Allied Telesis AT-FS724L - Manual (Page 14)

Overview
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This timer will delete a MAC address from the table if it does not see a frame
from the end-node with the address on any port after five minutes (300
seconds). The aging timer also helps to ensure that the table is correct should
an end-node be moved from one port on the switch to another port.
Store and Forward
These Fast Ethernet Switches use store and forward as the method for
receiving and transmitting frames. When an 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
In order 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 speed and duplex mode of the end-node and switch port. A twisted pair
port operating at 100 Mbps port and 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 end-nodes to stop sending data. When the
switch needs to stop a 100 Mbps, half-duplex end-node from transmitting data,
it forces a collision on the data link, which stops the end-node. Once the switch
is ready to receive data again, the switch stops forcing collisions. This is
referred to as backpressure.
A port operating at 100 Mbps and 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.
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.