Virtual Memory

What is virtual memory?

If your computer lacks the random access memory (RAM) needed to run a program or operation, Windows uses virtual memory to compensate.

Virtual memory combines your computer’s RAM with temporary space on your hard disk. When RAM runs low, virtual memory moves data from RAM to a space called a paging file. Moving data to and from the paging file frees up RAM to complete its work.

The more RAM your computer has, the faster your programs will generally run. If a lack of RAM is slowing your computer, you might be tempted to increase virtual memory to compensate. However, your computer can read data from RAM much more quickly than from a hard disk, so adding RAM is a better solution.

Virtual memory and error messages

If you receive error messages that warn of low virtual memory, you need to either add more RAM or increase the size of your paging file so that you can run the programs on your computer. Windows usually manages the size automatically, but you can manually change the size of virtual memory if the default size is not enough for your needs. For more information, seeChange the size of virtual memory.

Change the size of virtual memory

If you receive warnings that your virtual memory is low, you'll need to increase the minimum size of your paging file. Windowssets the initial minimum size of the paging file at the amount of random access memory (RAM) installed on your computer plus 300 megabytes (MB), and the maximum size at 3 times the amount of RAM installed on your computer. If you see warnings at these recommended levels, then increase the minimum and maximum sizes.

1.       Open System by clicking the Start button clicking Control Panel, clicking System and Maintenance, and then clickingSystem.

2.       In the left pane, click Advanced system settings.  If you are prompted for an administrator password or confirmation, type the password or provide confirmation.

3.       On the Advanced tab, under Performance, click Settings.

4.       Click the Advanced tab, and then, under Virtual memory, click Change.

5.       Clear the Automatically manage paging file size for all drives check box.

6.       Under Drive [Volume Label], click the drive that contains the paging file you want to change.

7.       Click Custom size, type a new size in megabytes in the Initial size (MB) or Maximum size (MB) box, click Set, and then click OK.

Increases in size usually don't require a restart, but if you decrease the size, you'll need to restart your computer for the changes to take effect. We recommend that you don't disable or delete the paging file.

Cache Memory

Cache memory is random access memory (RAM) that a computer microprocessor can access more quickly than it can access regular RAM. As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time-consuming reading of data from larger memory.

Cache memory is sometimes described in levels of closeness and accessibility to the microprocessor. An L1 cache is on the same chip as the microprocessor. (For example, the PowerPC 601 processor has a 32 kilobyte level-1 cache built into its chip.) L2 is usually a separate static RAM (SRAM) chip. The main RAM is usually a dynamic RAM (DRAM) chip.

In addition to cache memory, one can think of RAM itself as a cache of memory for hard disk storage since all of RAM's contents come from the hard disk initially when you turn your computer on and load the operating system (you are loading it into RAM) and later as you start new applications and access new data. RAM can also contain a special area called a disk cache that contains the data most recently read in from the hard disk.

                                                                             

                                           

Generation Of Computer

1.1 GENERATION OF COMPUTERS
The first electronic computer was designed and built at the University of Pennsylvania based on vacuum tube technology. Vacuum tubes were used to perform logic operations and to store data. Generations of computers has been divided into five according to the development of technologies used to fabricate the processors, memories and I/O units.
I Generation : 1945 – 55
II Generation : 1955 – 65
III Generation : 1965 – 75
IV Generation : 1975 – 89
V Generation : 1989 to present
First Generation (ENIAC - Electronic Numerical Integrator And Calculator

EDSAC – Electronic Delay Storage Automatic Calculator
EDVAC – Electronic Discrete Variable Automatic Computer
UNIVAC – Universal Automatic Computer
IBM 701)
􀂃 Vacuum tubes were used – basic arithmetic operations took few milliseconds
􀂃 Bulky
􀂃 Consume more power with limited performance
􀂃 High cost
􀂃 Uses assembly language – to prepare programs. These were translated into machine level language for execution.
􀂃 Mercury delay line memories and Electrostatic memories were used
􀂃 Fixed point arithmetic was used
􀂃 100 to 1000 fold increase in speed relative to the earlier mechanical and relay based electromechanical technology
􀂃 Punched cards and paper tape were invented to feed programs and data and to get results.
􀂃 Magnetic tape / magnetic drum were used as secondary memory
􀂃 Mainly used for scientific computations.

