Editing Physical layer

{{short description|Lowest-level electronic or optical transmission functions of a network}}
{{Redirect|PHY}}

{{OSI model}}

In the seven-layer [[OSI model]] of [[computer network]]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 connector]]s, 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]] (EDA), by a [[semiconductor intellectual property core|design block]]. In [[mobile computing]], the [[MIPI Alliance]] [[M-PHY|*-PHY]] family of interconnect protocols are widely used. 

Historically, the OSI model is closely associated with internetworking, such as the [[Internet protocol suite]] and [[Ethernet]], which were developed in the same era, along similar lines, though with somewhat different abstractions. Beyond internetworking, the OSI abstraction can be brought to bear on all forms of device interconnection in [[data communication]]s and computational electronics.

== Role ==
The physical layer defines the means of transmitting a stream of raw [[bit]]s<ref>{{cite web |url=http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/phy.html |archive-url=https://web.archive.org/web/20090618154921/http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/phy.html |archive-date=2009-06-18 |title=Physical Layer |author=Gorry Fairhurst |date=2001-01-01}}</ref> over a physical [[data link]] connecting [[network node]]s. 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]] transmission technologies of a network.<ref name="Fundamentals of Sensor Network Programming">{{cite book | last = Iyengar | first = Shisharama | title = Fundamentals of Sensor Network Programming | publisher = Wiley | year = 2010 | pages = 136 | isbn = 978-1423902454 | url = https://books.google.com/books?id=UD0h_GqgbHgC&q=network%2B+guide+to+networks}}</ref> 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.<ref>{{Cite news|url=https://interworks.com/blog/bfair/2011/07/30/physical-layer/|title=The Physical Layer {{!}} InterWorks|date=2011-07-30|work=InterWorks|access-date=2018-08-14|language=en-US}}</ref>

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.<ref>{{Cite web|url=https://www.networkworld.com/article/3239677/lan-wan/the-osi-model-explained-how-to-understand-and-remember-the-7-layer-network-model.html|archive-url=https://web.archive.org/web/20171204233851/https://www.networkworld.com/article/3239677/lan-wan/the-osi-model-explained-how-to-understand-and-remember-the-7-layer-network-model.html|url-status=dead|archive-date=December 4, 2017|title=The OSI model explained: How to understand (and remember) the 7 layer network model|last=Shaw|first=Keith|date=2018-10-22|website=Network World|language=en|access-date=2019-02-15}}</ref><ref>{{Cite web|url=https://www.researchgate.net/publication/288180515|title=DATA COMMUNICATION & NETWORKING|website=ResearchGate|language=en|access-date=2019-02-15}}</ref> The physical layer supports higher layers responsible for generation of logical [[data packet]]s.

== Physical signaling sublayer ==
In a network using [[Open Systems Interconnection]] (OSI) architecture, the ''physical signaling sublayer'' is the portion of the physical layer that<ref name="FS1037C">{{FS1037C}}</ref><ref>{{cite web |url=http://www.tiaonline.org/market_intelligence/glossary/index.cfm?term=%26%23TC%5DSR%3FN%0A |title=physical signaling sublayer (PLS) |access-date=2011-07-29 |archive-url=https://web.archive.org/web/20101227170125/http://tiaonline.org/market_intelligence/glossary/index.cfm?term=%26%23TC%5DSR%3FN%0A |archive-date=2010-12-27 |url-status=dead }}</ref>
* interfaces with the data link layer's [[medium access control]] (MAC) sublayer,
* performs [[symbol (data)|symbol]] encoding, [[transmission (telecommunications)|transmission]], reception and decoding and,
* performs [[galvanic isolation]].

== Relation to the Internet protocol suite ==
The [[Internet protocol suite]], as defined in [https://datatracker.ietf.org/doc/html/rfc1122 RFC 1122] and [https://datatracker.ietf.org/doc/html/rfc1123 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.<ref>{{Cite web|title=rfc1122|url=https://datatracker.ietf.org/doc/html/rfc1122|access-date=2021-07-28|website=datatracker.ietf.org}}</ref><ref>{{Cite web|title=rfc1123|url=https://datatracker.ietf.org/doc/html/rfc1123|access-date=2021-07-28|website=datatracker.ietf.org}}</ref>

