Data Communication

In twenty-first century, the key technology has been the information gathering, processing and distribution.
We are living in a century where this world has become a virtual family. We can talk to any person in any corner of the world from anywhere in the world. And the credit for all those goes to the communication revolution that has happened in this century especially in the last thirty years. Advances in the communication technology, coupled with rapidly evolving computer technology, have made possible nearly everything we dreamt of.

• It is only data communication that makes you take advantage of massive mainframe processing power if you turn your PC into a terminal of a mainframe.
• It is again communication that lets you chat with different people across the globe through an online service.
• It is again communication that allows you to access huge servers of information on virtually any subject via networks of computers.

Need of Data Transmission

The need for data transmission lies mainly to break the barriers of

• Distance
• Time
• Cost

This is because the communications via computers can be done to any distance in very short amount of time and in very cost-effective manner.
In fact, communicating via computer proves much cheaper and economic as compared to communicating via telephone lines. A normal telephone call ties up an expensive, dedicated circuit for the duration of the call, whereas access via a network ties up long distance lines only while data are actually being transmitted.
Data in any form be it text, pictures, sound, video, graphics etc. can be transmitted via computer networks. On the other hand, the telephone lines are mainly used for sound and some time text transmission.
Thus, we can say that the data transmission via computers is needed due to the following advantages:

• It breaks the barriers of distance, cost and time.
• Very cost effective as compared to telephone networks.
• All types of data viz. Text, Audio, Video, Pictures, and Graphics etc. can be transmitted through it.

Data Transmission Techniques

There are many ways of transmitting digital information through a medium. Making the choice between one technique and another is normally dependent upon performance of the techniques, in terms of the speed and accuracy of transmission and cost.
There are two major obstacles to successful transmission: Attenuation and Noise.
Noise can rise from a variety of sources in the environment and serves to distort the signal.
Attenuation is a measure of how much the strength of the signal is reduced in passing through the medium. It is proportional to the distance traveled.
For a particular medium, there will be a range of frequencies that can be transmitted through it.
In determining how much information can be sent through the cable, the most important aspect to consider is the width of this frequency range. This is known as the bandwidth of the medium.
Thus bandwidth is the range of frequency that can be transmitted through a particular medium of transmission.
Here there is a choice of baseband that carries a radio frequency signal on the cable.
In baseband modulation, interfaces are relatively inexpensive, as they require no special devices for generating the digital. Only one channel is available over the cable for communications and hence the signal can be transmitted at a single frequency at a time.
On the other hand, in the broadband modulation, interfaces generate radio frequency signals at different frequencies, making it more expensive.

Modes of Data Transmission

Communication signal can take any of the two forms: digital and analog. And they may be transmitted in parallel or serial mode and synchronous or asynchronous manner.

Digital and Analog Communication

A digital signal is a group of discrete electronic units i.e. sequences of 0s and 1s, transmitted in extremely rapid succession.
1. Digital transmission uses special equipment for transmission that is capable of transmitting data directly in binary form.
2. When digital signal is submitted, first it is translated into electrical signal compatible with the communication channel. This is usually done using a device called modem that is capable of converting a digital signal to analog and vice versa.
On the other hand, an analog signal consists of continuous electrical waves that are variable. Analog Transmission uses general-purpose communication channels, such as telephone lines.

Even if an analog signal is submitted, it assumes digital data. At propagation points data in signal are recovered and then transmitted. Repeaters are used to retransmit the new signal generated.

Parallel and Serial Communication

As you know that the unit of data in computers is byte (a group of 8 bits). In some computers, data are further organized into multiple byte words.

Parallel Communication

Sometimes data are transmitted a byte or a word at a time. This is done using a multiple wires, with one wire carrying each bit. This is called parallel communication.
Parallel communication is used primarily for transferring data between devices at the same time. For example, communication between a computer and a printer is most often parallel, so that an entire byte can be transferred in one operation.

Thus we can conclude, when data is transmitted using multiple wires with each wire carrying a bit, this is called parallel communication. Or we can say in parallel communication, data are transmitted byte by byte i.e. multiple bits combination together using multiple wires.

Serial Communication

In serial communication, the data is transmitted bit by bit using a single wire. The individual bits are sent one after another in a series along the same wire. Communication across computers is serial in nature.
Thus when the data is transmitted bit by bit in a series along the same wire, it is called as serial communication.

Synchronous and Asynchronous Transmission

In synchronous transmission, characters are transmitted as group of bits, preceded and followed by control characters. The transmission and receiving intervals between each bit are precisely timed that permits grouping of bits into identifiable characters.
Definition: When characters are transmitted as group of bits preceded and followed by control characters where transmission and receiving interval between each bit are precisely timed permitting grouping of bits into identifiable characters. Such a transmission is called Synchronous Transmission.

Features of Synchronous Transmission

• Thus in synchronous communication, groups of bits are sent one after the other al regular interval.
• The data form a continuous stream of bits spaced al equal intervals, with no space between consecutive bytes.
• A timing mechanism is involved which causes the receiving modem to read the stream at precisely the correct frequency. When the receiving modem has read the required number of bits to make up a character, it sends the character to the receiving computer.
• The major advantage of synchronous transmission is its speed, since fewer bits are needed to identify the beginning and end of the character coding.
• Its chief drawback is inaccuracy: when a receiver goes out of synchronization, losing track of where individual characters begin and end, correction of errors takes additional time.
• Synchronous communication requires high-quality communication channels.

Asynchronous Transmission

In asynchronous transmission each character is transmitted separately, that is, one character at a time. The character is preceded by a start bit, which tells the receiving device where the character coding begins, and is followed by a stop bit, which tells the receiving device where the character ends, after which there is interval of idle time on the channel. Then the next character is sent, start bit first, character's bit next, and stop bits last.

Features of Asynchronous Communication

• The start bits and stop bits allow the receiving and sending computers to synchronize the transmission. This is the most common mode of worldwide, especially for operation of interactive computer terminals.
• Its principal advantage is accuracy.
• Its main drawback is slow transmission time, caused by the great number of start and stop bits.
• It is slower than synchronous communication; it is typically used at communication rates lower than 2400 baud.
• It does not require the complex and costly hardware required for synchronous communication and is, therefore, the method used almost exclusively with microcomputers.

