Computer Network Note

 Computer Network

 A computer network is a system in which multiple computers are connected to each other to share information and resources.

Transmission modes 

• The way in which data is transmitted from one device to another device is known as transmission mode. 
• The transmission mode is also known as the communication mode. 
• Each communication channel has a direction associated with it, and transmission media provide the direction. Therefore, the transmission mode is also known as a directional mode.
• The transmission mode is defined in the physical layer. 

The Transmission mode is divided into three categories:

1. Simplex mode

•  In Simplex mode, the communication is unidirectional, i.e., the data flow in one direction.
• A device can only send the data but cannot receive it or it can receive the data but cannot send the data.
• This transmission mode is not very popular as mainly communications require the two-way exchange of data. The simplex mode is used in the business field as in sales that do not require any corresponding reply.
• The radio station is a simplex channel as it transmits the signal to the listeners but never allows them to transmit back. 
• Keyboard and Monitor are the examples of the simplex mode as a keyboard can only accept the data from the user and monitor can only be used to display the data on the screen.
• The main advantage of the simplex mode is that the full capacity of the communication channel can be utilized during transmission.

      2. Half-Duplex mode

• In a Half-duplex channel, direction can be reversed, i.e., the station can transmit and receive the data as well. 
• Messages flow in both the directions, but not at the same time.
• The entire bandwidth of the communication channel is utilized in one direction at a time.
• In half-duplex mode, it is possible to perform the error detection, and if any error occurs, then the receiver requests the sender to retransmit the data.
• A Walkie-talkie is an example of the Half-duplex mode. In Walkie-talkie, one party speaks, and another party listens. After a pause, the other speaks and first party listens. Speaking simultaneously will create the distorted sound which cannot be understood.

     3. Full-duplex mode

•  In Full duplex mode, the communication is bi-directional, i.e., the data flow in both the directions.
• Both the stations can send and receive the message simultaneously.
• Full-duplex mode has two simplex channels. One channel has traffic moving in one direction, and another channel has traffic flowing in the opposite direction.
• The Full-duplex mode is the fastest mode of communication between devices.
• The most common example of the full-duplex mode is a telephone network. When two people are communicating with each other by a telephone line, both can talk and listen at the same time.

Internet

It is a worldwide/global system of interconnected computer networks. It uses the

standard Internet Protocol (TCP/IP). Every computer in Internet is identified by a unique

IP address. IP Address is a unique set of numbers (such as 110.22.33.114) which

identifies a computer’s location.

A special computer DNS (Domain Name Server) is used to provide a name to the IP

Address so that the user can locate a computer by a name. For example, a DNS server

will resolve a name https://www.birajk.com to a particular IP address to uniquely

identify the computer on which this website is hosted.

Internet is accessible to every user all over the world.

Intranet

Intranet is the system in which multiple PCs are connected to each other. PCs in intranet

are not available to the world outside the intranet. Usually each organization has its own

Intranet network and members/employees of that organization can access the

computers in their intranet.

Each computer in Intranet is also identified by an IP Address which is unique among the

computers in that Intranet.

Extranet:

Extranet refers to network within an organization, using internet to

connect to the outsiders in controlled manner. It helps to connect businesses with their

customers and suppliers and therefore allows working in a collaborative manner.

Domain Name System:

When DNS was not into existence, one had to download a Host file containing host

names and their corresponding IP address. But with increase in number of hosts of

internet, the size of host file also increased. This resulted in increased traffic on

downloading this file. To solve this problem the DNS system was introduced.

Domain Name System helps to resolve the host name to an address. It uses a

hierarchical naming scheme and distributed database of IP addresses and associated

names

IP Address

IP address is a unique logical address assigned to a machine over the network. An IP

address exhibits the following properties:

•  IP address is the unique address assigned to each host present on Internet.
• IP address is 32 bits (4 bytes) long.
• IP address consists of two components: network component and host component.
• Each of the 4 bytes is represented by a number from 0 to 255, separated with dots. For example 137.170.4.124