The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model

OSI model

The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...

 of computer network

Computer network

A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

ing. The implementation of this layer is often termed PHY.

The physical layer consists of the basic hardware

Hardware

Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

 transmission technologies of a network

Computer network

A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.

The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link

Data link

In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver and the interconnecting data telecommunication circuit...

 connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium

Transmission medium

A transmission medium is a material substance that can propagate energy waves...

. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connector

Electrical connector

An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...

s, the frequencies

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 to broadcast on, the modulation

Modulation

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 scheme to use and similar low-level parameters, are specified here.

Within the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer

Data link layer

The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....

 into hardware-specific operations to affect transmission or reception of electronic signals.

Physical signaling sublayer

In a local area network

Local area network

A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

 (LAN) or a metropolitan area network

Metropolitan area network

A metropolitan area network is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the...

 (MAN) using open systems interconnection

Open Systems Interconnection

Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...

 (OSI) architecture, the physical signaling sublayer is the portion of the physical layer that:

• interfaces with the data link layer
  Data link layer
  The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
  
  's medium access control (MAC) sublayer.
• performs character
  Character (computing)
  In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
  
   encoding, transmission
  Transmission (telecommunications)
  Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
  
  , reception and decoding.
• performs mandatory isolation functions.

List of services

The major functions and services performed by the physical layer are:

• Bit-by-bit or symbol-by-symbol delivery
• Providing a standardized interface to physical transmission media, including
  • Mechanical specification of electrical connector
    Electrical connector
    An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
    
    s and cable
    Cable
    A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
    
    s, for example maximum cable length
  • Electrical specification of transmission line
    Transmission line
    In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
    
     signal level and impedance
    Electrical impedance
    Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
    
    
  • Radio interface, including electromagnetic spectrum
    Electromagnetic spectrum
    The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
    
     frequency allocation
    Frequency allocation
    Use of radio frequency bands of the electromagnetic spectrum is regulated by governments in most countries, in a Spectrum management process known as frequency allocation or spectrum allocation. Radio propagation does not stop at national boundaries...
    
     and specification of signal strength
    Signal strength
    In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
    
    , analog bandwidth, etc.
  • Specifications for IR over optical fiber
    Optical fiber
    An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
    
     or a wireless IR communication link
• Modulation
  Modulation
  In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
  
  
• Line coding
• Bit synchronization in synchronous serial communication
  Serial communication
  In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
  
  
• Start-stop signalling and flow control
  Flow control
  In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
  
   in asynchronous serial communication
• Circuit switching
  Circuit switching
  Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
  
  
• Multiplexing
  Multiplexing
  The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
  
  
  • Establishment and termination of circuit switched connections
• Carrier sense and collision detection utilized by some level 2 multiple access protocols
• Equalization
  Equalization
  Equalization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
  
   filtering, training sequences, pulse shaping
  Pulse shaping
  In digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
  
   and other signal processing
  Signal processing
  Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
  
   of physical signals
• Forward error correction
  Forward error correction
  In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
  
   for example bitwise convolutional coding
• Bit-interleaving and other channel coding

The physical layer is also concerned with

• Bit rate
  Bit rate
  In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
  
  
• Point-to-point, multipoint
  Multipoint
  Multipoint may refer to:* Multi-point fuel injection, an injection scheme for metering fuel into an internal combustion engine* Multipoint ground, a type of electrical installation which involves the creation of many alternate paths for electrical energy to find its way back to ground* Multipoint...
  
   or point-to-multipoint
  Point-to-multipoint
  Point-to-multipoint communication is a term that is used in the telecommunications field which refers to communication which is accomplished via a specific and distinct type of one-to-many connection, providing multiple paths from a single location to multiple locations.Point-to-multipoint is often...
  
   line configuration
• Physical network topology
  Topology
  Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
  
  , for example bus
  Bus network
  A bus network topology is a network architecture in which a set of clients are connected via a shared communications line, called a bus. There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet...
  
  , ring
  Ring network
  A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring...
  
  , mesh or star network
  Star network
  Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages...
  
  
• Serial
  Serial communication
  In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
  
   or parallel communication
• Simplex
  Simplex communication
  Simplex communication refers to communication that occurs in one direction only. Two definitions have arisen over time: a common definition, which is used in ANSI standard and elsewhere, and an ITU-T definition...
  
  , half duplex or full duplex transmission mode
• Autonegotiation
  Autonegotiation
  Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance...
  
  

List of protocols

• Telephone network modems- V.92
  V.92
  V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kb/s download and 48 kb/s upload rates. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream...
  
  
• IRDA
  Infrared Data Association
  The Infrared Data Association defines physical specifications communications protocol standards for the short-range exchange of data over infrared light, for uses such as personal area networks ....
  
   physical layer
• USB physical layer
• EIA
  Electronic Industries Alliance
  The Electronic Industries Alliance was a standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They developed standards to ensure the equipment of different manufacturers was compatible and interchangeable...
  
