Physical Layer in OSI Model

In this tutorial, you will learn the basic concepts of the physical layer in the OSI model. After reading this tutorial, you will be able to understand why the physical layer is used in computer networks, how it works on the network, and what are the responsibilities of the physical layer.

Contents:

1. Purpose of the Physical Layer
2. How does Physical Layer Receive Frame?
3. Responsibilities of Physical Layer
4. Representation and Synchronization of Bits
5. Data Rate and Line Configuration
6. Physical Topology
7. Transmission Media and Transmission Modes
8. Physical Components

Purpose of the Physical Layer

The physical layer is one of the seven layers of the OSI reference model. The main purpose of the physical layer is to connect devices physically on a network through media for communication. The connection of media depends on the size and type of network.

• The physical layer is responsible for converting the frames received from the data link layer into bits. Because the machine understands the low-level (binary) language.
• The physical layer tells you in what format your data travels through a cable.

How does Physical Layer Receive Frame?

The physical layer receives the frame from the data link layer and converts it into bits. But how does the physical layer receive the frame and process it? Let’s figure it out.

The figure below explains the encapsulation process at the layers of the OSI model.

• As shown in the above figure, the client requests a webpage from the server and this process will run at an application layer. So, the application layer will transmit the data to the transport layer.
• As you can see in the figure, as soon as the data is received at the transport layer, the transport layer will add a TCP header to the data which creates a segment, this process is known as encapsulation.
• Similarly, when the TCP segment is received by the network layer, the network layer adds an IP header to the segment that makes up the packet and is sent to the data link layer.
• The data link layer will receive the packet and add an Ethernet header to the packet that makes up the frame. And then this frame will be sent to the physical layer and the physical layer will convert this frame into bits and transmit to the physical layer of the receiver.

Responsibilities of Physical Layer

Now, you’ve got some basic information about how the physical layer receives frames from the data link layer and converts it into a stream of bits. So, let’s explore the responsibilities of the physical layer.

The physical layer has responsibilities as follows:

1. Representation and Synchronization of Bits
2. Data Rate and Line Configuration
3. Physical Topology
4. Transmission Media and Transmission Modes

Representation and Synchronization of Bits

Bits are in the form of “0” or “1”. It is known as binary language. Network Hardware devices and media are used at the physical layer to generate and transmit signals that represent bits.

Physical Layer provides two functionalities for representation and synchronization of bits. They are Encoding and Signaling.

• Encoding: The encoding method converts a stream of bits into a group of bits to form a pattern that is understandable by both the sender and the receiver. In short, it represents digital information that is encoded on the sender side.
• Signaling: The physical layer uses two types of media that are wired and wireless media. Both these media generate signals to represent bits.
• Wired media represents bits in the form of electrical signals or light pulses, while wireless media represents bits in the form of microwaves, radio waves, or infrared.
• When bits are converted into signals, the physical layer itself defines which signal represents “1” and which signal represents “0”.
• The physical layer adds a synchronization clock to the data for the synchronization purpose. So that, when the encoded data is received by the receiver, the data can be decoded correctly by the receiver.

Data Rate and Line Configuration

The data rate and line configuration are also very important responsibilities of the physical layer. Because the data rate tells you about the data flow, and the line configuration tells you the type of network.

Data Rate: The data rate is a very important factor because it tells you how fast the sender sends bits per second over the communication channel to the receiver.

• The data rate is measured in terms of bandwidth and bandwidth is the capacity of how many bits per second can be transferred on a network.

Line Configuration: The line configuration is simply called the communication path that is established between the communication devices. Point-to-point and multipoint connections are two types of line configuration.

• Point-to-Point Network: It is a simple and easy network which creates a dedicated path between a sender and a receiver for communication purpose.
• Multipoint Network: As the name suggests, there is one sender and many receivers. So, a multipoint network creates a shared link between one sender and multiple receivers for communication purposes.

Physical Topology

Topology is a way of visualizing the network before it is physically implemented. Mainly 5 types of topologies are used. They are as follows:

1. Bus Topology
2. Star Topology
3. Ring Topology
4. Mesh Topology
5. Tree Topology

• Bus Topology: In this type of topology, communication devices are connected through a cable to share resources.
• Star Topology: In this type of topology, communication devices are interconnected using a central device hub that forms a point-to-point connection.
• Ring Topology: Communication devices connect with each other to form a network like a ring. In the ring topology, each device is connected to two devices.
• Mesh Topology: In this type of topology, each communicating device is connected to all other communicating devices.
• Tree Topology: Current computer networks mostly use tree topology. All communication devices are connected in a tree format which forms a hierarchical format.

The below diagram describes the 5 types of network topologies.

The above diagram shows that 5 types of topologies are used in a network. They are Bus, Star, Ring, Mesh, and Tree topologies. It also shows how these topologies are used to connect devices on the network.

Transmission Media and Transmission Modes

For communication, a device must be connected to other devices on the network via media. Communication between devices cannot take place if the devices are not physically connected. So the physical layer provides the transmission media and transmission modes by which devices can connect and interact with each other.

Transmission Media: Transmission media provide a physical way to connect devices on a network. There are two types of transmission media that are wired and wireless.

• Wired (Guided) Media: Connect communication devices using cables and create physical paths between communication devices.
• Twisted pair cables, coaxial cables, and fiber-optics are categories of wired or guided media.
• Wireless (Unguided) Media: A communication device does not require a physical path to communicate with another device on a communication channel.
• Radio waves, microwaves, and infrared are categories of wireless or unguided media.

Transmission Modes: The physical layer provides three transmission modes to the media for successful communication between devices. In short, the transmission mode is used to define the mode of transmission to transmit data over a communication channel. Simplex mode, half-duplex, and full-duplex are the transmission modes.

• Simplex mode: This mode only provides unidirectional communication, so that data can be transmitted from only one device and one device can receive it. Television broadcasting and Radio broadcasting are common examples of Simplex mode.
• Half-duplex: In this mode, both sender and receiver can send or receive data but not at the same time. An example of a half-duplex is the walkie-talkie.
• Full-duplex: It provides bidirectional communication so that both the sender and receiver can send or receive data simultaneously. Telephone calls are an example of full-duplex mode.

The figure below describes the types of transmission media and transmission modes.

As you can see in the above figure, the physical layer provides the transmission media and transmission mode for communication. The diagram also shows that guided and unguided media are two types of transmission media that are further divided into different categories. Similarly, the transmission mode can be simplex, half-duplex, or full-duplex.

Physical Components

The physical layer provides physical components such as NICs (Network Interface Cards), cables, interfaces, etc., to represent the bits on the network.

• Every device has NIC (Network Interface Card), which is very important hardware for a device. Because by using NIC, a device can physically connect to the network.
• There are two types of NICs, Wired NIC, and Wireless NIC. If a device has a wired NIC then wired media will be used to connect the device to the network, and if the device has a wireless NIC then wireless media will be used. But where will this NIC connect in the network? So the answer is the intermediary device port.
• All intermediary devices have ports that help end devices to connect their NICs to ports.