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Analyzing a Basic Fax Call

Although the phases of a fax call provide a general overview of how fax calls work, the next step is to look into the actual T.30 messages and tones that make up these phases. Analyzing a basic call is the easiest way to get acquainted with fax signals. Figure 2-9 illustrates an example of a typical, two-page G3 fax transaction.

In Figure 2-9, the fax machine on the left is originating a call to the fax machine on the right. The CNG and CED signals are simple tones that occur in the very beginning of the call. These tones are followed by the T.30 messages that set up the parameters that will be used for the message transmission. Other than the CNG and CED signals (which are just single-frequency tones), all the other T.30 signals are modulated using V.21 at 300 bps; these are shown as low-speed messages in Figure 2-9. Fortunately, the message contents are relatively small, so the 300 bps speed used in the T.30 messages is adequate. The actual message or fax page information is sent using a modulation of a much higher speed. The exact modulation and speed is determined during exchange of the T.30 DIS and digital command signal (DCS) messages. Using a V.17 modulation, normal G3 faxes can transmit pages at 14400 bps.

The T.30 messages illustrated in Figure 2-9 are critical to every basic G3 fax transaction. Now we'll take an in-depth look at these signals along with some other important ones that are not included in the figure.

Figure 2-9 Two-Page G3 Fax Transaction

Figure 2-9 Two-Page G3 Fax Transaction

CNG (Calling Tone) 1100 Hz Every 3 Sec for .5 Sec >■

CED ( Called Terminal Identification) 2100 Hz Tone

DIS (Digital Identification Signal) with Optional NSF and CSI

DCS (Digital Command Signal) with Optional TSI

TCF (Training Check) High Speed Modulation Training

CFR (Confirmation to Receive)

Fax Page Transmission

MPS (Multipage Signal)

MCF (Message Confirmation)

Fax Page Transmission

EOP (End of Procedure)

MCF (Message Confirmation)

DCN (Disconnect)

CNG Tone

As the call is connecting, the originating fax machine starts playing a CNG or calling tone. The CNG signal is simply an 1100 Hz tone that plays for half a second, and then repeats every three seconds. As defined in the T.30 specification, it is permissible for the timing of the CNG tone to vary by ± 15 percent, and for the frequency to be within 38 Hz of the 1100 Hz value.

Call Setup/Tones

Low Speed

High Speed

The purpose of this tone is to notify the answering device or person that a fax machine is originating the call on the other end. People who have had the unpleasant experience of a fax machine mistakenly trying to continuously fax something to their house or office phone are probably familiar with this tone.

The CNG tone has other functions, too. The tone lets the answering fax machine know that the originating fax machine has a document to send and is awaiting a DIS to begin fax negotiation. The CNG is also used by some multifunction devices to determine whether an incoming call is fax or voice so that the call can be handled appropriately. Figure 2-10 illustrates the T.30 fax CNG tone.

Figure 2-10 T.30 Fax CNG Tone 1100 Hz

CED Tone

When the terminating fax machine answers the call, it sends a called terminal identification or CED signal. The CED signal is just a 2100 Hz tone that typically plays for about 3 seconds. As defined in T.30, the CED tone is "a continuous 2100 Hz ± 15 Hz tone for a duration of not less than 2.6 s and not more than 4.0 s." Figure 2-11 illustrates a T.30 fax CED tone.

Figure 2-11 T.30 Fax CED Tone

2100 Hz

The purpose of the CED tone is to disable any echo suppressors that are in the call path. The 2100 Hz CED tone defined in ITU-T T.30 is identical to the 2100 Hz tone defined in ITU-T Recommendation G.164. The G.164 specification covers echo suppressors and their operation and details a 2100 Hz tone as the signal to disable echo suppressors on a line. A G3 fax transmission requires that all echo suppressors in the fax path be disabled to prevent distortion in the modulation.

This tone can easily be heard by calling a fax from any phone. The CED should be the first tone that is heard, and it identifies the end device as a fax or a low-speed modem.

