Communication Basics

The physical layer is the lowest layer of the computer network architecture. It carries raw bit streams over a physical transmission medium. To understand how it works, you first need the basic concepts of communication.

1. Channel: the “road” for Data

Types of Channel

By Transmission Medium

A wired channel behaves like a motorway:

• Twisted pair: like an ordinary road, cheap but with a limited speed (telephone lines, network cables)
• Optical fibre: like high-speed rail, extremely fast and highly resistant to interference
• Coaxial cable: like an expressway, between twisted pair and optical fibre

A wireless channel behaves like an air route:

• Radio waves: broadcasting, television signals
• Microwaves: satellite communication, radar
• Infrared: remote controls, short-range communication

By Transmission Direction

This is the difference between a one-way road and a two-way road:

Characteristics of a Channel

Every channel has its own character, much as every road has its own properties:

1. Bandwidth: like the width of the road, it decides how many vehicles can pass
2. Transmission distance: like the length of the road, it affects signal attenuation
3. Interference resistance: like the quality of the road, it affects transmission stability
4. Delay: like the traffic conditions, it affects arrival time

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2. Signal: the “carrier” of Data

Analog Signals vs Digital Signals

These are two different ways of transporting data:

Analog Signals: a Continuous Curve

Characteristics:

• Vary continuously, smooth like a wave
• Can take infinitely many values
• Easily affected by interference

Everyday examples:

• 🎵 The audio signal of a traditional radio
• 📺 The antenna signal of an old television
• 🌡️ A thermometer reading (a continuous value)

Digital Signals: Stepping Stairs

Characteristics:

• Vary discretely, jumping like a staircase
• Can take only a finite number of values (usually 0 and 1)
• Highly resistant to interference

Everyday examples:

• 💻 Data inside a computer
• 📱 A text message sent from a phone
• 💡 The state of a switch (on/off)

Why Do Computer Networks Use Digital Signals?

Converting Signals

Sometimes an analog signal has to be converted into a digital signal, or the other way round:

Analog → digital (sampling, quantisation, encoding) is done by an ADC (analog-to-digital converter); digital → analog is done by a DAC (digital-to-analog converter).

Practical examples:

• 🎤 Recording: sound (analog) → MP3 file (digital)
• 🎧 Listening to music: MP3 file (digital) → sound (analog)
• 📷 Taking a photograph: light (analog) → image file (digital)

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3. Bandwidth: the “ceiling” on Transmission Capability

Two Meanings of Bandwidth

Physical Bandwidth (Frequency Bandwidth)

Unit: Hz (hertz)

Meaning: the range of frequencies the channel can pass

Examples:

• Telephone line: 300-3400 Hz, a bandwidth of about 3.1 kHz
• FM broadcast: 88-108 MHz, a bandwidth of 20 MHz
• WiFi 2.4GHz: 2400-2483.5 MHz, a bandwidth of 83.5 MHz

Data Bandwidth (Transmission Rate)

Unit: bps (bits per second)

Meaning: the amount of data that can be transmitted per second

Examples:

• Dial-up: 56 kbps
• Home broadband: 100 Mbps
• Fibre broadband: 1000 Mbps (1 Gbps)

The Relationship Between Bandwidth and Transmission Rate

What Bandwidth Means in Practice

Everyday analogy:

Practical impact:

• 📶 At 10 Mbps: downloading a 1GB film ≈ 13 minutes
• 📶 At 100 Mbps: downloading a 1GB film ≈ 1.3 minutes
• 📶 At 1000 Mbps: downloading a 1GB film ≈ 8 seconds

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4. Symbol, Baud, and Rate: the “units” of Transmission

These three concepts are easily confused, so take them one at a time.

