Latency considerations Stefan Buntz, Daimler AG Olaf Krieger, Volkswagen AG

Latency considerations
Stefan Buntz, Daimler AG
Olaf Krieger, Volkswagen AG
Supporters
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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
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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