DIS, NSF, and CSI Messages

Unlike the preceding CNG and CED signals, the digital identification signal (DIS), nonstandard facilities (NSF), and called subscriber identification (CSI) messages actually contain data and are transmitted using the V.21 modulation.

These three messages occur after the receiving side answers the incoming call and plays the CED tone. Each message is encapsulated in its own HDLC frame. The DIS message is the most important of these messages, and the ITU-T T.30 specification considers it mandatory. The NSF and CSI are optional, and all optional messages are transmitted before the mandatory DIS message. Figure 2-12 illustrates the arrangement of the NSF, CSI, and DIS frames.

Figure 2-12 NSF, CSI, and DIS Frame Arrangement

Preamble	NSF	CSI	DIS
	(optional)	(optional)	(mandatory)

The DIS message contains the capabilities of the terminating fax device. Parameters such as page modulation speed, image resolution, support of ECM, and page size are contained within the FIF of the DIS message.

Figure 2-13 diagrams the basic DIS frame and highlights some of the important DIS FIF bits. A comprehensive list of all the DIS FIF bits can be found in Table 2 of ITU-T Recommendation T.30. The FIF bits just define the capabilities of the terminating fax device. They let the originating fax device know what the terminating device can and cannot support. The originating fax device then analyzes these capabilities and decides what parameters to use for the fax transmission.

NOTE You can download ITU-T Recommendation T.30 from http://www.itu.int/rec/ T-REC-T.30-200509-I/.

Figure 2-13 T.30 DIS Frame

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11001000			01111110

Preamble	NSF	CSI	DIS
	(optional)	(optional)	(mandatory)

FCF - DIS 00000001

Important DIS FIF Bits
Bit Number	Parameter
11, 12, 13, 14	Modulation Capabilities
16	2-D Encoding Support (MR)
17, 18	Scan Line Recording Width
19, 20	Maximum Recording Length
21, 22, 23	MSLT (Minimum Scan Line Time)
27	ECM (Error Correction Mode)
31	T.6 Encoding Support (MMR)

FCF - DIS 00000001

The NSF or nonstandard facilities message allows fax capabilities to extend beyond what is defined in ITU-T Recommendation T.30. By using a specific country and vendor or terminal provider coding in the FIF field, a terminating fax device can signal that it is capable of vendor proprietary messaging and features. Assuming that the originating fax device is also enabled with this same capability, it will respond with a nonstandard facilities setup (NSS), and the two machines will proceed into a proprietary mode of operation that may deviate from the T.30 specification. Figure 2-14 illustrates the T.30 optional NSF frame.

Figure 2-14 T.30NSFFrame

Preamble	NSF	CSI	DIS
	(optional)	(optional)	(mandatory)

Figure 2-14 T.30NSFFrame

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11000000			01111110

1011 0101 (United States)

0000 0000 0101 0001 (Telogy)

1011 0101 (United States)

0000 0000 0101 0001 (Telogy)

Within the FIF of the NSF, there are at least 2 bytes of data consisting of country codes and terminal provider codes. The country codes are defined in ITU-T Recommendation T.35, and the terminal provider codes are defined by national organizations outside of the ITU. The sample NSF frame detailed in Figure 2-14 shows the country and provider codes set to United States and Telogy, respectively. These are the default settings seen when the NSF value is set by Cisco gateways.

Like the NSF, the CSI field is also optional. The purpose of the CSI is to identify the device that is being called. Some fax machines display this value when it is received so that the user can confirm their fax destination.

The CSI consists of 20 digits, and it should contain the full international phone number of the device, which includes the + character, the telephone country code, area code, and subscriber number. The exact binary encoding of these FIF values is defined in Table 3 of ITU-T Recommendation T.30. However, it is common to see ASCII text in this field as an alternative. Many fax devices support ASCII values in the CSI FIF, and this usually does not cause a problem even though ASCII text is not defined in T.30. Figure 2-15 diagrams the CSI frame format.