Symbol: the “atom” of a Signal

In plain terms:

• One high-to-low or low-to-high change in a digital signal is one symbol
• One symbol can carry one bit (0 or 1)
• It can also carry several bits (for example 00, 01, 10, 11)

Examples:

• Binary symbols: only two states, 1 symbol = 1 bit

  High level = 1
  Low level = 0

• Quaternary symbols: four states, 1 symbol = 2 bits

  High high = 11
  High low = 10
  Low high = 01
  Low low = 00

Baud Rate: the “speed” of Symbols

Unit: Baud

Calculation: Baud rate = number of symbols / seconds

Examples:

• A baud rate of 1200 Baud = 1200 symbols transmitted per second
• A baud rate of 2400 Baud = 2400 symbols transmitted per second

Bit Rate: the “speed” of Data

Unit: bps (bits per second)

The important relationship: Bit rate = baud rate × number of bits per symbol

Worked Examples

Comparison table:

The underlying rule: $n = \log_2 M$

where:

• $n$ = number of bits per symbol
• $M$ = number of symbol states

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5. Source and Sink: the “Start and End” of Communication

Source: the “Origin” of Information

Common sources:

• 📱 A phone: sends text messages and photos
• 💻 A computer: sends files and email
• 🎤 A microphone: captures sound
• 📹 A camera: captures video
• 🌡️ A sensor: captures temperature, humidity, and similar data

Sink: the “Destination” of Information

Common sinks:

• 📱 A phone: receives text messages and photos
• 💻 A computer: receives files and email
• 🔊 A speaker: plays back sound
• 🖥️ A monitor: displays images
• 📊 A database: stores data

The Communication Model

A complete communication process is like the movement of a parcel:

The role of each stage:

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6. The Complete Communication System Model

A complete communication system has several stages, just as a complete logistics system does:

Details of Each Part

1. Source

Role: produces the original information

Examples:

• A person speaking
• A computer generating a file
• A sensor collecting data

2. Transmitter

Role: converts the information into signals suited to transmission

Functions:

• Encoding: encodes the data into a digital signal
• Modulation: modulates the digital signal onto a carrier
• Amplification: increases the signal power

Examples:

• The transmit circuit of a phone
• The transmit module of a router
• The transmitter of a radio station

3. Channel

Role: the medium over which the signals travel

Types:

• Wired: twisted pair, optical fibre, coaxial cable
• Wireless: radio, microwave, infrared

Characteristics:

• Bandwidth limits
• Attenuation
• Delay
• Noise

4. Receiver

Role: turns the signals back into information

Functions:

• Demodulation: extracts the signal from the carrier
• Decoding: turns the signal back into data
• Filtering: removes noise interference

Examples:

• The receive circuit of a phone
• The receive module of a router
• The receive circuit of a radio

5. Sink

Role: receives and processes the information

Examples:

• A person hearing a voice
• A computer storing a file
• A monitor displaying an image

6. Noise Source

Types of noise:

Everyday analogies:

• 🚗 Making a call on a noisy street (environmental noise)
• 📻 Poor reception in the mountains (attenuation)
• 📱 No signal in a lift (shielding)

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Practical Examples

Example 1: Making a Phone Call

The complete communication process of a phone call:

1. Source: you speak (a sound signal, an analog signal)
2. Transmitting device: the microphone turns the sound into an electrical signal
3. Encoding: the phone converts the analog signal into a digital signal
4. Modulation: the digital signal is modulated onto radio waves
5. Channel: it travels over base stations and the fibre network
6. Demodulation: the other phone extracts the signal from the radio waves
7. Decoding: the digital signal is converted back into an analog signal
8. Receiving device: the speaker turns the electrical signal into sound
9. Sink: the other person hears your voice

Example 2: Browsing the Web

1. Source: you click a link
2. Transmitting device: the computer encodes the request into a digital signal
3. Channel: it travels over the network cable / WiFi
4. Relaying: it passes through several routers
5. Receiving device: the server receives the request
6. Processing: the server processes the request
7. Source: the server returns the page data
8. Channel: it travels back over the network
9. Receiving device: the computer receives the data
10. Sink: the browser displays the page

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Exercises

Exercise 1

What is a channel, and how can channels be classified?

Exercise 2

What is the difference between an analog signal and a digital signal, and why do computer networks mainly use digital signals?

Exercise 3

What is the relationship between baud rate and bit rate? If a system runs at 2400 Baud and uses quaternary transmission, what is the bit rate?

Exercise 4

Explain the role of each part of the communication system model, and give an example.