The storage capacity of a computer is measured in terms of Bytes
One byte includes a total of 8 individual units called as bits.
One bit can store either a 0 or a 1 in it.
Two kinds of memory
Semiconductor memory-Primary memory- RAM, ROM
Magnetic memory-Secondary memory-FLOPPY, HARD DISK, CD/DVD, PENDRIVE
Types of Computer Memory
Primary memories are two types
Random Access Memory (RAM), Read Only Memory ROM
Random Access Memory (RAM)
Ram is a Volatile memory
Information written into it is retained in it as long as the power supply is on.
Two types of RAM
Static RAM- retains in it as long as the power supply is on.
Dynamic RAM- loses its stored information in a very short time (few milliseconds)
Generally every 2 milliseconds dynamic Rams’ have to be refreshed.
Types of Computer Memory
It is non volatile. Used for permanent storage of information.
PROM: Programmable ROM. User can store permanent programs, data or any other kind of information in a PROM. User can write information in a PROM only once.
EPROM: Erasable PROM. Exposing it to high intensity ultraviolet light for about 20 minutes can erase the stored data in EPROM.
EEPROM: An Electronically Erasable Programmable Read Only Memory can be programmed through the use of special electrical pulses.
Secondary Storage Devices
Storage devices hold data, even when the computer is turned off.
The physical material that actually holds data is called a storage medium. The surface of a floppy disk is a Storage medium.
The hardware that writes data to or reads data from a storage medium is called a storage device. A floppy disk drive is a Storage device.
The two primary storage technologies are magnetic and optical.
Magnetic Storage Devices
Diskettes (floppy disks) (FDD)
Hard disks (HD)
High-capacity floppy disks (ZIP Disk)
USB flash drive
Hard Disk Drive
Long-term, nonvolatile storage
Large, inexpensive, slow level in the storage hierarchy
A magnetic disk consist of a collection of platters (1to 20 per disk) that rotate on a spindle.
Disk surface divided into tracks (1000 to 4800 per platter).
Tracks are divided into sectors (64 per track), which are the smallest unit that can be read or written.
Up to 10,000 rpm
Magnetic Storage Devices - Diskettes
Diskette drives, also known as floppy disk drives, read and write to diskettes (called floppy disks or floppies).
In disks the areas to save data are organized as a set of concentric circles called Tracks. Floppy disks have 80 tracks. The disks are further divided into pie-slice Sectors.
Floppy disk: coated with metal oxide and enclosed in vinyl or plastic covers
Three common sizes are 3 ½ inches 5 ¼ inches and 8 inches with storage capacity of 1.44 MB
Optical Storage Devices
Compact Disk Read-Only Memory (CD-ROM) - 0.7-0.9GB
Digital Video Disk Read-Only Memory (DVD-ROM)-4.7-17GB
Blueray Disc (BD) -25-128GB
Optical Storage Devices – How Optical Storage Works
An optical disk is a high-capacity storage medium.
An optical drive uses reflected light to read data.
To store data, the disk's metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently.
When an optical drive shines light into a pit, the light cannot be reflected back. This represents a bit value of 0 (off). A land reflects light back to its source, representing a bit value of 1 (on).
Optical storage devices
CD-ROM: compact disc ROM (Read Only Memory)
Capacity: 700 MB
CD-R: compact disc recordable
CD-RW: compact disc rewritable
CD-RW-Hewlett Packard has introduced CD-RW.
DVD: digital versatile disc
Capacity: 4.7GB – 17GB
DVD-RW (2 versions: + (plus) and
It is placed between the CPU and main memory
It is much faster than main memory
It increases the operating speed of the system
An operating system is an integrated set of specialized programs that is used to manage the overall operations of a computer.
It acts like an interface between the user, computer hardware and software.
Every computer must have an operating system to run other programs.
DOS (Disk Operating System), Unix, Linux and Windows are some of the common operating systems.
The compiler software translates the source program (user written program) into an object program (binary form).
Specific compilers are available for computer programming languages like FORTRAN, COBOL, C, C++ etc.
The utility programs support the computer for specific tasks like file copying, sorting, linking a object program, etc.
An Application Software consists of programs designed to solve a user problem.
It is used to accomplish specific tasks rather than just managing a computer system.
Application software are in turn, controlled by system software which manages hardware devices.
Some typical examples are : railway reservation system, game programs, word processing software, weather forecasting programs.
A machine language consists of the numeric codes for the operations that a particular computer can execute directly.