Figure 2-15 T.30 CSIFrame

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11000000			01111110

Preamble	NSF	CSI	DIS
	(optional)	(optional)	(mandatory)

FCF - CSI 00000010

Subscriber Number

DCS and TSI Messages

The digital command signal (DCS) and transmitting subscriber identification (TSI) messages are typically seen as a response to the DIS message. Like the DIS message, the DCS message is the important mandatory message, whereas TSI is similar to CSI and is optional. Figure 2-16 shows the arrangement of the TSI and DCS messages following the fax preamble.

Figure 2-16 TSI and DCS Frame Arrangement

Preamble	TSI	DCS
	(optional)	(mandatory)

The DCS message is transmitted by the originating fax device after it has analyzed the capabilities specified in the DIS message. Because the originating fax machine already knows its own capabilities, it can easily compare these settings with the settings received in the DIS. The DCS message then commands the terminating fax to use specific parameters for the fax transmission. The originating fax machine has the responsibility of commanding the terminating fax to only use a setting or parameter that was listed as an option in the DIS message.

The DCS FIF uses the same bits as the DIS FIF and these bit settings are defined in Table 2 of ITU-T Recommendation T.30. However, even with the same bits being used, the settings might be somewhat different. For example, when looking at the modulation bits (11, 12, 13, 14), the DIS bit settings specify a modulation like V.17 or V.29. Meanwhile, the DCS

can use the same bits to define a specific speed within that modulation, such as V.17 14400 bps or V.17 9600 bps. Figure 2-17 diagrams the T.30 DCS message and highlights some of the important FIF bit settings.

Figure 2-17 T.30 DCS Frame

Preamble	TSI	DCS
	(optional)	(mandatory)

Figure 2-17 T.30 DCS Frame

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11001000			01111110

FCF- DCS 11000001

FCF- DCS 11000001

Important DIS FIF Bits
Bit Number	Parameter
11, 12, 13, 14	Modulation Capabilities
16	2-D Encoding Support (MR)
17, 18	Scan Line Recording Width
19, 20	Maximum Recording Length
21, 22, 23	MSLT (Minimum Scan Line Time)
27	ECM (Error Correction Mode)
31	T.6 Encoding Support (MMR)

The optional TSI message carries the subscriber number of the originating fax device. Like the CSI, this value is programmed into the fax device by the user, or in some cases there is a default value already assigned. The terminating fax device may display the TSI information for the user so that the identification of the device that is sending the fax can be confirmed. Figure 2-18 illustrates the T.30 TSI message.

The T.30 specification defines other optional HDLC frames that can be sent along with DCS other than TSI. This includes such frames as NSS, subaddress (SUB), and sender identification (SID), among a few others. These optional frames are rarely seen; for additional information on these frames, refer to Table 2-3 or Section 5.3.6.1.3 of ITU-T Recommendation T.30.

Figure 2-18 T.30 TSIFrame

Preamble	TSI	DCS
	(optional)	(mandatory)

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11000000			01111110

FCF - CSI 11000010

Subscriber Number

TCF, CFR, and FTT Messages

The training check frame (TCF) and confirmation (CFR) are the last transactions that occur before the fax page transmission begins. Within the DCS message, a modulation and speed has been sent to the receiving fax device. Both fax devices are now prepared for a test sequence pattern of all 0s sent for 1.5 seconds using the modulation and speed specified in the DCS message. This test pattern of all 0s is known as the TCF and is sent by the originating fax machine. Unlike most other T.30 messages, it does not use an HDLC frame.

If the test pattern is received with little or no errors, the receiving fax machine sends a CFR response. Otherwise, a failure to train (FTT) message is sent. Upon receipt of a FTT, the originating device resends the DCS to retry the same training speed again, fall back to a slower speed within the same modulation, or even try a different modulation. The originating device then sends the TCF again using any new modulations or speeds dictated by the DCS. This process repeats itself until the receiving fax device gets a clean TCF and responds with a CFR. Figure 2-19 demonstrates a retraining and fallback scenario between two fax machines. In this scenario, notice that the originating fax machine even repeated trainings at 4800 bps and 2400 bps in an effort to get a successful TCF through.

