14.4
The PSTN fax call is divided into five phases of A, B, C1, C2, D, and E [ITU-T-T.30 (2005), McConnell et al (1999), URL (Audiocodes-fax), URL (SPRA073) , URL (Sage -fax)] as shown in Fig. 14.3 . Fax call messages for multiple pages of transmission are illustrated in Fig. 14.4. In this section, an overview of fax call flow messages is given. Complete details, several messages, various options, and errors are given in the T.30 recommendation Appendix A.
Phase A is the call establishment, and this is achieved either manually or automatically.
Phase B is the premessage procedure for call control and capabilities exchange. The major steps of this phase are as follows:
Figure 14.3. PSTN fax call phases.
Figure 14.4. Basic fax call phases in relation to messages, pages, and multiple page extensions.
• The calling and answering fax devices identify themselves and exchange capabilities
• Transmission parameters selection
Phase C1 is the in-message procedure for message transmission and controls, and it completes signaling for in-message procedures, namely in-message synchronization, error detection, correction, and line supervision:
• The calling unit sends a test pattern to determine the maximum data rate
• The answering unit either accepts the data rate or requests a lower rate
Phase C2 is the T.4/T.6 image transmission at an accepted data rate. Phase D is the postmessage procedure consisting of
• End -of-page (EOP) message is sent by the originating fax machine to indicate that all pages are transmitted. For a properly received page, the session will go to phase-E, and for a page with errors, the receiving fax machine may go to phase B waiting for the digital command signal (DCS) and training check.
• A multipage signal (MPS) is sent by calling the fax machine to indicate the end of the fax page and more pages to follow and returns to phase C2 to transmit the next page.
• End of message (EOM) indicates the end of the page, and both fax machines return beginning at phase B when digital identification signal (DIS) and DCS are exchanged again.
• Partial page signal (PPS) message is used in ECM to indicate the end of a partial block.
• Message confirmation (MCF) validates that the image is received and ready for more pages or to disconnect.
Phase E is for call release.
Phase-A Call Establishment. The fax call is established either through a manual process, in which the user dials a call and puts the machine into fax mode, or by automatic procedures, in which no human interaction is required. In both cases, the originating fax machine generates a 1100-Hz calling tone (CNG) and the terminating machine responds with a 2100-Hz called terminal identification tone (CED) or answering tone (ANS) at the beginning of the fax call. The SG3 V.34 fax machine generates an answer tone with an amplitude and 180 ° phase reversal modulation (/ANSam) tone instead of the ANS tone. These tones are used to recognize the presence of a fax call. At the end of phase-A, with a delay of 75 ± 20 ms, the called fax machine generates a mandatory V.21 signal for transmitting the fax call preamble and messages. In the
case, ANS is not available or is not properly interpreted, and V.21 preamble flags are used for fax call detection. Details on CNG, ANS family of tones, and V.21 preamble are given in the fax tones detection section. In the literature, the CED and ANS names are used interchangeably. Both names convey the same meaning. In this topic, both names are used.
Phase-B Premessage Procedure Capability Negotiations. During phase B, the fax machines negotiate the set of supported capabilities and agree to common fax capabilities and rates. The called fax machine transmits the mandatory DIS describing its capabilities to the calling fax. The calling fax then determines the common denominator for both machines and responds with a DCS to inform the called fax of the selected settings. The settings include
• Data rates supported
• Vertical resolution
• Image encoding
• Page width capabilities
• Maximum page length capability
• Handshake speed
• Error correcting mode
Optionally, both calling and called fax machines exchanges manufacturer-specific proprietary features through a nonstandard facility (NSF), from the called fax machine, and nonstandard facilities setup (NSS) from the calling machine. International phone numbers similar to caller ID are enchanged through called subscriber identification (CSI) and transmitting subscriber idenfication (TSI) from the calling fax machine. The modulation used for capability negotiation is defined in the V.21 modem.
