Contents
• Real-time communication
Lecture 6: Real-time communication
• Muddy-card evaluation
Mikael Asplund
Real-Time Systems Laboratory
Department of Computer and Information Science
Linköpings universitet
Sweden
The lecture notes are partly based on lecture notes by Jan Jonsson's course PDRTS.
These lecture notes should only be used for internal teaching purposes at Linköping
University.
Lecture 6: Real-time communication
Mikael Asplund
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Yesterday
Today
• >11000 electrical parts
• 61 ECUs
• 3 CAN buses
• 250 CAN
messages
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Communication scheduling
• Non-preemptive by nature
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Message delay
• Packetizing the message
• Queueing delay, waiting for medium access
• Distributed protocol required
• Transmitting
• Notifying receiver of message arrival
P1
• Depacketizing the message
Network
P2
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Communication media
• Electrical medium (wire)
Topologies
• Bus
• Optical fiber
• Wireless
• Ring
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Topologies
Message routing
• Packet switching
• Hypercube
• Circuit switching
• Wormhole routing
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Communication scheduling
• Two things to schedule: tasks and transmissions
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Integrated scheduling
• Suitable for:
– Homogeneous
• Integrated scheduling
– Hard real-time
– Requires compatible dispatching strategies
– Static allocation
• Separated scheduling
• Examples
– Allows different dispatching strategies
– Time driven dispatching + TDMA
– Static priority dispatching + CAN
– Static priority dispatching + Token Ring
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Separated scheduling
• Suitable for:
Medium access
• Contention-free communication
– Heterogeneous
– Time based, each processor gets a slot
– Dynamic allocation
– Examples: Time-division, multiple access (TDMA)
– Firm/soft real-time
• Collision-based
– Processors contend for the medium at run-time
• Why
– Examples: Ethernet, CAN
– Transmission time might be zero
• Token based
– Number of hops can vary
– A token is passed around that gives the right to send
– Different policies for different links
– Examples: FDDI, Token Ring
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TDMA
• Time-Division Multiple Access
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TTA /TTP
• The time triggered architecture [Kopetz et. al]
• Requires synchronized clocks!
• TDMA
– Allocates pre-defined slots within which pre-defined nodes
can send their pre-defined messages
• Offline scheduling
– Periodical architecture
• Temporal firewall
P1
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P2
P3
• Replication & failure detection
P4
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TTA
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Token-based
Timed-Token Protocol
• Target token-rotation time (TTRT)
• Real-time and non-real-time traffic
• Is token late? Yes => only RT.
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Feasability of Timed-Token Protocol
• Necessary:
Token Ring (IEEE 802.5)
• Token format:
P P
T
M
R R R
– R: reservation
SD AC ED
• Sufficient:
• Message format:
–The accumulated transmission quotas
should not exceed TTRT-Overhead
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P
– P: priority
–The deadline of each message transmission
must be at least 2*TTRT
SD AC ED
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packet data
error ctrl ED FS
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Token Ring
• Examine busy token, insert own priority in PPP if higher
than RRR
addresses
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Controller Area Network (CAN)
• Bus
• Identifier used as priority
• Grab free token only if priority higher than PPP
• Wired-AND
• Send message by adding to token
• Reset the priorities you have set yourself!
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Arbitration
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Real-time CAN
• With non-preemptive rate-monotonic
• With EDF and DM by partitioning the identifier field
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Reading
• Chapter 14 of Burns & Wellings
• Paper by Herman Kopetz, The Time-Triggered
Architecture
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