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. 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.
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.