Error control / correction   V42 (ITU)   LAP-M (?) -- subset of V42   LAP-B (proprietary)   MNP2-4   MNP10     -- can handle cellular connections during handoff, and                progressively vary packet lengthCompression   V42bis   MNP5Fax Signalling   14400bps     V.17		group 3 fax?   9600bps      V.29		group 3 fax?		V.27ter		V.21Data Signalling		Bell 103		V.21   1200bps	V.22   1200bps	Bell 212A   2400bps      V.22bis		V.23   4800bps      V.22ter   9600bps      V.32   14400bps     V.32bis   28800bps     "V.fast"   28800bps     V.fc		proprietary spec of Rockwell, preceded V34				unlike V34, does not do split speeds   28800bps     V34             Standard   56000bps     K56 / K56Flex / K56Plus				Rockwell, Lucent, Motorola, 3Com, others   56000bps     x2		USRobotics (now 3Com), TI, others   56000bps     V.90		StandardFeatures to aim forLAPM/V42 selective reject.  Not all modems do it, but it is good.Split speed V34.  Not all modems do it, but it is good.O'Reilly's reference dictionary is very good.From O'Reilly------------------------------LAPMLink Access Procedure for ModemsOne of the two error detection and correction protocols specified in V.42 to correct data communication errors occurring on the link between twomodems. Each frame of data holds up to 128 bytes of data and has a 16- or 32-bit CRC. Up to 15 (which is also the default value) frames can be sent before an acknowledgment is required (that is, the window size defaults to 15). Therefore,1,920 bytes of memory must be reserved for storing unacknowledged frames (since they may need to be retransmitted). See CRC, MNP, and V.42. ------------------------------MNPMicrocom Networking ProtocolA series of data communication-enhancing protocols that are usually embedded into modems (rather than being part of a PC's communicationsoftware). MNP levels 1 through 11 support features such as error correction, using a 16-bit CRC (MNP 4) and data compression (MNP 5). MNP 10 can keep aconnection open during the handoff time on a cellular telephone connection and adjust packet size according to communications circuit quality (packetsize starts small at the beginning of a call, and is rapidly increased if line conditions are good). During idle times, the two MNP 10 modems exchange linkmanagement idle packets to monitor line quality. MNP levels 4 and 5 use a block size of up to 256 characters. The following points apply to data communication links using modems which use MNP 5 data compression:       MNP 5 compresses data at up to a 2:1 ratio (so you can feed data at 19,200 bits/s into an MNP 5 9,600-bits/s modem), but the actual data      compression ratio depends on the data--ASCII text (which is only 7-bit data, and has lots of "e"s, for example) will be much more compressible      than an executable program file (which is 8-bit data and will likely have almost random bytes).       You should set the PC's (or whatever is sending the data) ports (at both ends of the link) to at least twice the modem data-pump speed (the bit      rate at which the modems actually are sending data). Otherwise, you don't get the full (or any) benefit of data compression.       You must configure the PC's serial port for flow control (either x-on/x-off or RTS/CTS) so that the PC does not send data too fast when the data      are not very compressible and to stop the data from the PCs while the modems retransmit during error correction. If the data are not compressible      at all (for example, because they were already compressed with a program such as pkzip or lharc), then MNP 5 actually makes the data bigger.      (V.42bis is smart enough to shut itself off if it detects that the data are not compressible.) MNP 4 and 5 are superseded by LAPM (which is part of the V.42 standard) and V.42bis. That is, you use MNP 4 and 5 only if they are all the modem atthe other end supports. Microcom has an WWW server at http://www.microcom.com. See CRC, HDLC, LAPM, Modem, V.42, and V.42bis. ------------------------------V.42A standard for error detection and error correction (through retransmission) that is often implemented in modems. V.42 actually specifies two error correction protocols: LAPM and MNP Level 4. MNP 4 is usually used only as a fallback (when the remote modem doesnot support LAPM). See CRC, HDLC, LAPM, MNP, Modem, and RPI. ------------------------------V.42bisA data compression capability that is often implemented in modems. (Both communicating modems must support V.42bis for it to be used--this isdetected and negotiated at the beginning of a dial-up modem connection.) The data compression algorithm:       Can compress data by more than an 8:1 ratio (though this high compression ratio would occur only for extremely redundant data, such as 10,000      consecutive "A"s in the data stream). The compression ratio is more often quoted as 4:1, to represent more real-world data, but is actually about      2.5:1 if you (and I mean you, not your modem supplier) actually measure it.       Requires V.42 (so that the link is known to be error-free).       