Data Communication

Written and reviewed by Rohit Singh, Computer Science & Commerce faculty at UniDrill (MCA, Osmania University · 6 years teaching). Based on NCERT Computer Science Class XII, Chapter 11.

How does a message reach a friend you have never wired to?

You tap Send and a photo appears on a phone in another city a second later. Nobody laid a private wire between the two phones. Instead, your photo was chopped up, passed through towers, cables and routers that follow shared rules, and rebuilt at the other end. This chapter explains each part of that journey: who sends, what carries it, how fast, in which direction, by which route and under which rules.

The five components

Data means text, images, audio, video and other multimedia. Communication means sending or receiving data. So data communication is the exchange of data between two or more connected devices that can send and receive over a communication medium. Every such exchange has exactly five components.

In computer networks the sender and the receiver are both called nodes. A router, a switch or a modem is network equipment, but it is not one of the five components.

How big is the pipe? Bandwidth and data transfer rate

The medium is also called a channel, and its capacity is measured in two ways that students often mix up.

Bandwidth is the difference between the highest and the lowest frequency in a composite signal:

$$\text{Bandwidth} = f_{\max} - f_{\min}$$

The units of bandwidth go up in thousands: $1\ \text{KHz} = 1000\ \text{Hz}$ and $1\ \text{MHz} = 1000\ \text{KHz} = 1000000\ \text{Hz}$. The units of data transfer rate go up in steps of 1024:

Watch the capital letters: Mbps is megabits per second, and MBps is megabytes per second. One byte is 8 bits.

Worked example: bandwidth of a signal. A composite signal contains frequencies between 500 MHz and 1 GHz. Find its bandwidth.

The common wrong step is adding the frequencies (1500 MHz), or subtracting 500 from 1 without converting the unit.

Worked example: required data rate. A user uploads 10 pages in 20 seconds. One page has 1600 characters and each character is 8 bits. What data rate does the channel need?

The common wrong step is forgetting that a character is 8 bits and dividing the characters, 16000 / 20 = 800, by mistake.

Worked example: unit conversion. Convert 18 Gbps to bits per second. Since $1\ \text{Gbps} = 2^{30}\ \text{bps} = 1073741824\ \text{bps}$, we get $18 \times 1073741824 = 19327352832\ \text{bps}$. Using 10⁹ instead of $2^{30}$ would give 18000000000 and be wrong for this chapter.

Beyond the textbook. For the exam: a 100 Mbps connection means 100 megabits per second, and 1 Gbps = 1024 Mbps. In reality: internet providers quote speed in decimal megabits, and a file size is shown in bytes. Divide by 8 to compare: a 100 Mbps plan can deliver at most about 12.5 MBps, and real downloads are slower because of protocol overhead and congestion.

Direction of communication: simplex, half duplex, full duplex

A road gives the best picture. Simplex is a one way street, half duplex is a narrow bridge where traffic can go either way but only one direction at a time, and full duplex is a two way road.

Full duplex shares the capacity of the link between the two directions. It is built either with two separate simplex lines (one to send, one to receive) or by dividing the capacity of a single channel.

Pause and predict: An optical fibre carries signals in only one direction. How can a phone call over fibre still be full duplex?

Check your answer: By using two fibres, one for each direction. This is exactly the "two separate simplex lines" method, and it is why two cables are needed for full duplex over optical fibre.

Switching: how data finds its route

Joining every pair of devices with a dedicated link (a mesh) or every device to a central one (a star) becomes very costly in a large network. Switching routes the data through intermediate nodes and forms a temporary route instead.

Each packet has two parts: a header with the destination address and other control information, and the main message part.

Beyond the textbook. For the exam: a telephone call is the example of circuit switching, and the Internet is the example of packet switching. In reality: many voice calls today are carried as packets as well. Voice over Long Term Evolution (VoLTE) on 4G networks and Voice over Internet Protocol (VoIP) apps send speech as small packets over the same shared network as data.

Transmission media

A transmission medium is anything that carries signals between the source (transmitter) and the destination (receiver). In guided (wired) media, the signal travels along a physical cable. In unguided (wireless) media, the signal travels through the air as electromagnetic waves using an antenna.

The classification in one line: Communication media → Wired (twisted pair, coaxial cable, optical fibre) or Wireless (radio waves, microwaves, infrared waves).

Wired media.

Wireless media.

The electromagnetic spectrum available for wireless communication runs from 3 KHz to 900 THz and is divided into radio waves, microwaves, infrared waves and visible light waves. The first three are useful for communication.

Signals weaken with distance, so repeaters are installed to regenerate them with the same energy.

Pause and predict: Your TV remote works from the sofa but not from the next room, while an FM radio plays in every room. Which waves explain each?

Check your answer: The remote uses infrared waves, which cannot penetrate walls. The radio uses radio waves, which are omnidirectional and can pass through walls.

Wireless technologies

Bluetooth is a low cost, short range technology that links mobile phones, mouse, headphones, keyboards, printers and computers with no cable. It uses the unlicensed 2.4 GHz band, covers about 10 metres and works at 1 to 2 Mbps. Devices within 10 metres form a personal area network called a piconet, working in a master slave arrangement.

Wireless LAN (Wi-Fi) is a local area network with the IEEE standard number 802.11. Laptops and phones connect through an access point (AP), a device that creates the wireless network by connecting to a wired router, switch or hub. Its benefits are that it can extend or replace wired infrastructure, it gives mobile devices more access, and it gives easy Internet access in public places. WiMax uses a larger spectrum and covers a larger area than Wi-Fi, so it suits metropolitan networks.

Beyond the textbook. For the exam: Bluetooth uses the unlicensed 2.4 GHz band. In reality: many Wi-Fi routers also work in the 2.4 GHz band, so Bluetooth devices and Wi-Fi can interfere with each other and slow down when both are busy nearby. That is why newer routers also offer a second, higher frequency band.

Mobile telephone generations

Each generation changed what the network could carry.

M2M means devices talking directly to each other with no human in between.

Protocols: the rules of the conversation

A protocol is a set of standard rules that the sender, the receiver and all the devices in between must follow. They are needed because networks differ in speed, format and location.

Worked example: who does what in TCP/IP? A file is sent across the Internet and reaches the receiver in the right order although the packets arrived by different routes. Which protocol gave the destination address, and which fixed the order?

The common wrong step is to say IP orders the packets. IP identifies and addresses nodes, TCP delivers and sequences the packets.

Exam traps

60-second revision

Where this chapter connects