Communication Channels

The cables that connect two or more workstations are called communication channels.

Twisted Pair Cable

The most common form of wiring in data communication application is the twisted pair cable consisting of two identical wires Wrapped together in a double helix. It is the basis for most internal office telephone wiring.
Features of Twisted Pair Cable

• The main advantages of twisted pair cable are its simplicity and ease of installation.
• It is physically flexible, has a low weight and can be easily connected.
• The data transmission characteristics are not so good. Because of high attenuation, it is incapable of carrying a signal over long distance without the use of repeaters.
• Its low bandwidth capabilities make it unsuitable for broadband applications.

Coaxial cable

Coaxial cable consists of a solid wire core surrounded by one or more foil or wire shields, each separated by some kind of plastic insulator.

The inner core carries the signal, and the shield provides the ground.

While it is less popular than twisted pair, it is widely used for television signals.
In the form of cable, it provides a cheap means of transporting multi-channel television signals around metropolitan areas.
Large corporations in building security systems also use it.

The data transmission characteristics of coaxial cables are considerably better than those of twisted pair. It is used in a shared cable network, with part of the band width being used for data traffic.

Optical Fibers

Optical fibers consist of thin strands of glass or glass like material, which are so constructed that they carry light from a source at one end of the fiber to a detector at the other end.
The light sources used are either light emitting diodes or laser diodes. The data to be transmitted is modulated onto the light beam using frequency modulation techniques. The signals can then be picked up at the receiving end and demodulated.

 

Features of Optical Fibers

• The bandwidth of optical fibers is potentially very high.
• The major problems with optical fibers are associated with installation. They are quite fragile and may need special care to make them sufficiently robust for an office environment.
• Connecting either two fibers together or a light source to a fiber is a difficult process.
• The major advantages of optical fibers over other media are their complete immunity to noise, because the information is travelling on a modulated light beam.
• A side effect of this noise immunity is that optical fibers are virtually impossible to tap. In order to incept the signal, the fiber must be cut and a detector inserted.

Micro wave

The microwave transmission is line of sight transmission where signal passes through the atmosphere, originates from a dish antenna, travels in a straight line free of material obstacles and received by another dish antenna.
In Microwave communication, parabolic antennas are mounted on towers to send a beam to other antennas tens of Km away.

Features of Microwave Transmission

• Microwave signals are used to transmit data without the use of cables.
• It proves cheaper than digging trenches for laying cables, and maintaining repeaters and cables if a variety of causes break cables.
• The microwave signals are similar to radio and television signals and are used for long distance communication.
• The higher the tower, the greater the range.
• The disadvantage is that the signal from a single antenna may split up and propagate by slightly different paths to the receiving antenna.
• Microwave propagation is also affected by thunder storms and other atmospheric phenomenon.

Satellite

In satellite communication the earth station consists of a satellite dish that functions as an antenna and communications equipment to transmit and receive data from satellites passing overhead.
Most communication satellites have multiple, independent reception and transmission devices known as transponders. A single transponder is usually capable of handling a full color, commercial television transmission, complete with audio.

Features of Satellite Transmission

The high frequency transmission through satellites is weather sensitive and prone to electronic interference.
The area coverage through satellite transmission is quite large.
The coding and decoding equipment usually provides the security in satellite transmission.

Points to Remember

• Noise can rise from a variety of sources in the environment and serves to distort the signal.
• Attenuation is a measure of how much the strength of the signal is reduced in passing through the medium. It is proportional to the distance traveled.
• A digital signal is a group of discrete electronic units i.e. sequences of 0s and 1s, transmitted in extremely rapid succession.
• On the other hand, an analog signal consists of continues electrical waves that are variable.
• When data is transmitted using multiple wires with each wire carrying a bit, this is called parallel communication. Or we can say in parallel communication, data are transmitted byte by byte i.e. multiple bits combination together using multiple wires.
• Thus when the data is transmitted bit by bit in a series along the same wire, it is called as serial communication.
• When characters are transmitted as group of bits preceded and followed by control characters where transmission and receiving interval between each bit are precisely timed permitting grouping of bits into identifiable characters. Such a transmission is called Synchronous Transmission ·
• In asynchronous transmission each character is transmitted separately, that is, one character at a time. The character is preceded by a start bit, which tells the receiving device where the character coding begins, and is followed by a stop bit, which tells the receiving device where the character ends, after which there is interval of idle time on the channel.
• The most common form of wiring in data communication application is the twisted pair cable consisting of two identical wires wrapped together in a double helix.
• Coaxial cable consists of a solid wire core surrounded by one or more foil or wire shields, each separated by some kind of plastic insulator.
• Optical fibers consist of thin strands of glass or glass like material, which are so constructed that they carry light from a source at one end of the fiber to a detector at the other end.
• The microwave transmission is line of sight transmission where signal passes through the atmosphere, originates from a dish antenna, travels in a straight line free of material obstacles and received by another dish antenna.
• In satellite communication the earth station consists of a satellite dish that functions as an antenna and communications equipment to transmit and receive data from satellites passing overhead.

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Computer Codes and Computer Arithmetic

 The previous chapter deals with pure binary numbers. The goal of this chapter is to present the formats used in computer memory to record data. We will see how these binary numbers are coded to represent characters in the computer memory.

Numeric data is not the only form of data that is to be handled. We often require to process alphanumeric data. An alphanumeric data is a string of symbols where a symbol may be one of the letters from A...Z, 0.9 or a special character (*,&,^,%,$,+,-) etc.

  Byte

In binary coding, every symbol that appears in the data is represented by a group of bits. The group of bits use to represent a symbol is called a byte. To indicate the number of bits in a group, sometimes a byte is referred as an n-bit byte where group contains n bits. However, the term byte is commonly used to mean an 8-bit byte because most of the modern computers use 8 bits to represent a symbol. Thus a byte is comprised of 8 bits.

Thus

1 byte = 8 bits

1 kilobyte = 1024 bytes

1 megabyte =1024 kilobyte (KB)

1 Gigabyte = 1024 megabyte (MB)

A floppy has a capacity of 1.44 MB

A hard disk varies from 10 GB to 40 GB nowadays. 

