About Auto-Negotiation - D-Link DFE-690TXD User Manual

Section 1 - Product Overview

About Auto-negotiation

The basic idea of auto-negotiation can be understood by reflecting for a moment on the familiar process of making a
dialup connection between two modems. You have probably heard some exchanges between your local modem and
a modem at the other end of a telephone line. (These exchanges are ordinarily played out through a speaker in your
local modem). As irritating as those few seconds of noise may be, they do let you know that your modem and the
remote modem are on the job, preparing for your intended communication with the remote computer.
The preparatory work of the two modems during those few seconds before you see the "connect" message is to
negotiate the best data communication scheme which is supported by both modems, and which is suitable for the
quality of the telephone-line connection between them. The parameters to be settled between the two modems include
best baud rate, compression method, and error correction method. When the two modems have tested the phone-line
quality and have switched to the combination of parameters which will provide the best data communication, then
you are given the "connect" message which signals the end of the inter-modem negotiation and the beginning of your
intended communication with the remote computer.
Auto-negotiation between devices within an Ethernet LAN is similar in concept, but much briefer. The two devices
involved in the auto-negotiation will be your DFE-690TXD and the switch or hub through which it is connected into
the LAN. (Switches ordinarily provide for auto-negotiation; traditional hubs do not.) The parameters to be negotiated
between the DFE-690TXD and its supporting switch or hub include speed (100Mbps = Fast Ethernet, or 10Mbps =
traditional Ethernet) and duplex mode (half-duplex or full-duplex).
Startup communication between the two devices occurs when both devices are operating, the cable connection
between them is good, and the connected notebook PC's network software is loaded. As soon as those conditions
are satisfied, the preparatory process of auto-negotiation between the DFE-690TXD and its supporting device begins
and proceeds automatically.
If the supporting switch or hub has auto-negotiation functionality, then it and the DFE-690TXD exchange a series
of messages in which each device signals its capabilities and listens for corresponding information about the other.
The auto-negotiation process requires only a few milliseconds, and the two devices select the best communication
parameters supported by both devices.
D-Link DFE-690TXD User Manual 6