Flow Control 157Flow ControlThe 3CR990 NICs utilize flow control technology to throttlethe incoming data packet stream and prevent the loss ofpackets. IEEE 803.2x flow control prevents the input buffersof a device from overflowing. By using pause frames tocommunicate buffer status between linked transmitting andreceiving devices (transmitters and receivers). A receiver sendsa pause frame to tell a transmitter to stop the transmission ofdata frames for a specified period, allowing the receiver’sinput port buffers to empty before receiving new packets.Pause FramesWhen a transmitter receives pause frames, it suspendstransmission for the specified period. When the receiver’sinput buffers can store packets again, it can either sendanother pause frame to tell the transmitter to resumetransmission, or wait for transmission to resume at the endof the specified period.With asymmetric flow control, only one of two linkeddevices can receive pause frames. With symmetric flowcontrol, both linked devices can send and receive pauseframes.Link NegotiationRelated to flow control is the auto-negotiation capability, inwhich linked devices advertise their flow control capabilitiesand automatically select the best common mode ofcommunication.Rare cases (for example, linking to a device that does notsupport auto-negotiation) may require that auto-negotiationbe disabled on a port, thereby enabling forced link on thatport. When forced link is enabled, linked devices must havematching flow control capabilities. For example, a port that isset for forced link and reception flow control can connectsuccessfully only with a port that is set for forced link andtransmission flow control.
