Physical Layer in OSI Model

The physical Layer is the bottom-most layer in the Open System Interconnection (OSI) Model which is a physical and electrical representation of the system. It consists of various network components such as power plugs, connectors, receivers, cable types, etc. The physical layer sends data bits from one device(s) (like a computer) to another device(s).

The physical Layer defines the types of encoding (that is how the 0's and 1's are encoded in a signal). It is responsible for the communication of the unstructured raw data streams over a physical medium.

Functions Performed by Physical Layer

The Physical Layer is responsible for sending raw data as bits over a physical medium. It converts data into signals that can travel through wires, fiber optics, or wireless channels (encoding) and turns these signals back into data at the receiver (decoding). It ensures signals are transmitted correctly and uses techniques like modulation to prepare the data for transmission and demodulation to retrieve it at the other end. This layer also decides how data flows (one-way, two-way alternately, or simultaneously) through transmission modes and controls the speed and timing of data transmission to keep everything running smoothly.

To read in detail about services offered by this layer, refer to Physical Layer Services.

Physical Topologies

Physical topologies describe the physical arrangement of devices and cables in a network. Let's take a look into different types of physical topologies :

Read more about Types of Network Topology.

Line Configuration 

• Point-to-Point configuration: In Point-to-Point configuration, there is a line (link) that is fully dedicated to carrying the data between two devices.
• Multi-Point configuration: In a Multi-Point configuration, there is a line (link) through which multiple devices are connected.

Modes of Transmission Medium

• Simplex mode: In this mode, out of two devices, only one device can transmit the data, and the other device can only receive the data.
  • Example- Input from keyboards, monitors, TV broadcasting, Radio broadcasting, etc.

• Half Duplex mode: In this mode, out of two devices, both devices can send and receive the data but only one at a time not simultaneously.
  • Examples- Walkie-Talkie, Railway Track, etc.

• Full-Duplex mode: In this mode, both devices can send and receive the data simultaneously.
  • Examples- Telephone Systems, Chatting applications, etc.

Read more about advantages, disadvantages and differences between simplex, half duplex and full duplex mode.

Protocols in Physical Layer

Typically, a combination of hardware and software programming makes up the physical layer. It consists of several protocols that control data transmissions on a network. The following are some examples of Layer 1 protocols:

• Ethernet (IEEE 802.3) – Widely used for wired networks.
• Wi-Fi (IEEE 802.11) – For wireless communication.
• Bluetooth (IEEE 802.15.1) – Short-range wireless communication.
• USB (Universal Serial Bus) – For connecting devices over short distances.

Advantages of the Physical Layer

• It ensures devices can transmit and receive raw data over physical mediums.
• It provides universal standards for cables, connectors, and signaling, ensuring compatibility.
• Support for Various Media: Works with wired (e.g., Ethernet) and wireless (e.g., Wi-Fi) technologies.

Limitations of the Physical Layer

• No Error Handling: Cannot detect or correct errors in data transmission.
• Susceptible to Physical Damage: Cables, connectors, and hardware failures can disrupt communication.
• No Data Interpretation: It only transmits bits and doesn't understand or process the actual data.