In this tutorial, we will learn about first layer of ISO / OSI Model – Physical Layer Protocol. We will see it’s different properties, functions etc. Then, we will see different questions on physical layer protocol.

First layer in the ISO/OSI stack is the physical layer. Physical layer of the ISO/OSI stack deals with the electrical, mechanical, functional and procedural characteristics of physical links i.e. what are the types of links and how to convert the signals.

Example – If physical link is a copper wire, the message has to be converted to electrical signals since data can be transmitted only in the form of electrical signals in a copper wire. To transmit data through optical fibre link it has to be converted to the light signals. The wireless link can transmit only electromagnetic waves.

The functional properties of physical layer include:

1. Types of Transmission
2. Network Topology
3. Decoding and Encoding
4. Data Rate
5. Synchronization of bits

Let’s see these properties in detail.

1. Type of Transmissions

Physical layer deals with types of transmission i.e. how the data will transferred, whether it is only one way communication or both way communication etc.

Different transmission types can be –

1. Simplex transmission: Only one way of transmission is possible. Eg. TV Antennae
2. Half-duplex transmission: Data can flow in both directions but one at a time. Example: Walkie-talkie
3. Full Duplex transmission: Data can flow in both directions and the sender and receiver can receive data at any point of time. Example: Mobile phone

With the advancement in technology, simple transmission and half duplex transmission are not in much use these days. We have shifted to full duplex transmission mediums.

2. Network Topology

Physical layer also deals with the topologies of computer networks. The topologies used have been discussed in greater detail in the previous tutorials.

3. Decoding And Encoding

Physical layer is also responsible for decoding and encoding. There are two types of encoding techniques used:

1. Manchester Encoding
2. Differential Manchester Encoding

Manchester Encoding

In manchester encoding, bits are represented as below –

Manchester Encoding

Example:
Signal 1010 waveform using Manchester encoding –

Manchester Encoding Example

Differential Manchester Encoding

In Differential Manchester Encoding, bits are represented as below –

Differential Manchester Encoding

Example:
Signal 1010 waveform using differential Manchester encoding –

Differential Manchester Encoding Example

For encoding, Baud Rate = 2 * (Bit Rate)
Where, baud rate = no of voltage changes per second

Therefore, in order to send one bit, two voltages are sent, high and low voltage. Eg. If 10 bits are sent per second, 20 bauds are sent.

4. Data Rate

The transmission rate ( the number of bits sent per second), is also defined by physical layer.

5. Synchronization of bits

The sender and receiver must have synchronization of bits i.e. sender and receiver clocks must be synchronized.

Till now, we have seen what physical layer is, it’s different functions etc. . Now, we will see some questions / exercises on physical layer protocol.

Exercises on Physical Layer Protocol

The physical layer concerns with ____________ delivery.

Physical layer provides:
a) mechanical specifications of electrical connectors and cables 
b) electrical specification of transmission line signal level
c) specification for IR over optical fibre
d) all of the mentioned

The physical layer is responsible for 
a) line coding
b) channel coding
c) modulation
d) all of the mentioned

The physical layer translates logical communication requests from the ______ into hardware specific operations. 
a) Data link layer
b) Presentation Layer
c) Network layer
d) Application layer

Wireless transmission can be done via 
a) Radio waves
b) Microwaves
c) Infrared

That’s was all about Physical Layer protocol in ISO / OSI model. Now, we will see about Data Link Layer Protocol.