Figure 2-19 Fax Retraining and Fallback Scenario

Figure 2-19 Fax Retraining and Fallback Scenario

CNG (Calling Tone) CED (Called Terminal Identification)

DIS DCS

FTT DCS

FTT DCS

FTT DCN

Call Setup/Tones Low Speed High Speed

The TCFs make sure that the transmission path can handle the modulation and speed before actual page data is sent. If the TCF can be sent across the transmission path cleanly, then usually the fax page data transmits clean, too, with little or no errors.

Both the CFR and FTT messages use HDLC frames, but they contain no FIF. Only the FCF identifying the message as a CFR or FTT is present. This occurs because these messages are used only to validate the TCF, and no additional information is necessary. Figure 2-20 diagrams a T.30 CFR frame.

Figure 2-20 T.30 CFR Frame

Figure 2-20 T.30 CFR Frame

FCF-CFR 00100001

After the first page has been sent using the modulation specified in the DCS, the page information is followed by a multipage signal (MPS) message. Like the rest of the T.30 messages, MPS is sent using the 300 bps V.21 modulation even though it immediately follows the higher-speed page data. MPS informs the receiving device that the page is completed and that the sender is waiting for a confirmation message before transmitting the next page.

Instead of the MPS, there could be an end of procedure (EOP) message following a fax page. This message lets the receiving fax device know that the preceding page is complete and that there are no further pages or other documents to be sent. The originating side is ready to disconnect the call after this page has been confirmed.

A less-used signal called end of message (EOM) can also be used in place of an MPS or EOP to indicate that a renegotiation of settings is required. This message is most commonly seen when the next page is of a different page resolution than the preceding page.

After receiving a fax page and an MPS or EOP message, the terminating fax device has a few common messages with which to reply. Usually the response is a message confirmation (MCF). MCF indicates that the previous page was received satisfactorily and that the receiving device is ready for additional pages.

If the page is received with some errors, usually a retrain positive (RTP) or retrain negative (RTN) message is sent rather than the MCF. RTP does indicate that the page was received satisfactorily but that there were probably some issues or small errors not severe enough to warrant a resending of the entire page. Instead, perhaps, retraining to possibly a different modulation will remedy any problems or errors for subsequent fax pages.

An RTN response means that the page was not received satisfactorily and a retrain must take place before further page information can be sent. Ideally, the page that was not received satisfactorily should be re-sent. However, this does not always happen because not all fax machines buffer the faulty page, making it impossible for this page to be retransmitted. In addition, even if the faulty page is re-sent, a mechanism does not exist for the receiving side to know whether the page following an RTN is a retransmission. Therefore, you might find inconsistencies in the ways that different fax machine vendors handle RTN messages.

Fax devices typically have predefined error thresholds for determining when an RTP or RTN message needs to be sent. The main type of error that is tracked is the number of invalid scan lines. When the number of bad scan lines for a page reaches the RTP or RTN threshold, one of these messages is triggered. These thresholds may be configurable on some fax devices, but they are usually preset to vendor-specific values and are not changeable.

The disconnect, or DCN message, is the last message seen in a fax transaction and indicates the initiation of Phase E, call disconnect. A response or acknowledgment is not needed for this message. This message signifies a graceful disconnect to the fax call, and both sides prepare for a new fax transaction by returning to an on-hook state.

Like the FTT and CFR messages, the MPS, EOP, MCF, RTP, RTN, and DCN message frames do not contain an FIF. Varying FCF values are the main criteria that distinguish these messages from one another. Figure 2-21 illustrates the common frame format shared among the MPS, EOP, MCF, RTP, RTN, and DCN messages.

