What is IEEE802.1AS-Rev?

Introduction

TSN uses the IEEE802.1AS_Rev (Timing and Synchronization) standard, which enables precise time synchronization between devices on a network, allowing actions to be taken at the exact time on each device.

What is 802.1AS-Rev?

802.1AS-Rev is an IEEE 802.1 standard related to Ethernet networking that defines a method for synchronizing time among devices in a network.
802.1AS-Rev is the successor to 802.1AS and is an upgrade from 802.1AS in several respects.

The Need for 802.1AS-Rev

The Network Time Protocol (NTP) method is one way for devices on a network to synchronize their time, but because this method uses software to process timestamps, its time synchronization performance is limited to the millisecond level (one-thousandth of a second).
802.1AS-Rev uses gPTP (Generalized Precision Time Protocol) as its time synchronization protocol, and performs time stamp processing in hardware, enabling highly accurate time synchronization in microsecond (one millionth of a second) units.
The adoption of 802.1AS-Rev is essential for Ethernet networks that require time synchronization performance greater than that of the NTP method (for example, in-vehicle, mobile, and factory networks).

How 802.1AS-Rev Time Synchronization Works - Part 1

Devices within the network synchronize their time on a peer-to-peer basis.
Using the BMCA (Best Master Clock Algorithm), a device is determined to be a GM (Ground Master) that distributes reference time information to the network. Devices other than GMs are either BCs (Boundary Clocks) or OCs (Ordinary Clocks). BCs have the function of receiving time information from higher-level devices and synchronizing the time (Slave function), and the function of transmitting time information synchronized with the higher-level device using the Slave function to lower-level devices (Master function). On the other hand, OCs are devices that have only one of the Master or Slave functions.
An example of an 802.1AS-Rev network configuration is shown in Figure 1. Time information is periodically distributed from the GM, and the BC successively corrects its own time information and transmits the time to downstream devices, enabling devices within the network to have the same time information with an accuracy of less than 1 μS. Time synchronization is achieved by exchanging messages specified in the gPTP protocol between upstream and downstream devices. This will be discussed later.

Figure 1. 802.1AS-Rev network configuration

How 802.1AS-Rev Time Synchronization Works - Part 2

The following four messages are used to synchronize the time of a subordinate device with the time of a superior device.

Sync Message
Follow_Up Message
Delay_Req Message
Delay_Resp Message

By sending and receiving the above four messages between the lower device and the higher device, the lower device can have the following four pieces of time information (t1 to t4).

t1 The time when the host device sent the Sync Message
t2 The time when the lower device received the Sync Message
t3 The time when the lower device sent the Delay_Req Message
t4 The time when the host device receives the Delay_Req Message

802.1AS-Rev specifies two methods for communicating the t1 time to downstream devices: the 1-Step synchronization method and the 2-Step synchronization method, as shown in Figures 2 and 3 below.
(802.1AS, the predecessor standard to 802.1AS-Rev, specifies only the 2-Step synchronization method.)
Time correction is performed periodically at regular intervals.

Figure 2.1 Step synchronization method
Figure 3. 2-Step synchronization method

The lower device corrects the time using the t1 to t4 time information to synchronize with the higher device.
Use the following formula to calculate the time correction value (= the time difference between the upper and lower devices) and correct the time on the lower device.

calculation formula

Time correction value (= time difference between upper and lower devices) = ((t2 - t1) - (t4 - t3)) / 2

If the sign of the calculation result is positive, it indicates that the time of the lower device is ahead of that of the upper device.
If the sign of the calculation result is negative, it indicates that the time of the lower-level device is behind that of the upper-level device.

Corrected time of the lower device (= time of the upper device) = (time of the lower device) - (time correction value)

Summary

In this column, we explained the 802.1AS_Rev (Timing and Synchronization) standard.
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