Group 3 Facsimile Communication

Copyright © 1997 Garret Wilson

Essay for CS4323 - Dr. Letcher


Facsimile machines, commonly known as, "fax machines," have been in use for many years — in fact, the first one was patented in 1843! However, fax machines were not widely used until the 1970's. The first fax machines were analog devices, and consequently they printed information at the exact time that they received it. The receiving machine would need to be at the correct place on the page to correspond with the information being sent. This required very accurate synchronization, and many times the resulting image was corrupted. Even on a single line, the printing was susceptible to "jitters."

Some of the earliest standards for fax machines used relatively slow communication with no compression. One standard, classified as, "Group 1," used a very straightforward frequency modulation to convey image information. Each frequency indicated a different color, and there were only two colors: 1300Hz represented a white pixel, and 2100Hz represented a black pixel. This method would normally take about six minutes to transmit a single page of information.

Group 2 changed the modulation method to amplitude modulation, and suppressed the sideband to compress bandwidth. White pixels were indicated by the maximum amplitude signal, and black pixels were indicated by the minimum amplitude signal. This resulted in a transmission time of approximately three minutes per page.

Currently, the most widely used type of facsimile transmission is Group 3. This type of fax communication uses a modulation method that combines amplitude and phase modulation. Furthermore, the digital data is compressed using run-length encoding, resulting in a transmission time of less than one minute per page. One other type of fax, Group 4, is used primarily with Public Data Networks. Here, we will deal with the required parts of Group 3, which is the most popular and is used over standard telephone lines.

Standards-Making Groups

The Group 3 specification was mostly formed by the International Telegraph and Telephone Consultative Committee (CCITT). The CCITT is part of the International Telecommunication Union (ITU), which is part of the United Nations. Participating countries have signed treaties which allow the resulting standards to be meaningful. In the United States. The Telecommunication Industries Association (TIA), which is part of the Electronic Industries Association (EIA), is responsible for handling telecommunication matters. The TR-29 group, Facsimile Equipment and Systems, eventually became the U.S. fax technical group for CCITT.

TR-29 has been very active in helping create fax standards. In fact, TR-29 produced the U.S. national standards for Group 3 and Group 4 before the CCITT standards were published. There are therefore separate standards numbers for EIA and CCITT: "Group 3 Apparatus for Transmission" is both EIA-465 and CCITT T.4, while "Procedures for Document Facsimile Transmission" is both EIA-466 and CCITT T.30.

Image Acquisition and Manipulation

Image data is typically acquired from a page using a Charge Coupled Device (CCD) which senses the brightness of the pixels on the page.

CCD -> A/D Converter -> MH/MR/MMR Compression

The CCD reads the data from the page using 1728 pixels per line. Two lines of 1728 pixels each are read into memory and manipulated at a time. These two lines are compressed using three compression methods: Modified Huffman (MH), modified read (MR), and modified modified read (MMR). These methods are referred to as run-length encoding. This encoding results in information that is 1.5th to 1/20th of its original size. This data can be further compressed by the digital modulation method used, which will be discussed later. The receiving procedure is the opposite of the transmission procedure.

Modified Huffman Coding

One of the compression methods used is a modification of the Huffman coding which was relatively simple to implement and royalty free when Group 3 was being developed. This is a run-length encoding method, which means that it uses special binary values to represent runs of various lengths. Usually, pixels are likely to be followed by a pixel of the same color — it is very unlikely that pixels will alternate between one black and one white pixel. Therefore, the substitution of certain predefined codes for black or white runs of certain lengths results in less bits. Redundant information is eliminated.

Modified Read and Modified Modified Read Coding

Similarly, there is usually a high incidence of black or white duplication in the vertical direction. Two lines are scanned in at a time, and many times the MR and MMR methods of compression, which use vertical duplication to compress data, result in a higher degree of compression than the Modified Huffman coding. Facsimile machines can thus choose among methods to achieve the highest compression.

