Fax machines have long been in use, but recently modems with fax capability have begun to take their place. Fax modems are considerably smaller and cheaper than fax machines. The original fax machines were classified as Group 1. Group 1 fax machines were very slow, taking 6 minutes to send a single page. In the late 70's, these were replaced by Group 2 fax machines. These machines converted the image into digital signals, which were then transmitted by modem. These digital signals were much more reliable, as they were affected much less by line noise than the analog signals sent by Group 1 machines. A mathematical compression formula was added as well, bringing transmission time down to 3 minutes per page. Group 3 faxes were given the option of two resolutions- 200x200 and 200x100. With the V.17 protocol, Group 3 faxes are capable of transmission at speeds up to 14,400 bps. The T.30 protocol is the method by which Group 3 faxes manage fax sessions and negotiate the capabilities supported by each fax in the connection. The T.4 protocol controls page size, resolution, transmission time, and coding schemes for Group 3 faxes. Group 4 faxes are designed for digital ISDN lines.
Class 1 fax modems are modems with extensions to their command sets that allow them to act as Group 3 fax machines. Class 1 fax modems leave almost all of the processing to the software. The official standard for Class 1 fax modems is EIA/TIA-578.
Class 2 fax modems handle much of the processing previously done by the CPU. It took a very long time for Class 2 to be approved, however, so many fax modems were produced before the standard was official. Thus, Class 2.0 is used for fax modems which apply strictly to the standards, while Class 2 applies to those fax modems that were produced before the standard was official.
Class 3 is proposed to handle the conversion of data streams into images. Not only would it handle the T.30 work done by Class 2 fax modems, it would also handle the T.4 work.
Class 4 fax modems have buffers, which allow the CPU to go for a short time without responding to the fax data.
Class 1 fax modems are modems with extensions to their command sets that allow them to act as Group 3 fax machines. Class 1 fax modems leave almost all of the processing to the software. The official standard for Class 1 fax modems is EIA/TIA-578.
Class 2 fax modems handle much of the processing previously done by the CPU. It took a very long time for Class 2 to be approved, however, so many fax modems were produced before the standard was official. Thus, Class 2.0 is used for fax modems which apply strictly to the standards, while Class 2 applies to those fax modems that were produced before the standard was official.
Class 3 is proposed to handle the conversion of data streams into images. Not only would it handle the T.30 work done by Class 2 fax modems, it would also handle the T.4 work.
Class 4 fax modems have buffers, which allow the CPU to go for a short time without responding to the fax data.
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