IT-162

FUNDAMENTALS OF Information Technology

Lecture #3

Lecture Outline

WEEK Topic Lecture

02 Introduction to Information and Communication SystemsTypes and forms of InformationAnalogue and Digital InformationInformation Systems communication modesComments for next class 

03

Data , Information, Messages and SignalsIt is important to distinguish between information, messages and

signals:

Data : A representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by human beings or by automatic means.

Signal: The actual entity (electrical, mechanical, etc) that is transmitted from sender to receiver (ex: electrical signals, sound waves, optical pulses)

Message: The content of the signal (ex: song, speech, binary code etc.)

Information: The content of the message, i.e. the knowledge that is communicated/received by the message.

Message and information are sometimes used interchangeably, however there is a subtle difference between the two.

Information/Data/Signals

4

Information Data Signal

001011101

Properties of a SignalData and Information is transformed in to electromagnetic or Optical Signal before transmission.

Three important characteristics of a signal are :

Amplitude : The Intensity of Signal or the instantaneous value of a signal at any time measured in volts.

Frequency : the number of repetitions of the period per second ,it is expressed in cycles per second or Hertz (Hz).

Phase : a measure of the relative position in time within a single period of a signal, measured in degrees.

As an example, consider the following scenario:

Yes, Mr. Faraday would like to meet you

at 4:00 p.m. today Sure, I’ll be there!

Electrical signal

Message (speech) Information Information

Analog and Digital Information The term analog is used to refer to the natural world,

where time is continuous, and most parameters (like light, sound intensity, position, etc. ) can vary smoothly and continuously over some range, taking on an infinite number of possible values

The term digital is used to refer to information representations for which both time and the value being measured move in discrete steps i.e. when there are a finite number of possible values

Analog Versus Digital

DISCRETE CONTINUOUS

When each discrete information value is converted to a binary value, the information becomes digitized.

Exists at SPECIFIC POINTS in space and time

Variables can have ONLY SPECIFIC values

Example: Clock hand that rotates in jumps (ticks)

Exists for ALL values of space and time

Variables can have ALL POSSIBLE values

Example: smoothly rotating hand on a clock

Analog Versus Digital

The Natural World is Analog

This continuous acoustical waveform can be detected by a microphone and

converted into an analog electrical signal for transmission over a piece of wire.

Human speech is an example of analog communication.Speech causes air to vibrate with varying amplitude (volume) and frequency (pitch).

The Computer World is Digital Digital computers communicate using 2 discrete values. In

other words, they speak in binary (0 and 1). Of course, 0s and 1s are not literally transmitted

In an electrical network, variations in voltage represent one of the two values.

In an optical network, pulses of light provide the discrete values.

The pulses of light or voltage variations are the “signal.”

Two values in different combinations sufficiently encode text, numbers, image, and sound!

Recall that the telegraph was an early example of communications using discrete, electrical pulse transmission.

Digital vs. Analog

Analog signals are susceptible to distortion and inaccuracy due to other signals (interference)

Digital information can be compressed for efficient transmission and storage

Digital information can be encrypted for increased security and multiplexed for increased capacity

Digital technology is much cheaper Digital signals can be accurately reproduced Digital signals are easier to detect There is opportunity for error detection and correction in

digital technology

A Problem with Analog

Analog signal

Noise

If an analog signal provides such a close representation If an analog signal provides such a close representation of information, why do we use digital?of information, why do we use digital?

Analog signal on magnetic tape. Random fluctuations in the magnetic tape add “noise” to the signal. The noise cannot be removed and becomes part of the subsequent versions of the analog signal.

Distorted Signal(unwanted electrical/electromagnetic energy)

Restoration of digital signals stored on magnetic tape. Random fluctuations in the magnetic tape add noise to the digital signal. A device, called a threshold detector, compares the signal to a threshold (dashed line) and decides that the data value is a 1 if the signal lies above the threshold, or a 0, otherwise.

Digital Signal Noise Distorted Signal

ThresholdDetector

Regenerated Digital SignalProcessor

The Digital Advantage

Communication Modes. It refers to the direction of flow of signals among linked devices. There are three ways for transmitting data from one point to

another

1. Simplex

2. Half-duplex

3. Full Duplex

1.SIMPLEX MODE :

Features:a) Unidirectionalb) Send-only or Receive-only devicesc) Sender is unaware about the status of transmission.d) Rarely used for data communication.

Example: Radio transmission

Sender Receiver

2. HALF-DUPLEX:

Features:a) Two-way communication but not simultaneously.b) At a moment it is unidirectional only.c) Can alternately send and receive data.c) Transmission direction can be changed whenever

required.d) Delay factor (Line Turnaround Time) is associated.

Example: i ) Wireless handsets used by military personnel ii) Data transmission to & from hard disk.

Sender

or Receiver

Receiver or

SenderOR

3. FULL DUPLEX:

FEATURES:a) Also known as Duplex Modeb) Communicates in both directions simultaneously.c) Line Turnaround time is eliminated.d) Much efficient mode.

Example: Telephone line

Sender and

Receiver

Receiver and

SenderAND

Comments for next class

Go over today’s lecture notes Download next week’s lecture notes Finish reading chapters 1 and 3