Physical layer
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In the seven-layer
OSI model The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...
of
computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...
ing, the physical layer or layer 1 is the first and lowest layer: the layer most closely associated with the physical connection between devices. 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 transmit on, the line code to use and similar low-level parameters, are specified by the physical layer. At the electrical layer, the physical layer is commonly implemented in a dedicated PHY chip or, in
electronic design automation Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing Electronics, electronic systems such as integrated circuits and printed circuit boards. The tools wo ...
(EDA), by a design block. In mobile computing, the MIPI Alliance *-PHY family of interconnect protocols are widely used.


Role

The physical layer defines the means of transmitting a stream of 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 consists of the
electronic circuit An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or Conductive trace, traces through which electric current can flow. It is a t ...
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

In a network 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
symbol A symbol is a mark, Sign (semiotics), sign, or word that indicates, signifies, or is understood as representing an idea, physical object, object, or wikt:relationship, relationship. Symbols allow people to go beyond what is known or seen by cr ...
encoding, transmission, reception and decoding and, * performs galvanic isolation.


Relation to the Internet protocol suite

The
Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...
, as defined i
RFC 1122
an
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.


Services

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 The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...
frequency allocation and specification of signal strength, analog bandwidth, etc. The transmission medium may be electrical or optical over optical fiber or a wireless communication link such as free-space optical communication or
radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...
. Line coding is used to 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. Other topics associated with the physical layer include: 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; and
autonegotiation Autonegotiation is a signaling mechanism and procedure used by Ethernet over twisted pair by which two connected devices choose common transmission parameters, such as speed, Duplex_(telecommunications), duplex mode, and Flow_control_(data), flow ...


PHY

A PHY, an abbreviation for ''physical layer'', is an
electronic circuit An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or Conductive trace, traces through which electric current can flow. It is a t ...
, usually implemented as an
integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
, required to implement physical layer functions of the
OSI model The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems inter ...
in a network interface controller. A PHY connects a link layer device (often called MAC as an acronym for medium access control) to a physical medium such as an optical fiber or copper cable. A PHY device typically includes both physical coding sublayer (PCS) and physical medium dependent (PMD) layer functionality. ''-PHY'' may also be used as a suffix to form a short name referencing a specific physical layer protocol, for example M-PHY. Modular transceivers for fiber-optic communication (like the SFP family) complement a PHY chip and form the PMD sublayer.


Ethernet physical transceiver

The Ethernet PHY is a component that operates at the physical layer of the OSI network model. It implements the physical layer portion of the Ethernet. Its purpose is to provide analog signal physical access to the link. It is usually interfaced with a media-independent interface (MII) to a MAC chip in a microcontroller or another system that takes care of the higher layer functions. More specifically, the Ethernet PHY is a chip that implements the hardware send and receive function of Ethernet frames; it interfaces between the analog domain of Ethernet's line modulation and the digital domain of link-layer packet signaling. The PHY usually does not handle MAC addressing, as that is the link layer's job. Similarly, Wake-on-LAN and Boot ROM functionality is implemented in the network interface card (NIC), which may have PHY, MAC, and other functionality integrated into one chip or as separate chips. Common Ethernet interfaces include fiber or two to four copper pairs for data communication. However, there now exists a new interface, called Single Pair Ethernet (SPE), which is able to utilize a single pair of copper wires while still communicating at the intended speeds. Texas Instruments DP83TD510E is an example of a PHY which uses SPE. Examples include the Microsemi SimpliPHY and SynchroPHY VSC82xx/84xx/85xx/86xx family, Marvell Alaska 88E1310/88E1310S/88E1318/88E1318S Gigabit Ethernet transceivers, Texas Instruments DP838xx family and offerings from Intel and ICS.


Other applications

* Wireless LAN or
Wi-Fi Wi-Fi () is a family of wireless network protocols based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital devices to exchange data by ...
: The PHY portion consists of the RF, mixed-signal and analog portions, that are often called transceivers, and the digital baseband portion that use digital signal processor (DSP) and communication algorithm processing, including channel codes. It is common that these PHY portions are integrated with the medium access control (MAC) layer in system-on-a-chip (SOC) implementations. Similar wireless applications include 3G/ 4G/ LTE/ 5G, WiMAX and UWB. * Universal Serial Bus (USB): A PHY chip is integrated into most USB controllers in hosts or embedded systems and provides the bridge between the digital and modulated parts of the interface. * IrDA: The Infrared Data Association's (IrDA) specification includes an IrPHY specification for the physical layer of the data transport. * Serial ATA (SATA): Serial ATA controllers use a PHY.


Technologies

The following technologies provide physical layer services: * 1-Wire * ARINC 818 Avionics Digital Video Bus * Bluetooth physical layer * CAN bus (controller area network) physical layer * DSL * EIA RS-232, EIA-422, EIA-423, RS-449, RS-485 * Etherloop * Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 1000BASE-T, 1000BASE-SX and other varieties * G.hn/ G.9960 physical layer * GSM Um air interface physical layer *
UMTS The Universal Mobile Telecommunications System (UMTS) is a 3G mobile cellular system for networks based on the GSM standard. UMTS uses Wideband Code Division Multiple Access, wideband code-division multiple access (W-CDMA) radio access technolog ...
air interface physical layer * LTE air interface physical layer * 5G air interface physical layer * Communications satellite technologies physical layers * IEEE 802.15.4 physical layers * IEEE 1394 interface * IRDA physical layer * ISDN * ITU Recommendations: see ITU-T * I²C, I²S * LoRa * Low-voltage differential signaling * Mobile Industry Processor Interface physical layer * Modulated ultrasound * Optical Transport Network (OTN) * SMB * SONET/SDH * SPI * T1 and other T-carrier links, and E1 and other E-carrier links * Telephone network modems — ITU-T V.92 * TransferJet physical layer * USB physical layer * PCI Express physical layer * 802.11
Wi-Fi Wi-Fi () is a family of wireless network protocols based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital devices to exchange data by ...
physical layer * Visible light communication co-ordinated under IEEE 802.15.7 * X10


See also

* Channel model * Clock recovery * Data transmission * Intrinsic safety * SerDes


References

{{reflist


External links


Physical Layer (Layer 1)


OSI model