UNIT-4

1. DATA COMMUNICATION

Data communication is the active process of transporting data from one point to another. Networks are communication system designed to convey information from a point of origin to a point of destination.

In data communication there are different components to communicate with other they are

i).Message: The message is the data information to be communicated. It can be text, numbers or audio or video or any combination of these types.

(ii).Sender: The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera etc..

(iii) Receiver: The receiver is the device that receives the messages. It can be a computer, workstation, telephone handset, video camera or television etc.

(iv).Medium: The transmission medium is the physical path by which a message travels from sender to receiver. It should be a twisted –pair wire, co-axial cable, fiber optic cable or Radio waves etc.

(v).Protocol: A protocol is a set of rules that govern the data communications. It represents an arrangement between the communicating devices. Without a protocol, two devices may be connected but cannot communicate.

Fig: Data communication

Modes of data communication:

There are three ways for transmitting data from one point to another as shown below

1. Simplex : In simplex mode the communication can take place in one direction. The receiver receives the signal from the transmitting device. In this mode the flow of information is Uni-directional. Hence it is rarely used for data communication.

2. Half-duplex: In half-duplex mode the communication channel is used in both directions, but only in one direction at a time. Thus a half-duplex line can alternately send and receive data.

3. Full-duplex: In full duplex the communication channel is used in both directions at the same time. Use of full-duplex line improves the efficiency as the line turn around time required in half-duplex arrangement is eliminated. Example of this mode of transmission is the telephone line.

Data and Signals

Data and signals are two of the basic building blocks of any computer networks. A signal is the transmission of data. Data is transmitted from one point to another point by means of electrical signals that may be in digital and analog form.

Analog signals:

In analog signal the transmission power varies over a continuous range with respect to sound, light and radio waves.

Fig: Analog signals

Digital signals:

On the other hand, a digital signal may assume only discrete set of values within a given range. Digital signals are discontinuous values to represent information. The data stored in the form of 0’s and 1’s. when the signal is at high point, its value is 1 and when signal is low its value is 0.

Fig: Digital signals

2. Types of data communication networks:

There are different types of data communications networks they are

Local Area Network Metropolitan Area Network Wide Area Network

Fig: Types of communication networks

Local Area Network (LAN)

It is also called LAN and designed for small physical areas such as an office, group of buildings

or a factory. LANs are used widely as it is easy to design and to troubleshoot. Personal

computers and workstations are connected to each other through LANs. We can use different

types of topologies through LAN these are Star, Ring, Bus, Tree etc.

LAN can be a simple network like connecting two computers, to share files and network among

each other while it can also be as complex as interconnecting an entire building.

LAN networks are also widely used to share resources like printers, shared hard-drive etc.

Fig: LAN

Applications of LAN

One of the computer in a network can become a server serving all the remaining

computers called clients. Software can be stored on the server and it can be used by the

remaining clients.

Connecting locally all the workstations in a building to let them communicate with each

other locally without any internet access.

Sharing common resources like printers etc

Metropolitan Area Network (MAN)It is basically a bigger version of LAN. It is also called MAN and uses the similar

technology as LAN. It is designed to extend over the entire city. It can be means to connecting a

number of LANs into a larger network or it can be a single cable. It is mainly hold and operated

by single private company or a public company. MAN is a medium-size network and they are

used to build networks with high data connection spreads for cities and towns. A MAN is

optimized for a larger geographical area than a LAN. It provides low speed data communication

and it is more expensive equipment.

Fig: MAN

Wide Area Network (WAN)

It is also called WAN. WAN can be private or it can be public leased network. It is used for the network that covers large distance such as cover states of a country. It is not easy to design and maintain. Communication medium used by WAN are PSTN or Satellite links. WAN operates on low data rates. WAN is a large geographical area such as a state or country.

Fig: WAN

3. Communication Media or Transmission Media

The group of interconnected computers through transmission media in-order to

communicate and share resources like hardware, data and software is known as communication

media. It is a system for communication between networks. The computers in the network may

be connected through cables, telephone-lines, beams etc.

Types of communication media

There are two main types of communication channels. They are:

1. Guided or Bounded or Wired Communication Media

2. Unbounded or Unguided or Wireless Communication Media

1. Guided or Bounded or Wired Communication Media

A transmission media where data signals are transmitted along a specific path through cable is

known as Guided Transmission Media. It transfers data from one place to another with the help

of wire. There are three types of cables used for wired network. They are:

1. Twisted Pair Cable

2. Co-axial cable

3. Fiber optic cable

Twisted Pair Cable

The twisted pair cable consists of a pair of insulated copper wire twisted around each other. The

number of twisted pair may 1, 2, 4 or more. The twisted pair cable is mostly used for connecting

computers on the network.

