Principles of

Engineering System Design

Dr T Asokan

asok@iitm.ac.in

INTRODUCTION TO SYSTEMS DESIGN

Dr T Asokanasok@iitm.ac.in

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

ApplicationPresentationSessionTransportNetworkData LinkPhysical

ApplicationPresentationSessionTransportNetworkData LinkPhysical

Physical Media

Network

Data Link

Physical

Host node Host node

Intermediate node

Communication in the OSI reference model

Layer Description Functions

Application Provides communication between the end user’s application processes and the application entity

Establish connection, transfer data, release connection

Presentation Defines data syntax for communication

Establish connection, transfer data, release connection

Session Provides connection controls for the host

Establish connection, transfer data, establish synchronization points, manage activity, release connection, report exceptional conditions

Transport Establishes transparent and reliable end-to-end transmission of data between host nodes

Establish connection, transfer data, provide error detection and recovery, release connection

Layer Description Functions

Network Determines establishment of connection and handles routing

Establish connection, transfer data, perform multiplexing, provide error control, release connection

Data link Establishes reliable transmission on the physical layer

Establish connection, negotiate quality of service, transfer data, provide flow control, reset connection, release connection

Physical Defines 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

Client Server

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 4th 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.

THANK YOU