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VOCAL's T.38 software include modules for proprietary and industry standard Group 3 facsimile. This module can be combined with VOCAL's SIP and network stacks to obtain seamless voice and/or facsimile relay operation from call startup to disconnect. Complete facsimile server call control is also available and can be combined with VOCAL's Group 3 facsimile store and forward module.
Alternatively, when integrated with VOCAL's SIP/T.30 modules and TCP/IP network stack, a standalone internet-ready facsimile end terminal can be realized. In this configuration the facsimile modulation layer is unnecessary and so interoperability with V.34fax based facsimile terminals can be achieved without the need of digital signal processing.
The purpose of T.38 and T.38 enabled devices is to enable seamless use of ordinary FAX machines on modern networks, permitting ATAs or other FoIP products to handle FAX calls through a VoIP service. Without the use of T.38, successful FAX calls can sometimes be placed, but are much less reliable than FAX through an ordinary telephone line. With the use of T.38, the user should not even know that a VoIP service is being used instead of a land line.
T.38 usage and capability varies greatly between VoIP providers. Some do not support it at all, while others support it fully, up to and including V.34 FAX. Despite the decline in conventional FAX due to internet and e-mail FAX, the clear industry trend is to transparently support FAX using T.38. Nearly all major VoIP services and devices now offer at least minimal support for T.38.
To help support manufacturers and VoIP developers, VOCAL provides T.38 modules for use with either VOCAL or other vendor-provided FAX pumps. The interface points are at the sample layer for the VOCAL FAX pump, or the FAX Class 1 command set for other vendor-provided FAX pump layers. The network interface is standard RTP. T.38 negotiation and setup is initiated by other modules, either VOCAL or vendor provided. A block diagram illustrating the relationship between these modules is available.
In a typical network environment where the SIP protocol is used to establish sessions between two (or more) entities, T.38 capability is usually indicated by SDP entries in the initial SIP INVITE message. This may be through a particular codec type in the audio stream or through an independent media stream different from the voice audio media stream.
After the initial INVITE, the session is typically established as an ordinary audio voice call, with the ability to switch to T.38 mode (with dedicated fax machines the initial INVITE may actually set up a T.38 connection first, which is easily handled by modifying the startup sequence to skip the autodetection phases). At this point, either the detection of a fax tone in the local audio codec stream or the receipt of a network event such as a SIP RE-INVITE or receipt of a T.38 RTP packet will force a transition to T.38 mode.