The automotive industry is swiftly moving towards Ethernet as the high-speed communication network for in-vehicle communication. There is nonetheless a need for protocols that go beyond what standard Ethernet has to offer in order to provide additional QoS to demanding applications such as ADAS systems (Advanced Driver-Assistance Systems) or audio/video streaming. The main protocols currently considered for that purpose are IEEE802.1Q, AVB with the Credit Based Shaper mechanism (IEEE802.1Qav) and TSN with its Time-Aware Shaper (IEEE802.1Qbv). AVB/CBS and TSN/TAS both provide efficient QoS mechanisms and they can be used in a combined manner, which offers many possibilities to the designer. Their use however requires dedicated hardware and software components, and clock synchronization in the case of TAS. Previous studies have also shown that the efficiency of these protocols depends much on the application at hand and the value of the configuration parameters. In this work, we explore the use of “pre-shaping” strategies under IEEE802.1Q for bursty traffic such as audio/video streams as a simple and efficient alternative to AVB/CBS and TSN/TAS. Pre-shaping means inserting on the sender side “well-chosen” pauses between successive frames of a transmission burst (e.g., as it happens when sending a camera frame), all the other characteristics of the traffic remaining unchanged. We show on an automotive case-study how the use of pre-shaping for audio/video streams leads to a drastic reduction of the communication latencies for the best-effort streams while enabling meeting the timing constraints for the rest of the traffic. We then discuss the limitations of the pre-shaping mechanism and what is needed to facilitate its adoption.

1. Pre-shaping Bursty Transmissions under IEEE802.1Q
as a Simple and Efficient QoS Mechanism
Nicolas NAVET, University of Luxembourg
Jörn MIGGE, RealTime-at-Work (RTaW)
Josetxo VILLANUEVA, Groupe Renault
Marc BOYER, Onera

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Use-cases for Ethernet in vehicles

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Main TSN QoS protocols on top of Ethernet
8 priority levels for streams
Benefits:
standard and simple
efficient at the highest priority
can be used with shaping in
transmission (“pre-shaping”)
Limitations:
 not fine-grained enough to
for all kinds of requirements
starvation at lowest priority
levels with bursty traffic
IEEE802.1Q
AVB / Credit-Based
Shaper (CBS)
TSN / Time-Aware
Shaper (TAS)
Two egress queues shaped +
6 priority levels below
Benefits:
 Perf. guarantee for AVB classes
 No starvation for best-effort
traffic
Limitations:
 Per class (not stream) shaping
 Not for control traffic
 Not flexible enough with
standard configuration (CMI)
TAS defines egress ports’
gate schedule (open/close)
Benefits:
 Strong time constraints can
be met
 Can be combined with AVB
Limitations:
 Hard to configure
 Rely on a global clock
 Task sched. must be tailored
to communication for best perf.
Temporal QoS = managing interfering traffic
Priority-based Traffic Shaping Time-triggered (TT)

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Main TSN QoS protocols on top of Ethernet
8 priority levels for streams
Benefits:
standard and simple
efficient at the highest priority
can be used with shaping in
transmission (“pre-shaping”)
Limitations:
 not fine-grained enough to
for all kinds of requirements
starvation at lowest priority
levels with bursty traffic
IEEE802.1Q
AVB / Credit-Based
Shaper (CBS)
TSN / Time-Aware
Shaper (TAS)
Two egress queues shaped +
6 priority levels below
Benefits:
 Perf. guarantee for AVB classes
 No starvation for best-effort
traffic
Limitations:
 Per class (not stream) shaping
 Not for control traffic
 Not flexible enough with
standard configuration (CMI)
TAS defines egress ports’
gate schedule (open/close)
Benefits:
 Strong time constraints can
be met
 Can be combined with AVB
Limitations:
 Hard to configure
 Rely on a global clock
 Task sched. must be tailored
to communication for best perf.
Temporal QoS = managing interfering traffic
Priority-based Traffic Shaping Time-triggered (TT)
In the picture too
 Frame-preemption (Qbu+3br)
 Asynchronous traffic shaping (Qcr)
 Cyclic Queuing & Forwarding (Qch)

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QoS support in the switches – on each output port
Traffic
Shaping
Priority-
Based
Scheduling
+
Frame
Preemption
Time-Triggered
Transmission
Up to 8 priority level overall [Figure inspired from Ashjaei2017]

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Under IEEE802.1Q – 3rd hop
High-priority streams
Best-effort
streams
High-
priority
streams
AVB SR-A
Best-effort streams
Under AVB/CBS – 3rd hop
Obtained by
simulation
in RTaW-Pegase