Figure 2-21 T.30 MPS, EOP, EOM, MCF, RTP, RTN, and DCN Frames

Preamble	HDLC
	Frame

Figure 2-21 T.30 MPS, EOP, EOM, MCF, RTP, RTN, and DCN Frames

Flag	Address	Control	Information	FCS	Flag
01111110	11111111	11001000			01111110

Message Type	Format
MPS	X111 0010
EOP	X111 0100
EOM	X111 XXXX
MCF	X011 0001
RTP	X011 0011
RTN	X011 0010
DCN	X101 1111

Other T.30 Messages

Table 2-3 provides a comprehensive list of the G3 fax messages defined in the T.30 Recommendation dated September 2005. Although many of the messages in Table 2-3 have already been discussed earlier in this chapter, many more have not been specifically addressed because they are rarely used. All are mentioned here for completeness and as a reference in case you ever encounter them.

Table 2-3 Comprehensive Listing of Fax T.30 Messages

Table 2-3 provides a comprehensive list of the G3 fax messages defined in the T.30 Recommendation dated September 2005. Although many of the messages in Table 2-3 have already been discussed earlier in this chapter, many more have not been specifically addressed because they are rarely used. All are mentioned here for completeness and as a reference in case you ever encounter them.

Table 2-3 Comprehensive Listing of Fax T.30 Messages

Message or Signal	Description
CNG (calling tone)	An 1100 Hz tone for 0.5 second duration occurring every
	3 seconds. Indicates a calling nonspeech terminal.
CED (called terminal identification)	A 2100 Hz tone lasting between 2.6 seconds and 4.0
	seconds that occurs when a called fax device answers.
	Disables echo suppressors in the transmission path.

Table 2-3 Comprehensive Listing of Fax T.30 Messages (Continued)

Message or Signal Description

Initial Identification: Messages from the called to the calling terminal

DIS (digital identification signal) Contains the capabilities of the called fax device.

Includes important parameters such as modulations, page resolutions, page compressions, and ECM ability.

CSI (called subscriber identification) An optional signal sent along with the DIS that indicates the identification (usually the international phone number) of the called fax terminal.

NSF (nonstandard facilities) An optional signal sent with the DIS that notifies the calling device of the ability to handle proprietary vendor encodings beyond what is defined in T.30.

Command to Send: Messages from the calling terminal wanting to be a receiver of fax information (also referred to as polling)

DTC (digital transmit command) Like a DCS message, this is a command response to a

DIS that defines the parameters that will be used for the fax transaction.

CIG (calling subscriber identification) Optional message that indicates the identification of the calling terminal. Functionally similar to TSI.

CIG (calling subscriber identification) Optional message that indicates the identification of the calling terminal. Functionally similar to TSI.

NSC (nonstandard facilities	Optional message response to an NSF. Signals the ability
command)	to handle proprietary vendor encodings beyond what is
	specified in T.30.
PWD (password)	Optional message used as a password for the polling
	mode.
SEP (selective polling)	Optional message that defines a subaddress for the
	polling mode or identifies a specific document number.
PSA (polled subaddress)	Optional message that defines a subaddress for polling.
CIA (calling subscriber Internet	Optional message that defines an Internet address for the
address)	calling fax device.
ISP (Internet selective polling	Optional message that defines an Internet address for the
address)	calling device in polling mode.
Command to Receive: Messages from the transmitter to the receiver

DCS (digital command signal) Mandatory message that defines the parameters for the fax transaction.

TSI (transmitting subscriber identification)

Optional message that identifies the calling terminal. It is usually the subscriber number of the calling device.

Table 2-3 Comprehensive Listing of Fax T.30 Messages (Continued)

Message or Signal

Description

NSS (nonstandard facilities setup)

Optional message sent as a response to an NSC or NSF signal. Used in setting up proprietary encodings beyond what is defined in the T.30 specification.

SUB (subaddress)

Optional message that specifies additional addressing or routing information for the terminating destination.

SID (sender identification)

Optional message that allows for the specification of a user-configured sender identity.

TCF (training check)

A modulated series of all 0s sent as a test to verify the transmission path before transmitting the actual page information.

