Introduction
With conventional video/audio equipment, almost all devices were connected one-to-one with CABLES. Also, because the type of CABLES depended on the interface, various CABLES were often routed around AV equipment, which could lead to unsightly and confusing installations due to the large number of CABLES. Ethernet attracted attention as a solution to this problem, but because it is a communication method developed based on the "best effort philosophy" that does not guarantee communication quality such as latency or bandwidth, it had the problem of being unsuitable for transmitting video/audio streaming data, which requires strong real-time performance.
This column provides an overview of AVB, one solution to this issue.
What is AVB?
AVB stands for Audio Video Bridging and is a technology for transmitting video and audio over Ethernet, established by the Institute of Electrical and Electronics Engineers (IEEE) as standard 802.1. It was established with the goal of achieving high reliability and low latency in the transmission of video and audio streaming data over Ethernet.
Why AVB?
There are three main reasons why AVB is necessary:
①To prevent interruptions in video and sound
Because Ethernet is a best-effort network, the Ethernet FRAMES carrying video/audio streaming data do not always reach their destination. This can cause video interruptions if the video data does not reach the monitor, or audio interruptions if the music data does not reach LOUDSPEAKERS. AVB employs a mechanism that prevents video/audio streaming data from being discarded.
②For smooth playback of video/audio
Because Ethernet is a best-effort network, Ethernet FRAMES carrying video/audio streaming data do not necessarily reach their destination within the required time. AVB can estimate the latency between the source and destination, preventing delays in video/audio playback.
3) To synchronize playback time
Ethernet networks have no concept of time. As a result, it is not possible to synchronize the playback times of multiple LOUDSPEAKERS or video devices, which can lead to playback lag. AVB employs a time synchronization function, which allows each device to have a synchronized time, preventing such playback lag.
AVB Use Cases
AVB is designed to connect devices using Ethernet CABLES and transfer video/audio stream data, and is used to integrate home AV equipment, professional audio and video system networks, and in-vehicle multimedia equipment, as shown in Figure 1 below.
AVB defines three ways to refer to devices:
①Talker
This refers to video/audio transmission equipment.
②Listener
This refers to video/music playback equipment.
③Bridge
This refers to relay devices (such as SWITCHES).
AVB guarantees a low latency of 2 ms at worst for a video/audio stream to reach the Listener from the Talker, even when passing through six Bridges, as shown below.
AVB Standards
AVB is made up of several 802.1 standards.
| Standard | Title | Contents |
|---|---|---|
| 802.1AS | Timing and Synchronization | Time Synchronization As a protocol for transmitting time, Uses gPTP (generalized Precision Time Protocol) |
| 802.1Qat | Stream Reservation Protocol | Stream transmission bandwidth reservation between Talker and Listener |
| 802.1Qav | Forwarding and Queuing Enhancement for Time-Sensitive Stream | Corrects uneven FRAMES transmission intervals Credit-based queuing ensures low latency |
Table 1 AVB standard group
Key features offered by AVB
Time Synchronization
AVB uses the IEEE802.1AS standard, which enables precise time synchronization between devices on a network.
This time synchronization allows actions to be performed at the correct time on each device.
Stream transmission bandwidth reservation
AVB uses the IEEE802.1Qat standard, which allows for the reservation of stream transmission bandwidth between Talker and Listener. The ability to reserve stream transmission bandwidth enables stable video and audio playback.
Corrects uneven FRAMES transmission intervals and guarantees latency
AVB uses the IEEE802.1Qav standard, which makes it possible to correct non-uniformity in FRAMES transmission intervals and guarantee latency. Correcting non-uniformity in FRAMES transmission intervals makes it possible to prevent FRAMES discards due to buffer overflow on the device receiving FRAMES. Also, by guaranteeing low latency, smooth playback of video and audio is possible.
Summary
In this column, we provided an overview of AVB (Audio Video Bridging).
We will explain IEEE802.1Qat (Stream Reservation Protocol) and IEEE802.1Qav (Forwarding and Queuing Enhancement for Time-Sensitive Stream) in turn.
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