Group 3 Modulation Methods

Group 3 can use several baud rates, which, based upon the different phase changes, can produce several data transfer speeds measured in bits per second (bps). Of these modulation methods, the CCITT Recommendation T.4 only requires support of two: V.27ter for sending data at 4800 and 2400bps, and V.21 for 300bps handshake signaling. V.21 is used mainly before and after each page; V.27ter is used for the transmission of actual image data. The other modulation methods are optional. Here, we will only discuss the two required types, V.27ter and V.21. V.21, being only a handshaking method, is not shown on the chart.

Bits per Second (BPS) Baud Rate Bits per Sample Type Carrier Frequency Bandwidth (in Hertz)
14400bps 2400 baud 6 V.17 1800Hz 550-3050
12000bps 2400 baud 5 V.17 1800Hz 550-3050
9600bps 2400 baud 4 V.29 1700Hz 450-2950
7200bps 2400 baud 3 V.29 1700Hz 450-2950
4800bps 1600 baud 3 V.27ter 1800Hz 950-2650
2400bps 1200 baud 2 V.27ter 1800Hz 1150-2450

V.27ter Modulation

This type of data modulation is the only one required for fax transmission. It uses phase changes to indicate different pixel states or, as we have seen, binary digits representing pixels in compressed format. When V.27ter changes phase 1200 times a second (1200 baud), it uses one of four different phases to indicate one of four different values, which is the same information encoded in two bits. Therefore, at 1200 baud, each change in phase holds the information of two bits, resulting in a doubling of the information being passed across the telephone lines — 2400bps.

V.27ter can also operate at 1600 baud. At this rate of phase change, each change represents one of eight possible phases, which is the same information that is encoded in three bits. Since each phase change sends three bits of information, the effective data rate is three times as much — 4800bps. Both baud rates of V.27ter are shown in the tables below.

Tribit ValuesPhase Change
0 0 1 0
0 0 0 45
0 1 0 90
0 1 1 135
1 1 1 180
1 1 0 225
1 0 0 270
1 0 1 315
Dibit ValuesPhase Change
0 1 0
0 0 90
1 0 180
1 1 270

V.21 Modulation

V.21 is not used to transmit normal image data; rather, it is used as a handshaking message modulation. This modulation technique is used to negotiate image data rate, image resolution, image compression, and other image and communication aspects. V.21 operates at the relatively slow speed of 300bps half duplex. Using Frequency Shift Keying (FSK), this method represents binary information by two frequencies. A binary zero is represented by 1850 Hz, and a binary one is represented by 1650 Hz.

Fax Call Connection Procedures

Two Group 3 fax machines have a certain procedure for connecting, transmitting one or more pages, and disconnecting. One fax machine takes on the job of initiating the connection; we will refer to this machine as the sender. The other machine, which we will refer to as the receiver, follows certain procedures for responding to the initiated connection and receiving the image being transferred. CCITT Recommendation T.30 lays out five procedures for this process, which are referred to as Phases A-E. They consist of the Call Establishment, Pre-message Procedure, Message Transmission, Post-message Procedure, and Call Release.

Phase A — Call Establishment

A fax call may be established manually or automatically. In this phase, the sender dials the number of the receiver, and immediately begins transmission of a fax announce tone, which is a 1100 Hz tone which lasts for ½ second, and is repeated every three seconds. This tone indicates that the incoming call is a fax transmission. Once the receiver goes off hook, it can recognize the fax announce tone and continue with the transmission. Furthermore, in manual operation, a human operator can recognize the tone as a fax transmission, and instruct the fax machine to continue with the transmission.

Once the receiver recognizes the incoming fax transmission as such, it transmits the fax answer tone. This tone is generated at 2100 Hz and lasts for three seconds. The sender will then know that the call has been recognized by the receiver, and the transmission can go to the next phase. If the call is being placed manually, the operator on the sender side will recognize the fax answer tone, and he/she can instruct the sending machine to continue with the transmission.

Phase B — Pre-message Procedure

In Phase B, the fax machines use V.21 at 300bps FSK to perform handshaking. First, the receiver sends a Digital Identification Signal (DIS), which specifies the unit's capabilities. The sender detects the DIS, and transmits a Digital Command Signal (DCS), which specifies the transmission parameters. The receiver then detects the DCS, and the transmission may advance to the next phase. There is also a Digital Transmit Command (DTC) which is a polling request for pollable documents and capability definition.

