Modem Standards ITU V.XX
V.22 bis
Synchronous/asynchronous data transmission, full-duplex operation over 2-wire leased or dialup lines; 1200-bps data rate (V22 bis, 2400 and 1200 bps).  Small businesses can beat the expense of leased lines by using V.22 modems with dialup lines.  If they later add a 2-wire leased line, they can still use this modem.  V.22 accommodates the equipment found in today's typical "hybrid" network; synchronous mainframes and terminals, and asynchronous PCs.  V.22 bis doubles data throughput (to 2400 bps) for quick service of  large file transfers. 
V.25 bis
Provides for automatic calling and answering circuitry for use on dialup lines.  V.25 defines a dialup parallel interface;  V.25 bis defines a dialup serial interface.  These standards enable any computer, sync or async, to perform autodialing functions with a V.25 compliant modem. 
Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup or 2-wire leased lines; 9600 bps data rate (fallback to 4800 bps).  The V.32 recommendation is the first universal standard for 9600 bps modems on dialup (private lines) or leased phone lines.   V.32 compliant modems are the industry standard for medium-speed networks.  Trellis-encoding modulation allows high data speeds and reduces errors.  Data can be sent over standard dialup lines, which are a lot less expensive to use than leased (private) lines.   And V.32 compliant modems will work anywhere in the world.  Any business, large or small, that handles large quantities of data transfer or huge data files can benefit from the fast, accurate data transfer of V.32 technology. 
V.32 bis Synchronous/asynchronous data transmission, full-duplex operation over 2 wire dialup or leased lines; 14,400, 12,000, 9600, 7200, 4800 bps data rates.  It offers two advantages over V.32.  First, a V.32 bis modem transmits data faster-up to 14,400 bps.  Second, V.32 bis redefines modem-connection negotiations (called training and retraining).   Training is the procedure two modems use to make a connection;  they "discuss" and "agree upon" a data rate.  Retraining is a subsequent negotiation after data exchange is under way:  Both modems "agree" to reduce transmission to a slower speed to overcome ambient line noise.  Retraining is triggered by the line noise.  After modems retrain, data transfer resumes.   V.32 bis also provides a new procedure called "fastrain",  which allows the modem to fall either backward to a slower speed or forward to a faster speed.  When two V.32 bis modems do a fastrain, they stop, determine that they can run faster, and then switch speed in a few milliseconds.  One advantage of V.32 bis over V.33 (see next column):  with V.33, you can send sync data at 14,400 bps over a 4-wire leased line, but with V.32 bis, you can do the same over a 2-wire dialup line - much less expensive to use than a leased line. 
Synchronous data transmission, full-duplex operation over 4-wire leased lines, 14,000 or 12,000 bps data rate.  A V.33 compliant modem uses the same signal modulation techniques that are used by V.32 modems, but you are restricted to using 4-wire leased lines.  If your network requires some kind of high speed transmission in sync fashion up to 14,000 bps, choose a V.33 modem. 
Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dial, and leased line with automatic fallback to compatible lower modems such as V.32 bis, V.32 and V.22 bis.   V.34 supports speeds from 2,400 to 28,800 bps
Provides for simulated half-duplex (switched carrier) control.  V.32 and V.33 compliant modems that support V.13 can be used in sync IBM RJE environments, so these networks, too, can take advantage of V.32/V.33 technology. 
V.42 bis
Protocols for Error Correction  
and Compression
Levels 1-4
Microcom Networking Protocol (MNP) developed by Microcom Systems, Inc., enables error-free async data transmission.  Although MNP is proprietary, it became an industry standard in the 80s because users demanded it from manufacturers.  Both modems in a connection must use the same MNP protocols. 
Level 5
Incorporate the first four levels and also applies a data-compression algorithm.  It "compresses" data in a rate of 2:1.  So you can double the amount of data you can send at the modem's top transmission speed.   For example, modems that transmit data at 4,800 bps would send, with MNP Level 5, an amount of data equal to an uncompressed 9,600 bps. 
V.42 bis
These "V.Series" procotols are internationally recognized standards for error control and data compression.   V.42 is the recommendation of the ITU for error control;  it contains two algorithms (LAPM, or Link Access Protocol, and MNP 1-4).  When two, V.42 compliant modems establish a connection, they use LAPM to control data errors and retransmit "bad" data blocks.  If one modem supports V.42 and the other supports only MNP, then the two negotiate to use MNP protocol.  In both cases, the error-control process is automatic and requires no special user action or software programs.   V.42 bis roughly corresponds to MNP Level 5.  The difference is the amount of data compressed:   V.42 can usually generate a 4:1 ratio of data compression, depending on the type of file transmited.  
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