BCD code

The binary coded decimal (BCD) is one of the early memory codes. It is based on the idea of converting each digit of a decimal number into its idea of converting each digit of a decimal number into its binary equivalent rather than converting the entire decimal value into a pure binary. The BCD equivalent of each decimal digit is shown in table below. Since 8 and 9 require 4 bits, all decimal digits are represented in BCD by 4 bits. The pure binary equal of 42₁₀ = 101010₂. Converting 42₁₀ into BCD, however, produces the following result:

42₁₀= 0100 (binary equal for 4) 0010 (binary equal for 2)

or 01000010 in BCD.

Note :

1. Each decimal digit is independently converted to a 4 bit binary number and hence the conversion process is very easy.
2. When 4 bits are used, altogether 16 configurations are possible 2⁴ = 16.

BCD equivalent of decimal digits

Decimal digit	BCD equivalent
0	0000
1	0001
2	0010
3	0011
4	0100
5	0101
6	0110
7	0111
8	1000
9	1001

 

You can see that only the first 10 of these combinations are used to represent decimal digits. The remaining 6 arrangements as given in table below are not used in BCD coding.

Decimal	Binary equal
10	1010
11	1011
12	1100
13	1101
14	1110
15	1111

 

That is, 1010 does not represent 1010 in BCD.

Instead

10₁₀ = 0001          0000
1                              0

OR 00010000 in BCD

In the above discussion, we have used a group of 4 bits to represent a digit (character) in BCD. 4-bit BCD coding system can be used to represent only decimal numbers because 4 bits are insufficient to represent the various characters used by a computer. Instead of using 4 bits with only 16 possible characters, computer designers commonly use 6 bits to represent characters in BCD code.

In the 6-bit BCD code, the four BCD numeric place positions are retained but the two additional zone positions are added. With 6 bits, it is possible to represent 64 (2⁶) different characters. This is a sufficient number to code the decimal digits (0-9, 10), letters (A-Z, 26) and other special characters (28).

Example:

Show the binary digits used to record the word BASE

B =110010
A=110001
S=010010
E=110101

So the binary digit

110010                  110001                  010010                  110101
     B                             A                            S                              E

Will record the word BASE in BCD.

EBCDIC Extended Binary Coded Decimal Interchange Code

The major problem with BCD code is that only 64 (2⁶) different characters can be represented in it. This is not sufficient for providing decimal number (10), lowercase letters (26), uppercase letters (26) and a fairly large number of special characters (28+).

Hence the BCD code was extended from a 6-bit code to an 8-bit code. The added 2 bits are used as additional zone bits, expanding the zone bits to 4 bits. The resulting code is called the extended binary coded decimal interchange code (EBCDIC). In this code it is possible to represent 256 (2⁸) different characters instead of 64 (2⁶). In addition to the various character requirements mentioned above, this also allows a large variety of printable characters and several non-printable control characters. The control characters are used to control such activities as printer vertical spacing, movement of cursor on the terminal screen etc. All the 256 bit combinations have not yet been assigned characters, so the code can still grow as new requirements develop.

 Because EBCDIC is an 8-bit code, it can be easily divided into two 4-bit groups .Each of these 4-bit groups can be represented by 1 hexadecimal digit. Thus the hexadecimal number system is used a short cut notation for memory dump by computers that use EBCDIC for internal representation of characters. Developed by IBM, EBCDIC code is used in most IBM models and in many other computers.

ASCII (American standard code for information interchange)

Another computer code that is very widely used is the ASCII (American Standard Code for Information Interchange). Several American computer manufacturers have adopted ASCII as their computer's internal code. This code is popular in data communication, is used almost exclusively to represent data internally in microcomputers, and is frequently found in the larger computers produced by some vendor.

ASCII is of two types: ASCII-7 and ASCII-8.

ASCII -7 is a 7 bit code that allows 128 (27) different characters. The first 3 bits are used as zone bits and the last four bits indicate the digit. ASCII-8 is an extended version of ASCII-7. It is an 8-bit code that allows 256 (2⁸) different characters rather than 128.

Computer arithmetic

Why a computer can understand only binary language?

Information is handled in the computer by electronic /electrical components such as transistors, semiconductors, wires etc. All these components can only indicate two states or conditions on (1) off (0).

1. Transistors are either conducting (1) or non-conducting (0)
2.  Magnetic materials are either magnetized (1) or non-magnetized (0)

A pulse or voltage is either present (1) or absent (0).

All information is represented within the computer by the presence or absence system, which has only two digits (0 and 1), is most suitable and is conveniently used to express the two possible states.

Besides, the computer circuits only have to handle two binary digits rather than ten decimal digits. The result is that the internal circuit design of computers is simplified to a great extent. This ultimately results in less expensive and more reliable circuits for computers.

Binary arithmetic

Addition

Binary addition is performed in the same manner as decimal addition. The complete table for binary addition is as follows:

0 + 0 = 0

0 + 1 = 1

1 + 0 = 1

1 + 1 = 0 plus a carry of 1 to next higher column.

Carry-overs are performed in the same manner as in decimal arithmetic. Since 1 is the largest digit in the binary system, any sum greater than 1 requires that a digit be carried over. For example 10 plus 10 binary requires the addition of two l's in the second position. Since 1 + 1 = 0 plus a carry-over of 1, the sum of 10 + 10 is 100 in binary.

Example 1: Add 101 and 10

101
+10
111

 Example 2: Add 101 and 11

Carry     111
101
+11
1000

 Example 3: Add binary number 100111 and 11011.

 Carry    11111

100111

+11011

1000010

In this we face a new situation (1+1+1) brought about by the carry-over of 1 in the second column. This can also be handled using the same four rules for binary addition. The addition of three 1's can be broken up into two steps.

1. First we add only two 1's giving 10.
2. In Second step we add 1 to 10 getting 11 as the result.

Subtraction

The principles of decimal subtraction can as well be applied to subtraction of numbers in other baese. It consists of four rules

0-0 = 0

 1-0 = 1

1-1 = 0

0-1 = 1 with a borrow from the next column.

Thus as in decimal 10 is borrowed, in binary 2 is borrowed, in octal 8 is borrowed and in hexadecimal 16 is borrowed.

