Allied Telesis AT-MC103ST/FS3, FS4 - Manual (Page 15)

AT-MC100 Series Installation Guide
5
Auto-negotiation Button
The auto-negotiation button, located on the front panel, disables the auto-
negotiation feature (IEEE 802.3u) of the media converter. The media
converter uses auto-negotiation to determine the duplex mode of the ports.
The duplex mode refers to the manner in which a node sends and receives
data on the network. Depending on its capabilities, a node can operate in
either half- or full-duplex mode. A node operating in half-duplex can either
send or receive data, but not both at the same time. However, a node
operating in full-duplex can send and receive data simultaneously. The best
network performance is achieved when a node can operate in full-duplex
mode.
In most configurations, you will want to leave the auto-negotiation button
activated so the unit can determine the appropriate duplex mode, based on the
capabilities of the end nodes. For example, the auto-negotiation feature on the
media converter should be left activated in situations where both end nodes
are also capable of auto-negotiation, or where both end nodes have been pre-
set to the same mode or are capable of operating in only one duplex mode, such
as half-duplex.
There is one situation where it may be necessary to disable the auto-
negotiation feature, and that is to prevent a mismatch from occurring between
the duplex modes of the end nodes. For example, Figure 2 shows two units
that have been connected with a media converter. Unit 1 is a repeater that is
capable of operating in half-duplex mode only. Unit 2 is a switch that can
operate in either half- or full-duplex mode, and will auto-negotiation the
duplex mode.
In attempting to auto-negotiate with Unit 1, the media converter will
determine that the unit is capable of half-duplex only and will set the port
connected to the unit appropriately. In auto-negotiating with Unit 2, the
media converter will determine that the unit can manage full-duplex and will
set the port connected to the unit to full-duplex. The result is a mismatch,
with one unit operating in half-duplex and the other unit operating in
full-duplex. This is referred to as a classic duplex mode mismatch and will
result in poor network performance between the end nodes.
Figure 2 Example of a Duplex Mode Mismatch
TX RX
MC103SC/FS3 FAST ETHERNET MEDIA CONVERTER
M/L ON
LNK TST
CLASS 1 LASER PRODUCT DO NOT STARE INTO BEAM 100Base-FX 100Base-TX
SINGLE MODE
FDX LNK
ACT M/L ON
PWR LNK
ACT A/N OFF MDI-X
A/N ON MDI
Unit 1
Unit
100Base-TX Repeater Media Converter 100Base-TX Switch