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