Example 4: subtract 01110₂ from 10101₂

Solution:

12

Borrow 0202

10101 (21₁₀ in decimal)

- 01110 (14₁₀ in decimal)

   00111 (7₁₀ in decimal)

The final result of subtraction is 00111₂

Additive method of subtraction

The direct method of subtraction using borrowing is easy on paper and pencil. But when subtraction is performed by digital components, this method becomes less efficient and we use another methods known as complementary subtraction method.

What is a complement?

Complement of a number is defined as the difference between the number and the base raised to the n^th power minus one, where n is the number of digits in a number.

Complement of x = [(base)" - 1] - X

Example 5: Find the complement of 57.

Solution

Complement =[(10)2 - 1] - 57

=[100 – 1] -57

=99 – 57

 =42

Thus, the complement of 57₁₀ = 42₁₀.

But in binary numbers, it is not necessary to go through the usual process of obtaining complement. Instead, a quick way is used where a complement of a binary number is obtained by transforming all its "0" to "1" and "1" to "0". For example the complement of 1011010 is 0100101. This circuit for obtaining complement of a number in binary system can be easily designed at very less expense.

Subtraction by complementary method

Subtraction by complementary method involves the following three steps.

1. Find the complement of the number you are subtracting (subtrahend)
2. Add this to the number from which you are taking away.
3. If there is a carry of 1, add it to obtain the result; if there is no carry, recomplement the sum and attach a negative sign to obtain the result.

Example 6: subtract 0111000 (56₁₀) from 1011100 (92₁₀) using complementary method.

Solution:

Step 1: complement of 0111000 is 1000111

Step 2: add the complement to the minuend (1011100)

1011100

+1000111 (complement of 0111000)

10100011 (result of sum)

Step 3: add the carry to the result of sum.

0100011 (result of sum)

____→_1 (add the carry of 1)

0100100

Result is 0100100₂= 36₁₀

Multiplication

Multiplication in binary arithmetic follows four rules.

1 x 0=0

0 x 1=0

1 x 0=0

1 x 1=1

The method is illustrated by an example.

Example 8: multiply 1010 by 1001.

      1010
   x 1001
      1010
    0000x
  0000xx
1010xxx
1011010 final product

Division

Binary division is again very simple. As in the decimal, division by zero is meaning less. Hence the complete table for binary division is as follows:

0 / 1=0

1/ 1 =1

The division process is performed in a manner similar to decimal division. The rules for binary division are as follows.

1.  Start from the left of the dividend.
2. Perform a series of subtractions in which the divisor is subtracted from the dividend.
3.  If subtraction is possible put a 1 in the quotient and subtract from the corresponding digits of dividend. 
4. If subtraction is not possible (divisor greater than remainder), record a 0 in the quotient.
5. Bring down the next digit to add to the remainder digits.
6. Proceed as before in a manner similar to long division.

 Example 9:

110/100001\ 0101 ( quotient)
        110 (step 1)
        1000 (step 2)
          110 (step 3)
            100 (step 4)
            110 (step 5)
            1001 (step 6)
              110 (step 7)
                11 (remainder)

 Description of steps:

1. Divisor greater than 100, so put a 0 in quotient.
2.  Add digit from dividend to group used above.
3. Subtraction possible so put 1 in quotient.
4. Remainder from subtraction plus digit from dividend.
5. Divisor greater so put a 0 in quotient. 6. Add digit from dividend to group.
6.  Subtraction possible so put 1 in quotient.

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Number System

We have seen that inside a computer, data is stored in a format that cannot be easily read by human beings. This is why I/O interfaces are required. Every computer stored numbers, letters and other special characters in a coded form. Before going into the details of the codes we must understand the number system.
Number system is of two types

1. Positional
2. non-positional

Non positional number system

In early days, human beings counted on fingers were not adequate, stone s, pebbles or sticks were used to indicate values. This method used an additive approach or the non-positional number system. In this approach we have symbols such as I for 1, II for 2, IIIII for 5 etc. Each symbol represents the same value regardless of its position in the number and the symbols are simply added to find out the value of a particular number. Since it is very difficult to perform arithmetic with such a number system, positional number systems were developed.

Positional number systems

In positional number system, there are only a few symbols called digits, and these symbols represent different values depending on the position they occupy in the number. The value of each digit in such a number is determined by three considerations

1. the digit itself
2. the position of the digit in the number
3. The base of the number system (where base is defined as the total number of digits available in the number system).

The number system that we use in our daily life is called decimal number system. In this system, the base is equal to 10 because there are altogether 10 symbols 0,1,2,3,4,5,6,7,8,9 used in the system.

Binary number system

 

The binary number system is exactly like the decimal system except that the base is 2 instead of 10. We have only two symbols or digits 0 and 1 that can be used in this number system. Note that the largest single digit is 1 (one less than the base). Again, each position in a binary number represents a power of the base (2). Ass system, the rightmost position is the units (2⁰) position, the second position from the right is the 2's (2¹) position and proceeding in this way we have 4's (2²), 8's (2³) position, 16's (2⁴) position, and so on. Thus the decimal equivalent of binary number 10101 (written as 10101₂) is

(1 x 2⁴) + (0 x 2³) +(1 x 2²) +(0 x 2¹) +(1 x 2⁰) = 16 + 0 + 4 + 0 +1 =21

In order to be specific about which system we are referring to, it is common practice to indicate the base as a subscript. Thus we write:

10101₂= 21₁₀

Binary digit or bit

"Binary digit" is often referred to by the common abbreviation "bit”. Thus, a bit in computer terminology means either a 0 or a 1. A binary number consisting of n bit is called an n-bit number. Following Table lists all the 3-bit numbers of along with their decimal equivalent. It may be seen that a 3-nit number can have one of the 8 values ranging from 0 to 7. In fact it can be shown that any decimal number in the range of 0 to 2ⁿ - 1 can be represented in the binary form as an n-bit number.

