3Com 3CR990-TX-95, 3CR990-TX-97, 3CR990SVR97 manual Flow Control, Pause Frames, Link Negotiation

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.