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TSN/TAS: coordinating gate scheduling tables
Sending node
Switch #1
Switch #2
White bands = transmission allowed
grey bands = not allowed

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Pre-shaping mechanism

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IEEE802.1Q with pre-shaping in transmission
– Pre-shaping = inserting “well-
chosen” minimum distance between
frames of a segmented message
on the sender side only – other
characteristics of traffic unchanged
– Objective is to spread out
transmissions to reduce latencies
of lower priority traffic
– Pre-shaping typically applies to
video streams to improve perf. of
best-effort
The last packet of the segmented
message must be received by the
deadline, typically 16.66ms for
60FPS camera

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Pre-shaping in practice
Setting idle-times by hand is not practical
– “PRESH” algorithm in RTaW-Pegase
automates it
– No need for dedicated HW unlike CBS &
TAS, implemented in SW in end-systems
– Not part of TSN but not forbidden!
– Find priorities and transmission pauses
between frames of segmented messages
such that
– all bursty frames subject to pre-
shaping meet their deadlines,
– while minimizing as much as
possible the latency of frames in
lower priority traffic classes

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Case-study

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Renault Ethernet prototype network
4 Cameras - 30 and 60fps 3 control units
3 domain
master
#Nodes 14
#Switches 5
#streams 41
Workload per
link
Min: <1%,
med:11%
max:60%
Link data rates 100Mbit/s and
1Gbit/s (1 link)

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Case-study: 4 types of traffic
With pre-shaping
in transmission
Pre-shaping parameters
for the 8 video streams

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Case-study: IEEE802.1Q priorities
Command & Control (C&C)
Audio Streams
File & data transfer, diag.
Top priority
Second priority
level
Best-effort
Third priority level
Decreasingpriorities
Video Streams

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Verification techniques
Probability
Response time
Simulation
max.
Upper-bound with
schedulability analysis
Q5Q4
(Actual) worst-case
traversal time (WCTT)
Easily observable events Infrequent events
Testbed &
Simulation
Long
Simulation
Schedulability
analysis
Used in
this study
 Long simulation here = 48 hours of driving  350 000 transmissions for 500ms frames
 Metrics: communication latencies, bandwidth usage and buffer occupancies

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Toolset & Techniques
– RTaW-Pegase: modeling / analysis /
configuration of Ethernet TSN (automotive,
avionics, industry) + CAN (FD)
– Developed since 2009 in partnership
with Onera
– Users across several industries, e.g; Daimler Cars, Airbus Helicopters, CNES, ABB
– Worst-case Traversal Time (WCTT) analysis – used for deadline constraints
– Timing-accurate Simulation – used for average & throughput constraints
– Optimization algorithms for setting the parameters of all supported protocols
Techniques used

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IEEE802.1Q with pre-shaping for Video
Average latencies for best-effort streams
IEEE802.1Q
IEEE802.1Q with pre-
shaping
AVB Tight Idle-Slope
Pre-shaping under IEEE802.1Q improves
average latencies for best-effort streams by
54% on average – up to 86% – similar
performance as using AVB custom classes
Best-effort streams only
Deadlines of C&C, Video, Audio met
– like without Pre-shaping
Averagelatencies(ms)

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IEEE802.1Q with pre-shaping for Video
Worst-case latencies for best-effort streams
IEEE802.1Q
IEEE802.1Q with pre-
shaping
AVB Tight Idle-Slope
Pre-shaping under IEEE802.1Q improves
worst-case latencies for best-effort streams
by 66% on average – up to 90% - similar
performance as using AVB custom classes
Worst-caselatencies(ms)
Best-effort streams only

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Discussion & conclusion

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Pre-shaping Pros and Cons
– Simple, compatible with standard IEEE801Q HW and as effective as AVB/CBS in our
experiments but
1. No protection against “babbling idiots” unlike CBS and TAS – per stream policing of Qci
could offer a solution
2. Adding frames to the system may require a reconfiguration of all flows subject to pre-
shaping (unlike AVB with standard parameters)
3. Setting pre-shaping parameters requires dedicated tool support
4. As there is no reshaping along a path, efficiency decreases with the number of hops
5. Pre-shaping is an additional specification to ECU suppliers which has a cost for OEMs,
but pre-shaping can be implemented on a subset of nodes only (e.g., 5 out of 14 in
our case-study)

21. Any questions? Contact us
nicolas.navet@uni.lu
jorn.migge@realtimeatwork.com