Second Generation (Manufacturers – IBM 7030, Digital Data Corporation’s PDP 1/5/8 Honeywell 400)

􀂃 Transistors were used in place of vacuum tubes. (invented at AT&T Bell lab in 1947)
􀂃 Small in size
􀂃 Lesser power consumption and better performance

Computer Organization
1. Introduction
􀂃 Lower cost
􀂃 Magnetic ferrite core memories were used as main memory which is a random-access nonvolatile memory
􀂃 Magnetic tapes and magnetic disks were used as secondary memory
􀂃 Hardware for floating point arithmetic operations was developed.
􀂃 Index registers were introduced which increased flexibility of programming.
􀂃 High level languages such as FORTRAN, COBOL etc were used - Compilers were developed to translate the high-level program into corresponding assembly language program which was then translated into machine language.
􀂃 Separate input-output processors were developed that could operate in parallel with CPU.
􀂃 Punched cards continued during this period also.
􀂃 1000 fold increase in speed.
􀂃 Increasingly used in business, industry and commercial organizations for preparation of payroll, inventory control, marketing, production planning, research, scientific & engineering analysis and design etc.


Third Generation (System 360 Mainframe from IBM, PDP-8 Mini Computer from Digital Equipment Corporation)
􀂃 ICs were used
􀂃 Small Scale Integration and Medium Scale Integration technology were implemented in CPU, I/O processors etc.
􀂃 Smaller & better performance
􀂃 Comparatively lesser cost
􀂃 Faster processors
􀂃 In the beginning magnetic core memories were used. Later they were replaced by semiconductor memories (RAM & ROM)
􀂃 Introduced microprogramming
􀂃 Microprogramming, parallel processing (pipelining, multiprocessor system etc), multiprogramming, multi-user system (time shared system) etc were introduced.
􀂃 Operating system software were introduced (efficient sharing of a computer system by several user programs)
􀂃 Cache and virtual memories were introduced (Cache memory makes the main memory appear faster than it really is. Virtual memory makes it appear larger)
􀂃 High level languages were standardized by ANSI eg. ANSI FORTRAN, ANSI COBOL etc
􀂃 Database management, multi-user application, online systems like closed loop process control, airline reservation, interactive query systems, automatic industrial control etc emerged during this period.

Fourth Generation (Intel’s 8088,80286,80386,80486 .., Motorola’s 68000, 68030, 68040, Apple II, CRAY I/2/X/MP etc)

􀂃 Microprocessors were introduced as CPU– Complete processors and large section of main memory could be implemented in a single chip
􀂃 Tens of thousands of transistors can be placed in a single chip (VLSI design implemented)
􀂃 CRT screen, laser & ink jet printers, scanners etc were developed.
􀂃 Semiconductor memory chips were used as the main memory.
􀂃 Secondary memory was composed of hard disks – Floppy disks & magnetic tapes were used for backup memory
􀂃 Parallelism, pipelining cache memory and virtual memory were applied in a better way
􀂃 LAN and WANS were developed (where desktop work stations interconnected)
􀂃 Introduced C language and Unix OS
􀂃 Introduced Graphical User Interface
Computer Organization
1. Introduction
􀂃 Less power consumption
􀂃 High performance, lower cost and very compact
􀂃 Much increase in the speed of operation

Fifth Generation (IBM notebooks, Pentium PCs-Pentium 1/2/3/4/Dual core/Quad core.. SUN work stations, Origin 2000, PARAM 10000, IBM SP/2)

􀂃 Generation number beyond IV, have been used occasionally to describe some current computer system that have a dominant organizational or application driven feature.
􀂃 Computers based on artificial intelligence are available
􀂃 Computers use extensive parallel processing, multiple pipelines, multiple processors etc
􀂃 Massive parallel machines and extensively distributed system connected by communication networks fall in this category.
􀂃 Introduced ULSI (Ultra Large Scale Integration) technology – Intel’s Pentium 4 microprocessor contains 55 million transistors millions of components on a single IC chip.
􀂃 Superscalar processors, Vector processors, SIMD processors, 32 bit micro controllers and embedded processors, Digital Signal Processors (DSP) etc have been developed.
􀂃 Memory chips up to 1 GB, hard disk drives up to 180 GB and optical disks up to 27 GB are available (still the capacity is increasing)
􀂃 Object oriented language like JAVA suitable for internet programming has been developed.
􀂃 Portable note book computers introduced
􀂃 Storage technology advanced – large main memory and disk storage available
􀂃 Introduced World Wide Web. (and other existing applications like e-mail, e Commerce, Virtual libraries/Classrooms, multimedia applications etc.)
􀂃 New operating systems developed – Windows 95/98/XP/…, LINUX, etc.
􀂃 Got hot pluggable features – which enable a failed component to be replaced with a new one without the need to shutdown the system, allowing the uptime of the system to be very high.
􀂃 The recent development in the application of internet is the Grid technology which is still in its upcoming stage.
􀂃 Quantum mechanism and nanotechnology will radically change the phase of computers.

