# A Perspective on Ethernet in Automotive Communications--Current Status and Future Trends
## 4 TSN Standards for Automotive Communications
### Bounded Low Latency  
Among the standards in this class, there are the previously discussed IEEE 802.1 Qat and IEEE 802.1 Qav amendments that belong to the AVB family of standards. In fact, TSN not only adds new features to the IEEE 802.1Q standard, but also retains the original functions, e.g., the ones introduced by AVB have been part of the IEEE 802.1Q standard since 2014.  
The main TSN standards that address bounded low latency are described below.  
The IEEE 802.1 Qbv-2015 [37] Enhancements for Scheduled Traffic introduces the transmission gate mechanism to support scheduled traffic, a traffic class that requires that frame transmissions occur following a predefined time schedule. The transmission gate mechanism is based on opening or closing the gates that are associated with each queue of a port of an IEEE 802.1Q switch to either allow or block the transmission of frames from each queue. Gate operations follow a given time schedule implemented as a a gate control list, i.e., a list of timed gate operations that repeats cyclically.  
The IEEE 802.1 Qbu-2016 Frame Preemption amendment [38] is also relevant to bounded low latency. In combination with the IEEE 802.3 br, the IEEE 802.1 Qbu implements  
Figure 2: Main TSN standards relevant for in-car communications.  
frame preemption to intersperse express frames (i.e., real-time frames) among best-effort ones. To realize frame preemption, a dual-MAC stack is used within IEEE 802.1Q switches in order to distinguish between express (i.e., time-critical and non-preemptable) and preemptable frames (i.e., best-effort) and handle them differently. The standard makes it possible to suspend the transmission of a preemptable frame and allow for one or more express frames to be transmitted. When the express frames have been transmitted, the transmission of the preempted frame is resumed. The advantage of preemption is the medium access delay reduction for time-critical frames, as they do not need to wait for the transmission of an entire best-effort frame, but can preempt it at some point. However, such a benefit becomes less significant at higher speeds, as the transmission time decreases.  
The IEEE 802.1 Qch [39] standard, Cyclic Queuing and Forwarding, introduces a mechanism for receiving and transmitting frames alternately for a fixed interval of time, called a cycle time. The real-time frames that are gathered during a cycle are then transmitted in the following one, to minimize transmission jitter and guarantee bounded end-to-end delays.  
The IEEE 802.1 Qcr-2020 [40] amendment to the IEEE 802.1Q standard defines Asynchronous Traffic Shaping (ATS) over full-duplex links with constant bit data rates. The protocol works over the port transmission queues and realizes traffic shaping at a stream level through a token bucket that limits the burst size of a given stream. Mixed real-time traffic types, such as periodic, rate-constrained, and event-driven (sporadic) are supported. An eligibility time is assigned to each received frame. When the current time is higher than or equal to the eligibility time of a frame, the latter can be enqueued to be transmitted.