Usually works with asynchronous data only (because of the necessity for flow control if the data are not compressible enough to match the DTE      speed used with the modem throughput). As a fallback, modems usually also support the older MNP Level 5, which has a maximum compression ratio of 2:1 and can actually increase the transfertime of files that are already compressed. Therefore, V.42bis is preferred. Uses a variation (developed by British Telecom) of the Lempel-Ziv data compression method, which is called BTLZ (not to be confused with abacon-lettuce-tomato sandwich, which is quite a different thing--though a BLT may require compression if there is too much lettuce). A dictionary (512 to 2,048 bytes in length) of frequently occurring strings of (usually up to six) characters is constructed by the transmitting modem andsent to the receiving modem. The transmitter then uses shorter bit patterns to represent the most-common strings (or sequences) of characters. The length of the dictionary and the speed at which data compression occurs are determined by the transmitting modem's design (and thereforedesigners). For example, while the V.42bis standard documents the compression algorithm by using the C language, a good implementation will re-codethis for efficiency and speed. Therefore, different (and compatible) V.42bis modems can have significantly different compression ratios (for the samedata!) and throughput (expressed in bits/s transferred) rates. Note that the serial port speeds of the PCs (or whatever is sending the data) at both ends of the link should be set to at least four times (which is themaximum compression ratio) the modem data-pump speed (the bit rate at which the modems are actually sending data); otherwise, you don't get the full(or any) benefit of data compression. Figure 1: V.42bisFor example, PCs using V.34 modems (with V.42bis data compression) should have their serial ports set to 4 x 28,800 bits/s = 115,200 bits/s, as shown inthe accompanying figure. Not all PCs support this speed. Also, the PC's communication software used must be configured to enable flow control (either x-on/x-off or RTS/CTS--whatever matches themodem's configuration). Flow control is required so that the PC does not send data too fast when the data are not very compressible. Also, flow control isrequired to stop the data from the PC while the modem retransmits during error correction. Finally, only very short EIA-232 cables (since EIA-232 is supposed to work only at speeds up to 20,000 bits/s) should be used, and the serial portUARTs should be 16550s (at both ends). See 16550A, Asynchronous, Data Compression, EIA/TIA-232, In-Band, LZW, MNP, Out-of-band, RPI, and V.42. ------------------------------V.34A modem modulation standard providing full-duplex, 28,800-bits/s data transfer over a standard one-pair, dial-up telephone line. Synchronous and asynchronous data communications are supported. A handshaking method based on V.8 provides faster (under 5 seconds) call setupnegotiation than V.32bis, V.FC, or V.terbo. Other enhancements over V.32bis (and its predecessors) include the following:       Line Probing (choosing a carrier frequency between 2,400 and 3,429 Hz) at call setup time. The choice is based on detected line impairments and      which frequencies the line best carries--perhaps because of telephone company digitizing. Previous modem modulation methods specify only a      single carrier frequency. V.34 modems are supposed to perform line probing continuously after a connection is established, to adjust to changing      line conditions. A good implementation will make these adjustments quickly.       Precoding--the receiving modem tells the transmitting modem about detected line distortion (for example, because of PCM digitization of the      analog phone line) so that the transmitting modem can precompensate for these deficiencies in the communications channel (for example, by      boosting the frequencies that are attenuated more than the others).       Retrain/Entrain during a data call to adjust the transmitting modem's precoding, receiving modem's equalization (compensating for the      communication line's deficiencies), or the data transmission rate (faster and slower) as required because of changing line conditions. Data transfer      is suspended during the few seconds that this occurs.       A half-duplex version (which could, for example, be used in Group III fax machines).       Smaller fallback (due to noisy lines) data transmission speed increments (or should that be decrements?) of 2,400 bits/s (many previous modems      have 4,800-bits/s increments). A total of 12 speeds (from 2,400 to 28,800 bits/s) are therefore available. The modulation scheme maps up to 9 data bits to each symbol (baud) sent over the telephone line. Therefore, a bit rate (data bits into the modem'sEIA-232 connector) of 28,800 bits/s results in a baud rate of 3,200 (9-bit) symbols per second out the analog phone line (since 3,200 x 9 = 28,800 bits/s). Implementation options are listed in the following table.  