802.1Qbu - Frame Preemption

The full title of this PAR is "Standard for Local and Metropolitan Area Networks-Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks - Amendment: Frame Preemption".

This amendment specifies procedures, managed objects, and protocol extensions that:

• Define a class of service for time-critical frames that requests the transmitter in a bridged Local Area Network to suspend the transmission of a non-time-critical frame, and allow for one or more time-critical frames to be transmitted. When the time-critical frames have been transmitted, the transmission of the preempted frame is resumed. A non-time-critical frame could be preempted multiple times.
• Provide for discovery, configuration, and control of preemption service for a bridge port and end station.
• Ensure that preemption is only enabled on a given link if both link partners have that capability.

The purpose of this amendment is to provide reduced latency transmission for scheduled, time-critical frames in a bridged LAN.

A large, non-time-critical frame may start ahead of time-critical frame transmission. This condition leads to excessive latency for the time-critical frame. The lack of transmission preemption severely inhibits the capabilities of an application that uses scheduled frame transmission to implement a real-time control network.