2 TYPES OF COMPUTERS

1. Super Computers
2. Main Frame Computers
3. Mini Computers
4. Micro Computers

1. Super Computers E.g.:- CRAY Research :- CRAY-1 & CRAY-2, Fujitsu (VP2000), Hitachi (S820), NEC (SX20), PARAM 10000 by C-DAC, Anupam by BARC, PACE Series by DRDO

􀂃 Most powerful Computer system - needs a large room
􀂃 Minimum world length is 64 bits
􀂃 CPU speed: 100 MIPS
􀂃 Equivalent to 4000 computers
􀂃 High cost: 4 – 5 millions
􀂃 Able to handle large amount of data
􀂃 High power consumption
􀂃 High precision
Computer Organization
1. Introduction
􀂃 Large and fast memory (Primary and Secondary)
􀂃 Uses multiprocessing and parallel processing
􀂃 Supports multiprogramming
Applications
􀂃 In petroleum industry - to analyze volumes of seismic data which are gathered during oil seeking explorations to identify areas where there is possibility of getting petroleum products inside the earth
􀂃 In Aerospace industry - to simulate airflow around an aircraft at different speeds and altitude. This helps in producing an effective aerodynamic design for superior performance
􀂃 In Automobile industry – to do crash simulation of the design of an automobile before it is released for manufacturing – for better automobile design
􀂃 In structural mechanics – to solve complex structural engineering problems to ensure safety, reliability and cost effectiveness. Eg. Designer of a large bridge has to ensure that the bridge must be proper in various atmospheric conditions and pressures from wind, velocity etc and under load conditions.
􀂃 Meteorological centers use super computers for weather forecasting
􀂃 In Biomedical research – atomic nuclear and plasma analysis – to study the structure of viruses such as that causing AIDS
􀂃 For weapons research and development, sending rockets to space etc

2. Main Frame Computers E.g.:- IBM 3000 series, Burroughs B7900, Univac 1180, DEC
􀂃 Able to process large amount of data at very high speed
􀂃 Supports multi-user facility
􀂃 Number of processors varies from one to six.
􀂃 Cost: 3500 to many million dollars
􀂃 Kept in air conditioned room to keep them cool
􀂃 Supports many I/O and auxiliary storage devices
􀂃 Supports network of terminals

Computer Organization
1. Introduction
Applications
􀂃 Used to process large amount of data at very high speed such as in the case of Banks/ Insurance Companies/ Hospitals/ Railways…which need online processing of large number of transactions and requires massive data storage and processing capabilities
􀂃 Used as controlling nodes in WANs (Wide Area Networks)
􀂃 Used to mange large centralized databases
3. Mini Computers E.g.:- Digital Equipments PDP 11/45 and VAX 11)
􀂃 Perform better than micros
􀂃 Large in size and costlier than micros
􀂃 Designed to support more than one user at a time
􀂃 Posses large storage capacities and operates at higher speed
􀂃 Support faster peripheral devices like high speed printers
􀂃 Can also communicate with main frames
Applications
􀂃 These computers are used when the volume of processing is large for e.g. Data processing for a medium sized organization
􀂃 Used to control and monitor production processes
􀂃 To analyze results of experiments in laboratories
􀂃 Used as servers in LANs (Local Area Networks)
4. Micro Computers E.g.:- IBM PC, PS/2 and Apple Macintosh

