Distributed embedded Systems and Real-time networks

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STREAM01 / Mastère SE course E10

Distributed embedded Systems and Real-time networks

Marie-Agnès Peraldi-Frati AOSTE Project UNSA- I3S-INRIA

December 2007

Course 2 :

Real-time Standards

Autosar standard

OSEK/VDX standard

Real-Time communications

ISO and realtime

Real time Protocols

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AUTOSAR

An Open Standardized Software Architecture for Automotive Industry www.autosar.org

Three main topics

Architecture : software architecture including AUTOSAR Basic software

Methodology: Exchange formats or description templates for seamless configuration process

Application interfaces: typical automotive interface (syntax and semantics)

Architecture

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Methodology

1a: SW component description

General characteristics, Communication properties : r-ports, p-ports, interfaces

Structure (sub-components, connections) Required HW resources (processing time, scheduling, memory)

1b: ECU resource description

General characteristics, Temperature, Available signal processing methods, Available programming capabilities, Available HW (uC, memory, interface CAN, LIN, Flexray), Periphery (sensor, actuator), Connectors : number of pins, SW below RTE

1c: Description of system

Bus systems CAN, LIN…, connected ECUs, Gateways, protocols, communication matrix and attributes, function clustering, function deployment (distribution to ECU)

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Methodology

2: Distribution of SW component description to ECU

Configuration on the basis of description

Consideration of ECU resources available and constraints given in the system description

3: Generation of required configuration for AUTOSAR infrastructure per ECU

Mapping of task, messages…

4: Generation of software executables based on the configuration on each ECU

Example of functional transferability : Speed Warning Use Case

SW-Component description get_v() describes a function to acquire the current vehicule speed and defines the necessary resources (memory, run-time and computing power requirements …)

Function v_warn() makes use of get_v()

Autosar virtual integration :

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Application interfaces

Syntax of interface

meta model,

software component template

Semantics of interface

Physical properties, units …

Supporting re-use across product lines

AUSTOSAR conclusion

A standards and a methodology based on component architecture design

Independent HW/SW development process

Interoperability and reuse of components

Interface and templates for AUTOSAR components

Open issues in AUTOSAR

RT in RTE mean Run Time … not Real Time

Timing issues ?

Safety ?

Predicatbility ?

….

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Timing issue example 1

Courtesy Kai Richter TIMMO project

Timing issue example 2

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Autosar OS and COM

Course 2 :

Real-time Communications

Autosar standard

OSEK/VDX standard

ISO and realtime

Real time Protocols

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OSEK/VDX overview

1993 OSEK group (Open Systems and the Corresponding Interfaces for Automotive Electronics)

University of Karlsruhe

Spare part designers

(Bosch, Siemens …)

Car manufacturers (

BMW, DaimlerChrysler, Renault, PSA, Opel etc

.)

1994 OSEK/VDX OS (Vehicle Distributed eXecutive)

Joined projects of OSEK and VDX (GIE PSA-Renault)

OSEK/VDX

Real-time OS for automotive embedded systems

Real-time constraints

Fault tolerance

Embedded hardware (RAM, ECU 8 et 16 bits)

Distributed Architecture

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OSEK-VDX Specifications

OS

: Sept 2001, OS Kernel event triggered

COM

: Jan. 2003, Services for communication

TIME

: OS kernel time triggered

FT

: Fault-tolerance manage redundancy and frame construction

NM

: May 2000, Network Management and Monitoring

OIL

: Jan. 2003, Application description language

http://www.osek-vdx.org

OSEK/VDX Operating system: Services

Taskmanagement

Activation, termination, state management, task switch

Synchronization

Resource management, Mutual exclusion, Event control

Interrupt management

Alarm

Relative and absolute alarms

Static (defined at compile-time) and dynamic (defined at run-time) alarms

Error management

Mechanisms supporting the user in case of various errors

Only static Objects : neither dynamic creation nor dynamic

suppression of objects

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Methodology

Task management

Interrupt tasks

System tasks

Application tasks

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Task scheduling

Multiple instance of the same task

Several tasks with the same priority

Problem of the memory space for context switching storage.