== Services ==
The major functions and services performed by the physical layer are:
The physical layer performs bit-by-bit or [[Symbol rate|symbol-by-symbol]] data delivery over a physical [[transmission medium]].<ref>{{Cite web|url=https://fossbytes.com/physical-layer/|title=Physical Layer Of OSI Model: Working Functionalities and Protocols|last=Shekhar|first=Amar|date=2016-04-07|website=Fossbytes|language=en-US|access-date=2019-02-15}}</ref> It provides a standardized interface to the transmission medium, including<ref>{{Cite book|url=https://books.google.com/books?id=cLwO-Hh6_VEC&q=The+physical+layer+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|title=Transmission and Distribution Electrical Engineering|last1=Bayliss|first1=Colin R.|last2=Bayliss|first2=Colin|last3=Hardy|first3=Brian|date=2012-02-14|publisher=Elsevier|isbn=9780080969121|language=en}}</ref><ref>{{Cite web|url=https://en.wikibooks.org/wiki/CCNA_Certification/Physical_Layer|title=CCNA Certification/Physical Layer - Wikibooks, open books for an open world|website=en.wikibooks.org|access-date=2019-02-15}}</ref> a mechanical specification of [[electrical connector]]s and [[Electrical cable|cables]], for example maximum cable length, an electrical specification of [[transmission line]] [[signal level]] and [[electrical impedance|impedance]]. The physical layer is responsible for [[electromagnetic compatibility]] including [[electromagnetic spectrum]] [[frequency allocation]] and specification of [[signal strength]], analog [[Bandwidth (signal processing)|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]].

[[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 (data)|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 (communications)|equalization]], [[training sequence]]s and [[pulse shaping]] may be used. [[Error correction code]]s and techniques including [[forward error correction]]<ref>{{cite book|last1= Bertsekas|first1= Dimitri |last2=Gallager |first2= Robert |title= Data Networks |url= https://archive.org/details/isbn_9780132009164|url-access= limited|publisher= Prentice Hall |year= 1992|isbn= 0-13-200916-1|page=[https://archive.org/details/isbn_9780132009164/page/61 61]}}</ref> may be applied to further improve reliability.

Other topics associated with the physical layer include: [[bit rate]]; [[point-to-point (telecommunications)|point-to-point]], multipoint or [[point-to-multipoint]] line configuration; physical [[network topology]], for example [[bus network|bus]], [[ring network|ring]], [[mesh network|mesh]] or [[star network]]; [[serial communication|serial]] or [[parallel communication|parallel]] communication; [[simplex communication|simplex]], [[half duplex]] or [[full duplex]] transmission mode; and [[autonegotiation]]<ref>{{Cite book|url=https://books.google.com/books?id=bwUNZvJbEeQC&q=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+Autonegotiation|title=Data Communications and Networking|last1=Forouzan|first1=Behrouz A.|last2=Fegan|first2=Sophia Chung|date=2007|publisher=Huga Media|isbn=9780072967753|language=en}}</ref><!--[[User:Kvng/RTH]]-->

== PHY ==
[[File:Elitegroup 761GX-M754 - Realtek RTL8201CL-5493.jpg|thumb|RTL8201 Ethernet PHY chip]]

[[File:DP83825I smaller.png|thumb|Texas Instruments DP83825 – 3&nbsp;×&nbsp;3&nbsp;mm 3.3&nbsp;V PHY chip]]{{Redirects here|PHY}}
A '''PHY''', an abbreviation for ''physical layer'', is an [[electronic circuit]], 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]] or [[copper cable]]. A PHY device typically includes both [[physical coding sublayer]] (PCS) and [[physical medium dependent]] (PMD) layer functionality.<ref>{{cite book|url=https://books.google.com/books?id=DRIryrLoxKkC&q=ethernet+PHY&pg=PA495 |title=Data Center Fundamentals |author1=Mauricio Arregoces |author2=Maurizio Portolani |year=2003 |isbn=9781587050237 |access-date=2015-11-18}}</ref>

''-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 [[small form-factor pluggable transceiver|SFP]] family) complement a PHY chip and form the [[Physical medium dependent|PMD]] sublayer.