􀂃 A microcomputer uses a microprocessor as its central Processing Unit. Microcomputers are tiny computers that can vary in size from a single chip to the size of a desktop model
􀂃 They are designed to be used by only one person at a time
􀂃 Small to medium data storage capacities 500MB – 2GB
􀂃 The common examples of microcomputers are chips used in washing machines, TVs, Cars and Note book/Personal computers.
Applications
Used in the field of desktop publishing, accounting, statistical analysis, graphic designing, investment analysis, project management, teaching, entertainment etc
􀂃 The different models of microcomputers are given below:-

a) Personal computers:- The name PC was given by the IBM for its microcomputers. PCs are used for word processing, spreadsheet calculations, database management etc.

b) Note book or Lap Top:- Very small in terms of size – can be folded and carried around – Monitor is made up of LCD and the keyboard and system units are contained in a single box. Got all the facilities of a personal computer (HDD, CDD, Sound card, N/W card, Modem etc) and a special connection to connect to the desktop PC which can be used to transfer data.

c) Palm Top:- Smaller model of the microcomputer- size is similar to that of a calculator – pocket size- It has a processor and memory and a special connection to connect to the desktop PC which can be used to transfer data.

d) Wrist PC:- Smallest type of microcomputer – can be worn on our wrist like a watch- It has a processor and memory and a wireless modem
Computer Organization
1. Introduction
Cost Speed Applications
Superfast Weapon design
Weather forecasting Super Computers
Aircraft design
fast Biomedical applications
Scientific calculations Main frame computers
Data Processing for large business
Teaching systems in Universities
medium
Large multi-user systems
Manufacturing processes Mini Computers
Hospital Administration
Teaching systems in Colleges
slow Office automation
Small business systems Micro Computers
Control applications
Teaching systems in schools
FUNCTIONAL UNITS OF A COMPUTER
Computer is a device that operates upon information or data. It is an electronic device which accepts input data, stores the data, does arithmetic and logic operation and outputs the information in desired format.
Even though the size, shape, performance, reliability and cost of computers have been changing over the years, the basic logical structure proposed by Von Neumann has not change. The internal architecture of computers differs from one system model to another. A block diagram of the basic computer organization specifying different functional units is shown below. Here the solid lines indicate the flow of instruction and data and the dotted lines represent the control exercised by the control unit.

1. Introduction

INPUT UNIT
Input unit accepts coded information from human operators through electromechanical devices such as the keyboard or from other computers over digital communication lines. The information received is either stored in the memory for later reference or immediately used by the Arithmetic and Logic circuitry to perform the desired operation. Finally the result is sent back to the outside through the output unit.
The keyboard is wired so that whenever a key is pressed, the corresponding letter or digit is automatically translated into its corresponding code and sent directly to either the memory or the processor.
Other kinds of input devices: Joy stick, track ball, mouse (pointing devices), scanner etc.
MEMORY UNIT
The memory unit stores program and data. There are two classes of memory devices :- Primary memory and Secondary memory.
Primary memory (Main memory)
􀂃 Contains a large number of semiconductor cells each capable of storing one bit of information
􀂃 These cells are processed in group of fixed size called words containing ‘n’ bits. The main memory is organized such that the contents of one word can be stored or retrieved in one basic operation.
􀂃 For accessing data, a distinct address is associated with each word location.
􀂃 Data and programs must be in the primary memory for execution.
􀂃 Number of bits in each word is called the word length and it may vary from 16 to 64 bits.
􀂃 Fast memory
􀂃 Expensive
􀂃 Time required to access one word is called Memory Access Time - 10nS to 100nS. This time is fixed and independent of the location.
E g. Random Access Memory (RAM)
Secondary storage
􀂃 They are used when large amount of data have to be stored (also when frequent access is not necessary)
E.g. Hard Disk, Compact Disk, Floppy Disk, Magnetic Tapes etc.
PROCESSOR UNIT
􀂃 The heart of the computer system is the Processor unit.
􀂃 It consists of Arithmetic and Logic Unit and Control Unit.
Arithmetic and Logic Unit (ALU)
􀂃 Most computer operations (Arithmetical and logical) are executed in ALU of the processor.
􀂃 For example: Suppose two numbers (operands) located in the main memory are to be added. These operands are brought into arithmetic unit – actual addition is carried. The result is then stored in the memory or retained in the processor itself for immediate use.
􀂃 Note that all operands may not reside in the main memory. Processor contains a number of high speed storage elements called Registers, which may be used for temporary storage of frequently used operands. Each register can store one word of data.
􀂃 Access times to registers are 5 to 10 times faster than access time to memory.
Control Unit
Computer Organization
1. Introduction
􀂃 The operations of all the units are coordinated by the control unity (act as the nerve centre that sends control signal to other units)
􀂃 Timing signal that governs the I/O transfers are generated by the Control Unit.
􀂃 Synchronization signals are also generated by the Control Unit
􀂃 By selecting, interpreting and executing the program instructions the program instructions the control unit is able to maintain order and direct the operation of the entire system.
The control unit and ALU’s are usually many times faster than other devices connected to a computer system. This enabled a single processor to control a number of external devices such as video terminals, magnetic taped, disk memories, sensors, displays and mechanical controllers which are much slower than the processor.
OUTPUT UNIT
􀂃 Counter part of input unit
􀂃 Output devices accept binary data from the computer - decodes it into original form and supplies this result to the outside world.
E.g. Printer, Video terminals (provides both input & output functions), graphic displays etc
Basic Operational Concepts:-
􀂃 Activity in a computer is governed by instructions
􀂃 To perform a given task, a set of instructions called program must be there in the main memory
􀂃 Individual instructions are brought from the memory into the processor which executes the specific operation.
􀂃 Data to be used as operands are also stored in the memory.
E.g. Add LOCA, R0
This instruction adds the operand at the memory location LOCA to the operand in the Processor R0 and places the sum into the register R0. Here the original contents of LOCA are preserved whereas those of R0 are overwritten.
Steps:-
1. Instruction is fetched from the main memory into the processor Memory access
2. Operand at LOCA is fetched operation
3. Add the contents to the contents of R0 ALU operation
4. Finally store the result in R0
Note: Data transfer between the main memory and the processor are started by sending the address of the memory location to be accessed to the memory unit and issuing the appropriate control signal by the control unit.
INTERNAL ORGANIZATION OF PROCESSOR
Processor contains a number of registers used for temporary storage of data other than ALU and Control circuitry
Instruction Register (IR) – holds the instruction that is currently being executed – its output is available to the control circuits which generate the timing signals that control the various processing elements involved in executing the instruction.