Task Scheduling

Fixed Priority Preemptive Scheduling

3 scheduling modes

Preemption mode: a task may be preempted by one with higher priority

Non preemptive mode: a task switch is only performed via an explicit system services (explicit point of rescheduling ,cooperative mode)

Mixed mode: a task have an attribute (preemptable or not) Advantages of a non-preemptive execution in a preemptive context :

In the case of a short execution time it is equivalent to a context switching

This mechanism needs few RAM consuming

Adapted to resource protection

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OSEK/VDX Application Task

Basic task

3 states : suspended / ready/ running

no waiting state, and thus only comprise synchronization points at the beginning and the end of the task

without deadlock

Moderate requirement regarding run time context (RAM).

Extended task

Can wait for an event waiting state

4 states : basic task states + waiting

OSEK/VDX Application Task

Group of tasks

A group is a set of tasks attached to an internal resource

Internal tasks

(from the same group) cannot preempt each others

External tasks

with a lowest priority than the highest

priority

task in a group cannot preempt a task in this group.

External tasks

with a highest priority than the highest

priority

task in a group can preempt all the tasks in this

group.

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OSEK/VDX Resource management

Resource management ensures that :

two tasks cannot occupy the same resource at the same time.

deadlocks cannot occur by use of these resources.

access to resources never results in a waiting state.

priority inversion cannot occurs.

OSEK/VDX Ressource management

Main problem : Deadlock

Priority Ceiling Protocol to solve these problems

Off line : Priority(Rj) = max {priority(Ti) using Rj}

On line

Priority{(Tk) using Rj}= Priority(Rj)

As Soon As Tk release Rj, Tk recover is initial priority

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Multiple resource access

T4 Priority >T3 Priority

T4 and T3 are using the same resource

Problem :

Abnormal suspension time for T4

Priority Ceiling Protocol

T5 Priority >T4 Priority

T4 and T3 are using the same resource

Solution : Priority inversion

between T1 and T4

since R1 is released

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OSEK-VDX COM

Objectives :

Portability, reuse and interoperability of application software

Same interface for internal or external communications

Independency on underlying protocol (CAN, TTP … )

Static configuration of communication

Communication Services based on objects

Writing a message : potentially multiple writers on a node

Reading a message : multiple readers on the same or on different sites

OSEK/VDX Communication

Osek API Sender/Receiver Periodic Message

Segmentation Recombination

Acknowledge Frame transfer

with CAN or TTP Monitoring Mode switching

Initialization

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OSEK-VDX COM

Asynchronous model of communication

Based on messages

Senders and receivers are tasks and/or ISR from OSEK OS

At the IL level :

Receiving/Sending Message objects (Int. and Ext. com.)

I-PDU (Ext. Com. only)

N,M Communications

Communications tasks for sendingcannot be suspended during transmission

Communications tasks for receivingcannot be lockedif the message is not available.

OSEK-VDX COM

Application layer manage its own copy

of a message A message can have

no data : it is a signal

The sizeof a message can be either static or dynamic (max boundary).

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OSEK-VDX COM

Management of the message object by

Blackboard (Update and multiple reads) Management of the message object by Queue (Read once)

Transfer characteristics :Triggered , pending Notificationmechanism is associated with a message

OSEK-VDX COM

I-PDU :

Only for external communication

Optimization: one or several messages in the same frame

Transmissionof a I- PDU on request, periodic, mixed mode

Temporal management (deadline, on sending and

receiving)

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OSEK-VDX Evolutions

Application model

OSEKtime OS : TTCAN, TTP/C, Flexray

Classical tasks : OSEK-VDX OS

TT Tasks : entry point, ending point – Suspended running preempted

Off line Static scheduling

Deadline monitoring

Free space time used for classical task execution

OSEK-VDX Conclusion

Maturity of the proposition

Industry products available : Motorala , WindRiver

A major step for the objectives of reuse and portability

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Course 2 :

Realtime Communications

OSEK/VDX standard

ISO and realtime

Real time Protocols

Main definition in data transmission

Architecture : OSI model-based architecture ?

