Allied-telesis AT 8000/8POE User Manual download pdf (Page 28)

Chapter 1: Overview

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.

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Allied-telesis AT 8000/8POE User Manual Page 28