Study on Effect of Degradation of Time Synchronization in IEEE 802.1AS Caused by Congestion on QoS of TAS and CBS

This paper quantitatively evaluates the effect of the degradation of the time synchronization accuracy of IEEE 802.1AS, a protocol for time synchronization over Ethernet, on the QoS of an in-vehicle network with IEEE 802.1TSN(Time-Sensitive Networking) by experiment. In IEEE 802.1AS, slave terminals synchronize the time to a master terminal by receiving the current time of the clock of the master terminal by gPTP. However, since it takes some time to transmit the time information due to the network delay, adjusting the time gap between the slave terminal and the master terminal is required. Since the adjustment is performed according to the measurement of the delay between a slave terminal and a master terminal, it will be affected by network congestion where gPTP messages are transmitted. To study the degradation of the time synchronization accuracy caused by congestion of in-vehicle networks, the authors evaluate the QoS of TAS and CBS, one of the QoS controls of IEEE 802.1TSN, over congested experimental networks. The experimental results show that the maximum delay and the coefficient of variation of the delay degraded owing to the congestion for TAS. However, it also indicates that the QoS of CBS is not always affected by the degradation of the time synchronization caused by the congestion.