Physical Layer: Computer Networks Class Notes

Updated: Aug 18

Mobiprep has created last-minute notes for all topics of Computer networks to help you with the revision of concepts for your university examinations. So let’s get started with the lecture notes on Computer networks.

  1. Computer Networks - Basics

  2. Network Devices

  3. Network Models

  4. Physical Layer

  5. Network Layer

  6. Transport Layer and Congestion control

  7. Application Layer

  8. Web Security

  9. Email and IP Security

Our team has curated a list of the most important questions asked in universities such as DU, DTU, VIT, SRM, IP, Pune University, Manipal University, and many more. The questions are created from the previous year's question papers of colleges and universities.

  1. what are the functions of physical layer?

  2. what are the different transmission media used for communication?

  3. explain various propogation modes.

  4. what are the different modes of data transmission?

  5. explain the causes of tranmission impairment?

  6. what are different types of noises responsible for transmission impairment?

  7. what is snr?

  8. what are data rates and what does it depends upon?

  9. what is shannon capacity?

Physical Layer

Question 1) What are the functions of physical layer?

Answer) The physical layer is responsible for the movement of data bits from one hop to another. The data in physical layer is in the form of bits (0 or 1). These data bits are encoded into signals (electrical or optical) for sending the bits through the transmission medium.

The following are the functions of physical layer:

  1. The physical layer decides the network topology. i.e. it decides how the devices are connected to each other in the network.

  2. It defines the mode of data transmission (simplex, half-duplex or full duplex)

  3. The physical layer encodes the data bits into signals for transmission over the medium. The physical layer defines the type of encoding. i.e. it decides whether the data bits have to be converted into electrical or optical signals.

  4. The physical layer is responsible for synchronization of the sender and receiver. It also decides the rate of data transmission.

  5. The physical layer connects the network devices to the transmission medium.


Question 2) What are the different transmission media used for communication?


Types of transmission media in computer networks class notes
Types of transmission media


Guided transmission is also known as wired transmission. Wired transmission media is usually used for short distance communication. It makes communication fast and secure. It is much expensive. The different types of guided media are explained below:


The twisted pair cable consists of two cables twisted to avoid interference or noise. It is used in telephone cables. The twisted pair cable is of two types: Unshielded and Shielded.

 twisted pair cable  in computer networks class notes
Twisted pair cable

1. Unshielded twisted pair (UTP)

The UTP cable does not contain a physical shield. It has the ability to block internal interference. But, it is susceptible to external interference. It is less expensive and has less capacity when compared to STP.

 Unshielded twisted pair (UTP) in computer networks class notes
Unshielded twisted pair (UTP)

2. Shielded twisted pair(STP)

The STP has a metal shield which helps in eliminating cross talk and external interference. It is immune to noise. STP cables are more expensive than UTP cables.

Shielded twisted pair(STP) in computer networks class notes
Shielded twisted pair(STP)


The coaxial cable consists of a central core or conductor which is surrounded by an outer plastic covering. It has two parallel conductors that are insulated separately. It is used in TV cables.

coaxial cable in computer networks class notes
Coaxial cable


The optical fiber transmits data in the form of light. The data is transmitted by the total internal reflection of light through the glass core of the cable. The core is surrounded by an insulation layer which is made up of glass or plastic. This layer is called cladding.

The optical fiber cables have a very large bandwidth and capacity. It is used for long distance transmission. It is immune to corrosion and interference.

optical fiber in computer networks class notes
Optical fiber


The unguided media refers to wireless transmission media. It does not require a physical medium for data transmission. It is less secure. The different types of unguided transmission media are given below:


The radio waves have a frequency range of about 3kHz – 1GHz. The radio waves are used for long distance transmission. The radio waves have the ability to penetrate through the objects or obstacles. It is used in FM radios and cordless phones.


The infrared waves have a frequency range of about 300GHz – 400THz. It is usually used for short distance transmission as it cannot penetrate through the obstacles. It is used in TV remotes, wireless mouse etc.


The microwaves have the frequency range of about 1GHz – 300GHz. It is used for line of sight transmission. It is used in mobile communication.


Question 3) Explain various propagation modes.

Answer) The mode of propagation of electromagnetic waves is classified into three types. They are:

a. Line of Sight propagation

In line of sight propagation, the electromagnetic waves travel in a straight line from source to destination. The EM waves travel in a direct path between the source and destination.

