Group 3 protocol

Updated: 11/12/2023 by Computer Hope
Brother fax machine

The Group 3 protocol is an international standard devised by the ITU (International Telecommunication Union) for sending and receiving faxes over standard phone lines.

It uses the T.30 specification to set the session-control procedures and then utilizes the T.4 data compression method as outlined by the CCITT for messages. Group 3 protocol has a 9.6 K Baud maximum transmission rate. Its output resolution, which is measured in DPI (dots per inch), is either 203 by 98 or 203 by 196.

How does Group 3 protocol work?

The Group 3 fax protocol can be broken down into five phases (usually listed as phases "A" through "E"):

  1. Call Establishment - Sending and receiving fax machines establish a connection over the telephone network. It involves the initiation of the call, including dialing, handshaking, and negotiation of capabilities.
  2. Pre-message Procedures - Involves additional setup procedures before the actual transmission of the fax message begins. It includes training, where the machines exchange information about their highest common capabilities for fax transmission.
  3. Message Transmission - The main phase, where the actual fax message is transmitted. The two machines send and receive the image data of the document. Compression techniques (mentioned above), such as modified Huffman coding reduce the amount of data transmitted.
  4. Post-message Procedures - After the message is transmitted, successful receipt of the message is confirmed, and any errors that occurred during transmission are resolved.
  5. Call Release - The fax machines terminate the call.

Fax, Group 4 protocol, Hardware terms, Resolution