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STREAM01 / Mastère SE
Distributed Embedded Systems and realtime networks
Embedded network TTP
Marie-Agnès Peraldi-Frati AOSTE Project
UNSA- CNRS-INRIA
January 2008
Abstract
Requirements for TT Systems
The Time Triggered Protocol
Objectives
Frame
Architecture
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Requirements for communication architecture
Time-triggered control system
Determinism: All activities are carried out at certain points in time know a priori at design time (based on a globally synchronized time base)
Transmission of messages
All nodes have a common notion of time
Monitoring of external states
Fault tolerance:
detection
monitoring
recovery
Composability , extensibility
Temporal : the temporal control of the communication network is determined predictable and independent from the application.
SAE Communication Classes
SAE: Society of Automotive Engineers
Three Communication System Classes
Class A
For systems with low speed networks
Soft Real-Time systems
Class B
For systems with high speed networks, but without safety-critical requirements
Class C
For systems with safety-critical requirements
Hard Real-Time systems
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Time-Triggered Protocol
TTP: Family of TDMA based, fault tolerant protocols
TDMA: Time Division Multiple Access
TTP/C: A communication protocol specifically designed for safety-related automotive applications
The development of TTP and TTP/C has been led by Prof. Hermann Kopetz, Technical University of Vienna
The commercial development of TTP/C tools and products is led by TTTech (www.tttech.com)
Existing protocols J1850 and CAN meet the bandwidth specification for an SAE Class C protocol, but not the fault tolerant requirements
Two TTP Protocols
TTP/A (Automotive Class A= Soft Real-Time)
A scaled-down version of TTP
A cheaper master/slave variant
TTP/C (Automotive Class C= Hard Real-Time)
A full version of TTP
A fault-tolerant distributed variant
Bandwith: 500kbit/s, 1Mbit/s, 2Mbit/s, 5Mbit/s, 25Mbit/s
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TTP Protocol Objectives
Message transport with low latency and minimal jitter
Support for composition
Provision of a fault-tolerant membership service
Fault-tolerant clock synchronization
Distributed redundancy management
Minimal overhead
Scalability to high data rates
efficient operation both on twisted wires and
on optical fibers
TTP/C Cluster
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Definitions
SRU: Smallest Replaceable Unit
A single node consisting of a TTP protocol processor and a Host processor
Shadow SRU : the unit emits in case of failure of the main node
FTUs: Fault Tolerant Units
Group of actively replicated units, each unit emits the same information
2 nodes : protection in temporal domain
3 nodes : protection in the value domain
CNI: Communication Network Interface
The Host programming interface to the Time-triggered network
TTP/C Bus Access Scheme
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TDMA: Time Division Multiple Access
A distributed static medium accessstrategy
The right to transmit a frame is controlled by the progression of real time
Requires that a fault-tolerant global time-base is available to all nodes
The channel capacity is statically dividedinto a number of slots
A unique sending slot is assigned to every node
A node can only send one frame in every TDMA round
If there are no data to send, an empty frame is transmitted
The sequence of sending slots within a group of nodes is called a TDMA round
The sequence of all different TDMA rounds is called a cluster cycle
TTP/C Frame Types: N-Frames
16 bit
I/N Message
Mode bit 1
Mode bit 2
Mode bit 3
4 bit Header
•Frame type
•Mode change request
Data = Application Data + Explicit C-State Or
Data = Application Data
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TTP/C Frame types: “Cold start frame”
16 bit
I/N Message
Mode bit 1
Mode bit 2
Mode bit 3
4 bit Header
C-State:
•Controller state
•Current clock
•Sender slot
•Current mode
Continuous state agreement : CRC Calculation
C-State is not emitted in each message
The CRC at the sender is calculated over the message contents concatenated with the sender C-State
At the receiver side the CRC is recalculated with the receiver C-State.
If CRC are different, the message has been corrupted or there is a disagreement on C-States.
Message must be discarded
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Continuous state agreement : CRC Calculation
CRC calculation at sender
Header Data Field C-State of Sender
Message on the network
Header Data Field CRC
CRC
Header Data Field C-State of
Receiver CRC calculation at receiver
CRC
TTP/C Frame validity
To be acceptable by the receiver node :
A frame must be valid according to the MEDL table of the receiver i.e.
Correct slot
Correct length
And correct
CRC sender side = CRC receiver side
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TTP/C frame transmission phases
slot i in the
« TDMA round » Slot duration for
the i node
Slot duration for the i +1 node
PRP idle PSP PRP idle PSP TP PRP idle PSP
slot i+1 in the
« TDMA round »
IFG Inter Frame Gap AT
Action Time PSP (Pre Send Phase)
TP (Transmission Phase) PRP (Post Receive Phase)
TP
TTP/C : C-State
C-State is a data structure generated by each node and transmitted during the slot node.
