1 Microprocessor-based Systems Course 12 Distributed Systems

2 Distributed Systems v.s. Centralized Systems  Advantages of distribution: the possibility to adapt the structure of the Information system to the distributed nature of an application (e.g. banking system, monitoring and supervision systems, etc.) increase of reliability and fault tolerance increase of the performance/price ration (the use of cheep computers instead of an expensive central one) improve the interactivity through multiple intelligent terminals (including mobile devices) new facilities: electronic information transfer/exchange, remote control of industrial processes, , e-publications, teleconferencing, etc. facilities for working in groups use of common resources (databases, expensive peripheral equipments, etc.)  Problems: the human thinking is sequential; it is difficult to write applications as a parallel set of threads/tasks there are not enough tools for expressing concurrency, synchronization, parallel execution, etc. the network generate delays and transmission errors the global state of the system is not exactly known because of the relativistic effect caused by the limited communication speed

3 Distribution forms  Multiprocessor systems: Parallel systems  nodes are strongly correlated at physical and logical level  the communication time can be neglected compared with the execution time Distributed systems  nodes are weakly connected at physical level but strongly connected at logical level (remote cooperation)  the transmission time is comparable with the execution time of tasks  nodes perform together a given functionality (e.g. banking service) Computers on a network  nodes are weakly connected at physical and logical level  small cooperation between nodes  What should be distributed? Distribution of equipment (hardware) Distribution of data Distribution of control (program)

4 Distribution forms  Classification of systems based on the distribution of physical resources: - uni-processor systems - multiprocessor systems with a common/shared buss - parallel systems - computers connected through a network  Classification of systems based on the distribution of data: - systems with a single database - system with distributed files but a single directory - system with replicated/duplicated database - system with partitioned database and decentralized directory  Classification of systems based on distribution of control: - single control point system - hierarchical system (with master/slave relations) - multiple control points that execute parallel threads of a single task - multiple autonomous points that execute tasks of a common service

5 Criteria to establish if a system is distributed:  must have multiple physical and logical resources  resources must be physically distributed  there should be a global vision of the whole system  cooperation and independence between the system’s components  the distribution should be transparent for the users or the system interface

6 Services required in a distributed system  Name service Goal: to offer a unique addressing mechanism for all the resources in the distributed system find resources (e.g. data, files, applications or equipments) based on symbolic names. Example: DNS (Domain Name Server) service in the Internet  Time service synchronization of local clocks Example: NTP (Network Time Protocol)  Replication service Replication is necessary for:  efficient/fast access to shared resources  fault tolerance Problem: preserving the consistency of data  Transaction service Goal: concurrent access to shared resources  Transparent access to distributed resources  Group communication

7 Computer-based Control systems  The use of computers in control applications: Advantages:  precizie mai mare în măsurarea şi procesarea informaţiilor culese  o mai mare imunitate la perturbaţii de diferite tipuri (ex: zgomote, variaţii ale condiţiilor de mediu sau ale surselor de alimentare, modificarea în timp a valorii componentelor, etc.); pot fi implementate scheme de autocalibrare şi autotestare, prin care se măreşte încrederea în sistemul de control  posibilitatea implementării prin program a unor scheme/metode complexe de reglaj, ca de exemplu: algoritmi neliniari de control, algoritmi adaptivi, scheme de reglaj cu mai mulţi poli  posibilitatea stocării datelor culese  posibilitatea vizualizării procesului pe mai multe nivele de detaliere  componentele unui sistem complex pot fi corelate prin comunicare digitală  se pot construi scheme ierarhizate de control Drawbacks:  semnalele de proces sunt discretizate în timp  Valorile semnalelolsunt cuantizate;  pentru schemele simple de reglaj, utilizarea unui calculator nu se justifică din punct de vedere economic;  schemele de reglaj implementate prin program sunt mai puţin vizibile pentru utilizatorul banal,  depanarea sau ajustarea dispozitivului de control necesită un personal de specialitate;

8 The schema of a computer-based control system

9 Probleme specifice privind controlul prin calculator al proceselor  Achiziţia semnalelor de proces Semnale digitale:  inchis/deschis, pornit/oprit,  de frecventa controlota,  Tren de impulsuri  impulsuri de latime controlata Semnale analogice:  De tensiune, de curent,  Semnale complexe – suma de semnale sinusoidale Se stabileste:  Frecventa de esantionare (citire)  Numarul de biti alocati pentru un semnal – plaja de variatie a semnalului  Regimul de achizitie: continu, sau la cerere

10 Canale analogice de intrare si de iesire  Canal analogic de intrare:  Canal analogic de iesire: CNAEE

11 Comenzi “analogice” prin semnale digitale – semnale PWM

12 Procesarea datelor  Off-line Procesarea nu se face in timpul achizitiei de semnale Nu sunt conditii de timp; procesarea poate fi oricat de complexa  On-line Procesarea are loc in timpul achizitiei semnalelor Restrictii de timp (perioada de esantionare=perioada de procesare a unui set de date)  Moduri de implementare a aplicatiei de control: Bucla de control Time driven- activarea unor taskuri de achizitie si procesare la anumite intervale de timp Event-driven – activarea unor taskuri ca reactie la aparitia unor evenimente Activarea taskurilor cu ajutorul unui planificator de timp-real  Planificare statica – inainte de lansarea aplicatiei  Planificare dinamica – in timpul executiei aplicatiei

13 Cerinte impuse sistemelor de control in medii industriale  Fiabilitate si toleranta la defecte Prin replicare spatiala si temporala  Imunitate mai mare la zgomote electro- magnetice  Lucru in conditii de mediu severe Variatii mari de temperatura Umiditate ridicata  Personal de deservire mai putin calificat  Restrictii: Dimensionale si de forma De cost De consum

14 Solutii  Sisteme dedicate si incapsulate  Retele industriale de comunicatii  Procesoare specializate pentru aplicatii de control (microcontroloare) si de procesare a semnalelor (procesoare de semnal)  Memorii nevolatile semiconductoare (ex: flash in locul discurilor)  Carcase robuste, protejate la praf si la umezeala

15 Nivele de control