Computer Organization
1. Introduction
Program Counter (PC) – It contains the address of the instruction currently being executed. During the execution of an instruction, the contents of the program counter are updated to hold the address of the next instruction to be executed. i.e. PC points to the next instruction that is to be fetched from the memory.
n General Purpose Registers (R0 to Rn-1) – Facilitates communication with the main memory. Access to data in these registers is much faster than to data stored in memory locations because the registers are inside the processor. Most modern computers have 8 to 32 general purpose registers.
Memory Address Register (MAR) – holds the address of the location to or from which data are to be transferred
Memory Data Register (MDR) – contains the data to be written into or read out of the address location.

Steps involved during operation:-
1. Program is stored in the main memory
2. PC is set to point to the first instruction of the program
3. Contents of the PC are transferred to the MAR and a Read Control signal sent to the memory
4. After the access time, the addressed word (in this case the first instruction) is read out of the memory and is loaded into the MDR
5. Contents of the MDR are transferred to the IR. Now the instruction is ready to be decoded and executed.
6. If the instruction involves an operation to be performed by the ALU, the required operands are to be fetched from the memory (or CPU registers). This is done by sending its address to the MAR and initiating a Read cycle.
Computer Organization
1. Introduction
7. Operands are read from the memory into the MDR and are transferred from MDR to the ALU.
8. ALU will perform the desired operation.
9. If the result is to be stored in the memory, then it is sent to the MDR.
10. The address of the location where the result is to be stored is sent to the MAR and a Write cycle is initiated.
11. At some point during the execution of the current instruction, the contents of the PC are incremented so that the PC now points to the next instruction to be executed.
12. As soon as the execution of the current instruction is completed, a new instruction fetch may be started.

Protocol Stacks

What is Protocol Stacks?

A set of network protocol layers that work together. The OSI Reference Modelthat defines seven protocol layers is often called a stack, as is the set ofTCP/IP protocols that define communication over the internet.

The term stack also refers to the actual software that processes the protocols. So, for example, programmers sometimes talk about loading a stack, which means to load the software required to use a specific set of protocols. Another common phrase is binding a stack, which refers to linking a set of network protocols to a network interface card (NIC). Every NIC must have at least one stack bound to it.

