Allied Telesis AT-FS708 - Manual (Page 17)

AT-FS708 Fast Ethernet Switch Installation Guide
17
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, 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-FS708 switch can operate in half- or full-
duplex mode for 10/100 Mbps. They are IEEE 802.3u-compliant and use
Auto-Negotiation to set the duplex mode setting for you automatically.
Note
In order for a switch port to successfully Auto-Negotiate its duplex
mode with a 10 or 100 Mbps end-node, the end-node should also be
configured for Auto-Negotiation. Otherwise, a duplex mode
mismatch can occur. A switch port using Auto-Negotiation defaults
to half-duplex if it detects that the end-node is not using Auto-
Negotiation. This results in a mismatch if the end-node is operating
at a fixed duplex mode of full-duplex.
Store and
Forward
An AT-FS708 switch uses 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 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 to allow 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.
How a switch signals an end-node to stop transmitting data differs
depending on the duplex mode of the end-node and switch port. A twisted
pair port operating in half-duplex mode stops an end-node from
transmitting data by forcing a collision. A collision on an Ethernet network
occurs when two end-nodes attempt to transmit data using the same data
link at the same time. A collision causes an end-node to stop sending data,
wait for a brief period of time, and then retransmit the same data. Once the
switch is ready to receive data again, the switch stops forcing collisions.
This is referred to as backpressure.