The framing of V.21 messages is created by HDLC [ITU - T- T.30 (2005) , URL (HDLC)]. Every message starts with a preamble of flag characters 01111110 in binary bits or 0x7E in hexadecimal number format. The pattern of 0x7E is repeated for 1 second ± 15%. The initiating fax machine maintains a timeout of 3 seconds ± 15% for responses. When timeout occurs, the initiating fax machine retransmits the frame, and after three unsuccessful attempts, it disconnects. After exchanging its capabilities to determine common modulation scheme and data rates, the calling fax machine sends a training check function (TCF) pattern. The TCF test pattern contains continuous zeros for 1.5 seconds ± 10% to test end-to-end transmission. The modulation and data rate for TCF data is V.17, V.29, and V.27ter as specified in the DCS message.
The answering unit responds to the TCF with confirmation to receive (CFR) for the correct received TCF pattern. Failure to train (FTT) is issued for the TCF test pattern identified as bad. If the calling unit receives an FTT, the calling fax machine may fall back to the next lower rate and then it sends a new DCS message followed by a TCF test pattern at the new data rate. This process is continued until a TCF test pattern is received OK (correctly) or all
data rates have been attempted. Depending on the manufacturer, few fax machines may retrain at the same data rate when they receive the FTT message. This process is continued for a maximum of three times, and then the call gets disconnected. The CFR or FTT messages are sent using V.21 modulations.
Phase-C Image Transfer. In some documents, phase-C is divided into C1 and C2. After testing the phone line and agreeing to a common data rate, the calling fax machine starts sending the page data to the receiving facsimile device. The exact rate and modem used depends on the negotiations and training check. Image data characteristics are data compression, resolution, paper size, and so on, as specified in DCS and any one of the fax rates from V.27ter (2400/4800bps), V.29 (7200/9600bps), V.17 (7200/9600/12,000/14,400bps), and V.34 (14 rates from 2400 to 33,600 bps).
Phase-D Postmessage Procedures. Phase-D goes through several conditional operations and options. Immediately after the image, the calling fax terminal sends an EOP, MPS, or EOM handshake message to indicate final image, more images to come, or a request for polling or to change parameters like a different resolution or paper size. The answering unit responds to the EOP, MPS, or EOM messages with an MCF if the image was received correctly, the re-train positive (RTP) image was received OK but with some errors and the phone line must be retested before receiving additional images, or the retrain negative (RTN) is applicable for unacceptable image. If the calling unit receives an MCF message, then it sends pending additional images or it sends a disconnect command (DCN) and disconnects. If the calling unit receives an RTP, then it resends a DCS/TCF sequence to retest the phone line at the same data rate before sending any more images. If the calling unit receives an RTP in response to an EOP command, then it sends the disconnect command and disconnects the call. If the calling unit receives an RTN, then it falls back to the next lower data rate, sends a new DCS/TCF sequence to retest the phone line, and retransmits the page for which the RTN was received.
Phase-E Call Release. When the fax transmission is complete, the call is disconnected with a DCN command. Several modes of phase-E exist. Some popular disconnect modes are as follows:
• When a response to a specific command is not received in time, the fax machine retransmits the signal and after three unsuccessful attempts or until timeout occurs and then it enters into phase-E by sending the DCN message. Any terminal can send this DCN message.
• I f a transmitter receives, receive not ready (RNR) continuously for a period of 60 ± 5 seconds in ECM mode, it may transmit DCN and enter phase-E. The digital response RNR is used only in the T.4 error correction mode.
• Procedure interrupt disconnect (PID) is the digital response indicating that a message has been received but that additional transmissions are
not possible. After correction of all outstanding pages or partial pages, the transmitter shall enter phase-E. • DCN is used to switch back to voice mode in fax over IP. More detail on fax-to-voice call switching is given in topic 15.
14.4.1
Multiple Pages and Fax Call Phases
The call phases for multiple pages are shown in Fig. 14.4 . The call phases B, C1, and C2 are repeated depending on the postmessage commands. Immediately after the first page, the calling facsimile device sends an MPS to indicate that more pages will follow. At the last page transmission, the calling facsimile device sends an EOP to indicate that this is the final page. The answering unit may respond with MCF, RTP, or RTN. A few example message exchanges of multipage transmission are given below.
