Switched Ethernet solutions for embedded real time
networks
Jean-Luc Scharbarg
Université de Toulouse - IRIT - ENSEEIHT/INPT
August 2015
ETR’2015
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Historical evolution of embedded networks
Early days: one single computer for all embedded functions
Then distribution of the functions on computing nodes
I
I
Transmission of data between these functions
Dedicated point-to-point links for deterministic transmissions
Two many links ⇒ shared communication medium
I
I
I
Timing constraints have to be insured
Specific technologies have been defined, e.g. CAN
Timing analysis have been developed
More and more functions ⇒ huge increase of communication needs
I
I
Specific technologies are no more sufficient
Ethernet technologies are candidate
F
F
Pros: large bandwidth, mature technology
Cons: Ethernet is not deterministic
Real-time Ethernet widely used for industrial communications, e.g.
Profinet
Candidates for embeded communications: AFDX, Ethernet-AVB,
TTEthernet, . . .
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Civilian aircraft communications
Avionics
A340
A/C Ops & CABIN
ARINC 429 - 664
IFE
ARINC 429- Ethernet
………
HF/VHF/SATCOM
Platform (s)
Connectivity
AIRLINE
Ground Information System (Airlines, Airbus,…)
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Avionics systems characteristics
Avionics: electronic systems installed in an aircraft
I
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Calculators and their software, sensors and actuators
Communication links between these elements
Example of avionic systems: autopilot, navigation, flight control, . . .
Constraints on the avionics systems
I
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Volume and weight limitations
Correct behavior in severe conditions: heat, vibrations, electromagnetic
interferences, . . .
Safety level required, depending on the system (ARP 4754):
catastrophic (failure = loss of the aircraft), hazardous, major, minor,
no effect
Segregation between critical functions
Aeronautical Radio INCorporated (ARINC): leadership in the
development of specifications and standards for avionics equipment
Airlines Electronic Engineering Committee (AEEC): development and
maintenance of specifications and standards (airlines, governments,
ARINC)
ADN: Aircraft Data Network
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Avionics systems evolution
In the 1950’s
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Very simple standalone systems
One single calculator for the execution of all the functions
In the 1960’s
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The beginning of modern avionics
Replacement of analog devices by their digital equivalent
Since then
I
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Huge increase in the complexity of avionics
More and more digital equipments for existing or new functions
New needs inherent to the evolution of civil aviation or better answers
to existing problems (e.g. fly-by-wire command system)
Increase in the number of embedded systems and functions ⇒
increase in communication needs
Integrated Modular Avionics (IMA) in the 1990’s: better sharing of
execution and communication resources
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I
First implementation of IMA for the Boeing 777, based on the ARINC
629 multiplexed data bus
IMA in the Airbus A380, based on the ARINC 664 (switched Ethernet)
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Classical avionics architecture (up to A340)
Each equipment (LRU) dedicated to a system function (braking,
flight computers, . . .): sensors, actuators, calculators . . .
