Duplex Mode; Store And Forward - Allied Telesis AT-GS950/16 Installation Manual

Chapter 1: Overview

Duplex Mode

Store and
Forward
22
Duplex mode refers to how an end node receives and transmits data. If an
end node can receive or transmit data, but not both simultaneously, the
end node is operating in what is referred to as half-duplex mode. If an end
node can both receive and transmit data simultaneously, the end node is
said to be operating in full-duplex mode. Naturally, an end node capable of
operating in full-duplex can handle data much faster than an end node that
can only operate in half-duplex mode.
The twisted pair ports on the AT-GS950/16 and AT-GS950/24 Gigabit
Ethernet Smart switches can operate in either half-or full-duplex mode.
They are IEEE 802.3u-compliant and use Auto-Negotiation to set the
duplex mode setting for you automatically.
For Auto-Negotiation to operate properly on a switch, the end nodes
connected to the switch should also use Auto-Negotiation. If an end node
does not have this feature and has a fixed duplex mode of full-duplex, the
result will be a duplex mode mismatch between the end node and a switch
port. A port on the Gigabit Ethernet switch connected to an end node with
a fixed duplex mode of full-duplex will operate at only half-duplex. This
results in the end node using full-duplex and the switch port using half-
duplex. This can produce network performance problems. If you
encounter this situation, you must configure the port on the end node to
use Auto-Negotiation or, if it lacks that feature, to half-duplex.
Note
Because the ports on the AT-GS950/16 and AT-GS950/24 Gigabit
Ethernet Smart switches operate in Auto-Negotiate mode only, the
end nodes connected to the switch must also be configured to
operate in the Auto-Negotiate mode. If an end node is configured to
a specific duplex in a manual mode, it will not respond to the Auto-
Negotiate protocol from the AT-GS950/16 or AT-GS950/24 Gigabit
Ethernet Smart switch. (The speed is determined from the link
pulses, however, so the speed is always detected correctly.) As a
result, the port setting on the switch will end up at half-duplex. If the
end node is manually configured to full-duplex, there will be a duplex
mismatch and data will be lost. If the end node is manually
configured to half-duplex, both ports will have the speed and duplex
match up correctly.
The AT-GS950/16 and AT-GS950/24 Gigabit Ethernet Smart switches use
store and forward as the method for receiving and transmitting frames.
When a Ethernet frame is received on a switch port, the switch does not
retransmit the frame out the destination port until it has received the entire
frame and stored the frame in a port buffer. It then examines the frame to
determine if it is a valid frame. Invalid frames, such as fragments or runts,
are discarded by the switch. This ensures that only valid frames are
transmitted out the switch ports and that damaged frames are not
propagated on your network.