Understanding IEEE's deterministic AV bridging standards
IEEE 802.1Qav Queuing and Forwarding Protocol (Qav):5 The job of the Queuing and Forwarding protocol is to ensure that legacy asynchronous Ethernet traffic does not and can not interfere with streaming AVB traffic. Fortunately for product designers, most of the burden of implementing 802.1Qav falls squarely on the shoulders of AVB-enabled Ethernet switches.
There are, however, defined pacing requirements that AVB media sources are constrained to follow. To further illustrate this point, imagine an AVB "talker" node in a live audio application. As analog audio is captured, digitized, and presented to the network, it is inherently isochronously paced. Such applications are natively suited for AVB transmission and will not be affected by 802.1Qav's pacing requirements.
On the other hand, problems could arise if ill-behaved stored media applications (such as PCs) were allowed to send large bursts of irregularly paced audio/video samples. Think of it this way: you can't send more than 1 Gbps down a gigabit Ethernet link. Because AVB guarantees the delivery of (up to) 750 Mbps of streaming data on that link, there is still capacity for 250 Mbps of asynchronous data. 802.1Qav defines how the link must be shared and how isochronous and asynchronous data are prioritized inside Ethernet AVB switches.
Previous attempts at solving what is inherently a shaping problem have relied on using memory to smooth bursted traffic but at what cost? Memory buffers are the enemy of low latency and low BOM cost. AVB networks lower costs by eliminating the need for costly buffers while guaranteeing low end-to-end latency of less than 2 milliseconds over seven network hops.
A more precise definition of 802.1Qav can be found on the IEEE website: "This standard allows bridges to provide guarantees for time-sensitive (i.e., bounded latency and delivery variation), loss-sensitive real-time audio video (AV) data transmission (AV traffic). It specifies per priority ingress metering, priority regeneration, and timing-aware queue draining algorithms. This standard uses the timing derived from IEEE 802.1AS. Virtual Local Area Network (VLAN) tag encoded priority values are allocated, in aggregate, to segregate frames among controlled and non-controlled queues, allowing simultaneous support of both AV traffic and other bridged traffic over and between wired and wireless Local Area Networks (LANs)."6
What a great description! Even better, it means that all the hard work and heavy lifting is accomplished inside the Ethernet switch. These same changes are already shipping in standards-based Ethernet AVB-capable switches and differentiate AVB from the single-sourced, expensive, and proprietary niche media networking technologies that have come before.
Where does AVBTP fit?
In short, the purpose of AVBTP is to act as a wire--with delay--to allow the logical connection of physically distant CODECs. More long windedly, AVBTP abstracts the underlying network transmission channel to enable the virtual connection of distributed audio and video CODECs over reliable Ethernet networks.
No article on networking would be complete without the obligatory network stack diagram. As shown in Figure 1, AVBTP sits above the IEEE 802.1 AVB plumbing and below the application layer. It acts as the conduit between an Ethernet MAC and a streaming application.
Streaming samples are connected to the application-side of the AVBTP interface. Supported media formats include a variety of raw and compressed audio and video including I2S audio, IEC 60958 SPDIF, MPEG2/4 and H.264 Transport Streams, Bt.601/656 raw video, and so forth. A complete listing of the supported media types and an overview of the synchronization mechanisms appears later in this article.
On the bottom of the AVB Transport Protocol, connection is made to IEEE 802.1 AVB-capable Media Access Controllers (MAC). In essence, media streams are fed into the top of AVBTP and IEEE 1722 Ethernet packets pop out the bottom--and vice versa.
Vendor-independent interoperability is provided by defining supported media types, their associated formats, and the location, organization, and orientation of media samples within the AVBTP Ethernet frame.
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