PDF of this tutorialThe ITU standards for fax specify the sequence of operations in a fax call, including the handshake messages to be exchanged, the format of such messages, procedures for testing the validity of handshake messages, error recovery procedures, timing, and so on. Most fax systems deviate from the ITU standards in one way or another. In order to be compatible with the worldwide installed base of fax devices, systems and networks should be tolerant of the many anomalies and standards violations that will be encountered. Some of the most common anomalies and standards deviations are described in the following sections.
Simple Fax Call with No Errors
For convenience, a single fax LAN is shown again in Figure 13.
Figure 13. Communication Between a Calling Fax Unit and an Answering Fax Unit
Common Timing Deviations
In addition to the format of the messages, the T.30 standard also specifies various timing requirements. For example, after dialing a number, the calling fax unit is supposed to listen for a response for 35 seconds before giving up. These time-out periods are as follows:
- T1—35 ± 5 seconds: the length of time two fax units attempt to identify each other
- T2—6 ± 1 second: a time-out used to start the sequence for changing transmit parameters
- T3—10 ± 5 seconds: a time-out used in handling operator interrupts
- T5—60 ± 5 seconds: a time-out used in Error Correction mode
These time-outs are sometimes misinterpreted. In addition, they are routinely ignored, and in many cases they should be. For example, after placing a call, the calling fax unit is supposed to wait for 35 seconds before giving up. If the answering unit does not answer on the first ring or if a voice answering machine is connected to the line, or if there are many delays through the network, then the delay before answer can be much longer than 35 seconds.
Fax units that support the error correction mode can respond to a post-image handshake message with a receiver not ready (RNR) message. The calling unit then queries the receiving fax unit with a receiver ready (RR) message. If the answering unit is still busy (printing for example), it will repeat the RNR message. According to the T.30 standard, this sequence (RR/RNR RR/RNR) can be repeated for up to the end of T5 (60 ± 5 seconds). However, many fax systems ignore the time-out and will continue the sequence indefinitely unless the user manually overrides.
All of the time-outs are subject to alteration, and in some cases, misuse. Extensive testing is required to verify that commonly encountered deviations are handled smoothly.
Intercarrier Gap Problems
The T.30 standard specifies a silent period of 75 ± 20 milliseconds (ms) between signals using different modulation modes—for example, between the end of a DCS handshake message and the start of a TCF test pattern or between the end of an image and the start of a post-image handshake transmission. This requirement is frequently violated, particularly during the silent period between DCS and TCF. Many fax systems routinely extend the duration of this silent period to well over 100 ms. Unfortunately, if this period is too long, it can interfere with the handshake message error recovery procedure when the DCS message is corrupted by lost packets or noise on the line. All systems should be tested to make sure that they stay within the prescribed T.30 limits and that if they exceed these limits, their ability to recover from error conditions is not impaired.
Other Timing Variations
Testing is also required to determine the ability of a fax system to handle variations in the duration of pauses between unacknowledged handshake message repetitions and also in the pauses between the receipt of a handshake command and the start of a response to that command. In order to reduce the total transmission time, many fax systems start sending a response message before the end of the command has been received.
Answer Tone Anomalies
The T.30 standard directs that the answering fax device send an answer tone of 2100 Hz for approximately 3 seconds before sending the first handshake message. Some fax machines send an 1850-Hz tone, some send an 1100 Hz tone, and some omit the answer tone altogether and just begin with the first handshake message. Testing is required to verify that a calling fax unit can complete the call, even in the absence of answer tone.
Other Deviations from ITU Standards
There are many other commonly encountered anomalies and deviations for which systems must be tested, including the following:
- frame sequence deviations
- preamble and flag sequence variations
- improper EOM usage
- unusual data rate fallback sequences
- common training pattern detection algorithms
- image transmission deviations
- echo protect tone usage
- image padding and short lines
- RTP/RTN handshake message usage
- long duration lines
- nonstandard disconnect sequences
- DCN usage