Topology : bus, ring, star, tree

Protocol

Synchronous /asynchronous

Event / Time trigerred

Protection from errors

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Architecture of an embedded network

Based on the OSI model with a limitation of the number of levels

Idea : reducing the number of layers (suppression of the network, transport, session and presentation layers)

Advantage : the duration to pass the layers is limited

Inconvenient : the associated services are not available !

Integration of temporal mechanisms in real time networks.

Architecture of real time embedded network

Real time network merges and simplifies the services of the different layers.

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Architecture of real time embedded network

Useful layers for real time networks ?

physical: transmission medium ,

physical link : optical, radio waves, coaxial cable, infra red …

shared between stations in full duplex accesses

Data link IEEE 802 :

Ensure the detection of error transmission

Includes the MAC sub layer which allows multiple accesses to physical medium

Depends on the topology

application:useful layer which provides services required for distributed information’s accesses. Services can be tagged with temporal information's.

Real time embedded network => layers 1, 2, 7

Architecture of real time embedded network

Unused layers?

Network andtransport: provide and control the routing of datagram's between users. The control for the safe transmission of data is ensured by the DLL layer.

Session : manage the dialogue before establishing a connection.

This layer is not implemented on embedded networks.

Presentation:data packaging. Few services of this layer are implemented.

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Topology

Good fit to long skinny systems – elevators, assembly lines, etc...

Flexible - many protocol options

Break in the cable splits the bus

Was prevalent for early desktop systems

Topology

Token ring

No contention over a shared wire

Nodes act as repeaters

Requires bypass for failed node

Popular choice for fiber optics

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Topology

Star

Can emulate bus functions

Easy to detect and isolate failures

Broken wire only affects one node

Broken hub is catastrophic

Requires more wiring; common for current desktop systems

Protocols

What’s services and protocol and at which level ?

Diffusion, point to point, multipoint

Protocol with or without connection

Connection is not adapted in a well-known environment.

The objective is to increase the exchange speed

Protocol with or without acknowledgement

Protocols provide negative or positive ack,

For periodic communications in real time embedded networks => no ack

For sporadic or aperiodic communications => ack

Main issue => recovery from errors

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Protocols

Synchronous or Asynchronous transmission

Synchronous : receiver and transmitter are sampling

the signal at significant instants to acquire transmitted bits.

Problem of clock drift between nodes => Characters based synchronization

Asynchronous

information is transmitted at non predefined instants. Synchronization between receiver and transmitter is done by the way of encapsulating data into a frame with start and stop information's.

Course 2 :

Realtime Communications

OSEK/VDX standard

ISO and realtime

Real time Protocols

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MAC Protocol

Protocol with concurrency :

CSMA (Carrier sense Multiple access) class protocol

Local access management

Protocol with temporal multiplexing :

frequency or time multiplexing

TDMA Time Division Multiple Access

Synchronous or asynchronous TDMA

Protocol with explicit control access

Token or arbiter

Temporal aspects

Propagation time

Few time in comparison to transmission time

Approximately Tp =5 microS/km for a coaxial wire

Transmission time

The duration for a station for the emission of the information bloc PDU (Protocol data unit).

Transmission time Tt of the n bits of the PDU on a circuit which bandwidth is C bits/s is Tt=n/C.

Transfer time

Ttr = Tt + Tp

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Temporal aspects

Latency time for emission:

Delay between a request for an emission (MAC_Data_Rq) an the beginning of the frame transmission at the sender node CNI level (Network Communication Interface).

Turnround time :

A characteristic of station with an access control protocol

The delay for a station to recognize that it is its turn to send or to receive

Response time :

Addition of transfer time and latency time

It represents the delay between the service demand of emission and the reception of the frame.

