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The physical layer defines the means of transmitting raw bits 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 by the physical layer.
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.
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 (or other) signals. The physical layer supports higher layers responsible for generation of logical data packets.
Physical signaling sublayer
- interfaces with the data link layer's medium access control (MAC) sublayer,
- performs symbol encoding, transmission, reception and decoding and,
- performs galvanic isolation.
Relation to the Internet protocol suite
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 a layer that deals exclusively with hardware-level specifications and interfaces, as this model does not concern itself directly with physical interfaces.
The major functions and services performed by the physical layer are: The physical layer performs bit-by-bit or symbol-by-symbol data delivery over a physical transmission medium. It provides a standardized interface to the transmission medium, including a mechanical specification of electrical connectors and cables, for example maximum cable length, an electrical specification of transmission line signal level and impedance. The physical layer is responsible for electromagnetic compatibility including electromagnetic spectrum frequency allocation and specification of signal strength, analog bandwidth, etc. The transmission medium may be electrical or optical over optical fiber or a wireless IR communication link.
Line coding is used convert data into a pattern of electrical fluctuations which may be modulated onto a carrier wave or infrared light. The flow of data is managed with bit synchronization in synchronous serial communication or start-stop signalling and flow control in asynchronous serial communication. Sharing of the transmission medium among multiple network participants can be handled by simple circuit switching or multiplexing. More complex medium access control protocols for sharing the transmission medium may use carrier sense and collision detection such as in Ethernet's Carrier-sense multiple access with collision detection (CSMA/CD).
To optimize reliability and efficiency, signal processing techniques such as equalization, training sequences and pulse shaping may be used. Error correction codes and techniques including forward error correction may be applied to further improve reliability.
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
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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
- Bit synchronization
- Channel model
- Clock recovery
- Data transmission
- Digital modulation
- Intrinsic safety
- Line code
- Pulse shaping
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