natural segregation between the equipments
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Data transmission in classical avionics
A network of equipments
F1
F2
F3
G1
G2
Each data is transmitted from its source to every equipment that
needs the data
One dedicated line for each data
Repetitive transmission of the data on each line
Each line: a mono-emitter ARINC 429 data bus
Each data individually identified by a label
Low bit rate: 12 Kbits/s to 100 Kbits/s
At most 20 receivers per line
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Classic avionics evolution
A310 (1983)
A320 (1988)
A340 (1993)
Avionic Volume
745 litres
760 litres
830 litres
A380 (2005)
Number of equipment
77
102
115
147
Embedding Software
4 Mo
10 Mo
20 Mo
100 Mo
Number of ARINC 429
136
253
368
1000 (AFDX)
Computing Power
60 Mips
160 Mips
250 Mips
90% of the avionic
Avionic repartition
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IMA characteristics
Hardware components
I
LRM (Line Replaceable modules): resources in common cabinets
F
F
F
I
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Core modules for the execution of the applications
Input/output modules for communications with non-IMA equipments
Gateway modules for communications between cabinets
LRU (Line Replaceable Unit): existing non-IMA equipments
Backplane databus: ARINC 659
Sharing of execution resources
I
I
An avionics subsystem: a partition with an assigned time window to
execute its application on a shared module
Guarantee isolation between subsystems ⇒ robust partitioning concept
F
F
I
Communications between partitions via ports (APEX: APplication
EXecutive)
F
F
I
Spatial isolation: limitation and protection of the address space of each
partition, e.g. by a MMU
Temporal isolation: static allocation of a time slice for the execution of
each partition, based on WCET
Sampling ports: only the last value of data is stored
Queueing ports: all the values of data are stored
Logical channel: multicast link between ports, independent of the
communication technology
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The reasons for Ethernet in avionics
ARINC 429 is no more sufficient: too many buses are needed
ARINC 629 is too expensive
Why Ethernet technology ?
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High throughput offered to the connected units (100 Mbits/s)
High connectivity offered by the network structure
A mature industrial standard
Low connection cost
But Ethernet CSMA/CD is not deterministic
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Potential collision on the physical medium
Binary Exponential Back off retransmission algorithm
The solution adopted for ARINC 664
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switched Ethernet technology: units directly connected by
point-to-point links to Ethernet switches ⇒ possible collision domain =
single link between two elements
Full duplex links ⇒ no more collisions
The problem is shifted to the switch level
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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From point-to-point to multiplexed communications
Before 1970, mechanical and hydraulic systems for engine control,
brake system, gear system, . . .
After 1970, huge increase in the number of electronic systems
Performance and reliability of hardware components + software
technologies ⇒ implementation of complex functions that improve
the comfort and safety (Antilock Braking System, Electronic Stability
Program, active suspension, GPS, doors, entertainment, . . .)
Early days of automotive electronics: 1 function = 1 Electronic
Control Unit (ECU: a microcontroller and sensors and actuators)
Exchanges of data between functions via dedicated links (e.g. speed
estimated by the engine control and used by the suspension control)
Engine
control
Air
conditioning
Suspension
Dash
bord
ABS
Gearbox
Central control
Seats
Interior
light
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From point-to-point to multiplexed communications
A function is either too complex or too small for a single ECU ⇒
functions distributed over several ECUs or several functions on the
same ECU
Too many information exchanges ⇒ need for a shared communication
medium.
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Reduces the weight of the electronic systems: reduction of 15 Kg with
the replacement of the dedicated links by a shared bus for the control
of the doors of a BMW
Necessary to guarantee that the communication delays can be bounded
First automotive bus: Controller Area Network (Bosch, 1980s)
First embedded CAN bus: Mercedes class S (1991)
Today: a complex architecture with several communication
technologies
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Automotive functional domains
Different applications with different domains
Powertrain
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Control of engine and transmission
Complex control laws implemented in microcontrollers with high
computing power
Stringent time constraints
Frequent data exchanges with other car domains
Closed-loop control systems ⇒ implentation is moving toward a
time-triggered approach
Chassis
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Control of the vehicle’s according to steering, braking and driving
conditions
Part of the vehicle active safety systems (Antilock Braking System,
Adaptive Cruise Control, . . .)
Communication requirements similar to those for the powertrain, more
emphasis on safety
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Automotive functional domains
Body and comfort
I
I
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Software-based systems that control dashboard, wipers, lights, doors,
windows, seats, mirrors, air conditioning
Many exchanges of small pieces of information
Activation of body functions triggered by occupants’ solicitations ⇒
event-triggered communication
Multimedia/infotainment
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Audio CD, DVD players, rear seat entertainment
Hand-free phones, in-car navigation systems, traffic information
systems, remote vehicle diagnostic, vehicle tracking, fleet management,
...
