Shulou(Shulou.com)06/01 Report--

IEEE 802.1Qbv Time Aware Shaper

Time-aware shaping (TAS) can be said to be a core protocol in TSN, and its application provides ultra-low latency and jitter guarantee for in-vehicle time-sensitive data. We know that Ethernet data is transmitted serially on the bus, as shown in figure 5, when multiple data are waiting to be transferred out of the switch exit (different data will be queued in different queues according to the priority in the VLAN Tag), who comes out first and who comes out determines the delay of the data.

Fig. 5 schematic diagram of Ethernet egress queue

TAS controls the transmission of data by opening and closing the door. As shown in figure 6, the opening and closing status of each queue at a certain time is controlled by the Gate Control list on the right. Take the T05 circled in the black box on the right as an example, at this time, the opening and closing status of queue 7 to queue 0 is CoCCoCCC (C for closing the door, o for opening the door). Data can only be sent when the door is opened, that is, only queue 6 and queue 3 can send data at that time.

Figure 6 TAS door control (partially quoted from IEEE 802.1Q)

Generally speaking, Gate Control List is cyclical, and before and during the sending of critical data (TC1 data in figure 7), we will turn off the transmission of other data to ensure that the TC1 data will not be affected. So as to provide ultra-low delay guarantee for TC1. At the same time, the characteristic of periodic door opening and closing also shows that TAS is more suitable for providing delay protection for periodic data.

Figure 7 example of TAS

IEEE 802.1 Qbr & IEEE 802.3bu frame preemption

Frame preemption is another protocol in the TSN family that provides delay protection. By modifying the preamble, the protocol divides normal Ethernet frames into two categories: Express MAC (EMAC high priority frames) and Preamble MAC (PMAC low priority frames). The high priority frame can interrupt the low priority frame that is being sent, thus reducing the waiting time of the high priority. Taking the data in figure 8 as an example, normally, the first blue PMAC frame is sent first, and even the subsequent EMAC frame must wait for the PMAC frame that is currently being sent to be sent. However, after applying frame preemption, the EMAC frame can interrupt the PMAC frame to send, and when the EMAC frame is sent, the remaining PMAC frames will be sent again. In this way, the waiting time of EMAC frames can be saved.

Fig. 8 example of frame preemption

Except that frame preemption can be used alone to reduce latency. It can also be used in conjunction with the TAS mentioned above to reduce the size of the Grandband (figure 9), thereby improving network utilization with frequent opening and closing of doors.

Fig. 9 frame preemption combined with TAS to reduce Grandband

Finally, there are three points to note when applying frame preemption:

Data is divided into two categories: PMAC frames and EMAC frames

Depending on the underlying support, the original MAC layer is divided into EMAC and PMAC.

The minimum frame length that a PMAC can be interrupted is that 124Bytes is less than that length and cannot be interrupted.

IEEE 802.1Qci Per-Stream Filtering and Policing

The PSFP protocol in the TSN protocol family is similar to the firewall mechanism, which can filter and filter the data before forwarding and control the data frames with specific identification. Taking figure 10 as an example, ECU1 and ECU2 normally send 20Mbps data respectively, but the 90Mbps data is sent because the ECU1 is affected by the network or because of its own error. At this time, since only 100m bandwidth is supported at the exit of switch, ECU2 can only send data of 10Mbps at most, so it will not work properly.

If PSFP is applied in the system, this mechanism can identify in advance that the data stream allowed by ECU1 can only be 20Mbps according to the characteristics of the data stream (VLAN,MAC address, IP address, etc.), so it will be handled in two ways:

PSFP limit: restricts the sending of data streams, down to the default value

PSFP blocking: completely blocking the wrong data flow

In addition, PSFP can also prevent the network (such as ARP***) and ensure that the filtered data is sent at a specific time through the gating mechanism.

Figure 10:PSFP as an example

IEEE 802.1CB Frame Replication and Elimination for Reliability (FRER)

802.1CB protocol is mainly responsible for redundant backup and transmission of data, and uses redundancy mechanism to solve the information errors or loss caused by CRC, line opening, connector disconnection and so on in the loop network topology. If autopilot is used and there is a higher functional safety level, then FRER is almost an inevitable choice.

Figure 11. R-tag and serial number

When sent, it tags and sequence the specific data (figure 11) and replicates the transmission on different paths. At the link convergence point, the replication information is identified and eliminated to prevent the upper application of the receiving node from processing multiple duplicate data. Its application can be roughly divided into two forms: the terminal node undertakes the FRER work (figure 12) or the Switch node completes the end FRER work (figure 13). Compared with the former, the latter is less difficult to implement, and the degree of design change and complexity is relatively small.

Figure 12. The terminal node undertakes the function of FRER

Figure 13. Switch assumes the FRER function

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