OSI Layer 1 – Physical Layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer. The implementation of this layer is often termed PHY.
The physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.
The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here.
Within 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 transmission or reception of electronic signals.Typical hardware on this layer: repeaters, hubs, cables, plugs,
• Bit-by-bit or symbol-by-symbol delivery
• Line coding
• Bit synchronization
• Start-stop signalling
• Circuit switching
• Carrier sense and collision detection
• Physical network topology, like bus, ring, mesh or star network
Popular Physical Layer Technologies
Avionics Digital Video Bus
Bluetooth physical layer
(controller area network) physical layer
also: EIA-422, EIA-423, RS-449, RS-485
Ethernet physical layer
10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and others
Um air interface physical layer
IEEE 1394 interface
Mobile Industry Processor Interface physical layer
Optical Transport Network
Telephone network modems