A modem modulation standard (still under development) providing full-duplex, 32,000- (or maybe 33,600-) bits/s data transfer over a standardone-pair, dial-up telephone line. Rather than being called a separate standard (V.34bis), this may end up being an enhancement to V.34 (and just another speed option within the V.34standard). See Modem and V.34. ------------------------------V.FCV.Fast ClassA modem modulation scheme (promoted by Rockwell International) that is based on an early, not yet standardized version of V.34. Transmits data at the same speeds as V.34, but a significant difference is that V.FC has a less powerful handshaking method (V.FC does not use the V.8scheme). Now made obsolete by V.34. See Modem, Rockwell International, V.8, and V.34. ------------------------------V.terboA nonstandard enhancement to V.32bis (promoted by AT&T Microelectronics). Transmits data at up to 19,200 bits/s (with fallback speeds down to 4,800 bits/s, in decrements of 2,400 bits/s). Now made obsolete by V.34. See AT&T, Modem, and V.34. ------------------------------V.8A method used by V.34 modems to negotiate connection features and options. Using V.21 modem modulation (which is 300 bits/s FSK--the same as the initial negotiation done by Group III fax machines), the calling modem sendsits calling menu (a message containing a list of the features it supports). The answering modem replies with a joint menu, which is a list of the features itsupports in common with the calling modem. Previous modem negotiations (for example, that used by V.32 modems) used tones. This method became too slow and inflexible for the large number ofoptions that V.34 needs to negotiate. See Fax, Modem, V.21, and V.34. ------------------------------V.21A modem modulation standard that provides full-duplex, asynchronous, data transfer at any speed from 0 to 300 bits/s (though usually at 300 bits/s) overa standard one-pair, dial-up telephone line. Currently used by fax machines and also by V.34 modems during their negotiations. Was also used (a long time ago) in Europe (as Bell 103 was in NorthAmerica) for everyday dial-up terminal to computer data communications (though the faster recent standards, such as V.32bis and V.34, have madeV.21 all but obsolete for this application). Similar to (in that they both use frequency shift keying modulation), but incompatible with, Bell 103 (they use different frequencies); that is, they arehomologous. (Now there's a word you don't get to use often.) See Asynchronous, Bell 103, Fax, Full-duplex, Modem, V.8, and V.34. ------------------------------Bell 103An old, very low-speed modem modulation method used in North America. A zero to 300 bits/s, full-duplex, two-wire asynchronous modem modulation standard for use on standard dial-up telephone lines. More than 20 yearsold but still widely used, for example, for retail store credit card authorization terminals. Similar in technology to, but incompatible with, V.21 (which was popular in Europe, though it is also used by fax machines). See Asynchronous, Fax, Modem, and V.21. ------------------------------FaxFacsimileThose amazing machines (or boards in PCs) that can send and receive images of letters and drawings over a telephone line. Currently, Group III machines are the most common (though other types of machines are standardized). The group specifies the digitization andcompression scheme, as shown in the following table.  Group       CCITT Standard                       Year Released                                     Transmission Time per Page[a]                                                                       Many nonstandard systems were used before Group I, primarily by the newspaper industry. I       T.2                       1968                                     4 to 6 minutes (depending on the length of the page), plus 30 seconds between pages (to change the page);                                     sends 180 lines per minute, since the drum (on which the page to be sent was mounted) rotated 180 times                                     per minute; scans at 98 lines per inch. II                               1976                                     2 to 3 minutes, plus 30 seconds between pages; sends 360 lines per minute and scans at 100 lines per inch. III       T.4 and T.30                       1980                                     9 to 50 seconds (at 9,600 bits/s), plus 15 seconds negotiation before first page. IV       T.6                       1984                                     3 to 12 seconds (at 64,000 bits/s). Footnotes:        [a] Depends on resolution selected and the image sent. Group III machines use the following modem modulations:       V.29 9,600 (with fallback to 7,200) bits/s       V.27ter 4,800 (with fallback to 2,400) bits/s       Sometimes V.17 14,400 (with fallback to 12,000) bits/s Also, half-duplex V.21 is used for the initial negotiation phase, as specified in T.30 and described below. Finally, 64,000-bits/s transmission over ISDNlines may become an option soon. The basic Group III resolutions are as follows:       1,728 pels in a scan width of 215 mm (this is about 8.5 inches and so is about 203 pels per inch)       3.85 or optionally 7.7 scan lines per mm (this is about 98 or 196 per inch) These are summarized in the table below.                              Resolution                                                                                                                                             Horizontal (pels/inch)                                 Vertical (lines/inch)                                                    Bits per Square Inch                                                                        Relative Size of Image Data Standard             203                                 98                                                    19,894                                                                        1 Fine             203                                 196                                                    39,788                                                                        2 Super Fine             203                                 392                                                    79,576                                                                        4 Ultra Fine[a]             406                                 392                                                    159,152                                                                        8 Footnotes:        [a] Standardized only for smaller page sizes, such as A6. A scan width of 2,560 pels is optional. This would support a width of up to 12.6 inches (for example, to fax a B size 11" x 17" page). Here is a summary of the sequence of a fax call:       After dialing the called fax machine's telephone number (and before it answers), the calling Group III fax machine (optionally) begins sending a      calling tone (CNG). This is a repeated 1/2-second duration 1,100-Hz tone, with a 3-second pause between tones, which can be used by an      automatic Voice/Data/Fax switch to connect the call as required.       