New Electronics -- Imagine you’re driving down the highway with the music blaring, enjoying the open road. Now imagine that the sound from your rear speaker system is delayed by a split second from the front; your enjoyment of the fancy in-car infotainment system comes to a screeching halt.
Ethernet is emerging as the network of choice for infotainment and advanced driver assistance systems that include cameras, telematics, rear-seat entertainment systems and mobile phones. A typical system is shown in fig 1. But standard Ethernet protocols can’t assure timely and continuous audio/video (A/V) content delivery for bandwidth intensive and latency sensitive applications without buffering, jitter, lags or other performance hits.
Audio-Video Bridging (AVB) over Ethernet is a collection of extensions to the IEEE802.1 specifications that enables local Ethernet networks to stream time synchronised, loss sensitive A/V data. Within an Ethernet network, the AVB extensions help differentiate AVB traffic from the non-AVB traffic that can also flow through the network. This is done using an industry standard approach that allows for plug and play communication between systems from multiple vendors.
The extensions that define the AVB standard achieve this by:
* reserving bandwidth for AVB data transfers to avoid packet loss due to network congestion from ‘talker’ to ‘listener(s)’
* establishing queuing and forwarding rules for AVB packets that keep packets from bunching and guarantee delivery of packets with a bounded latency from talker to listener(s) via intermediate switches, if needed
* synchronising time to a global clock so the time bases of all network nodes are aligned precisely to a common network master clock, and
* creating time aware packets which include a ‘presentation time’ that specifies when A/V data inside a packet has to be played.
Designers of automotive A/V systems need to understand the AVB extensions and requirements and how their chosen microcontroller will support that functionality.
AVB: a basket of standards
AVB requires that three extensions be met in order to comply with IEEE802.1:
* IEEE802.1AS – timing and synchronisation for time-sensitive applications (gPTP)
* IEEE802.1Qat – stream reservation protocol (SRP)
* IEEE802.1Qav – forwarding and queuing for time-sensitive streams (FQTSS).
In order to play music or video from one source – such as a car’s head unit – to multiple destinations, such as backseat monitors, amplifiers and speakers, the system needs a common understanding of time in order to avoid lags or mismatch in sound or video. IEEE802.1AS-2011 specifies how to establish and maintain a single time reference – a synchronised ‘wall clock’ – for all nodes in a local network. The generalised precision time protocol (gPTP), based on IEEE1588, is used to synchronise and syntonise all network nodes to sub-microsecond accuracy. Nodes are synchronised if their clocks show the same time and are syntonised if their clocks increase at the same rate.
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