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The physical layer consists of the electronic circuit transmission technologies of a network. It is a fundamental layer underlying the higher level functions in a network, and can be implemented through a great number of different hardware technologies with widely varying characteristics.
The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical data link connecting network nodes. The bitstream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a 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 line code to use and similar low-level parameters, are specified here.
Within the semantics of the OSI model, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to cause transmission or reception of electronic signals.
Physical signaling sublayer
In a local area network (LAN) or a metropolitan area network (MAN) using Open Systems Interconnection (OSI) architecture, the physical signaling sublayer is the portion of the physical layer that
- interfaces with the data link layer's medium access control (MAC) sublayer,
- performs character encoding, transmission, reception and decoding and,
- performs galvanic isolation.
Relation to the Internet Protocol
The Internet protocol suite, as defined in RFC 1122 and RFC 1123, is a high-level networking description used for the Internet and similar networks. It does not define an equivalent layer that deals exclusively with hardware-level specifications and interfaces, as this model does not concern itself directly with physical interfaces. Several RFCs mention a physical layer and data link layer, but that is in the context of IEEE protocols. RFC 1122 and 1123 do not mention any physical layer functionality or physical layer standards.
List of services
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The major functions and services performed by the physical layer are:
- Bit-by-bit or symbol-by-symbol delivery
- Providing a standardized interface to a physical transmission medium, 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
- Line coding
- Bit synchronization in synchronous serial communication
- Start-stop signalling and flow control in asynchronous serial communication
- Circuit switching
- 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 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
The following technologies provide physical layer services:
- ARINC 818 Avionics Digital Video Bus
- Bluetooth physical layer
- CAN bus (controller area network) 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
- G.hn/G.9960 physical layer
- GSM Um air interface physical layer
- IEEE 802.15.4 physical layers
- IEEE 1394 interface
- IRDA physical layer
- ITU Recommendations: see ITU-T
- I²C, I²S
- Low-voltage differential signaling
- Mobile Industry Processor Interface physical layer
- Modulated ultrasound
- Optical Transport Network (OTN)
- T1 and other T-carrier links, and E1 and other E-carrier links
- Telephone network modems — V.92
- TransferJet physical layer
- USB physical layer
- Varieties of 802.11 Wi-Fi physical layers
- Visible light communication co-ordinated under IEEE 802.15.7
- Clock recovery
- Data transmission
- Digital modulation
- Line code
- Pulse shaping
- Bit synchronization
- Channel model
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