=== Ethernet physical transceiver ===
[[File:Micrel KS8721CL on mainboard of Surf@home II-7778.jpg|thumb|[[Ray Zinn|Micrel]] KS8721CL – 3.3&nbsp;V single power supply 10/100BASE-TX/FX MII physical layer transceiver]]

The '''Ethernet PHY''' is a component that operates at the physical layer of the [[OSI model|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 [[Data frame|frame]]s; it interfaces between the analog domain of [[Ethernet physical layer | Ethernet's line modulation]] and the digital domain of link-layer [[Media Independent Interface|packet signaling]].<ref>{{cite web|url=http://electronics.stackexchange.com/questions/75596/what-is-the-difference-between-phy-and-mac-chip |title=microcontroller - what is the difference between PHY and MAC chip - Electrical Engineering Stack Exchange |publisher=Electronics.stackexchange.com |date=2013-07-11 |access-date=2015-11-18}}</ref>  The PHY usually does not handle MAC addressing, as that is the [[data link layer|link layer]]'s job.  Similarly, [[Wake-on-LAN]] and [[Network booting|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<ref>{{cite web |title=DP83TD510E Ultra Low Power 802.3cg 10Base-T1L 10M Single Pair Ethernet PHY |url=https://www.ti.com/lit/ds/symlink/dp83td510e.pdf?ts=1602524952891&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FDP83TD510E |website=Texas Instruments |access-date=12 October 2020}}</ref> is an example of a PHY which uses SPE.

Examples include the [[Microsemi]] SimpliPHY and SynchroPHY VSC82xx/84xx/85xx/86xx family, [[Marvell Technology Group|Marvell]] Alaska 88E1310/88E1310S/88E1318/88E1318S Gigabit Ethernet transceivers, Texas Instruments DP838xx family<ref>{{cite web |title=Ethernet PHYs |url=https://www.ti.com/interface/ethernet/phys/overview.html |website=Texas Instruments |access-date=12 October 2020}}</ref> and offerings from Intel<ref>[http://www.intel.com/content/dam/doc/brochure/ethernet-controllers-phys-brochure.pdf Intel PHY controllers brochure]</ref> and ICS.<ref>[http://netwinder.osuosl.org/pub/netwinder/docs/nw/PHY/1890.pdf osuosl.org - ICS1890 10Base-T/100Base-TX Integrated PHYceiver datasheet]</ref>

===Other applications===
* [[Wireless LAN]] or [[Wi-Fi]]: 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 code]]s. 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]]/[[3GPP Long Term Evolution|LTE]]/[[5G]], [[WiMAX]] and [[Ultra-wideband|UWB]].
* [[Universal Serial Bus]] (USB): A PHY chip is integrated into most USB controllers in hosts or [[embedded system]]s 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:<ref>{{Cite web|title=Physical Layer {{!}} Layer 1|url=https://osi-model.com/physical-layer/|access-date=2021-07-28|website=The OSI-Model|language=en}}</ref>
{{div col|colwidth=25em}}
* [[1-Wire]]
* [[ARINC 818]] Avionics Digital Video Bus
* [[Bluetooth]] physical layer
* [[CAN bus]] (controller area network) physical layer
* [[DSL]]
* [[Electronic Industries Alliance|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]] air interface physical layer
* [[LTE (telecommunication)|LTE]] air interface physical layer
* [[5G]] air interface physical layer
* [[Communications satellite]] technologies physical layers
* [[IEEE 802.15.4]] physical layers
* [[IEEE 1394 interface]]
* [[Infrared Data Association|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)
* [[System Management Bus|SMB]]
* [[SONET/SDH]]
* [[Serial Peripheral Interface Bus|SPI]]
* T1 and other [[T-carrier]] links, and E1 and other [[E-carrier]] links
* Telephone network [[modem]]s&nbsp;— [[ITU-T V.92]]
* [[TransferJet]] physical layer
* [[USB]] physical layer
* [[PCI Express]] physical layer
* [[802.11]] [[Wi-Fi]] physical layer
* [[Visible light communication]] co-ordinated under [[IEEE 802.15.7]]
* [[X10 (industry standard)|X10]]
{{div col end}}

== See also ==
* [[Channel model]]
* [[Clock recovery]]
* [[Data transmission]]
* [[Intrinsic safety]]
* [[SerDes]]

== References ==
{{reflist}}

== External links ==
* [http://www.tcpipguide.com/free/t_PhysicalLayerLayer1.htm Physical Layer (Layer 1)]
* [http://fmad.io/blog-10g-ethernet-layer1-overview.html 10G Layer 1 Walkthrough]

[[Category:OSI model]]