Physical Layer in OSI Model

Physical Layer in OSI Model

Explore the significance of the physical layer in OSI model. Know its functions, providing insights into its role in network communication and data...

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What is the physical layer in the OSI model? To answer that question, we have to understand what OSI is first.

The Open System Interconnection (OSI) model can be defined as a conceptual framework consisting of seven layers used by computer systems for communication and interoperability. The OSI model consists of seven layers, with each layer having a unique functionality and following a set of rules pre-defined to allow interoperability between different products and services in the network. These layers work collectively to transmit data from one person to another from anywhere across the globe. 

Layer 1 is called the physical layer in the OSI model. It is the first level connection between the devices and provides hardware and connectivity support to the whole network. To go into depth, we will require a better understanding of the whole model and understand the function of the physical layer in the OSI model.

What is the Physical Layer in OSI Model? 

Regarding network security and hardware support, the physical layer in OSI model is the basic level for the whole network. It identifies the equipment, including the wires, devices, frequencies, and pulses, required to connect between computers. The information is stored in bits and is transferred between devices through the nodes in this physical layer. 

Now that you understand the answer to questions like what is the physical layer in the OSI model?, you will also have to look at the significance it holds in the security of the whole network. The physical layer is required for network hardware visibility. The existing breed of software solutions often ignores Layer 1 in the OSI model. The lack of identifiability of the layer 1 devices may cause rogue devices to get implanted in the hardware and pose a security threat to the whole network. The physical layer identifies the devices and eliminates such bad actors. The layer also consists of a separate security procedure to ensure network safety. 

Functions of OSI Model in Physical Layer

1. Representation of Bits 

The physical layer in OSI model (Layer 1) takes the responsibility of transmitting individual bits from one node to another via a physical medium. It specifies the procedure for encoding bits, such as how many volts should represent a 0 bit and a 1 bit in the case of electrical signals.

2. Data Rate 

The data rate is maintained by the function of Physical Layer in OSI model. The number of bits sent per second is referred to as the data rate. It is determined by a variety of factors, including:

  • Bandwidth: The physical constraint of the underlying media.
  • Encoding: The number of levels used for signaling.
  • Error rate: Incorrect information reception due to noise.

3. Synchronization 

The function of physical layer in OSI model includes bit synchronization. The sender and receiver are bit-synchronized. This is accomplished by including a clock. This clock is in charge of both the sender and the receiver. Synchronization is achieved at the bit level in this manner.

4. Interface 

The transmission interface between devices and the transmission medium is defined by the physical layer in OSI model. PPP, ATM, and Ethernet are the three most commonly used frames on the physical interface. When considering the standards, it is common, but not required, that the physical layer be divided into two:

  • Physical Medium (PM) layer: The physical layer’s lowest sublayer.
  • Transmission Convergence (TC) layer: The high sublayer of the physical layer.

5. Line Configuration 

The function of physical layer in OSI models includes connecting devices to the medium or line configuration. Line configuration, also known as a connection, is the method by which two or more devices are connected to a link. A dedicated link connects two devices in a point-to-point configuration. A device can be a computer, a printer, or any device capable to send and receive data.

6. Topologies 

The physical layer in OSI model specifies how different computing devices in a network should be connected to one another. A network topology is a configuration by which computer systems or network devices are linked to one another. Topologies can define both the physical and logical aspects of a network. Mesh, Star, Ring, and Bus topologies are required for device connectivity.

7. Transmission Modes 

The physical layer in OSI model specifies the transmission direction between two devices. Transmission mode refers to the method that is used to transfer data from a device to another device. The physical layer in the OSI model primarily determines the direction of data travel required to reach the receiver system or node. Transmission modes are classified into three types:

  • Simplex mode
  • Half-duplex mode
  • Full-duplex mode
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Physical Topologies: 

Now that we know the basics of the physical layer in the OSI model, it’s time to explore the relevant physical topologies: 

1. Mesh Topology

It is a highly secure device connection where every device is connected through links to every other device in the network. A dedicated point-to-point connection is present, which is complex to form. 

2. Star Topology

In this kind of device connection, all the devices are connected with a dedicated point-to-point connection to a central hub. It is easy to install but has no fault tolerance.

3. Bus Topology

Here, all the devices are connected with a single backbone cable which is less costly and easily re-installable.

There are several other ways, too, through which the physical layer in OSI model works, such as the end-to-end configuration. 

4. Ring Topology

Here, a single device is connected to two separate devices, one from each side, forming a round ring-like structure. All devices are interconnected in a circular fashion and data is transmitted from one device to the other device in a single direction only. This transmission happens in the form of a ring. 

In case any fault takes place in any one device of this circular network, that particular device can easily be removed. This does not mean that other devices stop functioning, and so the entire system continues to function. If any error takes place in the transmission cable, then the entire network can be disturbed because all devices are connected to each other via that cable.

Point to Point Configuration

Only when two or more things are joined together, we say a connection has been established. One of the two types of connections is a point-to-point configuration, also known as P2P. It is a simple and effective connection between two devices. In this, two devices are connected to each other by a special link or cable and the link only serves to communicate data between the two devices.

Take the example of a telephone network, where communication can take place both back and forth but there are only two nodes involved, between which the link has been formed.

Multi-Point Configuration

Here, the channel or the cable being used for the data communication is not between just two devices, and serves for transmitting data between multiple devices. Numerous devices are connected to each other in this type of configuration. 

