Modem Speeds and Compression

Cisco.com

The speeds and compression rations shown assume ideal conditions.

The difference between the DCE-to-DCE modulation speed and DTE-to-DCE speed is often a source of confusion. The former represents how fast the modems communicate with each other across the telephone network. The latter represents how fast your computer communicates with the attached modem.

In an ideal situation, to gain full benefits from compression, the DTE (for example, a PC) must send to the DCE (a modem) at speeds matching the potential compression ratio. However, the EIA/TIA-232 serial interface commonly found on PCs and some Macintosh computers (the COM port) might operate considerably more slowly than the full potential speed of V.34. The problem is that some PCs and Macs use the EIA/TIA-232 serial interface with a combination of Universal Asynchronous Receiver/Transmitters (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-to-DCE speeds to match the established DCE-to-DCE speeds. This state is often called speed mismatch. To avoid speed mismatch, you must lock the DTE-to-DCE speed so that it remains constant, as originally configured. This speed-locking mechanism is called speed conversion (also known as port-rate adjustment or buffered mode).

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