Binary	Decimal
000	0
001	1
010	2
011	3
100	4
101	5
110	6
111	7

3-bit number with their decimal values

Every computer stores numbers, letters and other special characters in binary form. There are several occasions when computer professionals have to know the raw data contained in a computer’s memory. A common way of looking at the contents of a computer’s memory is to print out the memory contents on the line printer. This is called as memory dump. Because of the quantity of printout that would be required in a memory dump of binary digits and the lack of digit variety (0 and 1 only) two number system are used as short cut notation.

1. octal
2. hexa decimal

Octal number system

In the octal number system the base is 8. So in this system there are only eight symbols or digits:0,1,2,3,4,5,6,7 (8 and 9 do not exist in this system). Here the largest digit is 7. Each position in an octal number represents a power of the base (8). Thus the decimal equivalent of the octal number 2057 (written as 2057) is:

(2 x 8³) + (0 x 8²) +(5 x 8¹) +(7 x 8⁰) =1024 + 0 + 40 + 7 =1071

So we have,

2057₈= 10711₁₀

You can see that since there are only 8 digits in the octal number system, so 3 bits (23 = 8) are sufficient to represent any octal number in binary.

Hexadecimal number system

The hexadecimal number system is one with a base of 16. The base of 16 suggests choices of 16 symbols or character digits. The first 10 are the digits from 0 to 9 i.e. 0,1,2,3,4,5,6,7,8,9. The remaining six digits are denoted by A, B, C, D, E, F representing 10, 11, 12, 13, 14, 15 respectively. In the hexadecimal number system, therefore, the letter A has a decimal equivalent value of 10 and the hexadecimal F has a decimal equivalent value of 15. The largest single digit is F (15). Again, each position in a hexadecimal system represents a power of the base (16). Thus the decimal equivalent of the hexadecimal number 1AF (written as (1AF₁₆) is:

=(1 x 16²) + (A x 16¹) +(F x 16⁰)

= (1 x 256) + (10 x 16) +(15 x 1)

=256 + 160 + 15 =431

Thus, 1AF₁₆=4311₁₀

Since there are only 16 digits in the hexadecimal number system, so 4 bits (2⁴ =16) are sufficient to represent any hexadecimal number in binary.

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Input Output Devices

A computer system can be proved to be useful only when it is able to communicate with its external environment. The input-output (1/0) devices provide the means of communication between the computer and the outer world. They are also known as peripheral devices because they surround the CPU. Input devices are used to enter the data in the primary storage and output devices are used to accept the result from the primary storage and store them on the secondary storage area for further processing.
A wide variety of I/O devices are now available.

INPUT DEVICES

Keyboards

Keyboard, the most common input device operated with fingers provide various tactile responses. Various kinds of keys are present on keyboard which perform various functions. Mouse
A mouse is the standard pointing tool for interacting with a graphical user interface. You can select and open files and folders with this pointing device. All Macintosh computers require a mouse whereas on PCs mouse are recommended for additional help. A keyboard can do all the function which mouse can perform.

Trackballs

Trackballs are similar to mouse, except that the cursor is moved by using one or more fingers to roll across the top of the ball.

Touchscreen

Touchscreen are monitors that usually have a textured coating across the glass face. This coating is sensitive to pressure and registers the location of the user's finger when it touches the screen. They are excellent for multimedia applications in a kiosk, at a trade show or in a museum delivery system.

Magnetic card encoder and reader

Magnetic card setups are useful when you need an interface for a database application or multimedia project that tracks users. You need both a card encoder and a card reader for this type of interface. The encoder connects to the computer at a serial port and transfers information to a magnetic strip of tape on the back of the card. The card reader then rends the information encoded on the card.

Graphic tablets

Flat surface input devices are attached to the computer in the same way as a mouse or trackball. A special pen is used against the pressure sensitive surface of the tablet to move the cursor. Graphic tablets provide substantial control for editing finely detailed graphic element, a feature very useful to graphic artists and interface designers. They are used in 3D drawings.

Scanners

A scanner is an input device which scan images, photographs, text, pictures and images of items like stones, cloth etc. The images are stored in digital format at a resolution which can vary.

Digital cameras

Digital cameras such as apple quicktake, the dycam 10c, kodak Dc40 etc. can record up to 50 images on a reusable two inch floppy disk. Images can be played back directly from the camera to any standard TV monitor or used with a digitizer for computing input.
Software controls the image capture, image adjustment and save functions of the digitizer. Once the image is saved in the computer environment, it can be exported to various applications, incorporated into desktop publishing setups, used to enhance a database, or added as a graphic image to a multimedia presentation.

OUTPUT HARDWARE

Amplifiers and speakers

Speakers with built in amplifiers or attached to an external amplifier are important when your project will be presented to a large audience or in a noisy setting.

Monitor

A wide variety of monitors are available for Macintoshes and PCs. Serious multimedia developers will often attach more than one monitor to their computers, using add-on graphics board.

Printer

Till now we have seen everything on the screen. This copy of the document visible on the screen is called the soft copy. But, it is incomplete until we take a print of the document on the paper.
Hard copy of the document is the impression of the document on paper. Printer is a media or an output device, which performs this operation.
A most convenient and useful method by which the computer can deliver information is by mean of printed characters.
A printer falls into two broad categories.

• Impact Printers
• Non Impact Printers

IMPACT PRINTERS

In the impact printers, there is mechanical contact between the print head and paper.
The impact printers can further be classified into following categories:

• Line Printers
• Character Printers

Line printers

A line prints a complete line at a time printing speed vary from 150 lines to 2500 lines per minute with 90 to 100 characters on 15 inch line.
There are 2 types of line printers

• Drum Printer
• Chain Printer

Drum Printer

A drum printer consists of a cylindrical drum on which characters are embossed.
One complete set of characters is embossed for each and every print position on a line. Thus a printer that can print up to 132 characters per line and has a 96 character set will have on its surface 132 x 96 = 12672 characters embossed.
Working of a drum printer
1. The codes of all characters to be printed on one line are transmitted to a storage unit reserved for printer known as printer buffer.
2. The printer drum is rotated at a high speed.
3. A set offprint hammers one for each character in a line is mounted in front of a drum.
4. The position of each character on a bend of the drum surface is coded using its angular displacement from the origin.
5. Striking a hammer against the embossed character in the surface prints a character.
6. A carbon ribbon and paper are interposed between the hammer and the drum as the drum rotates the hammer waits and is activated when the character to be printed at the position appears in front of the hammer.
Thus the drum would have to complete one full revolution for a line to be printed.
This is called "on the fly". Printing as a drum continues to rotate at a high speed when the hammer strikes it.
Disadvantages of drum printer

• The hammer must strike very quickly and must be accurately synchronized with drum movement, if the hammer striking is missed time then the printed line looks wavy and slightly blurred.
• Printer drums are expensive and cannot be changed often.
• Drum printers have a fixed font.

