3Com 3CR990 manual Flow Control, Pause Frames, Link Negotiation

68APPENDIX A: SPECIFICATIONS

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.