Audio Video Bridging

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Digital protocols abound, but is AVB a standard for radio?

Not this kind of bridge.

If you have worked with AoIP or other digital audio transmission schemes you may have heard the acronym AVB bandied around. Just what is AVB, and how might it come in to play in broadcasting? That's our topic this time around.

We're all familiar with the big console/router manufacturer's systems - whether AoIP- or TDM-based. To a very large degree (although not completely) those systems remain proprietary. Audio Video Bridging (AVB) is the name given to the implementation of four protocols developed by the IEEE. One of the primary purposes behind all the work done was the development of a non-proprietary means by which different manufactures products could communicate with one another over Layer-2 (Ethernet) networks for purposes of media streaming. (The AVnu Alliance is a group of manufacturers that has formed with the express purpose of ensuring interoperability.) Another primary purpose behind AVB is to ensure multiple, related streams can be streamed across a network, with precision synchronization, and very low transit time.

To learn a little more about AVB we're going to look at its four facets in detail: IEEE 802.1BA Identification of Participating Devices; IEEE 802.1Qat Admission Controls; IEEE 802.1Qav Traffic Shaping for AV Streams; and IEEE 802.1AS Precise Synchronization.

As with any transmission scheme, there is a source and a destination. In the context of AVB the source is known as the talker and the destination is known as the listener. For the benefits of AVB to be realized all the intermediate points must be AVB capable. The intermediate points are ports on a Layer-2 switch that in this context will be known as AVB bridges.

Whether or not an AVB link can be established between a talker and listener is determined during the establishment of the Layer-2 connections - in other words when a port on an AVB bridge is brought up. Four specific (802.1BA) requirements are:

◊ The link is a full-duplex, 100baseT connection (or faster)
◊ 802.1AS protocol (which we'll discuss below) discovers exactly one peer on the port
◊ The maximum round-trip delay time from the port to the AVB peer meets requirements specified in 802.1AS
◊ An 802.1Qat "Stream Reservation Protocol" (SRP) request or acknowledgment is received on the port

- continued on page 2

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