Introduction to Computer Networks

A group of devices when connected together to share resources forms a computer network.

Computer Network are required so that the devices can:

  • can communicate with each other
  • share resources such as files, printers, databases and much more
  • transfer data from one to another.

Data Communication

The exchange of data between two devices via some form of transmission medium is known as data communication.

For data communication to happen, the communicating devices must be a part of the communication system. The communication system is made up of a combination of hardware i.e. the physical equipment and the software i.e the programs.

Characteristics of Data Communication

The effectiveness of data communication systems depends upon the following four characteristics:

  • Delivery
  • Accuracy
  • Timeliness
  • Jitter

Delivery

The computer system must deliver the data to the correct destination. The data must be received only by the intended device and by the intended user.

Accuracy

The computer system must deliver the data accurately i.e. the data must be unaltered. There should be no change in the integrity of the data. Data that has been altered during transmission and left uncorrected is unstable and can be harmful too.

Timeliness

The computer system must deliver data in a timely manner. Data that is delivered late is useless and may not be recognized at the destination. In case of a video or an audio file, timely delivery of data means that the data must be delivered as soon as they are produced and in the same order in which they are produced and also without any significant delay. This kind of delivery of data is known as real-time transmission.

Jitter

When there is a variation in the packet arrival time during a communication then it is known as jitter. It is the uneven delay in the delivery of audio or video packets during a real-time transmission.

For example, let us assume that video packets are sent every 20ms during the transmission of a live video but some of the packets arrive at the destination end with 5m delay i.e. some of the packets reach the destination end at an interval of 25ms while the others were arriving every 20ms, then there will be an uneven quality of the video as an result of this delay.

Components of Data Communication System

There are five major components of a data communication system. They are:

  • Message
  • Sender
  • Transmission Medium
  • Receiver
  • Protocol

(Basic Communication Model)

Message

The message is the information (data) that is to be communicated. The most popular forms of information includes text, numbers, pictures, videos, etc.

Sender

The sender is the device that sends the message (from the sender’s end). It can be a computer, video camera, telephone handset, workstation, and many others.

Transmission Medium

The transmission medium is the physical path through which the message travels from the sender to the receiver. Some examples of transmission medium are twisted-pair cables, fibre-optic cable, radio waves, etc.

Receiver

The receiver is the device that receives the message (at the receiver end). It can be a computer, television, telephone, etc.

Protocol

A protocol is a set of rules and instructions that govern data communication i.e. a protocol governs the flow of data during a data communication. It represents a consent between the communicating devices i.e. how the devices will communicate with each other. Without using a protocol, two devices may be connected with one another but not communicating with each other, just as a person speaking Sanskrit cannot be understood by a person speaking English, but they both are speaking to each other.

Data Flow

There are there types of data flow in a communication. They are:

  • Simplex → one direction only
  • Half-Duplex → either direction, but only one at a time
  • Full-Duplex → both directions at the same time.

Simplex

Simplex communication refers to the communication that occurs in one direction only. A simplex circuit is used when all the signals have to flow in only one direction. Such type of systems are often used in networks, where the receiver doesn’t need to send any data back to the sender. Simplex communications are not preferred because if there are any errors in the transmission then the receiver will not be able to communicate this issue back to the sender i.e. the sender will not know the integrity of data at the receiver’s end. The benefit of using simplex mode for data communication is that one can use the entire capacity of the channel to send data in one direction. The disadvantage of using simplex mode for data communication is that only one of the two devices on a link can transmit the data and other device can only receive the data.

Examples: Radio, Television Broadcast, Pager, etc.

Half-Duplex

Half-Duplex refers to the communication in which a sender and a receiver can communicate with each other, but not simultaneously. The communication can happen in only one direction at a time. When one device is sending the message, the other device can only receive and vice-versa. In this kind of transmission, the entire capacity of the channel can be utilized by any of the devices that would be transmitting at that time for each direction. It is possible to perform error detection in this kind of transmission and the receiver can request the send to re-transmit the information that did not arrive correctly at the receiver’s end.

Example: Walkie-Talkie and CB (Citizen Band) Radios

Full-Duplex

Full-Duplex refers to the communication in which a sender and a receiver can communicate with each other simultaneously. For a full-duplex transmission to occur, there must be two physically separate transmission paths, one for sending the message and one for receiving the message, or the capacity of the channel should be divided between the signals traveling in both directions so that the communication can occur simultaneously. In full-duplex mode, the communication is required in both the directions all time. Both the devices don’t have the wait for their turn to communicate.

Example: Telephone Network, Video Conferencing, etc.

Criteria for a Network

A network should meet the following criteria:

  • performance
    • transmit time
    • response time
  • reliability
  • security

Performance

The two major ways by which the performance of a network can be measured are: transit time and response time.

The amount of time required by a message to travel from one device to another is known as transit time.

The time elapsed between sending an inquiry and getting its response back is known as response time.

The performance of a network depends on several other factors like, number of users, the type of transmission medium, the capabilities of connected hardware and the efficiency of the software that manages the software.

Performance of a network is evaluated by two networking metrics. They are: throughput and delay. In real-time situations we need more throughput and delay.

Reliability

There are several factors that determine network reliability. They are: accuracy of delivery, frequency of failure, the time taken by a link to recover from a failure and the robustness of the network in a catastrophe.

Security

Network security issues includes protecting the data from unauthorized access, protecting the data from damage and modifications and implementing policies and procedures for recovery of data from security breaches and data losses.

Types of Communication/Connection

For any communication to occur, two or more devices must be connected in some way to the same network at the same time. A link is a communication pathway that transfers data from one device to another. There are two different types of connections:

  • Point-to-Point Connection → dedicated link between 2 devices
  • Multipoint Connection → more than 2 devices share a single link

Point-to-Point Connection

In point-to-point connection there is a dedicated link between only two devices and the entire capacity of the link is used for transmission between those two devices.

Example: sharing files over bluetooth, connection between infrared remote control and television’s control system, etc.

Multipoint Connection

In a multipoint connection, also known as multidrop, more than two devices share a single link. In a multipoint environment, the capacity of the channel is shared in two ways: spatially or temporally (time-shares).

If several devices can use the same link simultaneously, then it known as spatially shared.

If the users can access the link in turns i.e. one user can communicate at a time in a connection, then it is temporally shared.

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