                                       

List of network protocol stacks

Computer networks may be implemented using a variety of protocol stack architectures, computer buses or combinations of media and protocol layers, incorporating one or more of:

·                    ARCNET

·                    AppleTalk

·                    ATM

·                    Bluetooth

·                    DECnet

·                    Ethernet

·                    FDDI

·                    Frame relay

·                    HIPPI

·                    IEEE 1394 aka FireWire, iLink

·                    IEEE 802.11 aka Wireless LAN (Wi-Fi certification)

·                    IEEE-488

·                    Internet protocol suite

·                    IPX

·                    Myrinet

·                    OSI protocol suite

·                    QsNet

·                    RS-232

·                    SPX

·                    System Network Architecture

·                    Token ring

·                    USB

·                    X.25 protocol suite

OSI (OPEN SYSTEM INTERFACE/INTERCONNETION)

What is the OSI model ?

·                       Open Systems Interconnection model is fundamental to all communications between network devices. 

·                       Developed in 1974 by ISO after the American Department of Defence began using the TCP/IP suite of protocols. 

·                       Finally adopted in 1977. It is now the theoretical model for how communication takes place between network devices.

What are the seven layers ?

·                       Application layer

·                       Presentation layer

·                       Session layer

·                       Transport layer

·                       Network layer

·                       Data link layer

·                       Physical layer

Layers

·                       In the sense of purpose and responsibility, each layer is separate and independent 

·                       Each has its own function, but also provides a service to those layers above and below itself 

·                       The model should be considered an aid to understanding the nature of communication on the network and useful in sorting out troubles that might occur on a network 

·                       By providing, it allows both software engineers and hardware manufacturers ensure their products work together.

The Layers At Work

·                       When communicating, each OSI layer talks with the same layer in the other device 

·                       E.g. the Application Layer of Device A communicates with the Application Layer of Device B, by passing the data through the other layers 

·                       The Application Layer of each device is not concerned with how the other layers are functioning, but it does rely on them to do their job

How To Remembering The Layers?

Application                 All 

Presentation               People 

Session                      Seem 

Transport                   To 
Network                    Need 

Data Link                   Data 

Physical                      Processing

How Does Data Flow?

·                       When data is sent from the application on the source computer the following happens 

·                       Data in the form of a packet moves down through the layers 

·                       When it reaches the Physical Layer it is ready to be sent along the cable 

·                       At the Physical Layer the bits may be analogue or digital, in the form of electrical, light or radio waves

·                       The data is transmitted to the destination device 

·                       It travels up through the layers of the OSI model, reaching the user. 

·                       As data moves down through the layers it is encapsulated – ie additional information is added as headers or trailers 

·                       The data in the packet does not change 

·                       See the following diagram

Encapsulation

Application Layer

·                       ‘Closest’ layer to the user 

·                       Works with the applications you use to communicate over the network 

·                       E.g.. Services include SMTP, HTTP and FTP 

·                       Clicking on a link on a web page issues a command for the browser to retrieve the relevant information from the Internet 

·                       In this example your computer is the source, and the host of the web site information is the destination 

·                       The application completes your request and delivers the information to your computer

Application Services

·                       File Services 

·                       Electronic-mail Services 

·                       Network-printing Services 

·                       Application Services 

·                       Database Services

Presentation Layer

This layer has three fundamental functions 

Data Presentation 

·                       Enables receiving device to understand the information sent from the source 

·                       Converts data from native format (abstract syntax) to a common format (transfer syntax), e.g. ASCII

Data Compression

·                       By reducing the volume of data, transfers can take place in less time 

·                       Packets are examined and such things as spaces in text removed 

·                       The destination device returns the data to its original format before passing to the Application Layer 

Data Encryption

·                       Allows data to be converted to a form which hides its meaning, apart from those you wish to see it 

·                       Not all data is encrypted on its journey across the network 

·                       In order for decryption to occur at the destination device a ‘key’ is required

Session Layer

·                       Primarily responsible for handling the session between devices (beginning, maintaining and finishing) 

·                       Enforces order in the communication between devices 

·                       Regulates the flow of data 

·                       It takes responsibility for the following ….