Chain Printer

A chain printer consists of a steel band on which the characters are embossed.
Working of a chain printer

• To print a line, the characters in the line are transmitted in the memory to printer buffer.
• The band is rotated at a high speed.
• With the rotation of a band, a hammer is activated when the desired character as specified in the buffer register comes in front of it.
• Thus a printer with 132 characters per line, 132 hammers will be positioned to strike the carbon ribbon which is placed between the chain, paper and the hammer.
• There should be accurate synchronization between the hammer movement and chain movement as the bad synchronization leads to blurred lines.

Line printers are normally designed for heavy printing applications and can print continuously for a few hours.

Character printer

(Serial printer)

Character printer prints one character at a time, with the print had moving across a line, serial printer normally print 30 to 300 characters per second.
Broadly serial printers may be divided into 2 categories

• Dot Matrix Printer
• Letter Quality Printer

Dot matrix printer

It is the most popular serial printer.
In DMP's the printing head contains the vertical array of pins. Head moves across the paper, selected pins fire against and inked ribbon form a pattern of dots on the paper,

Features of a dot matrix printer

• In the DMP's the capital letter are formed by 5 x 7 matrix of dots i.e., 5 dot rows and 7 dot columns.
  Though there are 9 pins in the print head. The bottom 2 ping are used to form the descenders of small letters link f, g, q, w, e, r, t, y etc.
• Characters to be printed are sent 1 character at a time from the memory to the printer.
• The character code is decoded by the printer electronics that activates the appropriate pins in the print head.
• Many DMP's are bi-directional i.e. they can print the characters from either direction left or right. This speeds up the printing process.
• DMP's are very versatile since with the pin sets, characters of any language other than English can be printed, even the graphics can be printed with DMP's.
• DMP's with 24 pins in the print head are also available in the market.
• These DMP's have a better print quality. There are 80-column DMP's and 132-column dmp available in the market.
  

Letter quality printing

Daisy wheel printer

The letter quality printers print full characters. The most popular printer of such a type is daisy wheel printer.
Working of a daisy wheel printer

• A daisy wheel printer's print head resembles a daisy flower with its print arms appearing like the petals of the flower while the printing the character is sent to the printer buffer.
• The hub is continuously rotating at high speed and a hammer strikes the appropriate character when it is in the position.

The daisy wheel print can have speed up to 90 cps.

Non Impact Printers

The natural limitations of speed in electromechanical devices and cost considerations have led to the development of printers called non impact printers. These can primarily be categorized as follows.

Electromagnetic printers

By using magnetic recording technique, a magnetic image of what is to be printed can be written on a drum surface. Then this is pressed - onto the paper. Speeds of up to 250 characters per second are obtained in such systems.

Thermal printers

An electric pulse can be converted to heat on selected sections of a printing head or on wires or nibs. When this heat is applied to heat sensitive paper, a character is printed.

Electrostatic printers

For electrostatic printers the paper is coated with a non-conducting dielectric material, which holds charges when voltages are applied with writing "nibs”(heads). These heads write dots on the paper as it passes, as shown in the figure. Then the paper passes through a toner, which contains material with colored particles carrying an opposite charge to that written by the nibs; as a result, particles adhere to the magnetized areas, forming printed characters.

Inkjet printers

Some printers direct a high-velocity stream of ink towards the paper. This stream is deflected, generally by passing it through an electrostatic field such as that used to deflect beams in oscilloscopes. In some systems the ink stream is broken into droplets by an ultrasonic transducer.

Laser printers

These printers make use of office copier technologies. The de output image is written on a copier drum with the help of a light be controlled by a computer. With this certain parts of the drum surf get electrically charged, then this drum surface is exposed to the la beam. These laser exposed area attract a toner that form the image attaching itself (toner ink) to the laser generated charges on the drum the toner is then permanently fused on paper with hot or pressure.

Storage devices

Hard disk

A hard disk is a thin, circular metal plate/ platter coated on both the sides with a magnetic material. A hard disk jack consists of a number of disks. Thus a pack of 10 disks will have 18 surfaces to read and write. All these disks together comprise a hard disk, which has a very high capacity ranging from 2 GB to 40 GB (Giga byte). Each disk is divided into concentric circular tracks to store information.
Information is stored on the disk in the form of tiny invisible magnetic spots. The presence of a magnetized spot represents a 1 bit and the absence represents a 0 bit. A standard binary code called EBCDIC is used for recording data.
Hard disk is fixed permanently inside the system unit and hence it's non-portable. Though it can be taken out of the system unit by detaching it.

Floppy disk

The Floppy disk is also referred to as diskettes or floppies. IBM introduced them in 1972. A floppy is made up of flexible plastic material, which is coated with magnetic oxide. The flexible disk is enclosed within a square plastic or cardboard jacket, often referred to as a cartridge. The jacket gives handling protection to the disk surface. The disk is mounted on the disk drive along with the jacket cover and information is loaded and read through an aperture in the jacket.
Floppy disks are typically 3.5, 5.25 and 8 inches in size. But now a day’s floppies are not in used.
The capacity of a floppy is 1.44 MB. Floppy disks are cheap.

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Introduction to PC

A personal computer (PC) is a computer that can perform all of its input processing, output and storage activities by itself, i.e. it contains at least one input device, one output device, one storage device, memory and a processor. The processor called a microprocessor, is a central processing unit (CPU) on a single chip and is the basic building block of a PČ.
Two popular series of PCs are the IBM PC and the Apple Macintosh. These two types of computers have different processors and different operating systems. The PC and compatibles use the Windows operating system. Today the terms PC and compatibles are used to refer to any personal computer that is based on specifications of the original IBM PC computer. Companies such as Gateway, Compaq, Dell and Toshiba all sell PC compatible computers.
A computer is a composition of various parts, which are housed together in a box. A first glance, you will find a monitor, keyboard, mouse and a box. But when you will observe the box, you will find that the major organs of a computer are housed inside that box. That box is called as a system unit.