Frame

Temporal aspects

MAC-Data-Rq

Latency Time

Transmission Time Propagation Time

Response Time

Transfer Time Mac services

and medium

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PDU Frame encoding

• Manchester encoding

Data encoded by transition from high-to-low or low-to-high

“Self-clocking” scheme

PDU Frame encoding

NRZ = Non-Return-To_Zero

Few transitions (on average) = requires less oscillator drift

Bit stuffingrelaxes oscillator drift requirements

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Generic Message

Header

Routing information (source, destination)

Global priority information (which message gets on bus first?)

Data

Application- or high-level-standard defined data fields

Often only 1-8 bytes

Error detection

Detects corrupted data (e.g., using a CRC)

Detection is not perfect for high bit error rates

Embedded networks can have very high bit error rates

Central Issue: Message Priority

Local priority

Each node can transmit its highest priority message when it gets a turn on the bus

Or, it can implement some form of round-robin message transmission, etc.

Global priority

Which node gets the next turn on the bus?

Could be a function of round-robin selection of nodes

Could be a function of the node’s inherent priority

Could be a function of the priority of the highest message on the node -- a

“global message priority” scheme Fundamental tension:

Reducing latency for high-priority nodes/messages vs. Ensuring fairness for low-priority nodes/messages

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Access protocol : bus master approach

Bus Master can poll for messages

Master sends polling messages and waits for response

Problem: missing/slow slave

Master uses worst-case timeout waiting for response

If slave gets confused/is late, protocol fails

Problem: broken master

Master can send a time tick

Other nodes select response time from that time tick

Then becomes a form of time slice/time slot protocols discussed later

Polling

Operation

Centrally assigned master polls the other nodes (slaves)

Non-master nodes transmit messages when they are polled

Inter-slave communication through the master

Exemple : WorldFIP, Profibus

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Polling

Advantages

Simple protocol to implement;

historically very popular

Bounded latency for real-time applications

Disadvantages

Single point of failure from centralized master

Polling consumes bandwidth

Network size fixed during installation (not robust)– Or, master must discover nodes during reconfiguration

Prioritization is local to each node

But, can use centralized load balancing

Polling need not be in strict order; it could be, for example: 1, 2, 1, 3, 4, 1, 5, 1, 3, 1, 6, …(repeats)

TDMA Time division multiple access

Master node sends out a frame sync to synchronize clocks

Each node transmits during its unique time slot Exemple : TTP, FlexrayInterbus, ControlNet, Arinc

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TDMA

Advantages

Simple protocol to implement

Deterministicresponse time

No wasted time for Master polling messages Disadvantages

Single point of failure from the bus master

Wasted bandwidth when some nodes are idle

Requires stable clocks

Network size fixed during installation (not robust)

Prioritization is local to each node – (can use centralized load balancing) Variation: Variable Length TDMA (~Implicit Token)

Unused time slices are truncated to save time

More efficient use of bandwidth

CSMA/CD Transmit And Collide

CSMA : Carrier Sense Multiple Access

CSMA + CD Collision Detection

Transmit message if bus is idle;

if you get lucky network transites to “active”

If you get unlucky, you get a collision event (requires Collision Detection HW)

Vulnerability window is about 2 time slot

After collision, back off a certain time

Amount of time to back off should vary with network load

Exemple : ethernet

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CSMA/CD

Advantages

Small latency for low traffic load

Network initialization/configuration is not required

Node can enter or leave the network without any interruption

Supports many nodes

Probabilistic global prioritization is possible

Extensive installed base and support

Disadvantages

Designed for aperiodic traffic - not ideal for synchronized control loops

Collision detection is an analog process which is not always practical

Unbounded individual message latency

Poor efficiency under heavy loads

Explicit Tokens Protocols

Token ring

Nodes are connected in a ring using point-to-point links

This is not a circular bus – every wire is independent and operates concurrently

A token signal is passed from one node to another in a circular fashion

The token holder has the permission to access the media

This is a bit-at-a-time transfer protocol– Bits are shifted around the ring– All other protocols discussed deal with whole messages

