Phase E – Call Release
If the transmitting station follows EOP with the Disconnect Command (DCN), it and the receiving end will then both hang-up.
A typical page of ASCII-coded text will require a few thousand bytes of storage. ASCII is the most highly encoded form of image representation typically used in document storage, but it is limited to representing the defined ASCII symbols. Also, as we know, it must be translated to the Group 3 line-by-line representation of black and white pels before it is presented to the fax modem. For a typical page of text, a Huffman-encoded file will be roughly 10 times larger than its ASCII equivalent, or about 35,000 bytes. Of course, ASCII doesn’t support graphic images, so most fax software products support various graphic file types, with the most common being TIFF-F (Tagged Image File Format-F). TIFF-F is quite common and is exported by most scanners.
The tags in TIFF specify such things as page size and compression. Actually, the compression tag is often omitted, in which case the TIFF standard defaults to “class one storage”, which is no compression at all. Storage classes two, three, and four are variations of ITU-defined modified Huffman encoded data. One of these is the run-length encoding defined in T.4, and is what constitutes a TIFF-F (for fax) file. Commetrex uses an ordered TIFF-F file format, which is the output from its conversion or fax receive processes. There are no known incompatibilities between the Commetrex implementation and other products.
Others graphics file formats supported by various fax software products include PCL, PCX, GEM, IMG, etc. These are graphics formats developed by printer and graphics software manufacturers. The graphics formats must be supported by the fax software to allow direct use of the outputs of these packages.
Some fax products support what is called “conversion on the fly”. Typically, this means that the product will accept an ASCII file and convert it to the Huffman-encoded format required for transmission without requiring intermediate disk storage. Some products modify this by accepting the source file in its format, executing the conversion and spooling it to disk for transmission when the conversion is complete. Some PC add-in products can execute the ASCII-to-Huffman conversion on the fax board. This not only saves disk storage, but off loads the task from the host PC. Of course, if the source-file format is not handled by the board-level process, the conversion must be performed on the host PC. For fax-on-demand systems, where the bulk of the function is to transmit graphic files, the converted file is usually stored as the source file.