Allied Telesis AT 8000/8POE - Manual (Page 28)

Chapter 1: Overview
28
Back Pressure
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 speed and duplex mode of the end node and switch
port. A twisted pair port operating at 100 Mbps and half-duplex mode will
stop 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. When the switch is ready to receive
data again, the switch stops forcing collisions. This is referred to as back
pressure.
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.
The AT-8000/8POE Fast Ethernet Switch supports back pressure flow
control in half duplex mode and pause frame flow control in full duplex
mode.