Take the example of a broadcast network where the sending device transmits a data packet which is received and interpreted by all the devices in the network. If the device receiving the data checks that the data is not meant for it, it simply discards the packet. When the data reaches the concerned device, that device will store the data and give an appropriate response back to the sending device.

Modes of Transmission Medium: 

Well, now we know ‘what is the physical layer in the OSI model?’. However, what are the common modes of data transmission? 

How data is transferred between two interconnected devices, the direction of transfer and the time when this transfer happens all pertain to the mode or medium of transmission.

Here are the 3 common modes of transmission:

  • Simplex mode: Here, data is transmitted from one point to another only in one direction. The data flows only from one device in a single direction and the other device simply receives the data that is coming from this direction. Take the example of a computer in which the input devices like the keyboard can only send a signal to the monitor, which receives the data and displays the output.
  • Half-duplex mode: Here, the two devices involved in the data communication process can send as well as receive data. This happens only one at a time and not both at the same time. What this means is that while there is a bi-directional flow of data, data does not flow from both devices simultaneously. Take the example of a walkie talkie where signals are being sent as well as received by the same device, but both sending and receiving doesn’t happen at the same time.
  • Full-duplex mode: Here, the communication between two devices happens bi-directionally and both devices can send data or receive data at the same time. Two channels exist for this purpose as both sending and receiving can happen simultaneously. The most common example that you will see of a full-duplex mode is a mobile phone, where you can talk to someone and hear them at the same time.

What are the Layers in the OSI Model? 

An OSI model is made up of seven distinct layers that are typically described from top to bottom. The 7 layers in the OSI model are: application, presentation, session, transport, network, data link, and physical. These layers represent what happens within a networking system visually. Understanding the OSI model can assist in determining the source of networking issues, developing applications, and better understanding which networking products work with which layers. Each layer of the OSI Model is responsible for a specific function and communicates with the layers above and below it. DDoS attacks target specific network layers; application layer attacks target layer 7, and protocol layer attacks target layers 3 and 4.

1. Physical Layer 

In the Open System Interconnection (OSI) Model, the Physical Layer is the lowest layer. The physical layer in OSI model is in charge of transmitting data from one computer to another. It is not concerned with the data of these bits but rather with the establishment of a physical connection to the network. It interacts with actual hardware as well as signaling mechanisms.

2. Data Link Layer

The second layer of the OSI model is the data link layer. It is also known as layer 2. The data link layer controls the delivery of messages from node to node. The main goal of this layer is to ensure error-free data transfer from one node to another across the physical layer. The data link layer conceals the underlying hardware details and represents itself as the communication medium to the upper layer.

3. Network Layer

Network Layer
Source: Freepik

In the physical layer in OSI model, the network layer is the third layer. It serves two primary purposes. The “network layer” of the Internet communications process is where these connections are made by sending data packets back and forth between different networks. Furthermore, the network layer defines an addressing scheme in order to uniquely identify each device on the internetwork.

4. Transport 

Layer 4 of the OSI Model, known as the transport layer, provides transparent data transfer between end users while also providing reliable data transfer services to the upper layers. The transport layer is in charge of delivering an entire message from a source device application program to a destination device application program.

5. Session 

The Session Layer is the layer of the ISO Open Systems Interconnection (OSI) model that governs computer dialogues (connections). It is in charge of establishing, maintaining, synchronizing, and terminating sessions between end-user applications. It makes use of the transport layer’s services, allowing applications to establish and maintain sessions as well as synchronize them.

6. Presentation

The sixth layer in the Open System Interconnection (OSI) model is the Presentation Layer. It ensures that the message is delivered to the upper layer in a consistent format. It is concerned with the syntax and semantics of the messages. The data received from the Application Layer is extracted and manipulated in this layer so that it can be transmitted over the network.

7. Application 

Application Layer
Source: Freepik

The topmost layer in the Open System Interconnection (OSI) model is the Application Layer. The Application Layer contains a number of protocols that users frequently require. This layer also requests various types of information from its bottom layer, which is the presentation layer. This layer serves as the foundation for email forwarding and storage.


The physical layer in OSI model of the interconnectivity of devices has helped secure and seamless data transfer over devices and keeps the application services connected. It is all possible because of the seven layers that it consists of. The ground layer, the physical layer, provides all the hardware connections to the network and ensures that the next processes can occur without error. So the function of the physical layer in the OSI model is pretty significant, making it important for security and efficiency reasons.

Frequently Asked Questions (FAQs)

1. What does the Physical Layer Mean? 

Physical layer in OSI model is the OSI reference model’s lowest layer. It is in charge of transmitting data from one computer to another. It specifies the equipment needed to connect computers, such as wires, devices, frequencies, and pulses. For network hardware visibility, the physical layer is required.

2. What are the Physical Layer Protocols? 

A physical layer protocol is a set of rules that governs data transmission between computers in a network. Fiber cables, Integrated Services Digital networks, Ethernet, Universal Serial Bus (USB), Bluetooth, and other physical layer protocols are examples.

3. What Devices use a Physical Layer? 

Repeaters, hubs, network interface cards (NICs), cables, and connectors are examples of physical layer devices. Repeaters are used to regenerate electrical signals that have been attenuated (that is, weakened) due to distance. A hub is a device that connects twisted pairs or optical fiber devices in a local area network (LAN).

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