Pre-message handshaking uses 300bps FSK to transmit information in the following format: First, for each code (DIS, DCS, or DTC) there is a preamble which insures that the following data will be able to pass unimpaired. This is usually a series of flag sequences for one second. Then, the following information is passed from sender to receiver:

Flag (7Eh) Address (FFh) Control (C8h) Message Type (DIS, DTC or DCS) Fax Info (3 or more bytes) Frame Checking (2 bytes) Flag (7Eh)
8 bits
8 bits
Message Type
8 bits
Fax Info
Three or more bytes
Byte 1
Group 2 capabilities (00 if no Group 2 capability)
Byte 2
Data rate and resolution capabilities
Bytes 3
Paper size and scan time capability
Bytes 4-N
Error Correction Mode (ECM) and enhanced features support
Frame Checking
Two bytes

Phase C — Message Transmission

During the message transmission phase, the sender first sends phasing training information (referred to as the TCF pattern), which can be anywhere from 2400bps to 14400bps. This is necessary to establish at what speed both sender and receiver can reliably transmit the image data. If something goes wrong, the receiver will send back a Failure to Train (FTT) message, which indicates that retraining should occur. If the training occurred correctly, the receiver sends back a Confirmation to Receive (CFR), which indicates that the training was successful and the receiver is ready for the actual image data.

At this point, the sender sends the image data to the receiver using one of the modulation techniques described above (V.27ter, V.29, or V.17). As mentioned earlier, the image data can be sent faster than the baud rate because of two things: a) pre-sending image data compression, and b) phase modulation which allows multiple bits to be sent with each phase change.

Phase D — Post-message Procedure

After the digital image data has been send across the communication channel, the sender sends a Return to Control (RTC) code, which switches both faxes back to 300bps. The sender then sends another code, called the End of Procedure (EOP) signal. The receiver acknowledges the end of the transmission, and sends a Message Confirmation (MCF) which indicates that the receiver received the page successfully.

Phase E — Call Release

To complete the call, the sender sends another code, the Disconnect (DCN) signal to the sender. Then, both fax machines disconnect from the telephone line. At this point, another transmission can occur, starting at Phase A.

Error Control

The original Group 3 specification doesn't use any true error-detection or correction aspects. Recently, however, there have been two additions to the classic Group 3 transmission methods. One limits the effects of errors, and the other offers true error detection and correction.

The first method applies to the Modified Huffman coding. Each 1728-pixel line is divided into groups and encoded separately. This is supposed to limit any errors to a portion of the scan line instead of the entire line. However, the improvements from this technique are minor.

The second method is more widespread, and offers true error-correction. This method divides the image data into High-level Data Link Control (HDLC) frames of 256 bytes each. After each frame, a redundancy code is transmitted, which allows the receiver to detect any errors in the transmitted frame. After the entire page has been transmitted, the receiver requests the frames which contained errors. If the same frame is requested four times because of errors, the transmission will stop or a lower speed will be negotiated.

This second method uses 16-bit Cyclic Redundancy Checking (CRC) to detect errors, using the following polynomial generator: X16+X12+X5+1=10001000000100001. This method also uses "bit stuffing" in order to prevent the transmission of the flag character (7Eh). A zero is inserted after every five consecutive ones, except for the flag character. Therefore, one the receiving end, if the sixth character is a zero, it is thrown out. If it is a one, then the flag character has been transmitted.

The Future of Fax

Facsimile communication will continue to evolve. Additions will undoubtedly be made to the Group 3 specification, as has been done in the past. The rise of low-cost computers and communication equipment will certainly bring more improvements to facsimile procedures. The Internet has grown in popularity, and already there are already many ways to send faxes over this world-wide network of computers. The state of facsimile technology will continue to improve, getting "crisper" with every copy.


I would like to thank Robert Welch, Katie Hatfield, and the rest at Telegra Corporation for their readiness to assist me, going out of their way to get me information that I could use in my essay for my university course. Telegra has many fax-related materials and seminars which should prove invaluable to many who are in this area of technology. You can find more information on these resources at