Session Services

·                       Establishing a Connection 

·                       Ending the Connection 

·                       Handshaking – SYN and ACK packets 

·                       ‘Keep alive messages’ 

·                       Session must be terminated (otherwise one device will be still transmitting without any device actually listening) 

·                       Dialogue Control (simplex, half-duplex, full-duplex) 

·                       Dialogue Separation – checkpoints within the transmission which allow the detection of lost packets, and subsequent retransmission

Transport Layer

·                       Ensures reliable transport of packets from source to destination 

·                       Also manages the speed of transmission – flow control 

·                       There are two types of transmission (Connection-Oriented Transmissions and Connectionless Transmissions)

Connection-Oriented Transmissions

·                       Also known as ‘ Reliable Transport Method’ – uses acknowledgement (ack) packets on successful receipt of data. 

·                       Extra packets slows down communication 

Features are 

·                       Reliability 

·                       Slower Communication 

·                       Packets are re-transmitted if unrecognisable or not received 

·                       Once all the data is received successfully , the packet is re-assembled and the Transport Layer passes it to the Session Layer

Connectionless Transmissions

·                       In this mode the transmitting device does not require acknowledgements from the receiver, and continues to transmit on the assumption that the data was received 

Features are: 

·                       Little or No Reliability 

·                       Faster Transmission 

·                       Packets are not Re-transmitted

Flow Control

·                       Establishes the maximum speed at which both sender and receiver can communicate at 

·                       Transport Layer determines largest packet size which can be sent 

·                       Packets are numbered – to allow re-assembly in the correct order

Network Layer

·                       Responsible for the correct addressing and delivery of packets of data 

·                       These are known as datagrams 

·                       Uses the network address ( this is a logical address – and does not depend upon any hardware in the device, or the device’s physical location) 

The Network Layer does the following:

·                       Adds the address to the packet (encapsulation) 

·                       Maps the network address to the devices physical address 

·                       Determines the best path for the packet (routing) 

·                       Ensures that the packet is in the correct format for the destination

How does it work ?

·                       Encapsulation at the Transport Layer involves adding the address of the sender to the datagram 

·                       The destination address is now added. Both addresses are logical. 

·                       Both addresses are necessary for packets to move between end systems. 

·                       If a packet must move to another network, a routing protocol is required 

·                       If different packet lengths are used on the different networks, the Network Layer formats the data accordingly 

·                       The primary piece of hardware which works on this layer is the router.

Data Link Layer

Has two sub layers of its own: 

·                       Logical Link Control (LLC) 

·                       Media Access Control (MAC) 

·                       LLC acts between protocols such as Internet Protocol (IP) and the MAC method. 

·                       MAC is responsible for the connection to the physical media (eg cable)

MAC

·                       Each NIC has a unique number hard coded in to the card – its physical address 

·                       The first 6 digits denote the manufacturer, the next six are unique) – type “winipcfg” on your PC 

·                       When the MAC address is added to the packet it is now known as a frame 

·                       It now has all the information required to travel from the source to the destination

Physical Layer

·                       The lowest, bottom, layer – responsible for the physical connection between devices 

·                       The NIC converts the data (bits) in to transmission signals. 

·                       Transmissions may be analogue or digital 

·                       Responsible for the rate of transmission 

·                       Includes all components such as the type of connector (RJ-45, Token Ring, BNC, SC connector) 

·                       Devices at this level include NICs, repeaters, hubs and concentrators.

OSI Versus TCP/IP Model

·                       OSI model is an important concept 

·                       Protocol most in use on modern networks is TCP/IP 

·                       TCP/IP does not map its layers precisely to OSI model 

·                       OSI = 7 layers, TCP/IP = 4 layers (sometimes a 5th physical layer is referred to)

TCP/IP Model

·                       Application or Process Layer – concerned with how data at both ends is handled. 

·                       Transport Layer – manages flow of data 

·                       Internet Layer – consists of several protocols, primary protocol is IP (providing hierarchical addressing scheme 

·                       Data Link (or Network Interface) Layer – manages transmission of data within the network 

·                       Physical Layer – not really defined, TCP/IP leaves the physical connection to manage itself

Conclusion

·                       OSI open system interconnect model gives a layered approach to the network model and describes how the thing goes one by one in a particular order . 

·                       Each layer a perform a unit of work on each packet called packet data unit 

·                       Data Encapsulation Technique of wrapping the information on data unit at each layer . 

·                       TCP/IP Model contains four layers in comparison to OSI model it is also called DoD model .