Data compression results depend on the type of data being compressed. Some types, such as ASCII files, can be compressed quite a bit. Other types of data can compress only a little. Even though certain software applications can be used to achieve data compression, normally it's better to leave this operation up to the modem. This is because modem hardware compression algorithms are faster than the ones used by host software.

Compression normally works with error-correction algorithms. Error detection and correction techniques can be used to guarantee data integrity at any transmission speed. Two examples of such techniques are

• Microcom Networking Protocol (MNP)

• Link Access Procedure for Modems (LAP-M)

V.42bis and MNP5 are the compression algorithms that commonly operate over LAP-M or MNP4 correction. The V.42 and V.42bis compression algorithms can be implemented in V.32 and V.34 modems as well as in other equipment with lower speed capability. In theory, V.42bis can provide the 4:1 compression ration. However, in practice, this is rarely accomplished.

V.42bis compression is achieved when both communicating modems agree to use it. In such instances, the software compression option should be turned off. If hardware compression is used, the data transfer between the DTE and DCE can occur at a higher speed.

Modem Modulation Standards

ITU-T defines a number of modem modulation standards, as shown in the table below.

On top of various ITU standards, manufacturers have devised their own versions of modems. This causes some interoperability issues among different kinds of modems, even when the modems come from the same vendor:

When V.34 modems are properly configured, they can adapt to line conditions. Initially, two modems attempt to establish a call at 28.8 Kbps. If this transmission speed isn't possible because of line conditions, the modems can continue to reduce the speed in 2.4 Kbps increments all the way down to a minimum speed of 2.4 Kbps. By the same token, the modems try to increase the speed when line conditions improve.

In contrast, older modems can negotiate a fixed transmission rate only during handshaking, thus continuing transmission at the speed agreed to at the outset by the two modems. This situation might result in a connection failure if an older modem's line becomes particularly bad. If the line quality improves down the road, older modems still can't take advantage of greater bandwidth.

The access server is unaware of modulations, because it is only directly involved with DTE-to-DCE communication. However, the access server-to-modem speed must consider the modulation speed and compression ratio to achieve the best end-to-end performance.

The Relationship Between Modem Speeds and Compression Ratios

DCE-to-DCE speed is modem-to-modem communication speed across the telephone network. DTE-to-DCE speed is the communication speed between the computer and the modem attached to it. If you want to gain maximum benefits from compression, the PC should clock the modem at its speeds equal to the potential compression ratio. In a PC, the DTE should set the modem at its fastest rate to take advantage of compression.

The EIA/TIA-232 serial interface (COM port), found on PCs and some Macs, is sometimes used with Universal Asynchronous Receiver Transmitters (UARTs) and character-oriented communication packages. However, these features are unreliable at higher data rates, and the speed of the interface might fall a good deal short of the full potential of V.34.

If a modem isn't configured properly, it might automatically alter DTE-DCE speeds so that they match DCE-DCE speeds. This is often called speed mismatch. You can prevent speed mismatch by locking the DTE-DCE speed so that it remains the same as originally configured. This speed-locking procedure is called speed conversion. It is also known as port-rate adjustment or buffered mode.

The table below shows the maximum theoretical speeds possible for selected modem modulation standards. You can also see the possible speeds where V.42bis compression is used with the same standards.