OSI model, Physical Layer

The physical layer includes the basic network hardware transmission technologies of a network. It is a fundamental layer that underlies the logical data structures of the higher-level functions of a network.

The physical layer or layer 1 is the first and the lowest layer. The implementation of this layer is often called PHY.

Due to the large number of hardware technologies available with very diverse characteristics, this is perhaps the most complex layer of the OSI architecture.

The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link between nodes on the network.

The bit stream can be grouped into code words or symbols and converted into a physical signal that is transmitted through a hardware transmission medium.

The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. Here the shapes and properties of the electrical connectors, the frequencies to be transmitted, the modulation scheme to be used, and similar low-level parameters are specified.

In the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to affect the transmission or reception of electronic signals.

Services

• Bit-by-bit or symbol-by-symbol delivery
• Modulation
• Line coding
• Bit synchronization
• Start-stop signalling
• Circuit switching
• Multiplexing
• Carrier sense and collision detection

Protocols

• ARINC 818 Avionics Digital Video Bus
• Bluetooth physical layer
• CAN bus (controller area network) physical layer
• DSL
• Etherloop
• Ethernet physical layer 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and others
• GSM Um air interface physical layer
• G.hn/G.9960 physical layer
• IEEE 1394 interface
• ISDN
• IRDA physical layer
• ITU
• OTN Optical Transport Network
• SPI
• SMB
• Telephone network modems
• USB physical layer