The system unit

The system unit is a box like case that houses the electronic components of the computer that are used to process data. The system unit is made of metal or plastic and is designed to protect the electronic components from damage. On a desktop personal computer, the electronic components and most storage devices reside inside the system unit. Other devices, such as keyboard, mouse; monitor and printer normally are located outside the system unit. A laptop computer houses almost all of its electronic components in the system, unit.

Inside the system unit

The motherboard

It is the main printed circuit board, which contains the sockets that accepts additional boards. In a personal computer, the motherboard contains the bus, CPU and coprocessor sockets, memory sockets, keyboard controller and supporting chips.
Chips that control the video display, serial and parallel ports, mouse and disk drives may or may not be present on the motherboard If not, they are independent controllers that are plugged into an expansion slot on the motherboard.

The central processing unit

The CPU or central processing unit interprets and carries out the basic instructions that operate a computer, The CPU, also referred to as a processor, significantly imparts overall computing power and manages most of a computer's operations. That is, most of the devices connected to the computer communicate with the CPU in order to carry out a task. The CPU contains the control unit and the arithmetic/logic unit. These two components work together to perform the processing operations.
On larger computers, such as mainframes and super computers, the various functions performed by the CPU are spread across many separate chips and sometimes multiple circuit boards. On a personal computer, the CPU usually is contained on a single chip and sometimes is called a microprocessor.
In addition to the control unit and the arithmetic /logic unit, a microprocessor usually contains the registers and system clock. Each of these microprocessor components in the following sections.

The control unit

As you know, a program or set of instructions must be stored in memory for a computer to process data. The CPU uses its control unit to execute these instructions. The control unit, one component of CPU, directs and co-ordinates most of the operations in the computer. The control unit has a role much like a traffic cop: it interprets each instruction issued by a program and then initiates the appropriate action to carry out the instruction. For every instruction, the control unit repeats a set of 4 basic operations: 1) fetching an instruction 2) decoding the instruction 3) executing the instruction 4) storing the instruction. Together these 4 instructions comprise a machine cycle.

The arithmetic /logic unit

The arithmetic/logic unit (ALU), another component of the CPU, perform the execution part of a machine cycle. Specifically, the ALU performs the arithmetic, comparison and logical operations.

The system clock

The control unit relies on a small chip called the system clock to synchronize or control the timing of all computer operations. The system clock generates regular electronic pulses or ticks that set the operating pace of components in the system unit. Each tick is called a clock cycle. A CPU requires a fixed number of clock cycles to execute each instruction. The faster the clock, the more the instructions the CPU can execute per second. The speed at which a processor execute instructions is called clock speed or clock rate. Clock speed is measured in Megahertz which is equal to 1 million ticks of the system clock.
The strength of a CPU is determined by how fast it processes data. One of the major factors that affect this is the system clock. A higher clock speed means the CPU can process more instructions per second than the same CPU with a lower clock speed. Or example 700 MHz CPU is faster than 500 MHz.

Coprocessors

Another way to increase the performance of a computer is through the use of a coprocessor, who is a special processor chip or circuit board designed to assist the processor in performing specific tasks. Users running engineering, scientific or graphics applications, for instance, will notice a dramatic increase in speed with a floating -point coprocessor, provided the application is designed to take advantage of the coprocessor. Floating-point coprocessors are also called the math coprocessor or numeric coprocessor.

Parallel processing

Some computers use more than one processor to speed processing times. Known as parallel processing, this method uses multiple processors simultaneously to execute a program. That is parallel processing divides up a problem so that multiple processors work on their assigned portion of the problem at the same time.

Memory

While performing a processing operation, a processor needs a space to temporarily store instructions to be executed and the data to be used with those instructions. A computer's memory in the system unit is used to store data, instructions and information. The memory chips on the circuit board in the system unit perform this function. Memory stores three basic items:

1. The operating system
2. The application program
3. The data being processed

The number of bytes available for use measures the size of memory. A kilobyte is equal to 1024 bytes. The system unit contains two types of memory

1. volatile memory
2. Nonvolatile memory

Volatile memory is lost when the power is turned off whereas non-volatile memory are not lost when power is removed from the computer. RAM is an example of volatile memory. ROM, flash memory, CMOS is examples of non-volatile memories,

RAM

The memory chips in the system unit are called RAM (Random access memory. When the computer power is switched off, the data on the RAM is lost, when power is on, RAM contains, OS, Data and instructions and the application program. RAM is volatile, therefore data on RAM should be saved on secondary storage media like floppy or hard disk (discussed later).

Cache

Memory cache helps to speed the processor of the computer by storing frequently used instructions and data.

ROM

Read only memory is the name given to memory chips storing data that only can be read? That is, the data stored in ROM chips cannot be modified - hence the name read only. While RAM is volatile, ROM is nonvolatile; its contents are not lost when power to the computer is turned off. ROM chips contain data, instructions or information that is recorded permanently. For example, ROM contains the sequence of instructions the computer follows to load the operating system and other files when you first turn the computer on.
The data, instruction or information stored on ROM chips often is recorded when the chips are manufactured. ROM chips that contain permanently written data, instructions or information are called firmware.

Expansion slots and expansion cards

An expansion slot is an opening or a socket where a circuit board can be inserted into the motherboard. These circuit boards add new devices or capabilities to the computer such as more memory, higher quality sound devices, a modem or graphic capabilities. Many terms are used to refer to this type of circuit board: Expansion card, expansion board, and adapter card interface card, card, add-in and add-on.
Three types of expansion cards found in most of today's computer are a video card, a sound card and an internal modem.
A video card, also called a video adapter or graphics card, converts computer output into a video signal that is sent through a cable to the monitor, which displays an image on the screen.
A sound card is used to enhance the sound generating capabilities of a personal computer by allowing sound to the input through a microphone and output through speakers.
An internal modem is a communication device that enables computers to communicate via telephone lines or other means.

