Physical Layer OSI Model

In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first (lowest) layer. The physical layer consists of the basic networking hardware transmission technologies of a network.

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.

List of services

The major functions and services performed by the physical layer are:

• Bit-by-bit or symbol-by-symbol delivery
• Providing a standardized interface to physical transmission media, including
• Mechanical specification of electrical connectors and cables, for example maximum cable length
• Electrical specification of transmission line signal level and impedance
• Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc.
• Specifications for IR over optical fiber or a wireless IR communication link
• Modulation
• Line coding
• Bit synchronization in synchronous serial communication
• Start-stop signalling and flow control in asynchronous serial communication
• Circuit switching
• Multiplexing
• Establishment and termination of circuit switched connections
• Carrier sense and collision detection utilized by some level 2 multiple access protocols
• Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals
• Forward error correction[5] for example bitwise convolutional coding
• Bit-interleaving and other channel coding

The physical layer is also concerned with

• Bit rate
• Point-to-point, multipoint or point-to-multipoint line configuration
• Physical network topology, for example bus, ring, mesh or star network
• Serial or parallel communication
• Simplex, half duplex or full duplex transmission mode
• Autonegotiation

List of protocols

• Telephone network modems- V.92
• IRDA physical layer
• USB physical layer
• EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
• Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties
• Varieties of 802.11 Wi-Fi physical layers
• DSL
• ISDN etc.,