Large amounts of data to be exchanged within the vehicle and with the
external world
QoS of multimedia data stream, bandwidth, security
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Automotive network architecture in 2006
Diagnostic
CAN
CAN low
LIN
ECU
ECU
LIN
Gateway
ECU
Habitacle et confort
Gateway
CAN high
ECU
Gateway
ECU
ECU
ECU
ECU
ECU
ECU
ECU
ECU
ECU
Chassis
Fonctions pour
la surete
(Flexray)
Habitacle et confort
(MOST)
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Vision for future in-car networks
Diagnostic
Habitacle
CAN 1
CAN 2
...
LIN
Aide au
conducteur
Vision
panoramique
Ethernet
Multimedia
Information et divertissement
Divertissement
à l’arriere
Audio
TV
Radar
Cameras
Cam 1
Controle moteur
et chassis
Cam 2
Amplificateur
Cam 3
Ecran
Ecran
DVD
Cam 4
Flexray
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Shared Ethernet
A set of stations share a single communication channel
A frame from a station is received by all the other stations (eventually
through hubs)
Collisions may occur ⇒ CSMA/CD
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CSMA (Carrier Sense Multiple Access): listen to the medium, send
only when medium is detected idle
CD (Collision Detection): if a collision is detected during transmission
F
F
F
Transmission is stopped
Station waits a random delay (exponential backoff)
Station tries to transmit again
At least 64 bytes in order to detect a collision
Number of collisions: major impact on Ethernet performance
I
I
Performance decreases very quickly when number of collisions increases
Collision domain: the whole network
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Switched Ethernet
Division of the network with separate collision domains ⇒ more than
one frame being transmitted at a given time without collision
A switch:
I
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A set of input and output ports with buffers for pending frames
A switch fabric which “moves” each frame from its input port to its
destination output ports
A service discipline for the scheduling of frames in output ports
Two modes for a switch
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Store and forward: reception and verification of the whole frame before
it is forwarded to its output port(s)
Cut through: frame is transfered as soon as destination is decoded
F
F
Smaller latency
Invalid frame might be transferred
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Shared Ethernet Vs switched Ethernet
Shared Ethernet
Switched Ethernet
Collision domain
Full duplex links ⇒ no more collisions
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Full duplex switched Ethernet is not deterministic
Temporary congestion on an output port
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Increase of the waiting delay of frames in the Tx buffer
Frame loss by overflow of the Tx buffer
An illustrative example
f1
f2
f3
f4
f5
f6
f7
f8
f9
f10
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I
Switch
f1,f2,f3,f4,f5,f6,f7,f8,f9,f10
Five frames at the same time ⇒ one frame waits until the transmission
of the four other ones
More than five frames at the same time ⇒ at least one frame is lost
Addition of dedicated mechanisms to classical full duplex switched
Ethernet in order to guarantee the determinism of transmissions
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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The AFDX key characteristics
Static full duplex Switched Ethernet: no CSMA/CD, no spanning
tree, static 802.1D tables
Two FIFO buffers per output port (high and low priorities)
Traffic characterization based on the Virtual Link (VL) concept
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Static definition of the flows which enter the network
Multicast path with deterministic routing
Mono transmitter assumption
Sporadic flows (BAG : Bandwidth Allocation Gap)
F
F
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Minimum (Smin ) and maximum (Smax ) frame lengths for each VL
BAG and Smax define the maximum bandwidth allocated to a VL
F
I
Discrete values: 1, 2, 4, 8, 16, 32, 64, 128 ms
Traffic shaping on each emitting end system
Example: BAG = 16 ms and Smax = 128 bytes ⇒ 64000 bits/s
Scheduling of VLs on end systems ⇒ (bounded) jitter
e1
e2
e3
e4
v6,v8
v6,v8,v9
S1
v7,v9
vx,v1
S3
S2
v6,v7
vx,v2
