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.