Traffic shaping is important because it allows for a predictable amount of time for the stream to pass through a system. Think about it: a set of devices that use the public internet for transmission have to make use of a buffer so that they don't run out of data to convert to the actual stream output, while waiting for packets to arrive from the far end. Since the packets can arrive in bunches, followed by none for a time, the buffer has to be fairly deep.

However, if you can manipulate (or 'shape') the traffic so that it comes through consistently, you can make the buffer very short - and you can effectively speed up the time it takes for frames to propagate through the system.

The last aspect of AVB is that of precision timing (802.1AS). This is probably more important in video applications than radio. For example, if you have two streams, one for video, and one for its associated audio, then they have to be synchronized, otherwise anyone watching could notice a lip-sync problem. That's always annoying. AVB works to solve this problem. In an AVB network, one of the participating devices will become the grandmaster time source. (All other devices will be slaves.) The various devices that make up the AVB network periodically exchange time information. And, as I have mentioned previously, a talker knows how long frames take to propagate from end to end; so by adding that latency information to the frames themselves, along with a time stamp, the listener is able to synchronize the stream outputs by taking in to account the network latency. Differences in time between two associated media streams, caused by propagation delays through different paths, can be mitigated.

Broadcast use

SAS KDL-16

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So now that we've learned something about AVB, who is using it in our field? SAS offers a module for its 32KD system that allows the user to interface AoIP to the system, making use of the AVB standards. The KDL-16 offers 32 channels in, and 32 channels out of a 32KD network. Obviously, when connected to an AVB-capable LAN, any other device using AVB will have access to the 32KD system.

- continued on page 4