Principles of Engineering System Design Dr T Asokan

INTRODUCTION TO SYSTEMS DESIGN Dr T Asokan Interface Architecture Development Contd…..

Standards Standards ensure that an interface will enable the connection of two components. Each component is required to meet a given standard, and the interface is designed to meet the same standard. There are many benefits in having standards for interfacing.

Interchangeability Ability to interchange components with performance/cost characteristics Interoperability Ability to operate with a wider variety of external systems Portability Ability to run on various systems Reduces cost and risk Increased life cycle

Formal standards: Negotiated and promulgated by accredited standard bodies. ISO, ANSI, IEEE, EIA De-jure: (mandated by legal authorities) IDEF0 (by FIPS) De facto: (through popular usage) X windows, OS Standards have different levels of formality: formal, de jure, and de facto

Formal Standards: OSI (Open System Interconnection ) Architecture Introduced for network based communication between end-user nodes in a telecommunications network. In 1983, ISO and CCITT ( International Telephone and Telegraph Consultative Committee) approved a reference model for OSI Reference model contains seven layers- physical, data link, network, transport, session, presentation, and application

Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Physical Media Network Data Link Physical Host node Intermediate node Communication in the OSI reference model

LayerDescriptionFunctions ApplicationProvides communication between the end user’s application processes and the application entity Establish connection, transfer data, release connection PresentationDefines data syntax for communication Establish connection, transfer data, release connection SessionProvides connection controls for the host Establish connection, transfer data, establish synchronization points, manage activity, release connection, report exceptional conditions TransportEstablishes transparent and reliable end-to-end transmission of data between host nodes Establish connection, transfer data, provide error detection and recovery, release connection

LayerDescriptionFunctions NetworkDetermines establishment of connection and handles routing Establish connection, transfer data, perform multiplexing, provide error control, release connection Data linkEstablishes reliable transmission on the physical layer Establish connection, negotiate quality of service, transfer data, provide flow control, reset connection, release connection PhysicalDefines how the physical network is accessed in order to provide bit transparent transmission Determine presence of signaling pulses, Determine timing of signaling pulses

CORBA - Common Request Broker Architecture A standard that would permit programmers to integrate software modules resident on the same network by treating each application as an object. Developed in association with industries like HP, HYPERDESK, SUN etc.. Part of CORBA, interface definition language is a formal standard adopted by ISO. CORBA is a de jure standard in US Is a de facto standard in other parts of the world.

IDL is a universal notation for software interfaces defining a boundary between the client code (request for services) and the software objects that implement those services. Apart from IDL, the four additional categories of objects that comprise the CORBA are ORB (object request broker), CORBA facilities, CORBA services and CORBA domains ORB is the interface between client and server

Application Presentation Session Transport Network Data Link Physical ClientServer Object Request Broker (ORB) CORBA overlaid on OSI seven layer model

Interface Design Process I. Define interface requirements: Identify the items to be transported Emergency communications from the elevator to the building/emergency response team (ERT) Define the operational concept Bound the problem with an external diagram Define objectives hierarchy availability of interface, fidelity of communicated message, operational cost, deployment cost. Write the requirements

Interface Design Process Select a high level interface architecture of interface Identify several candidate architectures Telephone connection, dedicated communication system network to ERT, etc… Evaluate alternatives against requirements Dedicated network is too expensive Choose high level interface architecture: Telephone connection is chosen

Develop functional architecture of the interface Functional decomposition Fault detection and recovery Develop Physical architecture of interface Identify candidate architectures Eliminate infeasible candidates Develop operational architecture of interface Analyse behavior and performance alternatives

Summary Interfaces are the primary responsibility of System Engineers Interfaces are the most common failure point on systems. Standards play a major role in the design or selection of interface

CASE STUDY: PATH FINDER Path finder system that was deployed to the surface of Mars for a landing on 4 th July 1997 was a great success in many ways. Few days into the mission, operators on the ground noticed that total system resets were occurring that were causing the loss of data. A shared memory interface was used as the system interface between various sub systems. A priority system had been established for giving various activities access to this interface. Bus management, lengthy communications by spacecraft, meteorological data etc.. Mutual exclusion (mutex) locks were employed to give an activity access to the interface.

CASE STUDY: PATH FINDER Meteorological data was so voluminous that the activity had to obtain and release mutexes several times before it was finished. The long running, medium priority, communication activity would infrequently interrupt the meteorological activity during its pause and gain control of the interface.

Duration of these two tasks were sufficiently long to invoke a watchdog timer that was employed to ensure that the high priority bus management task was executing appropriately. In such rare cases, watch dog timer initiated a total system reset to prevent any further damage to the system. Jet Propulsion Lab. Engineers ran a pathfinder replica on earth till they reached the reset situation. It was found that the interface software (VXworks) had been programmed without a feature called ‘priority inheritance’. JPL engineers uploaded a short C programme and Pathfinder did not experience anymore system resets.

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