Avb specifications, P802.1as – Xilinx IP Ethernet AVB Endpoint v2.4 UG492 User Manual
Page 30
30
Ethernet AVB Endpoint User Guide
UG492 September 21, 2010
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®).