Handshake Definition

A handshake is the process by which two devices, such as modems, initiate communication with each other in order to establish a session, in which the devices can exchange any desired information.

This initial communication is necessary because the modems at each end of the line may have different capabilities (such as maximum data transfer speeds), and thus they need to inform each other of their respective capabilities (and settle on the highest transmission speed that both can use). Also, in the case of higher speeds, the modems have to determine the length of line delays so that their echo cancelers can be used properly.

Handshaking begins when one device sends a message to another device indicating that it wants to establish a communications channel. The two devices then send several messages back and forth that enable them to agree on a communications protocol. Evidence of handshaking can be heard in the various sounds that are made by a modem when dialing a call from a computer.

It is desired to keep the time required for handshakes as short as possible, as a shorter time can result in substantial savings. This is particularly true for Internet service providers (ISPs), which often handle many thousands of calls per day.

Fortunately, progress has been made on this in recent years. For example, the time required with V.92 modems, which were introduced in 1999, is at least 30 percent less than with V.90 modems. Ways in which this time have been reduced include the use of improved data compression algorithms and the building in of a learning capability which can simplify the procedure when the same number is redialed (which is frequently the case with modems).

Created November 9, 2005.
Copyright © 2005 The Linux Information Project. All Rights Reserved.