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8. PSTN Fax-Call Procedure This module will describe the stages of a standard fax call over the PSTN so that the processing required for a reliable fax transmission over a packet network can be explored. Fax machines in common use today implement the ITU recommendations T.30 and T.4 protocols. The T.30 protocol describes the formatting of non-page data, such as messages that are used for capabilities negotiation. The T.4 protocol describes formatting of page image data. T.30 and T.4 have evolved substantially over time and are now quite complex, given that they attempt to describe the behavior of an evolving set of fax machines. The timing related to the message interaction and phases of the call is critical and is one of the major causes of problems in the transmission of FoIP networks. The PSTN fax call is divided into five phases, as shown in Figure 9. This example assumes that the call is accomplished without errors. The procedure becomes somewhat more complicated if errors occur or if there is a need for modem retraining. The five phases are as follows: call establishment control and capabilities exchange page transfer end of page and multipage signaling call release Figure 9. PSTN Fax-Call Flow
Call EstablishmentThe fax call is established either through a manual process, according to which someone dials a call and puts the machine into fax mode, or by automatic procedures, according to which no human interaction is required. In both cases, the answering fax machine returns an answer tone, called a CallEd station IDentification (CED), which is the high-pitched tone that you would hear when you call a fax machine. If the call is automatically dialed, the calling station will also indicate the fax call with a calling tone (CNG), which is a short, periodic tone that begins immediately after the number is dialed. These tones are generated to allow a human participant to realize that a machine is present on the other end call. They are also, although not reliably, used to recognize the presence of a fax call. Control and Capabilities ExchangeThe control and capabilities exchange phase of the fax call is used to identify the capabilities of the fax machine at the other end of the call. It also negotiates the acceptable conditions for the call. The exchange of control messages throughout the fax call are sent using the low-speed (300 bps) modulation mode. Every control message is preceded by a one-second preamble, which allows the communication channel to be conditioned for reliable transmission. The called fax machine begins the procedure by sending a digital identification signal (DIS) message, which contains the capabilities of the fax machine. An example of a capability that could be identified in this message is the support of the V.17 (14,000 bps) data signaling rate. At the same time, the called subscriber information (CSI) and nonstandard facilities (NSF) messages are optionally sent. NSF are capabilities that a particular fax manufacturer has built into a fax machine to distinguish its product from others. These facilities are not required to be supported for interoperability. Once the calling fax machine receives the DIS message, it determines the conditions for the call by examining its own capabilities table. The calling machine responds with the digital command signal (DCS), which defines the conditions of the call. At this stage, high-speed modem training begins. The high-speed modem will be used in the next phase of the fax call to transfer page data. The calling fax machine sends a training check field (TCF) through the modulation system to verify the training and ensure that the channel is suitable for transmission at the accepted data rate. The called fax machine responds with a confirmation to receive (CFR), which indicates that all capabilities and the modulation speed have been confirmed and the fax page may be sent. Page TransferThe high-speed modem is used to transmit the page data that has been scanned in and compressed. It uses the ITU T.4 protocol standard to format the page data for transmission over the channel. End-of-Page and Multipage SignalingAfter the page has been successfully transmitted, the calling fax machine sends an end-of-procedures (EOP) message if the fax call is complete and all of the pages have been transmitted. If only one page has been sent and there are additional ones to follow, it sends a multipage signal (MPS). The called machine would respond with message confirmation (MCF) to indicate the message has been successfully received and that it is ready to receive more pages. Call ReleaseThe release phase is the final phase of the call, in which the calling machine sends a disconnect message (DCN). While the DCN message is a positive indication that the fax call is over, it is not a reliable indication, as the fax machine can disconnect prematurely without ever sending the DCN message.
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