Latency considerations Stefan Buntz, Daimler AG Olaf Krieger, Volkswagen AG Supporters • • • • • Stefan Buntz (Daimler) Natalie Wienckowski (General Motors) Olaf Krieger (Volkswagen) Efstathios Larios (Jaguar Land Rover) Doug Oliver (Ford) 15.05.2014 IEEE 802.3bp EMC ad hoc 2 Typical Automotive Latency Requirements • Automotive control loops require an E2E latency of the communication channel down to 1ms MAC+ PHY 12.5µs 25µs 25µs 12.5µs Host Switch Switch Host ∑15µs ∑ 15µs ∑ 15µs • The picture shows a typical network connection which fulfills the requirements (120µs << 1000µs) even if PHY needs 15µs for FEC 15.05.2014 IEEE 802.3bp EMC ad hoc 3 Automotive Connector Requirements • According to mechanical vibrations it is allowed for connectors to have a disconnection up to 1µs (LV214, USCAR-2) • 1µs is what todays connectors can provide and are tested for • If the channel is not robust enough to accept disconnections up to 1µs maybe there are mechanical designs (which maybe are more complex), alternatively, the test pass/fail criteria may need to be changed which would necessitate requalifying all existing connectors intended to be used for Ethernet 15.05.2014 IEEE 802.3bp EMC ad hoc 4 Recommendation • FEC with additional latency of 15µs is acceptable • Using a FEC which is robust enough to accept 1µs disconnections, would avoid requalification 15.05.2014 IEEE 802.3bp EMC ad hoc 5 Apendix 15.05.2014 IEEE 802.3bp EMC ad hoc 6 Latency definitions • • • • • • Transmission latency latency on the copper media (~5ns/m). Maximum is roughly 75ns = 0,075µs (@15m) TranmissionLatency = Length[m]*5ns/m (estimation) Frame latency latency for frame transmission (including Header & CRC = 18byte) FrameLatency= (Framesize[bytes]+18)*8 / Datarate [bit/s] Max. 12,144µs (@1Gbit/s; MTU=1500) or 121,44µs (@100Mbit/s; MTU=1500) PHY latency from x(G)MII to PMD (either up or down) 3a. PHYlatency_down (xGMII to PMD) 3b. PHYlatency_up (PMD to xGMII) Switch latency latency of the swtich (DataLink Layer, above xGMII) Hop latency Latency of a „hop“ in the network HopLatency = PHYLatency_down +FrameLatency+TransmissionLatency+PHYLatency_up Network Latency Latency of a network with N „hops“ NetworkLatency= N*HopLatency + (N-1)*SwitchLatency Overview 1000base-T1 stack in ISO/OSI Application Presentation Session Transport Network Data Link Data Link „Switch“ „switch“ latency LLC LLC MAC MAC MAC PCS PCS PCS FEC FEC FEC Scrambler Scrambler PMA PMA PMD PMD LLC Scrambler „hop“ latency PMA PMD Medium Physical Physical xMII „FEC“ latency