CTC (continue to correct)

Only used with ECM. A response message sent after the fourth PPR message indicating that the transmitter will continue to correct the previous message.

TSA (transmitting subscriber Internet Optional message that details the Internet address of the address) transmitting device. Used only when Internet capabilities were previously set in the DIS.

IRA (Internet routing address)

Optional message specifies an Internet address that can be used to provide additional routing information for gateways. Only sent if bit 102 in DIS/DTC is set.

Premessage Response Signals: Messages from the receiver to the transmitter

CFR (confirmation to receive)

Message that confirms the premessage procedure and the TCF. Page data can now be sent.

FTT (failure to train)

Message that rejects the TCF and requests a retrain.

CTR (response for continue to correct)

An ECM message that is the response to a CTC that indicates that the receiving device accepts the contents included with the CTC message.

CSA (called subscriber Internet address)

Optional message sent along with CFR that specifies the called device's Internet address. Only sent when Internet capabilities in DCS are enabled.

Post-Message Commands: Phase D messages from the transmitter to the receiver

EOM (end of message)

Indicates the end of a fax page and a return to Phase B.

MPS (multipage signal)

Indicates the end of a fax page and a return to Phase C.

Table 2-3 Comprehensive Listing of Fax T.30 Messages (Continued)

Message or Signal	Description
EOP (end of procedure)	Indicates the end of the fax page and that there is no
	further information to be sent. Proceed to Phase E.
PRI-EOM (procedure interrupt-end	Same as an EOM message along with the additional
of message)	request of operator intervention.
PRI-MPS (procedure interrupt-	Same as the MPS message along with the additional
multipage signal)	request of operator intervention.
PRI-EOP (procedure interrupt-end	Same as the EOP message along with the additional
of procedure)	request of operator intervention.
EOS (end of selection)	Optional message used in conjunction with SEP to
	indicate the end of the selected document.
PPS (partial page signal)	Indicates the end of a page or partial page of information
	during ECM.
EOR (end of retransmission)	Indicates the end of retransmission of error frames for the
	previous partial page during ECM.
RR (receive ready)	Indicates receiver status and flow control during ECM.
Post-Message Responses: Phase D messages from the receiver to the transmitter
MCF (message confirmation)	Indicates satisfactory message reception and that
	additional information may follow.
RTP (retrain positive)	Indicates satisfactory message reception and that
	additional information may follow but only after a
	retraining.
RTN (retrain negative)	Indicates that the preceding message has not been
	received satisfactorily. A retraining is needed before
	further information can be sent.
PIP (procedure interrupt positive)	Indicates that the previous message has been received
	and that operator intervention is needed before further
	transmissions are possible.
PIN (procedure interrupt negative)	Indicates that the previous or in-process message was not
	received satisfactorily and operator intervention is
	needed before further transmissions are possible.
PPR (partial page request)	Indicates that there are errors in the previous partial page
	during ECM. The frames specified must be re-sent.
RNR (receive not ready)	Indicates that the receiver is not ready for more data
	during ECM.
ERR (response for end of	Response for the EOR message.
retransmission)

Table 2-3 Comprehensive Listing of Fax T.30 Messages (Continued)

Message or Signal	Description
FDM (file diagnostics message)	Optional message that can be used in place of MCF when
	binary file transfers are being sent.
Other Line Control Signals: Messages for handling errors and maintaining control
of the connection
DCN (disconnect)	Indicates call release and the beginning of Phase E. No
	response is required.
CRP (command repeat)	Optional message indicates that the previous command
	was received in error and it needs to be retransmitted.
FNV (field not valid)	Optional message indicates that one of the following
	messages is not accepted or is invalid: PWD, SEP, SUB,
	SID, TSI, PSA, or secure fax signal.
TNR (transmit not ready)	Optional message available in flow control mode that
	indicates the transmitter is not ready.
TR (transmit ready)	Optional message available in flow control mode that
	requests the transmitter status.

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