Allied-telesis AT-GS908 Manual de usuario descargar pdf (Pagina 19)

AT-GS908 Gigabit Ethernet Switch Installation Guide

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 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.

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Allied-telesis AT-GS908 Manual de usuario Pagina 19