As the waves travel only upto a distance of sight, this mode of propagation is used for short distance communication. If there is any obstacle in the path, the transmission will not be smooth. Infrared waves or microwaves are used for line of sight transmission.

line of sight propagation in computer networks class notes
Line of sight (LOS) propagation

b. Ground wave propagation

Ground wave propagation is also called direct wave propagation. The ground waves follow the contour or curvature of the earth. The space between the earth’s surface and the ionosphere is used for ground wave propagation.

Ground wave propagation has a longer range than the line of sight propagation. It can penetrate through the obstacles on its transmission path.

Ground wave propagation in computer networks class notes
Ground wave propagation

c. Sky wave propagation

The sky wave propagation is used for long distance communication. Here, the radio waves transmitted from the surface of the earth are reflected back by the ionized topmost layer of the ionosphere and then directed towards the receiving antenna.

The signal can have any number of hops between the earth’s surface and ionosphere. Hence, sky wave propagation is not limited by the curvature of the earth.

 sky wave propagation in computer networks class notes
Sky wave propagation (2 to 30MHz)


Question 4) What are the different modes of data transmission?

Answer) The transmission modes are of two types: Parallel transmission and Serial transmission.

Data Transmission Modes in computer networks class notes
Data Transmission Modes

Parallel Transmission

In parallel transmission, several bits are transmitted simultaneously. For transmitting ‘n’ bits at a time, ‘n’ transmission channels are required. So, parallel transmission is much expensive. It enables fast transmission of data.

Serial Transmission

In serial transmission, one bit is transmitted at a time. i.e. the bits are transmitted one after the other in a serial fashion. Here, the bits are transmitted over a single channel. It is less expensive and slower than parallel transmission of data.


Question 5) Explain the causes of transmission impairment?

Answer) Practically, the transmission media are not perfect. This imperfection of the transmission media causes the deterioration of the signal passing through it. The signal at the beginning of the transmission is not same as the signal at the end of transmission. This is called transmission impairment. The causes of transmission impairment are given below:

a. Attenuation

Attenuation represents the decrease in signal strength. The signal strength decreases due to loss of energy over time. As long as the signal passes through the transmission medium, the signal loses energy gradually. So, the signal must be amplified at regular intervals of time during transmission.

The unit of attenuation is decibels (dB). Attenuation is given by the following formula:

Attenuation (in dB) = 10log10(P2/P1)

b. Distortion

Distortion represents the change in the shape of the signal. Distortion causes change in the relationship between different frequency components of the signal. Distortion leads to signal degradation.

c. Noise

Any unwanted signal is called noise. As the signal travels through the transmission media, some unwanted signals get mixed up with the original signal. The noise signal causes transmission impairment.


Question 6) What are different types of noises responsible for transmission impairment?

Answer) The different types of noises that are responsible for transmission impairment are:

a. Thermal noise

Thermal noise is caused by the random motion of electrons in the conductor due to thermal agitation. This causes distortion in the original message signal by adding some unwanted extra signal component to it. The thermal noise is also known as white noise.

b. Induced noise

Induced noise is the noise generated by some other circuit which adds up with the original message signal sent by the transmitter. The noise generated by motors, or by other external appliances is called induced noise.

c. Crosstalk

Crosstalk is caused by the interference of signal in one wire with the signal in another wire (or circuit). It is caused by an unwanted coupling of two wires. Here, one circuit causes an undesired effect on another circuit.

d. Impulse noise

An impulse is a signal with very high energy and a very short period. Such kind of spikes in the original message signal is called impulse noise. It introduces bit errors in the message signal.


Question 7) What is snr?

Answer) The Signal to Noise Ratio (SNR) is the ratio of signal power to noise power. The SNR is used to measure the signal strength with respect to the background noise. The unit of SNR is decibels.

SNR= Signal PowerNoise Power


Question 8) What are data rates and what does it depends upon?

Answer) Data rate represents the speed of transmission of data. It refers to the amount of data transmitted over a channel in a given time. It is measured in bits/sec.

The data rate depends on:

1. Channel bandwidth

Channel bandwidth represents the maximum amount of data that can be transmitted through the channel. So, data rate of the channel depends upon the channel bandwidth.

2. Channel quality

If the channel is noisy, then the bit rate of the channel is low. Hence, data rate depends on the noise immunity of the channel.

3. Number of levels in digital signal

The number of levels in digital signal decides the number of data bits transmitted over the channel per unit time.


Question 9) What is shannon capacity?

Answer) The Shannon capacity is used to determine the maximum data rate of the given channel. It is used to find out the theoretical limit of data rate that can be achieved. The Shannon capacity of a channel can be determined using the formula given below:

Capacity, C=BW+ log2(1+SNR)

Where, BW is the bandwidth of the channel, and SNR is the Signal to Noise Ratio of the channel.

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