e5
v2,v3
v6,v8,v9
vx,v6,v7
S4
v5
v2,v5
e7
e8
e9
v1,v3
v1,v3,v4
S5
e10
v4
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e6
28 / 47
Worst-case end-to-end delay on an AFDX network
Three parts in the delay
I
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Transmission times on links (known): link bandwidth, frame size
Switching latency (known)
Waiting time in FIFO queues (unknown)
mi1
mi2
mi3
mi4
v1
v2
v3
v4
v1 ,v2
v1 ,v3,v4,v5
v2
mi6
mi7
v3,v4
mi5
v5
Upper bound on the waiting time in FIFO queues
I
Network calculus
F
F
I
based on modelling with arrival curves and service curves
Certification of AFDX on board A380 and A350
Trajectory approach
F
Maximum workload of interfering frames on the considered path
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An industrial AFDX configuration
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An industrial AFDX configuration
2 redundant networks, 9 switches per network, 24 ports per switch,
123 end systems
984 VLs, 6412 paths
100 Mbits data rate
BAG and maximum frame length
Bag
(ms)
2
4
8
16
32
64
128
Number
of VL
20
40
78
142
229
220
255
Smax
(bytes)
0-150
151-300
301-600
601-900
901-1200
1201-1500
> 1500
Number
of VL
561
202
114
57
12
35
3
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An industrial AFDX configuration
Length of VL paths
Number of crossed switches
1
2
3
4
Number of paths
1797
2787
1537
291
Load of the links
70
Number of links
60
number of link
50
40
30
20
10
0
0
0.05
0.1
0.15
output link load
0.2
0.25
0.3
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Need for QoS support in embedded networks
in the context of avionics
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AFDX is purely event-triggered with no global synchronization
AFDX only support two priority levels which are not significantly used
AFDX is lightly loaded ⇒ share the network with non avionic flows ⇒
non avionic flows should not disturb avionic ones
In the context of automotive
I
Hetergeneous flows with heterogeneous constraints ⇒ needs for
differentiated service disciplines
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
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Ethernet AVB (IEEE Audio Video Bridging)
Set of standards that provide the specification for time-synchronized
low latency streaming services through IEEE 802.1 networks
I
IEEE 802.1AS: timing and synchronization for time-sensitive
applications
F
F
I
IEEE 802.1Qat: stream Reservation protocol
F
I
Variation of IEEE 1588
Precise time synchronization with an accuracy better than 1 µs
Resevation of resources along the path between the talker and the
listener
IEEE 802.1Qav: forwarding and queueing enhancements for
time-sensitive streams
F
F
F
F
Different classes for time-critical and non-time-critical traffic
Two Stream Reservation classes for time-critical traffic: SR-A and SR-B
Credit-Based Shaper (CBS) to prevent traffic bursts
SR-A and SR-B can start transmission only if their credit is not negative
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Ethernet-AVB Credit-Based Shaper
Credit decreases at sendslope rate during the transmission of frames
of the associated class
Credit increases at idleslope rate when frames of the associated class
are waiting or no frame of the associated class is waiting, but credit is
negative
Credit reset to 0 when credit greater than 0 and no frame of the
associated class is waiting
Ent1
Ent2
Ent3
Sortie
idleslope
sendslope
Credit
SR−A 0
Ent1
Ent2
Ent3
Sortie
BE 1
SR−A 1 SR−A 3
SR−A 2
SR−B 1
BE 1
t0 t1
SR−A 1 SR−A 2 SR−B 1 SR−A 3
t2
t3
t4
t5
t6
t7
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Scheduled traffic over AVB
Ongoing project within the Time Sensitive Networking Task Group
I
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Procedures to suspend the transmission of a non-time critical frame
and allows for one or multiple critical frames to be transmitted
Policies to support scheduled traffic
Scheduled traffic is a traffic class that
I
I
Requires to schedule frame transmission based on time
Is time-sensitive and requires a bounded latency
F
Ex: delay-sensitive command and control traffic which requires
deterministic and very small delays
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Latency assessments for time-sensitive traffic
Without interfering traffic