Example Situation-1. If the answering terminal responds with an MCF as a response to an image followed by an EOP, then the originating fax machine enters into phase-E. In this example, the call flow goes through the normal phases of phase-A, B, C1, C2, D, and E. This situation occurs only a simple page transfer without any errors. Upon receiving the MCF message as a response to an image followed by an MPS command, the originating fax machine enters into phase-C2 to transmit the next page as shown in Fig. 14.4. The call flow phases for multiple pages are phase-A, B, C1, C2, D, C2, D, . . . , C2, D, and E. This call flow indicates that all pages are transmitted completely without any errors. If any one of the pages is received with errors and it requires retransmission, then the call phases may change for multiple pages. These cases are explained in examples 2 and 3.
Example Situation-2. If the answering unit responds with an RTP as a response to the image followed by an MPS, then the originating fax machine enters into phase-B. The additional messages follow after retransmission of a DCS and training at the same or lower rate. The rate may fall back if fallback support is enabled in the DIS and DCS commands. In this case, the call flow phases for multiple pages are phase-A, B, C1, C2, D, C2, D, . . . , C2, D, B from DCS onward (DCS, training), C1 and C2 (page retransmission), D, and E. If the answering unit responds with an RTP as a response to the image followed by an EOP, then the originating fax machine will go to phase – E.
Example Situation-3. If the receiving terminal responds with an RTN as a response to the image followed by an EOP/MPS, the originating fax machine enters into phase-B. The additional messages will follow the retransmission of a DCS and training at a lower rate. In this example, the call flow phases are the same as in example-2 [i.e., phase-A, B, C1, C2, D, B from DCS onward
(DCS, training signal at a lower rate), C1 and C2 (page retransmission), D, and E]. The difference between the response to RTP and RTN is that on receiving an RTP, the originating terminal enters phase-E if it has completed the transmission of all pages. If transmission is not complete, it will enter phase-B and subsequently transmit the next page. On the contrary, with RTN, the originating terminal enters phase-B and then retransmits the current page for which RTN was received as shown in Fig 14.4.
Example Situation-4. In a few situations, the originating fax machine may change the mode or rate for the subsequent pages by sending an EOM instead of an EOP/MPS. In such cases, both fax machines will go to the beginning of phase-B. The receiving fax machine responds with an MCF in response to the EOM and enters into phase-B. Upon receiving the MCF in response to the EOM, the originating fax machine enters into phase-B. In this example, DIS and DCS are exchanged again after the MCF. The call flow phases are phase-A, B, C1, C2, D, B (DIS, DCS, Training, CFR), C1 and C2 (page retransmission),
D, and E.
14.4.2
Fax Call Timeouts
Fax communication takes place through complex and noisy networks where a packet can be corrupted or lost. To compensate for these impairments, the T.30 protocol specifies the retransmission handshaking messages. If a response to the message is not received within a specified time usually of within 3 seconds, then the fax machine resends the messages again. The messages are repeated up to three times or until an acknowledgment is received. The unacknowledged DIS messages are repeated every 3 seconds until the end of the 35-second timeout. The delay between the transmissions of the DIS signals shall be 4.5 seconds ± 15% for a manual receiving terminal and shall be 3 seconds ± 15% for automatic fax terminals. The delay between two consecutive messages is 75 ± 20ms [ITU-T-T.30 (2005)].
14.4.3
Fax Call with ECM
ECM is the error correction mode. The switched telephone network is prone to error during the transmission of fax data. These errors are created because of several impediments, such as telephone line noise from long-distance analog lines and from fax machine interaction. When such a transmission error occurs in a PSTN, page data may be corrupted. Two error control options are defined by the ITU. Before the optional ECM was approved in 1988 by the ITU [McConnell et al. (1999)], error concealment techniques were used to minimize the effect of transmission errors in the PSTN. The error concealment technique is possible when EOL code is transmitted between two scan lines. This technique was considered for MH and MR coding techniques used by G3 fax machines. In this technique, if the receiver does not detect the EOL code
at the expected location in the line data stream, it indicates that an error has likely occurred on the line after the recent correctly received EOL code. This option limits the effect of errors without correcting them.
The second option employs true error correction technology called ECM that is an optional transmission mode built into G3 fax machines. The ECM is mandatory when SG3 fax machines are used with a V.34 fax modem. The MMR line encoding requires the use of ECM as it does not use any EOL between scan lines. In ECM, errors are detected at the receiving end, but they are not corrected automatically; the receiving end reports these errors to the transmitting end. The transmitting fax machine will retransmit only those frames that were reported to be received with errors. Eventually, the errors are corrected by repetitive retransmissions of such frames. The T.30 Annex-A defines the error control mechanism to cope with transmission errors over PSTN for T.4 image data. The ECM is based on a half-duplex page-selective automatic repeat request (ARQ) technique.
