Fax Protocols
Group 3 fax devices conform to the ITU-T T.4 and T.30 Recommendations, which manage the procedures used for
image conversion and fax transmission. VOCAL offers a comprehensive and fully optimized Group 3 Fax solution
for a wide variety of platforms. This includes the T.4, T.6, and T.30 protocol stacks. Included as well is
the T.30E module, which is necessary for the transmission and reception of colour fax.
Group 3 fax "high speed" modulations include V.17 (14.4 kbps, 12.0 kbps, 9.6 kbps, and 7.2 kbps), V.29
(9.6 kbps and 7.2 kbps), and V.27ter (4.8 kbps and 2.4 kbps). V.33 is no longer specified as an option under T.30.
T.30 specifies an additional 14 modulation rates between 2400 bps and 33600 bps (in increments of 2400 bps) that can
be used from the V.34 modulation. This is referred to as V.34 Fax. As with V.34 data modulation procedures, V.8 startup
is required. V.34 Fax includes procedures for full and half duplex data channel operation; half duplex is mandatory for
V.34 Fax support.
In non-V.34 Fax, V.21 is used to transport T.30 control information frames. V.34 half-duplex procedures define a 1200
bps and 2400 bps control channel. In V.34fax, this control channel is used to transport T.30 control information frames. The
use of error correction mode for V.34 Fax is mandatory.
T.30
ITU-T T.30 defines the procedures necessary for fax communication and document transmission in the
general switched telephone network, international leased circuits, and the Integrated Services Digital
Network (ISDN). There are five distinctive and consecutive phases necessary for fax transmission:
- Call set-up which initiates fax connection between two sides
- Pre-Message procedure where each side identifies their capabilities and negotiates session parameters
- Fax message transmission, including any necessary phasing and synchronization
- Post-message procedures which include indication of end-of-message, confirmation, and multi-page signaling
- Call release with asynchronous termination
These procedures manage fax communication by controlling call establishment and release, compatibility checking,
status and control commands, monitoring line conditions, and fax operator recall.
T.30E
ITU-T T.30-E uses an encoding methodology for continuous-tone (multilevel) images based on the lossy mode of encoding (based on the
Discrete Cosine Transform DCT) of JPEG (T.81) image encoding standard. T.30E uses Forward Error Correction defined in T.42 and adopts
a device-independent color space representation, the CIELAB space, which allows unambiguous exchange of color information. T.30E also
indicates the procedure for negotiation of the capabilities for transmission of continuous-tone color and gray-scale images. It
specifies the definitions and the specifications of new entries to the Facsimile Information Field of the DIS/DTC and DCS frames of
Recommendation T.30.
Information is specified pertaining to image digitization resolution (in bits/pel), spatial resolution, sampling ratio of
color components, JPEG capability, color capability, and image data scaling that is subject to negotiation in the
pre-message phase of the T.30 protocol.
The technical features of encoding and decoding the continuous-tone color and gray-scale image data are
described in Annex E/T.4. It describes two modes of image encoding (lossy gray-scale and lossy color) which
are defined using Recommendation T.81.
HDLC and ECM
The binary coded signalling system is based on a High Level Data Link Control (HDLC)
format developed for data transmission procedures. This provides for frame labelling, error checking,
and confirmation of correctly received information. Error correction mode is optional for some fax image
formats (T.4) and mandatory for other compressions (T.6).
Image Conversion
Other features of Group 3 Fax include image compression algorithms such as T.4 1-D and 2-D coding,
T.6 compression, JPEG gray scale and color modes. Image format conversion, although not specified by T.30, is an optional
feature of facsimile service class 2 command sets. Binary file transfer (BFT), is a mechanism to transport a
T.454 format file under error correction.
Features
- Fully compliant with ITU Recommendations
- Supports control of facsimile high speed modulations up to V.34fax 33,600 bps.
- T.30 session control and negotiation
- Supports fax polling operation
- Supports procedure interrupts
- Optional Error Correction Mode (ECM) available
- Optional Binary File Transfer (BFT) available
- T.4 1-dimensional Image Decoding/Reencoding
- T.4 2-dimensional/ T.6 Image Decoding/Reencoding available
- In-band data format conversion support available
- Multi-tasking environment compatible
- Multichannel implementation
Configurations
- DAA interface using linear codec at 7.2 kHz, 8.0 kHz or 16.0 kHz sample rate
- Direct interface to 8.0 kHz PCM data stream (A-law or u-law)
- North American and International Dialing/Telephony dialing functions available
- Facsimile protocol layer (T.30 with ECM/BFT) available
- Facsimile service class 1 command sets (T.31 and EIA-578) available
- Facsimile service class 2 command sets (T.32, EIA-592 and industry standard fax class 2) available.
- System can be combined with V.34fax facsimile modulations and startup procedures (V.8 and V.8bis).
- Configurable for standalone operation (no command set layer required).
- T.38 Fax Relay/Fax over IP configurations available
- Error Correction Mode memory can be maintained off-system.
ITU Recommendation T.4
ITU Recommendation T.6
ITU Recommendation T.30
