Group 3 Protocols

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What Does Group 3 Protocols Mean?

Group 3 Protocols are universal protocols used to send fax documents across telephone lines. They specify CCITT T.4 data compression with a maximum transmission rate of 9600 baud. The different levels of resolution provided are 203 * 196 and 203 * 98.

Techopedia Explains Group 3 Protocols


The session control procedure for Group 3 protocol depends on T.30.T.30 sets. A call is divided into five different phases:

Phase 1: This relates to call setup.

Phase 2: This deals with pre message procedures.

Phase 3: This relates to image and message transmission.

Phase 4: This is all about post message procedures.

Phase 5: This deals with call release.

Session control procedures control phases from 2 to 5 and use HDLC frames at 300 bits per second.

Group 3 protocols use modified Huffman codes for one dimensional compression and modified READ for two dimensional compressions. The first phase verifies that fax terminals are present at each end of transmissions. As these protocols transmit over voice networks, calling and called fax terminals send tones at the start of a fax call. Calling terminals transmit a calling tone that identifies the fax terminal. Called fax terminals also respond with called station identification at around 2100 Hz tone, which stays for 3 seconds.


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Margaret Rouse
Senior Editor
Margaret Rouse
Senior Editor

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.