The codes are strings of 0s and 1s, or binary digits (“bits”), which are frequently converted both from and to hexadecimal (base 16) for human viewing and modification.
Assembly language is one level above machine language.
It uses short mnemonic codes for instructions and allows the programmer to introduce names for blocks of memory that hold data.
One might thus write “add pay, total” instead of “0110101100101000” for an instruction that adds two numbers.
High Level Language
Like machine language, assembly language requires detailed knowledge of internal computer architecture.
The first important algorithmic language was FORTRAN (formula translation), designed in 1957 by an IBM team led by John Backus.
Important points-Computer Languages
ALGOL (algorithmic language) -- 1958–60
LISP (list processing) was developed about 1960 by John McCarthy
The C programaming language was developed in 1972 by Dennis Ritchie and Brian Kernighan at the AT&T Corporation
COBOL (common business oriented language) has been heavily used by businesses since its inception in 1959.
BASIC (beginner’s all-purpose symbolic instruction code) was designed at Dartmouth College in the mid-1960s by John Kemeny and Thomas Kurtz
About 1970 Niklaus Wirth of Switzerland designed Pascal
In the early 1990s, Java was designed by Sun Microsystems, Inc., as a programming language for the World Wide Web (WWW).
A computer network is a group of interconnected computers.
It allows computers to communicate with each other and to share resources and information.
First Network : The Advanced Research Projects Agency (ARPA) funded the design of the "Advanced Research Projects Agency Network" (ARPANET) for the United States Department of Defense
Type of Networks
By Structure / Functional Relationship
Client / Server
Peer to Peer (P2PN)
Nodes and servers share data roles
Nodes are called clients
Servers are used to control access
Access to data controlled by server
Server is the most important computer
Peer to peer networks (P2PN)
All nodes are equal
Nodes access resources on other nodes
Each node controls its own resources
Most modern OS allow P2PN
Distributed computing is a form
No need for a dedicated server
Does not provide the security available on a client
generates data to be transmitted
Converts data into transmittable signals
Converts received signal into data
Takes incoming data
Data transmission involves data transfer from one computer to another through a communication medium, such as telephone, microwave relay, satellite link or physical cable
Network Cables-Twisted-pair cabling
Most common LAN cable
Called Cat5 or 100BaseT
Four pairs of copper cable twisted
May be shielded from interference
Speeds range from
1 Mbps to 1,000 Mbps
1 Mbps to 1,000 Mbps
Similar to cable TV wire
Has a single copper conducter at his center
One wire runs through cable
Shielded from interference
Speeds up to 10 Mbps
Data is transmitted with light pulses
Glass strand instead of cable
Immune to interference
Hard to work with Speeds up to100 Gbps
Data transmitted through the air
LANs use radio waves
WANs use microwave signals
Easy to setup
Difficult to secure
¡ One direction
÷ e.g. Television
÷ Half duplex
¡ Either direction, but only one way at a time
÷ e.g. police radio
÷ Full duplex
¡ Both directions at the same time
÷ e.g. telephone
- Circuit Switching
A switching technique in which a dedicated path is established between end points for the entire duration of information transfer.
- Packet Switching
A switching technique in which large size data is broken as packets and transferred across networks.
- Message/Cell Switching
A switching technique in which information is cast in the form of small cells and transported over broadband ISDN networks
Also called linear bus
One wire connects all nodes
Failure of one or more nodes does not affect the bus
Terminator ends the wires
Easy to setup
Small amount of wire
Easy to crash
All nodes connect to a hub
Packets sent to hub
Hub sends packet to destination
Easy to setup
One cable can not crash network
One hub crashing downs entire network
Uses lots of cable
Most common topology
Nodes connected in a circle
Tokens used to transmit data
¡ Nodes must wait for token to send
¡ Time to send data is known
¡ No data collisions
¡ Lots of cable
Combines characteristics of linear bus and star topologies
By Size or Scale
Local Area Network (LAN)
Contains printers, servers and computers
Systems are close to each other
Contained in one office or building
Organizations often have several LANS
Bus and Ring topologies are used
Wide Area Networks (WAN)
Two or more LANs connected
Over a large geographic area
Typically use public or leased lines
The Internet is a WAN
Metropolitan Area Network (MAN)
Large network that connects different organizations
Shares regional resources
Star and Ring topologies are generally used.
v a global computer network providing a variety of information and communication facilities, consisting of interconnected networks using standardized communication protocols
v The Internet consists of a worldwide interconnection of governmental, academic, public, and private networks based upon the networking technologies of the Internet Protocol Suite.
v It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the U.S. Department of Defense in 1969
v In 1989, Tim Berners-Lee invented the World Wide Web, an Internet-based hypermedia
E-mail - Electronic mail is usually used to exchange messages and data files. Each user is assigned an electronic mail box. Using mail services, one can scan a list of messages that can be sent to anyone who has the proper email identification. The message sent to any one resides in the mailbox till it is opened. Many other features of standard mail delivery are implemented in email.