Token bus

A token signal is passed from a node to node on a bus (virtual

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Explicit Tokens Protocols

Advantages

Bounded latency for real-time control applications

High throughput during heavy traffic

On-the-fly reconfiguration Disadvantages

Token passing latencies under light traffic conditions

Prioritization local to each node

Lengthy reconfiguration process

Token initialization, loss, and duplication recovery overhead

Collisions may occur during initialization and reconfiguration

Complex protocol (especially at MAC sublayer)

Example : profibus, telway, map

Implicit Tokens protocols

Length of waiting period is used as a time-domain implicit “token”

Owner of bus determined by what time it is instead of explicit token message

Time slices -- waiting period is a whole message long

TDMA, TTP

Time slots -- waiting period is as short as possible

~ 2tpd

CSMA/CA

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CSMA/CA (Implicit Token)

IDLE: Active station transmits immediately

After each message, reserve S slots for N nodes

BUSY: Transmit during your assigned slot

– If S=N, no collisions - known as Reservation CSMA

– If S<N, statistical collision avoidance

CSMA/CA Tradeoffs

Advantages

Small latency for light traffic

Good throughput under heavy traffic

Global prioritization through fixed slots – prioritized implicit token passes

Bounded latency through rotating slots – non-prioritized implicit token passes Disadvantages

Restarting time slots from an idle bus can be difficult

Send dummy messages to avoid idle state

Collisions can occur

Node complexity in mapping Sth slot to Nth node

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Binary Countdown (Bit Dominance)

Each node is assigned a unique identification number

All nodes wishing to transmit compete for the channel by transmitting a binary signal based on their identification value

A node drops out the competition if it detects a dominant state while transmitting a passive state

Thus, the node with the dominantidentification value wins

Examples : CAN, VAN, LIN

Binary Countdown

Bit encoding

Self-clocking schemes are simpler, but require more bandwidth

Bit-stuffed schemes require extra bits for stuffing, result in nondeterministic message lengths

Collision-based protocols

An unbounded number of collisions results in unbounded worst-case latency Idea: use collision to signal start of a reservation CSMA protocol – works well. In

general not constrained by bit speed/network length ratio (but IS constrained by message speed/network length ratio)

Bit dominance/binary countdown protocols

Excellent efficiency – But must have compatible network medium

Constrained by network bit speed/network length ratio

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Conclusion

Protocols are optimized for different operating scenarios

Collision-based

High number of possible transmitters

Low number of active transmitters

Arbitration overhead proportional to activity

Token-based, Time-multiplexed & Polled

Moderate number of total transmitters

Handles worst case activity without a problem

Arbitration overhead relatively constant

Binary countdown

Moderately large number of message types

Arbitration overhead constant

Global prioritization (but no mechanism for fairness)

Conclusion

General embedded network issues

Dynamic tension among efficiency, latency, determinism Classes of protocols

Time-multiplexed (polled/time-triggered)

Token (implicit/explicit)

Binary countdown

General tradeoff overview

Global vs. local priority (and, priority vs. fairness)

Efficiency vs. dynamic flexibility

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References

H. Kopetz,

Real-Time Systems : Design Principles for Distributed Embedded Applications, Kluwer, 1997.

CIA Web site http://www.can-cia.org/can/protocol/

OSEK/VDX operating system specification V2.2.2 , http://www.osekvdx.org/osekvdx_OS.html

OSEK/VDX Communication specification V3.0.3 , http://www.osekvdx.org/mirror/OSEKCOM303.pdf

ITEA2 Timmo projectwww.timmo.org

Systèmes temps réel 1 & 2 Ed N. Navet , Hermès edition

Distributed embedded Systems and Real-time networks

STREAM01 / Mastère SE course E10