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Introduction to V.34 High-Speed Fax

Posted By: Ben Needles | Posted On: 2008-11-04 | Article ID: 111458

This article discusses the unique features of the V.34 facsimile standard, which delivers higher performance and reliability, and how the rapid adoption of high-speed fax results in increased economy and efficiency for businesses.

With advancement in facsimile technology, enterprises face the decision to upgrade their fax solutions to high-speed fax, namely V.34. Dubbed V.Fast, the V.34 fax standard is a significant enhancement in fax technology with many advantages over legacy fax solutions. The adoption of the V.34 standard allows:
* Data rate of up to 33.6Kbps, more than twice the speed of its predecessor, V.17 (14.4Kbps)
* Support for fast handshaking, which cuts session setup and management time significantly over legacy solutions
* High-speed transmission enables transport of color fax data

In addition, fax devices supporting V.34 offer more robust and reliable fax transmissions for varied line conditions as a result of the mandatory ECM protocol and the more sophisticated modulation/demodulation scheme. These improvements result in cost savings from reduced fax phone bills especially for long-distance scenarios.

The ITU-T V.34 Fax Standard
The V.34 fax standard was derived from the V.34 data modem standard established by the International Telecommunications Union (ITU). The V.34 data modem standard is a full-duplex implementation for sending and receiving data across telephone lines with a maximum data rate of 33.6Kbps. Certain elements of the V.34 data modem standard were eliminated for V.34 fax while new features, such as a control channel and mandatory ECM, were added to enable fast and reliable fax transmission.

The V.34 Fax Connection and Session
In order to understand the benefits of the V.34 fax standard, it is first necessary to understand how a fax transmission works. V.34 session management and setup were designed with a similar mechanism to legacy handshaking procedures. The first step of a fax session is to establish a handshake between the sending and the receiving devices. During handshaking, the sending and receiving devices negotiate key parameters for how the fax call should be set up such as determining what is the highest transmission speed supported by both devices. The handshaking process itself is performed at 300bps in legacy devices. In V.34 fax capable devices, handshaking is performed at a much faster data rate of 1.2Kbps. The result is a handshake time that is reduced from approximately 16 seconds of legacy systems to 9 seconds for V.34.

The V.34 Fax Connection and Session
In order to understand the benefits of the V.34 fax standard, it is first necessary to understand how a fax transmission works. V.34 session management and setup were designed with a similar mechanism to legacy handshaking procedures. The first step of a fax session is to establish a handshake between the sending and the receiving devices. During handshaking, the sending and receiving devices negotiate key parameters for how the fax call should be set up such as determining what is the highest transmission speed supported by both devices. The handshaking process itself is performed at 300bps in legacy devices. In V.34 fax capable devices, handshaking is performed at a much faster data rate of 1.2Kbps. The result is a handshake time that is reduced from approximately 16 seconds of legacy systems to 9 seconds for V.34.
After handshaking is complete, the next stage of a fax session is the transmission of the actual fax page data. The retraining and re-synchronization process takes place after each page is transmitted in legacy schemes, where capabilities such as supported modulation and transfer are renegotiated. In case of error in the transmission, entire pages may need to be retransmitted. This cycle of page data retrain and retransmit repeats until the fax call is completed, and account for significant inefficiency of legacy fax machines. V.34 provides the most extensive range of supported data transmission rates, allowing it to optimize both speed and reliability over a wide range of line conditions. With V.34, fax page data is transmitted at 33.6Kbps, twice the speed of V.17. In addition, V.34 uses ECM (Error Correction Mode) as a mandatory feature that handles page transmission error in a much more efficient way.

ECM as a Mandatory Feature
ECM is a mandatory feature for V.34 fax as opposed to V.17, where it is optional. The ECM protocol was designed to automatically detect and correct errors in the fax transmission process caused by factors such as telephone line noise. The page data to be transferred is divided into small blocks of data called Octets. Once all octets are received, they are examined using check-sums.
If any errors in the checksums are detected, the receiving fax device signals the transmitting fax device to retransmit the octets that were received incorrectly. The transmitter then retransmits only the needed blocks rather than the whole page. Once, all octets are received correctly, they are ordered and the page data is reconstructed by removing the octet frame and signaling flags. Generally, this results in a faster and more successful fax transmission than in a scenario where entire page data is retransmitted once or multiple times.

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Ben Needles

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