The classification of FoIP is given in Fig. 15.1. Store-and-forward fax is based on the T.37 recommendation [ITU-T-T.37 (1998)]. It is used for off-line fax transmission. Store-and-forward fax is similar to e-mails, but at both the end terminations, fax machines and computers are used to interface for fax pages. In the store-and-forward mode, the caller sends the fax messages stored on the simple mail transfer protocol (SMTP) server to another SMTP server, and
Fax over IP functional classification.
Figure 15.1. Fax over IP functional classification.
finally reaches the destination VoIP fax interface. At the destination, a fax can be delivered as an e -mail message either with attachment or as a fax to a standard PSTN-based fax machine. The caller is notified on the fax delivery status. Store-and-forward fax is used with both computers and fax machines. Store-and-forward fax does not give users some fundamental advantages of fax technology. It lacks guaranteed delivery because store -and -forward fax services treat fax transmissions like e-mail without any guarantee of delivery. Psychological satisfaction will be missing, because of off-line operation. Store-and-forward fax allows retrieval of fax later similar to e-mails offering this as one of the service advantage. Store-and-forward fax details are not given in this topic.
Real-fime fax over IP works similar to a regular fax call. Fax machines synchronize and send data over the IP link between the two connections. When the fax is busy, the caller gets a busy signal and the user has the option to retry sending fax later. Internet-aware fax (IAF) devices may have the option to revert to store-and-forward mode on identifying the busy conditions. The
simple analogy can be given in relation to Internet Protocol (IP) phones. IP phones are having built-in phone and VoIP functions. IAF is a complete fax machine with built-in VoIP functions. The popular real-time modes of sending real-time fax on VoIP are fax pass -through and T.38 fax relay. Fax pass-through is similar to a G.711 -based VoIP voice call [ITU-T-G.711 (1988)] with additional care in modifying certain functionality of echo cancellation (EC), voice activity detection/comfort noise generation (VAD/CNG), packet loss concealment (PLC), dual-tone multifrequency (DTMF) rejection, signal gain-loss settings, and jitter buffer. A fax call based on T.38 [ITU-T-T.38 (2005)] is a true real-time FoIP call. T.37 is a store-and-forward equivalent of T.38. FoIP with T.38 or T.37 uses fax modules of ITU-T-V.21, V.27ter, V.29, V.17, and V.34 [ITU-T-V.21 (1988), ITU-T-V.27ter (1988), ITU-T-V.29 (1988), ITU-T-V.17 (1991), ITU-T-V.34 (1998)]. These modules are used to extract bits for delivering payload on FoIP using Internet facsimile protocol (IFP) packets. These V.series modules are also known by the name “data pump.” The management of a fax call is done through VoIP signaling protocols such as SIP, T.38, and T.30. A minimal part of T.30 [ITU-T-T.30 -2005)- is usually built inside the T.38 to handle T.30 timeouts and to simulate spoofing techniques, and hence, T.30 is usually not mentioned separately in VoIP fax calls. In the absence of T.38 and fax data pump support in the VoIP gateway or if interoperation of T.38 and data pump cannot progress with the call, a fax call is established in G.711 fax pass-through mode. Fax pass-through is used when there are no major network impediments and available bandwidth is more. It will be closely matching to a PSTN- based pulse code modulation (PCM) u.-law (PCMU) PCM A-law (PCMA) voice call under best end-to-end packet and signal transmission characteristics. Pass-through is closely related to the VoIP voice call. Refer to topic 2 for a VoIP voice modules overview. The T.38 standard is used for facsimile transmission in real time over the IP network, in which the T.30 fax from the PSTN is demodulated at the sending gateway. The demodulated bits of fax content are encapsulated into IFP packets. The IFP packets are sent over the network and the T.38 relay on the receiving gateway de-packetizes, performs the remodulation, and it sends the T.30 fax through the telephone interface to the answering fax device. At both fax machines, analog fax signals will appear very similar to a fax that is coming from PSTN lines.
As shown in Fig. 15.1 of fax calls classification, the T.38-based FoIP call makes use of user datagram protocol (UDP)-or transmission control protocol (TCP)-based transport protocols to deliver IFP packets on the IP network. TCP is a session-based, confirmed delivery service that does not need error control techniques such as redundancy and forward error correction (FEC) [Perkins et al. (1997), Rosenberg and Schulzrinne (1999)]. In TCP transport, the IFP payload is encapsulated in a transport protocol data unit packet (TPKT). The TPKT header as defined in RFC1006 [Rose and Cass (1987)] precede the IFP packet in TCP implementation. TCP implementation is more effective when the bandwidth for facsimile communication is limited and
packet impediments are minimal. The usage of UDP with UDPTL or real-time transport protocol (RTP) may be more effective when the bandwidth for facsimile communication is sufficient.
In UDP transport, IFP packets are encapsulated using either RTP or UDP transport layer protocol (UDPTL). Sometimes RTP [Schulzrinne et al. (2003)] is shown as a separate method. RTP makes use of UDP for voice and fax transport and is shown under UDP in main classification. The T.38 fax over UDP with UDPTL for transport is the most popular and well- established method used in the current deployments. UDPTL makes use of redundancy and FEC techniques for error correction as per ITU-T-T.38 recommendation. Parity FEC algorithms takes care of burst packet losses based on the number of FEC messages encoded. The IFP with RTP and G.711 fax pass-through encapsulation uses the redundancy and FEC techniques as defined in RFC2198 and RFC2733, respectively, for error correction. The format for RTP headers is different for redundancy and FEC schemes. The error recovery techniques with UDPTL make use of procedures given in the ITU-T T.38 recommendation. The payload and packet formats are given in topic 16.

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