I
Maximum latency of a time-sensitive frame mainly depends on
F
Transmission time of the time-sensitive frame on links (computed from
the frame size)
TS
TS
ES1
S1
TS
S2
ES2
TS
ES1−>S1
S1−>S2
TS
TS
S2−>ES2
TS
TS latency
Good news since time-sensitive flows typically feature small frames
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Latency assessments for time-sensitive traffic
With interfering traffic
I
Maximum latency of a time-sensitive frame also depends on
F
Maximum size of the interfering frames, due to non-preemptive
scheduling at the egress port of each network device
TS, F1
TS, F2
ES1
TS, F3
S1
F2
S2
F1
ES2
F3
F2
TS
ES1−>S1
S1−>S2
F1
TS
F2
TS
S2−>ES2
F3
TS
TS latency
Bad news since non time-sensitive frames might be large
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Time-Aware Shapers (TAS)
Time-Aware Shapers have been defined to deal with interfering frames
Based on the knowledge of the next arrival time for scheduled
time-sensitive frames
I
I
Synchronization is provided by IEEE 802.1AS
Time-sensitive traffic typically follows a regular pattern
TAS blocks any lower priority frame that would interfere with an
upcoming time-sensitive frame
I
Time-sensitive traffic mapped onto SR Class A
TS, F1
TS, F2
ES1
TS, F3
S1
F2
S2
F1
F3
ES2
F2
F1 TS
ES1−>S1
TS
F1
F2
S1−>S2
TS
F2
F3
S2−>ES2
TS
F3
TS latency
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An alternative solution to support scheduled traffic
TAS alone are not enough to support scheduled traffic
I
I
I
Only two real-time traffic classes (A and B) ⇒ time-sensitive control
traffic often has to share Class A with other real-time traffic
It might interfere significantly with time-sensitive frames
Increased effect due to credit-based shaping
AVB ST (Scheduled Traffic) approach
I
I
I
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A new ST on top of AVB SR class A and B
ST traffic is handled in a separate queue
ST traffic does not undergo credit-based shaping
Offline scheduling of ST frames (a priori known periods and frame
sizes)
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AVB ST
ST queue
TAS
CBS
SR Class B queue
Static priority
CBS
SR Class A queue
...
BE queue
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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TTEthernet very short summary
Time is shared between time-triggered and event-triggered traffic
Three types of traffic
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TT flows: scheduled flows, possible thanks to the clock synchronization
Rate Constrained flows (RC): very similar to AFDX VLs
Best Effort flows (BE)
Priority order: TT, then RC, then BE
RC and BE frames might interfere with TT frames
Three solutions when a RC or BE frame is ready before a TT slot
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RC or BE frame starts transmission and might delay TT frame
RC or BE frame starts transmission and is preempted if TT frame is
ready
RC or BE frame starts transmission only if there is enough time before
TT slot
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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Agenda
1
Context
Avionics network architecture
Automotive network architecture
2
Ethernet
Ethernet Evolution
AFDX
Ethernet-AVB
TTEthernet
3
Some open issues
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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Some open issues
Comparative analysis of candidate switched Ethernet solutions
Design and analysis of heterogeneous network architectures
I
Interconnection of fieldbuses through an Ethernet backbone
Probabilistic analysis
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Up to now, certification process based on guaranteed delay upper
bound
Worst-case delay scenario: very rare event
Leads to over-dimensionning of the network
Do we really need a guaranteed upper bound?
Is it ok if the probability to exceed the deadline is 1012 ?
Avionics functions designed to behave correctly even if some (rare)
frames are lost ⇒ computing a probabilistic upper bound makes sense
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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Thank you for your attention
Jean-Luc ScharbargUniversité de Toulouse - IRIT
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