Group 4 Protocols

What Does Group 4 Protocols Mean?

Group 4 protocols are a protocol suite used to send fax documents over ISDN networks supporting images up to 400 dpi resolution. It is designed for FAX transmissions between ISDN 64 kbps systems and for using protocols.


Techopedia Explains Group 4 Protocols

Group 4 Protocols include the following protocols:

  • T.6
  • T.62
  • T.70
  • T.72
  • T.411
  • T.412
  • T.414
  • T.415
  • T.416
  • T.417
  • T.503
  • T.521
  • T.563

While fax protocols in groups 1 to 3 are analog in nature and designed to use analog telephone lines, the group 4 use digital transmissions of fax images requiring an ISDN or digital connections to the destination system. Group 4 also uses the CCITT data compression protocol termed as T.6.

Group 4 faxing connections between source and destination are digital in nature. For instance, an office may use a PBX base telephone system which is digital and the telephone carrier might use digital connections. In such cases, it is always a better option if there is at least a little analog communication in the circuit between local telephone service and PBX. Until telephone systems are converted to digital networks, the group 4 fax systems cannot replace Group 3 systems.

The group 4 encodings are compression algorithms designed to encode one bit image data. Document and fax formats (including TIFF) support Group 4. They replace group 3 in many conventional document image storage systems. The encoded data is half the size of one dimensional group 3 encoded data. Though Group 4 encoding is difficult, it still encodes and decodes faster than group 3. Because they were designed for use on data networks, they do not encapsulate synchronization codes for error detection. This is because they are not commonly used for image transfers. Group 4 protocol is identical to MMR; both use the same algorithm and achieve identical compression results.


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Margaret Rouse 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 explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…