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T.38 FoIP & T.38 Fax Software

T.38 FoIP  and T.38 Fax software support transmission of proprietary and industry standard real time Group 3 fax/facsimile over IP networks using the T.38 protocol. Our T.38 fax software may be licensed by developers as a library. Custom solutions are also available for T.38 FoIP applications.

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VOCAL’s T.38 fax software is optimized for execution on ANSI C and leading DSP architectures from TI, ADI and other vendors. Contact us to discuss your specific T.38 fax over IP / fax relay application requirements.

Real-Time Super/Group 3 Facsimile

The purpose of T.38 FoIP and T.38 fax software enabled devices is to allow the use of ordinary group 3 facsimile as well as high speed V.34 machines on modern internet protocol (IP) networks, permitting ATAs or other FoIP products to relay calls through a VoIP service. Without T.38 protocol, successful calls can be placed, but these can be less reliable than facsimile transmissions through an ordinary telephone line. With T.38 protocol, the user should not even know that a VoIP service is being used instead of a land line connection.

T.38 Fax over IP usage and capabilities can vary greatly between VoIP providers. Some provide minimum support, if at all, while others fully support the protocol up to and including high speed V.34 FAX. Despite the decline in conventional FAX due to internet and e-mail services, the clear industry trend is to provide for transparent fax relay support using the T.38 protocol.

T.38 Fax Software

T.38 Fax software can be combined with VOCAL’s SIP and network stacks to obtain seamless voice and/or fax relay operation from call startup to disconnect. Complete facsimile server call control is also available and can be combined with our real-time Group 3 fax store and forward module.

Alternatively, when integrated with VOCAL’s SIP and T.30 protocol modules and TCP/IP network stack, a standalone internet-ready T.38 protocol capable facsimile end terminal can be realized. In this configuration the modulation layer is unnecessary and so interoperability with high speed V.34 fax based terminals can be achieved without the need of digital signal processing.

VOCAL’s T.38 Fax over IP modules are available for use with either the VOCAL or other vendor-provided FAX pumps. The interface points are at the sample layer for the VOCAL pump or the Class 1 command set for other vendor-provided pump layers. The network interface is either RTP or UDPTL. The protocol negotiation and setup is initiated by other modules, either from VOCAL or vendor provided. A block diagram illustrating the relationship between these T.38 fax software modules is available.

T.38 Fax Call Flow

In a typical network environment where the SIP protocol is used to establish sessions between two (or more) entities, the T.38 fax call flow is normally 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 fax relay mode. With dedicated facsimile machines the initial INVITE may actually set up a T.38 connection first, which can be easily handled by modifying the startup sequence to skip the autodetection phases. At this point, either the detection of a tone in the local audio codec stream or the receipt of a network event such as a SIP RE-INVITE or the receipt of a RTP packet will force a transition to the fax relay mode.


  • V.34 fax data channel – full and half duplex modulation for data rates from 2400 bps to 33600 bps.
  • V.34 fax control channel – 1200/2400 bps modulation for use with half duplex procedures
  • V.17 - 7200, 9600, 12,000 and 14,400 bps trellis coded modulation
  • V.29 - 7200 and 9600 bps modulation
  • V.27ter - 2400 and 4800 bps modulation
  • V.21 Channel 2 - 300 bps modulation


  • Fully integrated Group 3 Facsimile processing system
  • Configurable to support both TCP/IP and/or UDP/IP modes of data transmission
  • Configurable type/amount of redundant information for UDP/IP service
  • Configurable to support UDPTL forward error correction messages
  • Configurable high speed training mechanism – Local TCF generation or TCF transmitted between end-terminals
  • Optimized for high performance on leading edge DSP architectures
  • Multichannel implementation
  • Multi-tasking environment compatible


  • SIP-based system available for industry standard and proprietary Fax over IP networks
  • Facsimile/Voice Distinction available for seamless facsimile/voice system operation
  • Direct interface to 8.0 kHz PCM data stream (A-law or μ-law), run-time time-slot selection
  • Analog DAA interface using linear codec at 8.0 kHz sample rate
  • North American/International Telephony supports DTMF generation, dialing procedures, call progress, Caller ID, etc.
  • Group 3 Facsimile store-and-forward features available
  • Group 3 Facsimile End Terminal operation with direct T.38 protocol interface available
  • Complete VoIP systems available (speech codecs, DTMF tokens, silence tokens, etc.)
  • Speech codecs include V.34 fax, G.723, G.729, G.729A, GSM, G.728, G.726/G.721
  • Complete data modem systems available (through V.90/V.42bis and V.34fax/T.30)
  • Multiple ports can be executed on a single DSP



VOCAL’s fax software is available for the following platforms:

  • Texas Instruments - C6000 (TMS320C62x, TMS320C64x, TMS320C645x, TMS320C67x), C5000 (TMS320C54x, TMS320C55x), OMAP3 (OMAP35x, OMAP36x), OMAP4 (OMAP44x), Sitara (AM37x)
  • Analog Devices – Blackfin, TigerSHARC, SHARC
  • ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM, ARM Cortex-A8
  • AMCC – PowerPC
  • MIPS – MIPS32, MIPS64, MIPS4Kc
  • AMD x86
  • Intel

Please contact us for specific T.38 Fax over IP and Real Time Group 3 Facsimile supported platforms.

Operating Systems

VOCAL’s T.38 Fax over IP and Real Time Group 3 Facsimile software is available for the following Operating Systems:

  • Google Android
  • Apple iOS / iPhone / iPad & MacOS
  • Unix,  Linux, μClinux, BSD
  • Microsoft Windows ACM / RTC / CE / Mobile
  • Symbian
  • eCOS / eCOSPro
  • Wind River VxWorks
  • Green Hills Integrity
  • Micrium μCOS

Please contact us for other supported Operating Systems.

Additional Information

VOCAL Technologies, Ltd.
520 Lee Entrance, Suite 202
Amherst, New York 14228
Phone: +1-716-688-4675
Fax: +1-716-639-0713