Today's IP Networks



General Fax Technical Documentation



General Fax Tech Tips and FAQ's






Passthrough simply involves the transport of fax information over a low compression codec, such as G.711. In most respects it is like a typical voice call.


Fax/Modem Passthrough (NSE-based)



Cisco ATA

At this time the Cisco ATA only officially supports NSE-based passthrough. On IOS voice gateways, you must use the "modem passthrough" configuration command to successfully send faxes to the ATA.


Fax Pass-through (protocol-based)





Relay demodulates fax communications and passes the fax data across the IP network using special relay protocols. Cisco voice gateways generally support two types of fax relay, standards-based T.38 fax relay and the pre-standard, propriatary Cisco fax relay.


T.38 Fax Relay



Cisco Fax Relay



T.37 Store-and-Forward Fax


Unity Integration


A Cisco voice gateway running T.37 onramp and/or offramp can be integrated directly with Unity.



Fax Servers


Cisco Fax Server


The Cisco fax server is simply an OEM of the Captaris RightFax product with a Dialogic fax engine (made up of a hardware-based fax board or the software-based SR140 solution). Integrating via T.38 with Cisco voice gateways and Unified CM, the Cisco fax server handles the faxing piece of a Unified Communications solution.




Windows Fax Server

Windows Fax Service is an off-the-shelf component from Microsoft. When installed on Windows Server 2003/2008 the Service is available for all computers in the domain. There is third pary software that integrates Windows Fax Service with T.37-capable integrated services routers, see for details.

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