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AT-GS900/16, AT-GS900/24 Fanless Gigabit Ethernet Switches Installation Guide
19
Duplex Mode Duplex mode refers to how an end-node receives and transmits data. If an
end-node can either 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-GS900/16 and AT-GS900/24 switches
can operate in half- or full-duplex mode for 10/100 Mbps and full-duplex for
1000 Mbps. They are IEEE 802.3u-compliant and use Auto-Negotiation to
set the duplex mode setting for you automatically.
Store and
Forward
An AT-GS900/16 or AT-GS900/24 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 from
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 process insures that only valid frames are transmitted
from 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.
A port operating in full-duplex mode uses PAUSE frames, as specified in
the IEEE 802.3x standard, to stop the transmission of data from an end-
node. Whenever the switch wants an end-node to stop transmitting data, it
issues this frame. The frame instructs the end-node to cease transmission
for a period of time specified within the frame. The switch continues to
issue PAUSE frames until it is ready again to receive data from the end-
node. This is referred to as flow control.