Physical Layer
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In the seven-layer OSI model of
computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections ar ...
ing, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the
electrical connector Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection betwee ...
s, the frequencies to broadcast on, the
line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmission (telecommunications), transmitted down a communication channel or written to a storage medium. This repertoire of signals ...
to use and similar low-level parameters, are specified by the physical layer.


Role

The physical layer defines the means of transmitting a stream of raw
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented a ...
s 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 traces through which electric current can flow. It is a type of electri ...
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 The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. 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, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different conc ...
encoding, transmission, reception and decoding and, * performs galvanic isolation.


Relation to the Internet protocol suite

The Internet protocol suite, 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 connector Components of an electrical circuit are electrically connected if an electric current can run between them through an electrical conductor. An electrical connector is an electromechanical device used to create an electrical connection betwee ...
s and cables, for example maximum cable length, an electrical specification of
transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
signal level and impedance. The physical layer is responsible for electromagnetic compatibility including
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
frequency allocation Frequency allocation (or spectrum allocation or spectrum management) is the allocation and regulation of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and t ...
and specification of signal strength, analog bandwidth, etc. The transmission medium may be electrical or optical over
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
or a wireless communication link such as free-space optical communication or
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a transm ...
. 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 Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream contai ...
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 In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
. 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 code In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea i ...
s and techniques including forward error correction may be applied to further improve reliability. Other topics associated with the physical layer include:
bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
; point-to-point, multipoint or point-to-multipoint line configuration; physical
network topology Network topology is the arrangement of the elements ( links, nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, including command and contr ...
, for example
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
, ring, mesh or star network; serial or parallel communication;
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension ...
, half duplex or
full duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
transmission mode; and autonegotiation


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 traces through which electric current can flow. It is a type of electri ...
, usually implemented as an integrated circuit, required to implement physical layer functions of the OSI model 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 An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
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 Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pr ...
(like the SFP family) complement a PHY chip and form the PMA 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 Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
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 A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office building ...
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 local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio w ...
: 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 LTE may refer to: Science and technology * LTE (telecommunication) (Long-Term Evolution), a telephone and mobile broadband standard ** LTE Advanced, an enhancement *** LTE Advanced Pro * Compaq LTE, a line of laptop computers produced by Compaq * ...
/ 5G,
WiMAX Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options. The WiMA ...
and UWB. *
Universal Serial Bus Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply ( interfacing) between computers, peripherals and other computers. A broa ...
(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 ARINC 818: Avionics Digital Video Bus (ADVB) is a video interface and protocol standard developed for high bandwidth, low-latency, uncompressed digital video transmission in avionics systems. The standard, which was released in January 2007, has b ...
Avionics Digital Video Bus *
Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limit ...
physical layer *
CAN bus A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for ...
(controller area network) physical layer * DSL * EIA
RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' ('' data terminal equipment'') suc ...
, EIA-422, EIA-423, RS-449, RS-485 *
Etherloop Etherloop is a kind of DSL technology that combines the features of Ethernet and DSL. It allows the combination of voice and data transmission on standard phone lines. Under the right conditions it will allow speeds of up to 6 megabits per secon ...
* 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 * IEEE 802.15.4 physical layers * IEEE 1394 interface * IRDA physical layer *
ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wor ...
* ITU Recommendations: see
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
*
I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is w ...
, I²S * LoRa * Low-voltage differential signaling * Mobile Industry Processor Interface physical layer * Modulated ultrasound * Optical Transport Network (OTN) * SMB *
SONET/SDH Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting ...
*
SPI SPI may refer to: Organizations * Indian Protection Service (''Serviço de Proteção ao Índio''), Brazil * Shotmed Paper Industries, an Egyptian paper manufacturers * Simulations Publications, Inc., a former US board game publisher * Sony P ...
* T1 and other
T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell ...
links, and E1 and other
E-carrier The E-carrier is a member of the series of carrier systems developed for digital transmission of many simultaneous telephone calls by time-division multiplexing. The European Conference of Postal and Telecommunications Administrations (CEPT) o ...
links * Telephone network
modems A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more carr ...
 — V.92 * TransferJet physical layer * USB physical layer *
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common m ...
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 local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio w ...
physical layer * Visible light communication co-ordinated under IEEE 802.15.7 * X10


See also

* Channel model * Clock recovery *
Data transmission Data transmission and data reception or, more broadly, data communication or digital communications is the transfer and reception of data in the form of a digital bitstream or a digitized analog signal transmitted over a point-to-point o ...
* Intrinsic safety * SerDes


References

{{reflist


External links


Physical Layer (Layer 1)


OSI model