Allied Telesis AT-GS908 - Manual (Page 19)

AT-GS908 Gigabit Ethernet Switch Installation Guide
9
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.