AVB – Traffic Classes

AVB – Traffic Classes

The traffic in an AVB Network is classified into Class-A, Class-B, Control Traffic, and the rest as Best-Effort. These streams are identified by, unique (application generated) Stream-ID, destination Multicast MAC address, and classified based on VLAN ID and priority. Each stream defines the worst-case packet-size, frequency of such packets so that the intermediate nodes know the amount of bandwidth to be reserved to deliver them to the listener(s).

Needless to say, the Audio/Video streams are mapped to the highest priority traffic classes A/B, which are:

  • Class-A streams: Produce frame(s) at 8 KHz (or 125 micro-seconds) and support an end-to-end latency of 2 milliseconds.
  • Class-B streams: Produce frame(s) at 4 KHz (or 250 micro-seconds) and support an end-to-end latency of 50 milliseconds.

This is followed (in priority) by Control Traffic. The control-plane traffic is important for all the state-machines and signaling, however, are not as much latency-bounded as the AV streams. The control traffic is also mostly event-based, majorly non-periodic and much smaller in volume when compared to the AV and other data-traffic. This includes PTP control frames, MSRP BPDUs and other protocol exchange like IGMP and multicast etc.

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The remaining data-traffic in an AVB network is treated as Best-effort traffic with no guarantees of bandwidth or latency or even loss-less delivery. However, the AVB network tries to serve the AV vs best-effort in a 75:25 ratio so that the best-effort does get some share of bandwidth on each port and is not totally starved out.

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