What is IEEE802.1Qat?

Introduction

TAVB (Audio Video Bridging) uses the IEEE802.1Qat (Stream Reservation Protocol) standard, which makes it possible to reserve transmission bandwidth for audio and video streams between Talker and Listener over an Ethernet. This column provides an overview of IEEE802.1Qat.

IEEE802.1Qat is one of the IEEE 802.1 standards related to Ethernet networking. This standard reserves bandwidth for streams between Talkers and Listeners on an Ethernet network, enabling stable playback of Audio/Video streams sent from Talkers by Listeners.

Why IEEE802.1Qat is needed

Because Ethernet is a best-effort network that does not guarantee communication quality, if congestion occurs on the path from Talker to Listener, Ethernet FRAMES carrying the Audio/Video stream may be discarded. This makes it impossible to guarantee stable playback of the Audio/Video stream at the Listener. Therefore, to prevent the Audio/Video stream from being discarded on the path from Talker to Listener due to congestion, it is necessary to reserve the Audio/Video transmission bandwidth in advance between Talker and Listener, following the procedures of the IEEE802.1Qat standard, before starting to send the Audio/Video stream from the Talker to the Listener.

Congestion refers to a situation in which excessive traffic concentrates on network equipment, making communication difficult.

Procedure for reserving bandwidth for audio/video stream transmission

This section explains the procedure for reserving transmission bandwidth between Talker and Listener.
As an example, let's consider a case where there are three bridges between Talker and Listener.

Talker refers to the device that transmits the Audio/Video stream.
A Listener is a device that plays back audio/video streams.
A bridge refers to a relay device (such as SWITCHES).

①The Talker sends an Advertise message to the Listener containing information about the transmission bandwidth it wishes to reserve.

②If each Bridge on the Talker⇒Listener route can reserve the transmission bandwidth requested by the Talker, it will relay the Advertise message as is.

③After receiving the Advertise message, the Listener sends a Ready message back to the Talker.

④Each Bridge relays the Ready message.

⑤After receiving the Ready message from the Listener, the Talker starts sending the Audio/Video stream to the Listener.

⑥Each Bridge uses the reserved transmission bandwidth to relay the Audio/Video stream and plays the stream on the Listener.

The explanations above for steps 1 to 6 are successful examples of reserving transmission bandwidth between Talker and Listener, but there are also cases where it fails due to insufficient bandwidth. The following failure example shows a case where Bridge 2 does not have enough bandwidth and is unable to reserve the transmission bandwidth requested by the Talker. Bridge 2 stops relaying the Advertise message and sends a Fail message to the Talker to inform it that it has failed to reserve transmission bandwidth. By receiving the Fail message, the Talker realizes that a Ready message has not been returned from the Listener and can immediately take new action.

Combination with IEEE802.1Qav (QavForwarding and Queuing Enhancement for Time-Sensitive Stream)

Strictly speaking, reserving transmission bandwidth between Talker and Listener according to the IEEE802.1Qat standard does not guarantee stable playback of Audio/Video streams at the Listener in all cases. The IEEE802.1Qat standard does not specify the transmission interval for Ethernet FRAMES carrying Audio/Video streams. Therefore, for example, if a burst of Ethernet FRAMES is sent continuously from one Bridge to the next Bridge in a short period of time, some of the Ethernet FRAMES may be discarded depending on the receiving capabilities of the next Bridge. For this reason, it is necessary to adopt the IEEE802.1Qav standard, which specifies the transmission method and transmission interval for Ethernet FRAMES carrying Audio/Video streams, in conjunction with IEEE802.1Qat.

For more information about IEEE802.1Qav, please see the separate page.

Summary

In this column, we have provided an overview of IEEE802.1Qat (Stream Reservation Protocol). In the next column, we will explain IEEE802.1Qav (Forwarding and Queuing Enhancement for Time-Sensitive Stream).

What is IEEE802.1Qat?

Introduction