Data Communication Lecture 1, Lecture notes of Data Communication Systems and Computer Networks

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Data Communications

Introduction

Lecture - 1

Data Communication

Data communication is defined as exchange of data between 2 devices over a

transmission-medium.

• A communication-system is made up of

→ hardware (physical equipment) and

→ software (programs)

For data-communication, the communicating-devices must be part of a

communication-system

Components of a Communication

System

Five components of a communication-system

1)Message

2. Sender

3. Receiver

4. Transmission-Medium

5. Protocol

1. Message

• Message is the information (or data) to be communicated.

Message may consist of

→ number/text

→ picture or

→ audio/video

2. Sender

Sender is the device that sends the data-message.

Sender can be

→ computer and

→ mobile phone

Components of a Communication

System

Examples of wireless medium:

→ radio waves

→ microwaves

→ infrared waves (ex: operating TV using remote control)

5. Protocol

• A protocol is a set of rules that govern data-communications.

In other words, a protocol represents an agreement between the communicating-devices.

Without a protocol, 2 devices may be connected but not communicating

Components of a Communication

System

• Five different forms of information:

1. Text

Text is represented as a bit-pattern. (Bit-pattern  sequence of bits: 0s or 1s).

Different sets of bit-patterns are used to represent symbols (or characters).

• Each set is called a code.

The process of representing symbols is called encoding.

• Popular encoding system: ASCII, Unicode.

2. Number

Number is also represented as a bit-pattern.

• ASCII is not used to represent number. Instead, number is directly converted to binary-form

Data Representation

Data Representation

ii) Color Image

• There are many methods to represent color images.
• RGB is one of the methods to represent color images.
• RGB is called so called ‘.’ each color is combination of 3 colors: red, green & blue.

4. Audio

• Audio is a representation of sound.
• By nature, audio is different from text, numbers, or images. Audio is continuous, not discrete.

5. Video

• Video is a representation of movie.
• Video can either

→ be produced as a continuous entity (e.g., by a TV camera), or

→ be a combination of images arranged to convey the idea of motion.

Three ways of data-flow between 2 devices :

1. Simplex

2. Half-duplex

3. Full-duplex

Direction of Data Flow

1) Simplex

The communication is unidirectional

(For ex: The simplex mode is like a one-way street).

On a link, out of 2 devices:

i) Only one device can transmit.

ii) Another device can only receive.

For example (Figure 2(a)):

The monitor can only accept output.

Entire-capacity of channel is used to send the data in one direction.

Direction of Data Flow

Both the stations can transmit as well as receive but not at the same time.

(For ex: The half-duplex mode is like a one-lane road with 2 directional traffic).

When one station is sending, the other can only receive and vice-versa.

For example (Figure 2(b)): Walkie-talkies

Entire-capacity of a channel is used by one of the 2 stations that are transmitting

the data.

Direction of Data Flow

Networks

• A network is defined as a set of devices interconnected by communication-links.
• This interconnection among computers facilitates information sharing among

them.

• Computers may connect to each other by either wired or wireless media.
• Often, devices are referred to as nodes.
• A node can be any device capable of sending/receiving data in the network.

For example: Computer & Printer

• The best-known computer network is the Internet.

Networks

2. Reliability

• Reliability is measured by

→ frequency of network-failure

→ time taken to recover from a network-failure

→ network's robustness in a disaster

More the failures are, less is the network's reliability.

3. Security

Security refers to the protection of data from the unauthorized access or damage.

• It also involves implementing policies for recovery from data-losses.

Network Criteria

Two types of connections (Figure 3):

1. Point-to-Point

Only two devices are connected by a dedicated-link (Figure 1.3a).

Entire-capacity of the link is reserved for transmission between those two devices.

• For example: Point-to-Point connection b/w remote-control & TV for changing the channels.

2. Multipoint (Multi-Drop)

• Three or more devices share a single link.

The capacity of the channel is shared, either spatially or temporally (Figure 1.3b).

i) If link is used simultaneously by many devices, then it is spatially shared connection.

ii) If user takes turns while using the link, then it is time shared (temporal).

Type of Connection