2.1 Modem Functions
2.1.9 Error control and data compression

Error-detection and error-correction methods were developed to ensure data integrity at any speed. Some widely used methods include Microcom Networking Protocol (MNP) and Link Access Procedure for Modems (LAPM).

MNP actually has ten different levels. They are as follows:

MNP1 Asynchronous, half-duplex transfer
MNP2 Error correction, asynchronous, full duplex
MNP3 Error correction, synchronous
MNP4 Error correction, better throughput than MNP2 and 3; this modification rides on top of MNP2 or 3 to improve throughput
MNP5 Simple data compression, about 2:1
MNP6 Statistical duplexing and Universal Link Negotiation; with V.29, modems can emulate full-duplex operation; also supports fallforward operation between two MNP modems
MNP7 Data compression, about 3:1
MNP8 MNP7 for pseudo-duplex modems
MNP9 Data compression, about 3:1; includes V.32 technology
MNP10 Dynamic fall-back and fall-forward adjusts modulation speed with link quality; intended for use with cellular technology

Compression algorithms typically require error-correction algorithms, so V.42bis and MNP 5 compression usually run over LAPM or MNP 4 correction. V.42 and V.42bis are not limited to V.32 and V.34 modems. They can also be implemented in lower-speed equipment. The 4:1 compression ratio provided by V.42bis is theoretical and is rarely achieved.

How well the modem compression works depends on the kind of files being transferred. In general, you will be able to achieve twice the speed for transferring a standard text file (like the one you are reading right now). Decreasing by 50 percent means that you can double the throughput on the line so that a 9600-bps modem can effectively transmit 19200 bps. However, V.42bis and MNP5 modems cannot compress a file that is already compressed by software. In the case of MNP5, it will even try to compress a precompressed file and actually expand it, thus slowing down the file transfer! If you have ever downloaded files from a BBS or online service, you know that almost all files are in a compressed format. 

The modern data-compression technique is analogous to the video-compression or disk-packing algorithms used in computers. The compression efficiency is highly dependent on data content. Some data (such as ASCII files) compress readily, whereas others can be compressed only a little.

Although some application software supports data compression, it is usually better to let the modem compress transmitted data. Data-compression algorithms that run in modem hardware are faster than those performed by host software. If two modems have agreed on V.42bis compression, you need to turn off the compression capability of the application. This means transferring data at a higher speed on the interface between the DTE and the DCE.

Confusion often arises between the DCE-to-DCE modulation speed and DTE-to-DCE speed. DCE-to-DCE represents how fast the modems communicate with each other across the telephone network. DTE-to-DCE represents how fast your computer communicates with the attached modem.

To gain full benefits from compression in an ideal situation, the DTE (for example, a PC) must send to the DCE (for example, a modem) at speeds matching the potential compression ratio (as shown in Figure ). However, the EIA/TIA-232 serial interface (COM port) that is widely found on PCs and some Macintosh computers might operate considerably slower than the full potential of V.34. The problem is that some PCs and Macs use the EIA/TIA-232 serial interface with a combination of UARTs and character-oriented communications software packages, which are not reliable at higher data rates. In a PC, DTE should be set to clock the modem at its fastest rate to take advantage of compression.

An improperly configured modem might automatically adjust DTE-DCE speeds to match the established DCE-DCE speeds. This is often called speed mismatch. To avoid speed mismatch, you must lock the DTE-DCE speed so it remains constant, as originally configured. This speed-locking mechanism is called speed conversion (also known as port-rate adjustment or buffered mode).

Figure shows the maximum theoretical speeds that are possible for selected modem-modulation standards. Also shown are the possible speeds if V.42bis compression is used with the same standards.