C-State of a node N may contains :
Clockof the N node (master clock only)
Slot numberassociated with the N node in the current TDMA round
Demand for a mode switchingat the next cluster cycle Local vector of the Membership.
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Membership
The node membership vector
contains as many bits as there are nodes in a cluster
each node is assigned to a specific bit position
“True” indicates that the node was operating successfully during the last sending slot
“False” indicates that the node was not operating successfully
The position bit of the bit membership indicates the position point for the node to send or receive.
Update every SRU slot after a CRC checking on the received messages (PRP phase).
The delay for updating all membership information is at most one TDMA round
Consequence : a node is considered operational or not until its following membership point in the next TDMA round
Membership
A node which doesn’t receive any correct message assumes that the sending node has crashed and it
eliminates the sending node from its membership vector
If however the conclusion is different for the other nodes, from this moment two cliques have formed that cannot communicates with each other because they don’t have the same C-State
In such conflict, TTP ensure that the majority view wins, and the nodes with the failed input port is eliminated from the membership.
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TTP/C System Architecture
TTP Node Configuration
Host Processor Dual Port
RAM
Protocol Processor Bus Guardian
ROM TTP/C Control
Data (MEDL)
TTP/C Controller CNI:
Communication Network
Interface «global clock tick»
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Bus guardian
Open the bus access at determined slots
Slots are specified in the MEDL
Protection from desynchronized units
Protection from “babbling idiot” unit
Clock synchronization
Not managed with additional traffic
A minimum of 4 Master Clocks (MC) nodes
Maximum jitter between MC is 10-4s/s
Each node compares its clock with the one of the sender node (if it is a master clock node)
If the difference is greater than a precision, the reception node disconnect from the network
Otherwise, the reception node updates its clock and the associated data with the a fault tolerant synchronization algorithm.
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Conceptual Layers of TTP/C
Application Software in Host
FTU Membership
Permanent value management
Redundancy Management
SRU Membership Clock Synchronization
Media Access: TDMA
Host Layer
FTU CNI FTU Layer
RM Layer
SRU Layer
Data
Link/Physical Layer
Basic CNI
The Basic CNI Structure
Global Internal Times SRU-Time (part of C state)
MEDL (part of C state)
Membership
(part of C state)
Status Information
Status Registers Control Registers
Watchdog Timeout Register Mode Change Request Reconfiguration Request External Rate Correction
Dual Port Ram
Updated by TTP Controller
Updated by Host
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Communication Network Interface - CNI
•CNI : Dual Port RAM + registers
•Consistent Data Transfert :
• Arbitration of the DPRAM access
• Host may derive read access interval from the global time base and the MEDL
• Host may access the RAM arbitrarily to read/modify the communication objects.
In that case a Non Blocking Write Protocol is provided to ensure integrity of data . The TTP controller is never delayed.
The Message Descriptor List (MEDL)
SRU-Time Address Attributes
D L I A message
time
D: Direction – input/output message L: Length of message
I: Initialization – Initialization or normal message
A: Additional parameter – protective information concerning mode change MEDL
Message Area
Message
The MEDL’s of a cluster are generated automatically by a cluster compiler
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Operating Modes
Different operating modes require different message schedules
• Accelerating vs. cruise might need different information
• Operation vs. diagnosis need emphasis on different aspects of the vehicle
• Failure recovery might need access to different message traffic
TTP solution: use multiple schedules
• Precompute a different MeDL for every possible situation
•Currently used on TTP/A; but could be used on TTP/C with special care )
Fault-tolerant Node
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Conclusion : TTP/C Properties
Static Scheduling
Guaranteed delivery times with known variance (jitter)
Clock Synchronization
All nodes synchronized to within one microsecond each TDMA round
Composability
TTP/C nodes are temporally composable as well as functionally composable
Fail Silent
The bus guardians ensure transmission only during the correct timeslot in all cases
Membership
Every node’s membership is available during each TDMA round
Advantages/Disadvantages of TTP
Advantages
Simple protocol to implement
Deterministic response time
No wasted time for Master polling message
Disadvantages
Wasted bandwidth when some nodes are idle
Fixed network size after installation
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TTP/TTA References
Real-Time Systems Research Group at the Vienna University of Technology
http://www.vmars.tuwien.ac.at
TTA Group Forum (the open industry consortium for time-triggered systems today)
http://www.ttagroup.org/
TTTech Computertechnik AG
supplier of technology in the field of time-triggered systems and TTP® (Time-Triggered Protocol).
http://www.tttech.com/
- [1] H. Kopetz course and from its textbook:
“Real-Time Systems – Design Principles for Distributed Embedded Applications”
Chapter 8: “The Time-Triggered Protocol”
Chapter 14: “The Time-Triggered Architecture”
- [2] P. Koopman Course
(http://www.ece.cmu.edu/~ece540/lecture/) -[3] Slides TTPtech (http://www.tttech.com/ )
