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MULTILINK ML810 MANAGED EDGE SWITCH – INSTRUCTION MANUAL
FLOW-CONTROL, IEEE 802.3X STANDARD
CHAPTER 4: OPERATION
4.3
Flow-control, IEEE 802.3x standard
Multilink ML810 Switches incorporate a flow-control mechanism for Full-Duplex mode. The
purpose of flow-control is to reduce the risk of data loss if a long burst of activity causes
the switch to save frames until its buffer memory is full. This is most likely to occur when
data is moving from a 100Mb port to a 10 Mb port and the 10Mb port is unable to keep up.
It can also occur when multiple 100Mb ports are attempting to transmit to one 100Mb
port, and in other protracted heavy traffic situations.
Multilink ML810 Switches implement the 802.3x flow control (non-blocking) on Full-Duplex
ports, which provides for a “PAUSE” packet to be transmitted to the sender when the
packet buffer is nearly filled and there is danger of lost packets. The transmitting device is
commanded to stop transmitting into the ML810 Switch port for sufficient time to let the
Switch reduce the buffer space used. When the available free-buffer queue increases, the
Switch will send a “RESUME" packet to tell the transmitter to start sending the packets. Of
course, the transmitting device must also support the 802.3x flow control standard in order
to communicate properly during normal operation.
NOTE
Note
When in Half-Duplex mode, the ML810 Switch implements a back-pressure algorithm on
10/100 Mb ports for flow control. That is, the switch prevents frames from entering the
device by forcing a collision indication on the half-duplex ports that are receiving. This
temporary “collision” delay allows the available buffer space to improve as the switch
catches up with the traffic flow.
