System Analysis and Design Chapter 1

                 System Analysis and Design                 Chapter 1

1.1 What is a System ?

“A collection of interrelated components that work together to achieve some objectives”

Real world system

 •There are millions of systems in operation in the world     around us at every moment in time.
•Note : Not all systems are dependant on the involvement of computers
•Examples
•Family unit, school unit
•Digestive system
•Information systems•Radar systems

•A system has three stages...1.Something feeds into the system (the input)2.The system does something with the input (the process)3.The process gives a result (the output) 

 

1.1.2 Factors to be considered 

Factors to be considered when developing a system•Objective setting : What the system supposed to do.•System boundary : this defines the components that make up the system•System environment : anything outside the system boundary

What is subsystem? 

•“A system that is part of some larger system”•Each subsystem carries out part of the system function•Subsystems communicates by passing  messages between themselves•A good system will made up of highly independent subsystems with minimal flow between them•Advantages
•Help to handle system complexity
•Improve the understanding of the system

What is an Information System ? 

“Information system is an arrangement of people, data, processes, communications, and information technology that interact to support and improve day-to-day operations in a business, as well as support the problem-solving and decision-making needs of management and users ”

•Information systems are crucial to the success of modern business organizations

•New systems are constantly being developed to achieve

•Operational Excellence : efficiency, productivity
•New product services and business models : create opportunities to for products, services, and new ways to engage in business

•Customer and supplier intimacy : improved communication with customers and suppliers •Improved decision making : accurate and timely 
•Competitive advantage : effective and efficient information system improve profit than competitors.
•Survival : necessity of doing business 

 •The key to successful system development is through systems analysis and design••System analysis and design is used to analyze, design and implement improvements in the functioning of businesses that can be accomplished through the use of computerized information systems.

Software Development Process/ Systems Development Life Cycle (SDLC)

•Process used in the system development
•The process (as a whole) of developing system or software to meet certain requirements.
•SDLC is a process of gradual refinement/modification, meaning that it is done through several development phases.

4 fundamental phases 

 •4 fundamental phases which are common to all software processes

1.Software specification2.Software development3.Software validation
4.Software evolution
•Different software processes decomposes these activities in different ways

Waterfall Model 

 •The waterfall Model illustrates the software development process in a linear sequential flow.
•Any phase in the development process begins only if the previous phase is completed
•Does not define the process to go back to the previous phase to handle changes
•So suited for projects where the requirements can be clearly defined 

•The linear cycle is usually with structured teams and documentation systems••Documents flows also highly structured
••Documents produced at the end of the phase must be available for the input of next stage.

Waterfall Model 

Principal stages of the model map onto the fundamental development activities

•Requirement analysis and definition

•System and software design :

•Implementation and unit testing

•Integration and system testing

•Operations and maintenance 

•Advantages and disadvantages of waterfall model 

ISO-OSI Reference Model and IEEE Standards

OSI Overview

1. OSI Layered framework for the design of network systems that allows communication across all types of            

    computers system.

2. The OSI 7 layers.(Brief functional overview.)

3. Vertical and horizontal communication between the layers using interface.(defines what information and                                         service should the layer provide to the layer above it.)

4. Data Encapsulation
         a) PDU conception-each protocol on the diff. layer has it's own format.
         b) Headers are added while a packet is going down the stack at each layer.
         c) Trailers are usually added on the second layer.

Comparison with TCP/IP

• Pretty similar to OSI.
• TCP/IP has less layers(four)
• Main difference in layers is after layer 4.

Layers

                   

• Application layer(Layer 7)

Application layer interact with application program and is the highest level of OSI model. application layer of OSI model marks the spot where users actually communicate to the computer.

The application layer contains variety of protocols that are commonly needed by user.

One widely used application protocols is HTTP which is basis for www. when browser wants a web page.

Other application protocols are used for file transfer (FTP), E mail ( SMTP).                                                              

           ◦Provides user interfaces and support for services

◦Resource sharing and device redirection

◦Remote file access

◦Remote printer access

◦Inter-process communication

◦Network management

◦Directory services

◦Electronic messaging (such as mail)

◦Network virtual terminals

•  Presentation layer(Layer 6)                     

A presentation layer present data to the application layer and is responsible for data translation and cord formatting.

A successful data transfer technique is to adapt the data into a standard format before transmission. by providing translation service the presentation layer ensure that data transfer ed format. The application layer of one system can be read by the application layer of another one.

The OSI has protocol standards that define how standard data should be formatted. Task like data compression, decompression, encryption and decryption are associated with this layer.

This is the layer at which application  programmers consider data structure and presentation.     Example: GIF, JPEG, TIFF, etc.

       ◦Translation (connects different computer systems)

                      ◦Compression (transmission efficiency)

      ◦Encryption (SSL security)  

• Session layer (Layer 5)

 Session layer provides mechanism for controlling the dialogue between the tow end system. It define how to start, control and end conversation (call session) between applications.

It coordinate communication between systems, and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex.

An other service that is offered as a part of the session layer might include data synchronization.

 ◦Session establishment, maintenance and termination (Deciding who sends, and when.)

   ◦Session support  (security, name recognition, logging ) 

• Transport layer (Layer 4) 

Transport layer segments and reassemble data into a data stream.Service located in the transport layer both segment and reassemble data from upper-layer applications and unite it onto the some data stream.

They provide end-to-end data transport service and can establish a logical connection between the sending host and destination host on an network.

◦Connectionless and connection-oriented services

◦Process-Level Addressing

◦Multiplexing and Demultiplexing

◦Segmentation, Packaging and Reassembly

◦Connection Establishment, Management and Termination

◦Acknowledgments and Retransmissions

◦Flow Control

• Network layer(Layer 3) 

The network layer manage device addressing, tracks the location of device on the net work, and determines the best way to move data, which means that the network layer must transport traffic between devices that aren't locally attached.

Routers are specified at the network layer and provide the routing service within an inter network. 

  ◦Logical Addressing

◦Routing (where the packet is destinated to)

◦Datagram Encapsulation  

◦Fragmentation and Reassembly (handling too big packets )

◦Error Handling and Diagnostics ( using status messages for example )

• Data link layer (Layer 2)

Data link layer provides the physical transmission of the data and handles errors notification, network topology, and flow control. This means the data link layer will ensure that massages are delivered to the proper device on a LAN using hardware addresses, and translate massages from network layer into bits for the physical layer to transmit.

The data link layer formats the massage into pieces, each called a data frame, and adds a customized header containing the hardware destination and source address. 

  2.1. Logical Link Control (LLC )

   ◦Establishment and control of logical links between local devices on a network.

      2.2. Media Access Control (MAC)

   ◦The procedures used by devices to control access to the network medium.

   •Frame sequencing

   •Frame acknowledgment

   •Addressing

   •Frame delimiting

   •Frame error checking

   •PDU: frame  

• Physical layer (Layer 1)

Physical layer specifies the electrical, mechanical, procedural and functional requirement for activating a physical link between end system.

Physical layer does tow thin-

•  It sends bits and receives bits. Bits come only in value of 1 and 0. The physical layer communicate with various type of actual communicate media. Different kind of media represent these bit values in different way.
• Specific protocols are needed for each type of media that describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media's attachment interface.

The role of the physical layer is to encode the binary digits that represent data link layer frames into signals and to transmit and receives these signals across the physical media that connect network devices.

Copper cable

Fiber

Wireless 

     •Definition of Hardware Specifications                                                                      

   •Encoding and Signaling (bit representation)

   •Data Transmission and Reception (half duplex, full duplex )   

   •Topology and Physical Network Design (mesh, ring, bus)

   •PDU: bit