What is the physical layer in the OSI model?

The physical layer is the first and, therefore, the bottommost layer of the OSI model. It allows for physical connections to be created, monitored and deactivated.

The OSI model is a reference model that defines standards for communication between two systems. The aim of this model is to allow different hardware and software to interact. Development of the reference model began in 1977, and it was published for the first time in 1983. It has seven different layers that are layered one on top of the other, with each layer having its own clearly defined tasks.

For any type of data processing or transfer to be a success, both the sender and the receiver must work according the OSI model's rules. The model’s first and bottom layer is the physical layer. The layers are ordered as follows:

1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Presentation layer
7. Application layer

The main task of the physical layer is to allow for a physical connection between two units within a network. It ensures that connections can be created and terminated. Additionally, it facilitates the monitoring of connections while data is being transferred. The most important elements are bits, which are the smallest units of information that are transferred.

Alongside the actual transfer, the physical layer also regulates the bit structure, their meaning, and the individual methods used to transfer them. The data is transferred, prepared, boosted and, if necessary, changed bit by bit. During this process, the physical layer does not distinguish between user dater and control information, nor does it correct errors.

The physical layer only creates the physical connection, transferring all data as energy in the form of bits. Once it has completed this task, it deactivates the connection. The physical layer also takes on a few management functions.

The physical layer also provides information about how the binary digits should be physically represented. This can, for example, be electrical, electromagnetic, optical or acoustic. The physical layer also checks the direction of the transfer. Directions can be simplex (in one direction), half duplex (changing between both directions but not at the same time) or full duplex (occurring at the same time in both directions).

The physical layer provides information to the other layers, making seamless connections possible. This information could, for example, be in the form of radio, light or electronic signals. When choosing suitable hardware for a network and deciding on the correct network type, the physical layer should be taken into account.

This is because the parameters set out in the physical layer have an influence on the other layers. Among these include the choice of transmission medium, the function of individual transmission lines, the transfer speed and the direction of the transfer. Likewise, the pin layout, the properties of the plug and cable as well as physical quantities such as energy supply and voltage are all relevant to the physical layer.

Different hardware components ensure that physical layer requirements are met. These components can be broadly divided into passive and active components. Some of these components can have a direct influence on the next layer. The following hardware can be categorized as passive components:

• Terminating resistors
• Antennas
• Connectors
• Wiring
• Plugs
• T-pieces

The following types of hardware can be categorized as active components in the physical layer:

• Hubs
• Network cards
• Repeaters
• Transceivers
• Amplifiers

There are a wide range of technologies that contribute to the physical layer and operate on the OSI model. Among those include:

• 1-Wire: A series of interfaces that can be used as a power supply or act as sending and receiving lines
• Bluetooth: The industry standard for transferring data over short distances
• DSL: A different physical layer standard for data transfer that uses copper wiring and has a high transfer rate
• E-carrier: A carrier system for the transfer of different telephone calls at the same time
• Ethernet: The transfer of data through cables within a local network
• FireWire: An earlier range of interfaces with a high transfer rate
• GMS: A cell phone standard for transferring mobile data
• IEEE 802.15.4: A standard developed to transfer data within WPAN networks
• IrDA: An association created to standardize infrared receivers
• ISDN: An international standard for digital telecommunication networks
• PCI Express: A standard for the connection between peripheral devices with a primary processor
• SONET/SDH: A multiplex technology for synchronized optical transfers using fiber-optic cables
• USB: A data transfer system between computers and external devices
• Wi-Fi: WLAN devices and networks using the IEEE 802.11 standard
• X10: A protocol for building information using switch signals