Usenet News Groups: Electronic discussion groups. User network abbreviated as Usenet is essentially a giant disbursed bulletin board. Electronic discussion groups that focus on specific topic forms, computer forums.
Mailing list: Email based discussion groups combining E-mail, news groups and mailing lists send messages on a particular subject. Automatically messages reach the mailbox of that group.
FTP: File Transfer Protocol, abbreviated as FTP is used for the net user for transferring files around the world. The transfer includes software, games, photos, maps, music and such other relevant materials.
Telnet: Telnet is a protocol that allows the user to connect to a remote computer. This feature is used to communicate a microcomputer with mainframe.
History of the OSI Model
Open Systems Interconnected Model
was created in the 70’s by the ISO (although the CCITT came up with their own model)
10 different people got together and considered all functions of communications
was created because people realized that our computers needed to talk to each other(and there was no one dominant computer system)
ISDN was a big factor
7 layers = 4 upper and 3 lower
OSI Reference Model in Practice
The OSI reference model is used in many ways:
To provide assistance when troubleshooting network problems
To provide a common terminology and framework for networking technology developers
To facilitate the development of connectivity standards needed for flexible open architectures
To enable the development of protocol stacks that allow network nodes to communicate with one another
The OSI Model
LAYER 7– APPLICATION
The visual interface level between the user and the network, or computer. (Ex Word, Excel, Access, Email)
LAYER 6-- PRESENTATION
This layer is responsible for converting the visual interface into a code that is then sent through the computer or network. For example, this layer may convert ASCII code (what many applications like Word use) to an 8-bit code.
LAYER 5 – SESSION
This layer keeps track of whose turn it is to receive traffic, basically it is a dialog control. This is the level that acknowledges receipt of a transmission as well as sends the message to the network.
LAYER 4 – TRANSPORT
All streams of data are received and combined into one single stream so that data may be sent through the network. Multiplexing and demultiplexing occurs on this layer.
The OSI Model
Bottom / Lower Layers
Layer 3 - Network
On this level the router exists to determine if the message is meant for the system or if it needs to be redirected to its final destination. This is done by a header system, which is programmed to accept or reject depending if the header is the one used by the network.
Layer 2 – Data Link
There is where a check of the message occurs. That is the message is checked for the proper frame, formation, synchronization, power level, voltage, and wavelength. If the test is a successful, the message is sent to the network layer to determine if the message is truly meant for the system.
Layer 1 - Physical
Where the actual physical makeup of the message is identified. That is the duration of bits, the right number of bits, and the right wavelength is checked. As with the other two layers, if this test is successful, the message is sent up the chain.
TCP/IP Protocol Suite
The TCP/IP suite provides insights into the inner workings of the Internet
Like the OSI model, the TCP/IP suite is layered
Because the protocols found at each layer are independent of those at the other layers, a given protocol can be modified without affecting those found at other layers
Integrated Services Digital Network
Public networks are used for a variety of services
Public Switched Telephone Network
Private Lines (leased)
Packet Switched Data Networks
Circuit Switched Data networks
The telephone network has evolved into a digital one with digital exchanges and links
The signalling system has become a digital message-oriented common channel signalling system (SS#7)
The term ‘Integrated Digital Network’ is used to describe these developments
An ISDN is a network, in general evolving from telephony IDN, that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multipurpose user-network interfaces.
Benefits to Subscribers
Single access line for all services
Ability to tailor service purchased to suit needs
Competition among equipment vendors due to standards
Availability of competitive service providers
First, ISDN is an infrastructure to support a wide variety of services and is not network designed for any specific service.
Benefits to Subscribers
Second, the end-to-end digital connectivity implies that the digitisation process begins right at the user premises.
Third, it should be possible to support every conceivable service on ISDN, for any such service is either a voice or non-voice service.
Finally, a small set of carefully chosen interfaces should enable the support of all possible services. The users of ISDN should not be burdened-with too many specialised interfaces, but at the same time, an expensive universal interface should be avoided