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CT420 Realtime Systems
Dr. Hugh Melvin, Discipline of IT, NUI,G 1
Dr. Hugh Melvin/Dr. Michael SchukatRoom 406/402 Ext 3716/2031
[email protected]@nuigalway.ie
See Web resources
Course Structure
• 2 hrs lectures / week for both semesters– Wed 9-11 IT304 Semester 1
• Structured and unstructured labs
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• Assignments– Lab based work
• Technical Reports
– Linux/Windows based
Course Structure• Content updated
– Course development in realtime ! Subject to change
• Lecture notes on web in advance of lectures
A t
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• Assessment– Exam + Assignments
• Typically 70/30 split
• Site Visit ?– Moneypoint (ESB) / Shannon ATC / Aer Lingus
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Main Textbooks
• Real-Time Systems , Krisha & Shin, McGraw Hill
• Real-Time Systems Design & Analysis, Laplante, Wiley
• Programming for the Real World POSIX.4,
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Programming for the Real World POSIX.4, Gallmeister, O Reilly
• Advanced Unix Programming, Rochkind, Addison Wesley
• Real-Time Systems & Programming Languages, Burns & Wellings, Pearson
Course Outline
• Classification of RTS• Time Synchronisation for RTS• Soft RTS
– Multimedia & Networks
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• Scheduling for RTS• Memory Management for RTS• RT Programming Languages• RTOS & POSIX.4 RT • Fault Tolerance for RTS
RTS
• Definition of RTS?– Difficult due to the extent and scope of RTS
– System where a substantial fraction of the design effort goes into making sure that
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design effort goes into making sure that deadlines are met (Krishna/Shin)
• Classification of RTS?– Hard – Firm – Soft
• Differ by the consequences of missed deadlines
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Hard RTS
I/O
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I/O I/O
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I/O
CPU
AS101 AS133AS112 AS131AS017AS013
Boiler BMSTurbine
I/O
CPU
R30
CPU
I/O
MEC-RTData Storage Unit
VDU 1
VDU 2
VDU S\E’sOffice
VDU Prog. Room
Tape DriveStorage Unit
Operators Keyboard 1
Operators Keyboard 2
S\E’s Keyboard 1
Programmers Keyboard
Annunciator
Printer 1
Printer 2
Printer Prog. Room
CS 275 B B 0Bus 1
HW Analog Inputs
HW Binary Inputs
Bus Coupler
Dr. Hugh Melvin, Discipline of IT, NUI,G 8Teleperm ME System Overview
VAXMaintenance Management
System
I/O
CPU
I/O
CPU
AS231Unit Alarms System
PG750Maintenance
Computer
OS254OperationsComputer
Turbine Boiler
VDU 1 VDU 1
VDU 2
VDU 3
Operator’sKeyboard
MEC-RTData Storage Unit
Programmers VDU & Keyboard
VDU 2
VDU 3
Operator’sKeyboard
MEC-RTData Storage Unit
Programming Room Printer
Fault Log Printer
Con
trol
Roo
m
Control R
oom
LPA0
LPA1
Local Printer
VDU 1
VDU 2
Operator’sKeyboard
5 1/4”FDDStorage Unit
Alarms Printer
Annunciator
I/O
CPU
Local T
erminals
CS 275 Bus Bus 0
S\E’s VDU & Keyboard
12:45:01 23:01:97
Master Clock
Remote OperatorStation
OIS Server IBM NetFinity 5510
Bus ABus B
BusCoupler
U/I
CS275Bus 0
Synogate Interfaceto CS275
CS275Bus 1
U/I
Bus ABus B
BusCoupler
U/I
CS275Bus 0
Synogate Interfaceto CS275
CS275Bus 1
U/I
Bus ABus B
BusCoupler
U/I
CS275Bus 0
Synogate Interfaceto CS275
CS275Bus 1
U/I
Unit 1 Unit 2 Unit 3
Dr. Hugh Melvin, Discipline of IT, NUI,G 9Control Room Unit 3
Operator Station
IBM NetFinity 5510
Control Room Unit 1 Operator Station
Control Room Unit 2 Operator Station
Station LAN
MP002
GatewayR30 Room Gateway
Communications Room
MP001
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Burner Management System
CPU 2CPU 1 CPU 3
N8 N8
U/I U/I
Bus ABus B
CS275
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.. .. ........
2 out of 3
Non-Redundant Control Circuits
Double RedundantSafety Circuits Triple Redundant Safety Circuits
Other Hard RTS
• Fly-by-wire control systems
• Robotics
• Medical Devices
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• ABS / Airbag Protection Systems
Boeing 777
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Soft RTS: Multimedia
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Firm RTS
• Somewhere in the middle ?– Eg. Private IP Network governed by SLA
(Service Level Agreement)• SLA specifies jitter/delay/loss/availability
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SLA specifies jitter/delay/loss/availability– 99.999% Availability
• Lack of adherence results in :– Poor customer satisfaction
– Penalties imposed on provider
– Consumer Devices• Mobile phone / Cameras etc…
Time Synchronisation for RTS• Time of Day and Timing
– Two distinct issues
• Both often critical for RTS– Synchronising 2v3 systems (timing)– Precise data logging for distributed system (time)
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gg g y ( )– Skew problems in POTS / VoIP / IPTV
• Clock Terminology– Accuracy , Stability, Offset, Skew, Drift
• Synchronisation Algorithms– Network Time Protocol (NTP)
• Limitations of conventional OS Timing• Timer granularity & Time Services
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Time & Timing
• Time– UTC– Evolved from Greenwich
Mean Time GMT– Time Synchronisation
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Time Synchronisation• How closely 2 clocks
agree on Time of Day
• Timing– Relates to frequency– Timing Synchronisation
• How closely 2 entities operate at same frequency
Greenwich• Home of Time & the
Prime Meridian• Longitude 0 & GMT
agreed in 1884– GMT driven by
expansion of railways
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p yand need for national/international consistency
• “Timeball” rises and falls at 13.00 hrs– Then site of London
docks– In service since 1833
Greenwich
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Soft RTS: MM and Networks
Real Time Communications– Best Effort Internet default service
– Recent QoS Initiatives to differentiate between traffic streams
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between traffic streams• Eg. Voice over IP
– Requires M2E delay typically < 150 msec
– Can cope with small degree of packet loss
» Soft RTS
– How can delay/loss reqds be met?