The twisted pair cable comes into two categories:

Unshielded Twisted Pair (UTP): A UTP cable is one of the most popular LAN cables. This

cable consists of 4 twisted pairs of metal wires (means there are 8 wires). Adding RJ-45

connector at both ends of the cable, it becomes a LAN cable that we generally use.

Shielded twisted Pair (STP): A shielded twisted pair cable (also known as IBM Type I) is

similar to UTP but it has metallic covering placed just underneath the plastic casing. It is more

expensive than UTP cables. STP cable offers the best protection from interference. It can support

data transfer rate from 16 to 500 Mb/s (millions of bit per second).

Fig: Twisted Pair Cable

Co-axial cable

Coaxial cable has wide bandwidth and noise immunity. These are widely used in long distance

telephone lines. Its transmission speed is much higher than twisted pair cables.

Fig: Co-axial cable

Fiber optic cable

Fiber optic cables are made up of plastic or glass fibers and gives high quality

transmission of signal at a very high speed. Fiber optic cable transmissions are not affected by

electromagnetic interference. These can be used to communicate either analog or digital signals.

These are most commonly used for point to point one way communication.

Fig: Fiber optic cable

2. Unbounded or Unguided or Wireless Communication Media

The way to transfer data without the help of wire is called unguided media. The three types of

wireless media are:

1. Micro waves

2. Radio waves

3. Satellite

Microwave: The way to transfer data straightly from one point to another point in the

way of light in the universe is called microwave system.

Radio wave: Radio wave transmissions are used for communication between computers

in inaccessible locations or for short range communications. The other benefit is the

responsibility of reaching rural and hilly area which are not covered by land telephone

line.

Satellite: Satellite communications are like microwave and have relay stations in the sky.

Transponders on the satellite are used to receive and re-transmit signals sent from earth

stations. A transponder has a very high capacity and can handle more than 400 channels.

Fig: Satellite

4. Concepts of Computer Networks

Network Topology

Network Topology is the schematic description of a network arrangement, connecting various nodes (sender and receiver) through lines of connection. The term “Topology” refers to the way in which the end points or stations/computer systems, attached to the networks, are interconnected. There are different types of network topologies. They are

1. Bus Topology2. Ring Topology3. Star Topology

4. Mesh Topology5. Tree Topology

Bus Topology

Bus networks share a common connection that extends to all devices. This network

topology is used in small networks, and it is simple to understand. Every computer and network

device connects to the same cable, so if the cable fails, the whole network is down, but the cost

of setting up the network is reasonable. This type of networking is cost effective. However, the

connecting cable has a limited length, and the network is slower than a ring network.

Fig: Bus Topology

Ring Topology

Each device in a ring network is attached to two other devices, and the last device

connects to the first to form a circular network. Each message travels through the ring in one

direction clockwise or counterclockwise through the shared link. Ring topology that involves a

large number of connected devices requires repeaters. If the connection cable or one device fails

in a ring network, the whole network fails. Although ring networks are faster than bus networks,

they are more difficult to troubleshoot.

Fig: Ring Topology

Star Topology

A star topology typically uses a network hub or switch and is common in-home networks.

Every device has its own connection to the hub. The performance of a star network depends on

the hub. If the hub fails, the network is down for all connected devices. The performance of the

attached devices is usually high because there are usually fewer devices connected in star

topology that in other types of networks. 

A star network is easy to set up and easy to troubleshoot.

The cost of setup is higher than for bus and ring network topology, but if one attached device

fails, the other connected devices are unaffected. 

Fig: Star Topology

Mesh Topology

Mesh network topology provides redundant communication paths between some or all

devices in a partial or full mesh. In full mesh topology, every device is connected to all the other

devices. In a partial mesh topology, some of the connected devices or systems are connected to

all the others, but some of the devices only connect to a few other devices.Mesh topology is

robust and troubleshooting is relatively easy. However, installation and configuration are more

complicated than with the star, ring and bus topologies.

Fig: Mesh Topology

Tree Topology

Tree topology integrates the star and bus topologies in a hybrid approach to improve network scalability. The network is setup as a hierarchy, usually with at least three levels. The devices on the bottom level all connect to one of the devices on the level above it. Eventually, all devices lead to the main hub that controls the network.

This type of network works well in companies that have various grouped workstations. The system is easy to manage and troubleshoot. However, it is relatively costly to set up. If the central hub fails, then the network fails.

Fig: Tree Topology