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Flow Control 157
Flow Control
The 3CR990 NICs utilize flow control technology to throttle
the incoming data packet stream and prevent the loss of
packets. IEEE 803.2x flow control prevents the input buffers
of a device from overflowing. By using pause frames to
communicate buffer status between linked transmitting and
receiving devices (transmitters and receivers). A receiver sends
a pause frame to tell a transmitter to stop the transmission of
data frames for a specified period, allowing the receiver’s
input port buffers to empty before receiving new packets.
Pause Frames
When a transmitter receives pause frames, it suspends
transmission for the specified period. When the receiver’s
input buffers can store packets again, it can either send
another pause frame to tell the transmitter to resume
transmission, or wait for transmission to resume at the end
of the specified period.
With asymmetric flow control, only one of two linked
devices can receive pause frames. With symmetric flow
control, both linked devices can send and receive pause
frames.
Link Negotiation
Related to flow control is the auto-negotiation capability, in
which linked devices advertise their flow control capabilities
and automatically select the best common mode of
communication.
Rare cases (for example, linking to a device that does not
support auto-negotiation) may require that auto-negotiation
be disabled on a port, thereby enabling forced link on that
port. When forced link is enabled, linked devices must have
matching flow control capabilities. For example, a port that is
set for forced link and reception flow control can connect
successfully only with a port that is set for forced link and
transmission flow control.