PORTS

A cable, often attaches external devices such as a keyboard, monitor, printer, mouse and microphone, to the system unit. The interface or points of attachment to the system unit is called a port. Most of the time, ports are located on the back of the system unit, but they can also be placed on front.

Serial ports

A serial port is one type of port. The transmit data bit by bit, it is usually used to connect devices that don't require test data transmission rates, such as a mouse, keyboard or modem. In a PC, serial port circuits are built into the motherboard.

Parallel port

Unlike serial port, a parallel port is an interface used to connect devices that are capable of transferring more than one bit at a time. Parallel ports originally were developed as an alternative to the slower speed serial ports. Many printers to the system unit use a parallel port with a 25-pin connector. This parallel port can transfer 8 bits of data (one byte) simultaneously through eight separate lines in a single cable.

Power supply

Many PCs are plugged into standard wall outlet, which supply an alternating current (AC) of 115 to 120 volts. The power supply is the component in the system unit that converts the wall outlet AC power into DC power.

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Computer Languages

Computer programming languages are developed with the primary objective of facilitating a large number of people to use computers without the need to know in detail the internal structure of the computer.
Languages are matched to the type of operations to be performed in algorithms for various application Languages are also designed to be machine-independent.
In the other words, the structure of a programming language would not depend upon the internal structure of a specified computer. Ideally, one should be able to execute a program on any computer regardless of who manufactured it or what model it is.
A language is a system of communication. The process of writing a program in any language is called programming. There are three broad categories of computer languages

1. Machine Language
2. Assembly Language
3. High Level Language

Machine language (I generation Language)

Computers depend upon the instructions provided by others to accomplish data processing tasks, and a set of such instructions is commonly called a programming language. Many of these languages were developed as prototypes and therefore could only see on the machines for which they were designed. These languages are called machine-dependent programming languages or Low level languages.
Machine language is the fundamental language of the computer. It is written in combinations of O's and 1's. A computer can understand this language without using any translation program. The circuitry of a computer is wired in such a way that it immediately recognizes the machine language and converts it into the electrical signals needed to run the computer.

Advantages of machine language

1. The computer can execute programs written in machine language very fast. This is mainly because there is no need of translation program like interpreter or compiler.
2. The instructions of the machine language program are immediately executable.
3. Machine language instructions can be used to manipulate the individual bits in a byte of computer storage.

Disadvantages of Machine Language

1. Machine language requires high level of programming skill the language is machine dependent. Because the in design of every type of computer is different from every other type of computer is different from every other type of computer and needs different electrical signals to operate; the machine language also is different from computer to computer. The actual design or construction of the ALU, the control unit and the size as well as the word length of the memory Unit determines it. Hence suppose after becoming proficient in the machine code of a particular computer, a company decides to change to another computer, the programmer may be required to learn a new machine language and would have to rewrite all the existing programs.
2. Difficult to write programs. Although easily used by the computer, machine language is difficult to write into programs. Besides dozens of code for every letter, a programmer has to keep track of data and instruction and hardware structure of the computer.
3. Error prone. For writing programs in machine language, since a programmer has to remember the OPCODES and he must also keep track of the storage location of the data and instructions, it becomes difficult for him to concentrate fully on the logic of the program. This frequently results in program errors. Hence, it is easy to make errors while using machine code.
4. Difficult to modify. It is difficult to modify and correct the machine language programs. Checking errors is very time consuming and tedious.

Assembly Language

Although ML programs are easily stored in memories of computers, humans do not easily program them. Writing instructions in 0 and 1 is boring, tedious and prone to errors. Thus, to improve programmer productivity, II Generation language called ASSEMBLY LANGUAGE were developed.
These Programming Language substitute alphabetic or numeri symbols for binary code of machine language, thereby enabling programmers to express computer instructions more easily.
One of the first steps in improving the program preparation process was to substitute letter symbols pneumonic for the numeric operation codes of machine language. A mnemonic is any kind of mental trick we use to help to summarize. Mnemonics comes in various shapes and sizes. For example the mnemonic for subtraction can be "SUB".
The language which substitute the binary combination with symbols and letter is called assembly language or symbolic language. The translator programs which converts an assembly language program into machine language equal is called an assembler.

Advantages of Assembly Language (II GL)

1. Assembly Language is easier to use than Machine Language.
2. An assembler is useful for detecting programming errors.
3. Programmers do not have to remember the addresses of data values.
4. Assembly Language encourages programming in modules.

Drawbacks of II generation language (Assembly Language)

1. Assembly Language programs are not immediately executable.
2. There is almost a one for one correspondence between Assembly Language and Machine Language instructions.
3. Assembly Language is machine dependent.
4. Programming in Assembly Language requires a high level of programming skills.

High level language (III GL)

Both Machine Language and Assembly Language were machine dependent.
High level language was developed to allow application program to be run on a variety of computers. These languages are machine independent and procedure oriented.
One statement in a high level language would be translated into many statements in its machine language equivalent.
III GL or High Level Programming Language (HLPL) was developed in part to avoid the expense and trouble of machine-dependent programming. e.g. BASIC, FORTRAN)
III Generation Languages, a language such as C, COBOL and FORTRAN OR PASCAL that provides developers with considerable flexibility at cost of ease use)
Most III GL can handle such detailed tasks memory manipulation a physical disk assess which most IV GL's cannot handle.
III GL's however complex and require more development time for application. The problem with most III GL is that they are procedural programming languages i.e. lang.'s that focus on programming procedures rather than results. A term applied to languages designed for interacting with programmer, often used to define languages used with relational data basis. The interest to imply that such languages are a step up from standard high level programming lang. such as C, PASCAL, COBOL.IV GL's are generally preferable for creating data base applications and for use with popular development tools.

IV Generation Language

1. Interactive code: A programmer enters English like phrases or on menu choices to formulate an inquiry or define a task.
2. Limited training required: Using IV GL's a novice (user) can often design a simple program including everything from describing a data file to designing a formatted data screen with just one training.
3. Emphasis on end results: The thrust of these languages is on specifying the dimensions of a problem rather than a complicated set of procedures with which to solve the problem. These speeds programming because the problem is almost always what the user understands best and therefore can best specify.
4. Increase productivity.
5. Increased CPU memory requirement.

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