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9033630 Flow Control 33
APPENDIX B. FLOW CONTROL
Flow control is a mechanism which allows you to protect the switch from
overload conditions and to keep additional traffic off the network when
excessive congestion will result. Figure B-1 illustrates flow control for both
half- and full-duplex applications.
Figure B-1. Flow Control for Half- and Full-Duplex Applications
Each port of the switch has a transmit queue which buffers frames to be
sent out on that port. In this example, large amounts of data are being
sent from Workstation A (connected to port X) and other ports on the
switch to Workstation B (connected to port Y).The queue on port Y starts
filling up with data until it reaches a determined threshold. The packet
which causes the threshold to be exceeded triggers the flow-control
function on the port from which the packet entered the switch, in this case
port X.
Since port X is configured with flow control set to Enabled, the switch
responds by initiating a pause frame (full-duplex applications) or back-
pressure mechanism (half-duplex applications). The pause frame causes
Workstation A to stop sending packets. After a certain amount of time has
elapsed, determined by a value in the pause frame, Workstation A will
resume sending data. Similarly, the back-pressure mechanism forces
Workstation A to stop sending packets by inducing collisions on port X.
The pause-frame flow-control mechanism supported by the
VH-4802 switch conforms with the IEEE 802.3x specification for full-
duplex flow control.
For the full-duplex pause-frame mechanism to work, the device
connected to the switch port must also support IEEE 802.3x flow control.
Flow control is configurable per port in the Port Configuration Menu of the
console menus, using the on-board Web agent, or via SNMP.
ELS100-48tx2m.book Page 33 Thursday, November 16, 2000 4:59 PM