T.38 is an ITU standard for sending FAX accross IP networks in a real-time mode.

Asterisk does not support T.38, CallWeaver and Gridborg HMP Server (SIP/H.323) do.

FAX messages are sent as UDP or TCP/IP packets.

• The IETF RFC RFC 3362 implements a media type called image/t38 for T.38 faxes.

From RFC 3362:

• Commetrex runs a free interoperability test lab and keeps a list of tested equipment. Commetrex T.38 Interop Lab
• Brooktrout White paper: http://www.brooktrout.com/whitepapers/pdf/fax_over_ip.pdf (link no longer works as Brooktrout became part of Cantata Technology, which is now itself part of Dialogic). The document can be found at www.viatechnology.es/Documents/Productos/fax_over_ip.pdf

T.38 with Asterisk (initial work)

From version 1.4, Asterisk supports T.38 negotiation for SIP users, and the related passthrough of UDPTL T.38 data. This allows many T.38 nodes to communicate through an Asterisk box. Asterisk does not, however, understand the T.38 protocol. It cannot terminate T.38 calls, or act as a T.38 PSTN gateway without external support - i.e. by passing the T.38 data to something which can perform those functions.

T.38 with Asterisk (Attractel commercial solution)

A full fledged t.30 and t.38 stack for asterisk (both passthrough, termination and gateway) is available from Attractel
As the software is not under GPL, this can be used together with other non GPL parts of asterisk, such as g729 or iLBC.
The software comes with ITU t.30 modems : v.21, v27ter, v29, v17 and includes standalone FAX transmit and receive programs for use in fax2email and email2fax gateways.

T.38 with CallWeaver (supported)

Callweaver has rapidly maturing support for T.38 passthrough, T.38 gateway and T.38 termination. Passthrough simply means forwarding T.38 between two other T.38 entities. Gateway operation provides interworking between the PSTN and the T.38 world. Termination makes Callweaver act as a T.38 FAX machine. Callweaver can also act as a traditional PSTN type FAX machine.

Many T38 implementations are quirky, and work is ongoing to extend the number of T.38 platforms which interoperate smoothly with Callweaver. At present it has been successfully used with products from Cisco, Patton, Quintum and many others.

For further information, see the Callweaver home page at www.callweaver.org

T.38 with Gridborg HMP Server (supported)

Gridborg HMP Server supports T.38 fax origination, fax termination and fax pass through. All of these options are available for both H.323 and SIP protocols. Gridborg HMP Server stores received documents in a multitude of formats and also enables sending of fax documents saved in TIFF and other formats.

Gridborg HMP Server has been successfully used with VoIP gatways from many vendors like Cisco, Planet, Patton, Sipura, and many others.

T.38 with T38modem

What is t38modem www.openh323.org?

• It is not a modem.
• From a FAX application's view point (e.g. HylaFAX) T38modem looks like a class 1 FAX modem pool.
• From an IP network view point it's a H.323 endpoint with T.38 FAX support. Recent versions also support SIP.
• From your view point it's a gateway between a fax application and an IP network.

See also

• Asterisk T.38 Bounty
• Asterisk T.38
• Uniqall Gridborg HMP Server
• FAX
• FAX over IP
• T.37
• Asterisk fax
• Steve Underwood Faxing over IP networks Article - excellent explanation of issues
• quintum VoIP gateways T.38 ready
• Patton VoIP gateways T.38 ready
• Traffic analysis of two T.38 devices (link now working at Cantata Technology website)
• Axacore FaxAgent T.38 Fax Server