When the called fax machine answers, it replies with a 3-second 2,100-Hz tone, the Called Station Identification (CED).       The two fax machines then communicate using 300-bits/s, V.21 modulation. This is an old, slow, reliable, full-duplex modem modulation that uses      Frequency Shift Keying (FSK). FSK which uses one tone to send a "1" and another for a "0"--this produces a distinctive warbly sound when      carrying data. HDLC framing is used, with a 16-bit CRC and 256 (the default) or (optionally) 64-byte frames.       The communication begins with the called (answering) fax machine sending the calling fax a 20-character identifying message. The standard      allows this message to carry only the numbers, "+", and a space, but some fax machines support letters as well. This message is called the Called      Subscriber Identification (CSI). It is manually programmed into each fax machine and is supposed to be the answering fax machine's telephone      number in international format (for example +1-416-555-0641). The calling fax machine usually shows this on its LCD display.       The called fax machine then sends a 32-bit Digital Identification Signal (DIS), which requests the following:             Bit rate to be used for fax transmission             Time required to print a scan line (defaults to 20 ms but can also be specified as 0, 5, 10, or 40 ms; 0 ms is specified if the receiving fax            machine can receive into buffer memory, to speed up the fax transmission)             Fax resolution             Maximum paper size; support for specifying the A5 and A6 paper size is included in a larger (40-bit instead of 32-bit) DIS       The calling fax sends the called fax a Calling Subscriber Identification (CIG, though sometimes people make up new acronyms, and call this a      Transmit Station Identification, or TSI). It is (typically) a 25-character company or user name (as configured in advance through cryptic      button-pushes on the calling fax). The receiving fax may show this information on its LCD display and include it in a log that can later be printed.      Along with other information, such as time, date, and page count, the calling fax usually includes the CIG in the fax image sent to the called fax;      this is called the TTI. U.S. law may soon require displaying the date and time sent and the sending business's or individual's name and telephone      number at the top of each received fax page (though the significance of this with portable PCs with fax modems is questionable).       The calling fax then sends the called fax machine a Digital Command Signal (DCS), which confirms which options requested in the DIS will be      used for the call; that is, the called and calling fax machines do a one-round-trip negotiation to agree on the options. These options are based on      those requested and the capabilities of the called fax machine.       A test data transmission is then done at the agreed bit rate, and the fax machines switch back to 300-bits/s mode to confirm that the transmission      was successful. If not, then a lower speed is used. Otherwise, page transmission begins. After each page, the fax machines switch back to 300-bits/s mode to determine whether there are more pages to be sent. Group III fax machines usually negotiate a scan width of 1,728 pels per row, which is used for a scan width of 215 mm (this is about 8.46"). This is usedfor both North American standard A-size 81/2" x 11" paper and Metric A4 (210 mm x 297 mm) paper. These 1,728 pels are compressed using a data compression scheme that first looks for sequences of pels set to the same value in the horizontal rasterscans of the source document and produces a count of the number of repetitions of that value (this is called Run Length Encoding--RLE). Then a lookup table is used that produces bit patterns that are shorter to represent the more commonly expected counts. This is called Modified HuffmanEncoding and is often called MH in fax machine specifications. Almost all fax machines support this. The table is designed to compresses long sequences of white space better than black (since this is what documents typically have). For example, anall-white row requires only nine bits. This is called one-dimensional compression. At fine Group III resolution, a single page is about 3,800,000 bits (464kbytes). By using only one-dimensional compression, this can be compressed to 20 to 50 kbytes, depending on the image. Group III fax machines can also use two-dimensional compression (though not all machines implement it), in which only the difference between thecurrent and the previous (one-dimensional compressed) scan row is sent. This is called Modified Read (MR) or Modified Modified Read (MMR) in faxspecifications. To reduce the impact of scan rows lost due to line noise, this is limited to two rows for standard resolution and four rows for fine resolution(following a one-dimensionally compressed row). For example, at standard Group III resolution, a single page is about 250 kbytes; by using two-dimensional compression, this can be compressed toabout 25 to 80 kbytes (10:1 to 3:1 compression). Since Group IV fax machines always use error correction, there is no limit to the number of rows that can be two-dimensionally compressed (this isspecified in T.6). Both one-dimensional and two-dimensional compression are examples of lossless data compression. At the receiving end, if after decompressing each row of received data, the receiver does not get exactly 1,728 pels, then it knows that a datatransmission error occurred. The corrupted row is ignored, and in its place, either the previous (good) row is repeated or a blank (white) row is used. Some fax machines will request a retransmission of the entire page if that page had more than 32 or 64 (depending on the implementation) corruptedrows. Most transmitting fax machines ignore these requests (since the paper has already been scanned and the fax did not keep a copy to retransmit). The T.30 standard also specifies an optional Error-Correcting Mode (ECM), which supports retransmitting corrupted rows (rather than the entire page).Since ECM slightly increases fax transmission time and most fax calls have very few errors, most fax machines with ECM can be set to disable thefeature. Group IV machines require 56,000- or 64,000-bits/s communications (a switched 56 or ISDN B channel), and have selectable resolutions of 200 x 200,300 x 300, and 400 x 400 dots per inch. A fax board's service class specifies how much of the work is done by a Group III fax board (the rest being done by the PC) and is also an extension tothe Hayes AT command set, since it describes the commands supported by a fax modem. Class 1 fax boards are defined in EIA/TIA-578 (Service Class 1 Asynchronous Facsimile DCE Control Standard) and the subsequent EIA TechnicalSystems Bulletin 43 (TSB-43). ITU-T's T.31 is an international version of the standard but includes some extra functions. Class 1 fax boards performonly the simplest functions such as:       Converting the asynchronous data from the PC to synchronous HDLC data       Generating and detecting the handshaking tones before a fax transmission