The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

8.4. PAUSE Function

The PAUSE function is used to implement flow control on full duplex Ethernet links. PAUSE operation uses the MAC Control architecture and frame format just described. (In fact, it is the only function ever implemented using that general-purpose architecture and frame format.) The operation is defined only for use across a single full duplex link; that is, it cannot be used on a shared (half duplex) LAN, nor does it operate across or through intervening switches. It may be used to control data frame flow between:

• A pair of end stations (i.e., a simple, two station network)

• A switch and an end station

• A switch-to-switch link

The PAUSE function is designed specifically to prevent switches (or end stations) from unnecessarily discarding frames due to input buffer overflow under short-term transient overload conditions. Consider a device designed to handle the expected steady-state traffic of the network, plus an allowance for some time-variance of that load. The PAUSE function allows such a device to avoid discarding frames even when the short-term load increases above the level anticipated by the design. The device can prevent buffer overflow by sending PAUSE frames to its partner on the full duplex link; the reception of the PAUSE frame by the partner will cause the partner to stop sending data frames. This gives the first device time to reduce its buffer congestion either by processing frames in the queue that are destined for the device (end station operation) ...