Feature                              Provides Asymmetrical Transmission Rates                              Sending and receiving data rates can be different so that they can each be as high as the communication circuit                              supports. Auxiliary Channel                              Supports a 200-bits/s in-band data channel for diagnostics, line quality monitoring, and modem configuration.  Baud Rate                              The standard baud rates are 2,400, 3,000, and 3,200 symbols per second. Optional rates of 2,743 and 2,800 baud                              may be the highest that work for links that use ADPCM (for example, satellite data links usually can't handle the                              3,000-baud rate). This provides a higher data rate than if the 2,400-baud rate was used.                               An optional baud rate (implemented by Motorola) of 3,429 9-bit symbols per second results in a data rate of 33,600                              bits/s. Nonlinear Encoding                              Allows better operation over PCM-digitized analog communication circuits by using amplitudes that are less                              sensitive to quantization noise. Precoding                              Reduces high-frequency noise and reduces inter-symbol interference--important for noisier analog                              communication circuits. Trellis Coding                              Modems must be able to transmit using 16-, 32-, and 64-state codes. Modems only need to be able to receive                              using any one (though optionally two or all three) of these Trellis codes.                               Higher-state codes provide better operation on noisier communication circuits--ideally, avoiding the need to drop                              to a lower baud rate.                               Trellis coding specifies which points of a constellation are allowed next, given the current point (which is the                              combination of phase and amplitude defining the symbol). Therefore the circuit will be more immune to noise.Note that vendors can call the modem a "V.34 modem" if it implements none, any, or all of these options. Also options will be used only if both modems ina session support the option. Other implementation options (which are entirely up to the vendor and not part of any standard) include the following:       How the modem's software (sometimes called firmware, because it is more like hardware that is built in to the modem) is upgraded (Flash ROM      with downloading from a bulletin board system is best; plugging in a new EPROM is next best)       Configuration by the front panel, a management system, or remotely (in addition to AT Commands)       Distinctive ringing detection       Caller ID detection Other differences between V.34 modems include the quality of the data compression algorithm (not that errors will occur, but that the data will not becompressed as much as possible) and the processing power available (if it is inadequate, transmission may slow significantly if data are sent full-duplex). See the discussion about synchronous data communications support in the entry for V.32, as this applies to all modems from Bell 212A through V.32. During its lengthy standards development process, V.34 was called "V.fast" (since it was expected to be the modem modulation standard that providedthe fastest possible data rate over standard POTS analog telephone lines). See ADPCM, AT Command Set, Baud, BBS, EIA/TIA-232, In-Band, Modem, PCM, POTS, V.8, V.32, V.34bis, V.42, V.42bis, V.FC, and V.terbo. ------------------------------V.32A modem modulation standard providing full-duplex, 9,600-bits/s data transfer over a standard one-pair, dial-up telephone line. Asynchronous (through the use of a built-in asynchronous-to-synchronous converter) and synchronous data communications are supported. However, to save money, some modems (low-end) do not provide synchronous user-data support, which at first glance seems strange, since themodem uses synchronous data communications to the remote modem and requires (and has) extra circuitry to handle asynchronous datacommunications. One would expect that it would be easy to provide synchronous user-data support (by simply bypassing the asynchronous converter).The bottom line is that it costs more money to support synchronous user-data, since it requires extra signals--the clocking--on the EIA-232 interface.And very few users require it, especially in the home-PC market, in which saving money is more important than seldom-used features. Also, internal modems (the modem is a card in the PC rather than an external box) usually do not support synchronous data communications. This isbecause the built-in COM port required by internal modems uses a standard PC UART, and these UARTs do not support synchronous datacommunications. The above discussion concerning synchronous data communications support applies to all modems from Bell 212A through to V.34. A fallback speed of 4,800 bits/s is included in the V.32 standard. See Asynchronous, EIA/TIA-232, Modem, Synchronous, UART, V.42, and V.42bis. ------------------------------V.34bis