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In the seven-layerOSI model ofcomputer networking, thephysical layer orlayer 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 theelectrical connectors, the frequencies to transmit on, theline 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 dedicatedPHY chip or, inelectronic design automation (EDA), by adesign block. Inmobile computing, theMIPI Alliance*-PHY family of interconnect protocols are widely used.
The physical layer defines the means of transmitting a stream of rawbits[2] over a physicaldata link connectingnetwork nodes. Thebitstream may be grouped into code words or symbols and converted to a physicalsignal that is transmitted over atransmission medium.
The physical layer consists of theelectronic circuit transmission technologies of a network.[3] 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.[4]
Within the semantics of the OSI model, the physical layer translates logical communications requests from thedata link layer into hardware-specific operations to cause transmission or reception of electronic (or other) signals.[5][6] The physical layer supports higher layers responsible for generation of logicaldata packets.
In a network usingOpen Systems Interconnection (OSI) architecture, thephysical signaling sublayer is the portion of the physical layer that[7][8]
TheInternet protocol suite, as defined inRFC 1122 andRFC 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.[9][10]
The major functions and services performed by the physical layer are:The physical layer performs bit-by-bit orsymbol-by-symbol data delivery over a physicaltransmission medium.[11] It provides a standardized interface to the transmission medium, including[12][13] a mechanical specification ofelectrical connectors andcables, for example maximum cable length, an electrical specification oftransmission linesignal level andimpedance. The physical layer is responsible forelectromagnetic compatibility includingelectromagnetic spectrumfrequency allocation and specification ofsignal strength, analogbandwidth, etc. The transmission medium may be electrical or optical overoptical fiber or a wireless communication link such asfree-space optical communication orradio.
Line coding is used to convert data into a pattern of electrical fluctuations which may bemodulated onto acarrier wave orinfrared light. The flow of data is managed withbit synchronization in synchronousserial communication orstart-stop signalling andflow control inasynchronous serial communication. Sharing of the transmission medium among multiple network participants can be handled by simplecircuit switching ormultiplexing. More complexmedium access control protocols for sharing the transmission medium may usecarrier sense andcollision detection such as in Ethernet'sCarrier-sense multiple access with collision detection (CSMA/CD).
To optimize reliability and efficiency, signal processing techniques such asequalization,training sequences andpulse shaping may be used.Error correction codes and techniques includingforward error correction[14] may be applied to further improve reliability.
Other topics associated with the physical layer include:bit rate;point-to-point, multipoint orpoint-to-multipoint line configuration; physicalnetwork topology, for examplebus,ring,mesh orstar network;serial orparallel communication;simplex,half duplex orfull duplex transmission mode; andautonegotiation[15]
APHY, an abbreviation forphysical layer, is anelectronic circuit, usually implemented as anintegrated circuit, required to implement physical layer functions of theOSI model in anetwork interface controller.
A PHY connects alink layer device (often called MAC as an acronym formedium access control) to a physical medium such as anoptical fiber orcopper cable. A PHY device typically includes bothphysical coding sublayer (PCS) andphysical medium dependent (PMD) layer functionality.[16]
-PHY may also be used as a suffix to form a short name referencing a specific physical layer protocol, for exampleM-PHY.
Modular transceivers forfiber-optic communication (like theSFP family) complement a PHY chip and form thePMD sublayer.
TheEthernet PHY is a component that operates at the physical layer of theOSI 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 amedia-independent interface (MII) to a MAC chip in amicrocontroller 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 Ethernetframes; it interfaces between the analog domain of Ethernet's line modulation and the digital domain of link-layerpacket signaling.[17] The PHY usually does not handle MAC addressing, as that is thelink layer's job. Similarly,Wake-on-LAN andBoot ROM functionality is implemented in thenetwork 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[18] is an example of a PHY which uses SPE.
Examples include theMicrosemi SimpliPHY and SynchroPHY VSC82xx/84xx/85xx/86xx family,Marvell Alaska 88E1310/88E1310S/88E1318/88E1318S Gigabit Ethernet transceivers, Texas Instruments DP838xx family[19] and offerings from Intel[20] and ICS.[21]
The following technologies provide physical layer services:[22]
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22.