Xilinx UG492 manual AVB Specifications, P802.1AS, Overview of Ethernet Audio Video Bridging

Chapter 3: Overview of Ethernet Audio Video Bridging

To understand the requirements of this network, we must differentiate between certain types of data:

•Audio and Video streaming data, referred to in this document as AV traffic. Requires a good quality of service to avoid, for example, TV picture breakup, and must be transferred reliably and with guaranteed low latency.

•Other data, referred to in this document as legacy traffic. Does not have the strict requirement of AV traffic: data can be started, stopped and delayed without serious consequence for example, a PC surfing the internet.

For these reasons, an important aspect of the AVB technology is therefore to prioritize the audio/video streaming data (AV traffic) over that of standard data transfer (legacy traffic).

AVB Specifications

The IEEE802.1 Audio Video Task Group is currently working on new specifications which combine to define this technology:

P802.1AS

This specification defines how to synchronize a common time base across an entire AVB network, utilizing functionality from IEEE1588 (version 2), and known as Precise Timing Protocol (PTP). This common time base is in the form of a Real Time Clock (RTC), effectively a large counter which consists of a 32-bit nanoseconds field and a 48-bit seconds field. A single device on the network is designated as the clock master (by automatic resolution) using a Best Master Clock Algorithm (BMCA). All other devices resolve to be slaves. Using the P802.1AS PTP, all slave devices will regularly update their own RTC to match that of the network clock master.

This common time base has various applications:

•It can be used to synchronize media clocks (audio clocks or video pixel clocks) across the entire network to match audio and video data rates between talkers and listeners.

•It can be used by an Ethernet AVB Endpoint System, that is, configured as a "talker", to time a class measurement interval for an SR stream. (The class measurement interval for a stream depends upon the SR class associated with the stream: SR class A corresponds to a class measurement interval of 125 microseconds; SR class B corresponds to a class measurement interval of 250 microseconds). The class measurement interval for a stream is used to limit the number of data frames that are placed into the stream's queue per class measurement interval.

•It can be used by higher layer applications (for example IEEE1722) to provide presentation time stamps for audio and video data. This is used, for example, to synchronize the lip sync on a TV set so a viewer hears the words at the same time as they see the lips move.

The P802.1AS specification is implemented in the Ethernet AVB Endpoint using a combination of hardware and software. The hardware components are incorporated into the core, and the software component is provided with the core in the form of drivers. These drivers should be run on an embedded processor (MicroBlaze™ or PowerPC®).

30	www.xilinx.com	Ethernet AVB Endpoint User Guide
		UG492 July 23, 2010