Chapter 1: Product Description
18
Duplex Mode
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, it is
operating in a half-duplex mode. If an end-node can 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-716L and AT-FS724L Fast Ethernet
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.
Note
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 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. Should 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.
Store and
Forward
Both 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. Then it
examines the frame to determine if it is a valid frame. Invalid frames, such
as fragments or runts, are discarded by the switch. This insures that only
valid frames are transmitted out the switch ports and that damaged frames
are not propagated on your network.
Backpressure and
Flow Control
To maintain the orderly movement of data between the end-nodes, an
Ethernet switch may periodically need to signal an end-node to stop
sending data. This can occur under several circumstances. For example, if
two end-nodes are operating at different speeds, the switch, while
transferring data between the end-nodes, might need to instruct the faster
end-node to stop transmitting data allowing the slower end-node to catch
up. An example of this would be when a server operating at 100 Mbps is
sending data to a workstation operating at only 10 Mbps.