• See ITU G.1010
Mouth-to-Ear Delays
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Real Time Communications• LAN based developments
– Switched Networks (Embedded RTOS)• Also Fast/Gigabit Ethernet : Overprovisioning?• What Andy giveth, Bill taketh away !
– New difficulties for wireless environment • Access Protocols CSMA
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• WAN based developments– IntServ– DiffServ – MPLS : Layer 2.5
• Also development of RTP/RTCP/RTSP Protocols to support packet based MM traffic
Real Time Communications
• Endpoint Issues– Increased complexity in terminals
– OS / Driver / Application Software / Hardware issues
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issues • Increased delays due to complex and/or poorly
written s/w
• Hardware Issues – eg. skew = lack of synchronisation (timing)
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Clock Skew : Buffer Issues
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Audio-System Clock Skew
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RT Programming Languages– Consider the following Fortran Loop statement
DO 100 I=1,5<block of text>
100 Continue
– but the following is also validDO 100 I=1 5
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DO 100 I=1.5<block of text>
100 Continue
– Fortran interpreted DO 100 I as a previously undeclared float variable assigned value 1.5 NASA mishap
– Ada is most recognised RT Prog. Lang.• Boeing 777 control written in Ada
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RTOS
• Windows / Unix / Linux – Designed for time sharing of resources rather than meeting
hard deadlines– Designed for good average performance– Not suited to Hard RTS typically
• Can be used after careful design considerations
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– Examples• M’pt : User Interface provided by Linux/Win 2000
» BUT not core control system
• Shannon ATC– Unix powers main Thales ATC systems
– More suited to Soft RT functionality – POSIX.4 improves RT performance of some OS
RTOS• Hard RTS
– Good Average performance is of little use– At 35000 ft when cabin pressure is lost– When a Power station at full output is suddenly disconnected from the
National Grid
• RTOS– Can better meet RTS requirements
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q• Different design to conventional OS• Scheduling/Process priorities are key issues
– VxWorks, QNX, Lynx, RTLinux,
– Cyclic Executive approach still widely used • Software runs ‘bare’ on hardware• No OS !
– Used a lot with Soft RTS and some hard RTS• Mobile phone OS: Symbian is market leader
Fault Tolerant TechniquesKey Characteristic of Hard RTS1. Fault-tolerance via Redundancy
• Full/Partial Fault Tolerance
2. Graceful degradation– Eg.1: Citreon CX
• Common hydraulic system for Steering, Brakes and Suspension• Order of failure??
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• Order of failure??– Eg. 2: Power Plant
• Faulty sensor Alarm Overload AS station switch to manualbut holds its state and will still respond to safety events/manual control
3. Failsafe operation• Eg. Power Plant: Steam inlet valves spring loaded to shut on
loss of power
Many Hard RTS combine all 3 techniques
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Citreon CX
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Fault Tolerant Techniques
• Redundancy– Hardware Redundancy
• Voting schemes
– Software Redundancy• Code developed by different teams
Oft bi d ith h d d d
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• Often combined with hardware redundancy
– Time Redundancy• Build in time slack or roll back
– Information Redundancy • Error Detection + Correction • Note: TCP-IP based Error Detection via CRC relies on Time
Redundancy for TCP traffic. Not acceptable for UDP based MM traffic
Fault Tolerant Techniques: Airbus
• 3 Main Flight Controllers
• 2 Backup Flight Controllers
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• Software developed by different teams and on different platforms
• Seamless transfer
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Fault Tolerant Techniques
CPU 1 CPU 2 CPU 3
2-out-of-3 2-out-of-32-out-of-3ZV1 ZV2 ZV3
DB-In DB-In DB-InDB-Out DB-Out DB-Out
Uni
t
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2-out-of-3 2-out-of-3 2-out-of-3
2-ou
t-of
-3
Inputs Outputs
Extension Units
EAVn
ZV1 ZV2 ZV3
Cen
tral
I/
O L
evel
RAMEPROM
1
RAMEPROM
2
RAMEPROM
3
Course Outline
• Case Study Approach
• Assessment– Assignment Possibilities
Ti S h i ti
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• Time Synchronisation
• Timing Services (Windows versus Linux)
• Internet Multimedia
• Scheduling
• Ada tutorial
• POSIX RT Features
Resources
• Various text books
• Research papers
• Websites
• Details will be posted on course website
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Details will be posted on course website
• Possible site visits ?– ESB Moneypoint
– Shannon ATC
– Aer Lingus facility