   RS-232
  RS-232
  In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
  
  , EIA-422
  EIA-422
  RS-422 is a technical standard that specifies electrical characteristics of a digital signalling circuit. Differential-mode signals can be sent at rates as high as 10 million bits per second, or may be sent on cables as long as 1200 metres. Some systems directly interconnect using RS 422 signals,...
  
  , EIA-423
  RS-423
  RS/EIA/TIA-423 is a standard for serial communications. It defines an unbalanced interface , with a single, unidirectional sending driver, and allows for up to 10 receivers . It is normally implemented in integrated circuit technology and can also be employed for the interchange of serial binary...
  
  , RS-449
  RS-449
  The RS-449 specification, also known as EIA-449 or TIA-449, defines the functional and mechanical characteristics of the interface between data terminal equipment and data communications equipment....
  
  , RS-485
• Ethernet physical layer
  Ethernet physical layer
  The Ethernet physical layer is the physical layer component of the Ethernet family of computer network standards.The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed...
  
   Including 10BASE-T
  10BASE-T
  Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
  
  , 10BASE2
  10BASE2
  10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors...
  
  , 10BASE5
  10BASE5
  10BASE5 was the original commercially available variant of Ethernet.For its physical layer it used cable similar to RG-8/U coaxial cable but with extra braided shielding. This is a stiff, diameter cable with an impedance of 50 ohms , a solid center conductor, a foam insulating filler, a shielding...
  
  , 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties
• Varieties of 802.11 Wi-Fi
  Wi-Fi
  Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
  
   physical layers
• DSL
  Digital Subscriber Line
  Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
  
  
• ISDN
  Integrated Services Digital Network
  Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
  
  
• T1 and other T-carrier
  T-carrier
  In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
  
   links, and E1 and other E-carrier
  E-carrier
  In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
  
   links
• SONET/SDH
• Optical Transport Network
  Optical Transport Network
  ITU-T defines an Optical Transport Network as a set of Optical Network Elements connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals...
  
   (OTN)
• GSM Um radio interface
  Um Interface
  The Um interface is the air interface for the GSM mobile telephone standard. It is the interface between the mobile station and the Base transceiver station . It is called Um because it is the mobile analog to the U interface of ISDN. Um is defined in the GSM 04.xx and 05.xx series of...
  
   physical layer
• Bluetooth
  Bluetooth
  Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
  
   physical layer
• ITU
  International Telecommunication Union
  The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
  
   Recommendations: see ITU-T
  ITU-T
  The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
  
  
• IEEE 1394 interface
  IEEE 1394 interface
  The IEEE 1394 interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, frequently used by personal computers, as well as in digital audio, digital video, automotive, and aeronautics applications. The interface is also known by the brand...
  
  
• TransferJet
  TransferJet
  TransferJet is a new type of close proximity wireless transfer technology initially demonstrated publicly in early 2008. By touching two electronic devices, TransferJet allows high speed exchange of data...
  
   physical layer
• Etherloop
  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...
  
  
• ARINC 818
  ARINC 818
  ARINC 818: Avionics Digital Video Bus is a video interface and protocol standard developed for high bandwidth, low latency, uncompressed digital video transmission in avionics systems...
  
   Avionics Digital Video Bus
• G.hn
  G.hn
  G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
  
  /G.9960 physical layer
• Controller Area Network
  Controller Area Network
  Controller–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....
  
   (CAN) physical layer

Hardware equipment (network node) examples

• Network adapter
• Repeater
  Repeater
  A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
  
  
• Network hub
• Modem
  Modem
  A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
  
  
• Fiber Media Converter
  Fiber media converter
  Fiber media converters are simple networking devices that make it possible to connect two dissimilar media types such as twisted pair with fiber optic cabling. They were introduced to the industry nearly two decades ago, and are important in interconnecting fiber optic cabling-based systems with...
  
  

Relation to TCP/IP model

The TCP/IP model, defined in RFC 1122 and RFC 1123, is a high-level networking description used for the Internet and similar networks. It does not define an equivalent layer that deals exclusively with hardware-level specifications and interfaces, as this model does not concern itself directly with physical interfaces. Several RFCs

Request for Comments

In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...

 mention a physical layer and data link layer, but that is in context of IEEE protocols. RFC 1122 and 1123 do not mention any physical layer functionality or physical layer standards.

See also

• Clock recovery
  Clock recovery
  Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...
  
  
• Ethernet physical layer
  Ethernet physical layer
  The Ethernet physical layer is the physical layer component of the Ethernet family of computer network standards.The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed...
  
  
• Data transmission
  Data transmission
  Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
  
  
• Digital communication
• Digital modulation
• Line code
  Line code
  In telecommunication, a line code is a code chosen for use within a communications system for baseband transmission purposes...
  
  
• Pulse shaping
  Pulse shaping
  In digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
  
  
• Bit synchronization
• Channel model

External links

• Physical Layer (Layer 1)