An HDLC frame structure is used for all binary-coded facsimile message procedures. More details on framing are given in the section “V.21 HDLC Framing and Deframing.” Specific to HDLC framing in ECM mode, the facsimile information field (FIF) is a length of 257 or 65 bytes and is divided into two parts: the frame number (1-byte) and the facsimile coded data (FCD) field
(256 or 64 bytes).
In error correction mode, each page data is divided into 64 or 256 byte frames. These frames are transmitted as payload inside HDLC frames with a unique frame number and frame check sequence (FCS) that allows the receiver to detect any errors in the transmitted frame. Several frames constitute a block or partial page. Each partial page contains a maximum of 256 frames. The exact number and frame size is negotiated during phase-B using DIS and DCS messages. The receiving terminal must be able to receive 256 or 64 byte frames up to a maximum block size of 256 frames.
The transmitting terminal may send the block whose size is less than 256 frames at the end of each partial page if page data contain more than 256 frames or for simple page data. This block is called a short block. The frame size should not be changed during a transmission of one page. The T.30 fax call flow with ECM for multiple pages is shown in Fig. 14.5. After sending the partial page data, the delineation is obtained by the transmission of the return to control for partial page (RCP) frame. This transmission tells the T.4 modulation system to drop off the line and be replaced by binary coded modulation with a delay of 75 ± 20 ms for postcontrol message command exchange. The receiving fax machine initiates the postmessage command reception if it detects at least one RCP frame. At the end of each block or partial page, the originating fax machine sends the T.30 PPS message and waits for MCF from the receiving fax machine. The originating fax machine can send any of the following messages after each partial page. The partial page signal-NULL (PPS-NULL) is transmitted between partial pages or a block of page data. The partial page signal-multipage signal (PPS-MPS) is transmitted between pages.
Figure 14.5. Fax call flow with ECM for multiple pages.
The partial page signal-end of procedure (PPS-EOP) is transmitted at the end of the last partial page of the last page data. The partial page signal-end of message (PPS-EOM) is transmitted at the page boundary to change the modulation speed and frame size.
The receiving fax machine unit responds to these postmessage commands with either MCF if the partial page was received correctly, or the partial page request (PPR) if the partial page is not received correctly, or the RNR if the receiver is not ready to receive more data. When the partial page is not received satisfactorily, the receiver requests the retransmission of bad frames by frame number. The receiver respond with a PPR to seek retransmission of the frames received with errors. When a PPR is received, the transmitting
terminal retransmits the requested frames specified in the PPR information field. Multiple PPRs are possible. When a PPR is received four times for the same block because of errors, either an end of retransmission (EOR) command is transmitted or a continue to correct (CTC) command is sent for continuous retransmission. The calling fax terminal decides to terminate the retransmission of error frames in response to the fourth PPR received; it sends the EOR command. The calling unit transmits the next partial block upon receiving the ERR command in response to the EOR command. As indicated in Fig. 14.5 , the command CTC comes from calling (transmitting) the fax terminal in response to the fourth PPR received, and it indicates that the transmitting terminal will send the frames requested in the PPR information field. When the transmitter receives PPR four times, the modem speed may either fall back or continue at the same speed in accordance with the decision of the transmitting terminal using a CTC command. The receiving terminal can accept the CTC message and can respond back with a CTR response to continue to correct. The fax call with two pages of data with ECM is shown in Fig. 14.5. The first page is divided into two partial blocks (0,0) and (0,1), and the second page has only one partial page block (1,0). As illustrated in Fig. 14.5, the receiver can respond to a postmessage sequence with RNR indicating that the receiver is not ready to receive any more data. The calling terminal then queries the receiving fax terminal with a receiver ready (RR) message. If the receiver unit is still busy (e.g., busy with printing), then the receiving terminal repeats the RNR message. The sequence RNR/RR, RNR/RR, RNR/RR can be repeated up to60 ± 5 seconds as defined in the T.30 recommendation.
