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The Physical Layer is the first and lowest layer in the seven-layer OSI model
OSI model

The Open Systems Interconnection Reference Model is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection initiative....
 of computer network
Computer network

A computer network is a group of interconnected computers. Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories and also presents the basic components of a network....
ing.

The Physical Layer comprises the basic hardware transmission technologies of a network. 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.






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The Physical Layer is the first and lowest layer in the seven-layer OSI model
OSI model

The Open Systems Interconnection Reference Model is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection initiative....
 of computer network
Computer network

A computer network is a group of interconnected computers. Networks may be classified according to a wide variety of characteristics. This article provides a general overview of some types and categories and also presents the basic components of a network....
ing.

The Physical Layer comprises the basic hardware transmission technologies of a network. 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 implementation of this layer is often termed PHY
PHY

PHY is a common abbreviation for the physical layer of the OSI model.A PHY connects a link layer device to a physical medium such as an optical fibre or copper cable....
.

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 digital information....
 connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal
Signal

Signal, signals, signaling, or signalling may refer to:...
 that is transmitted over a hardware transmission medium
Transmission medium

A transmission medium is a material substance which can wave propagation energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound....
. 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 a Electrical conductor for joining electrical circuits together. The connection may be temporary, as for portable equipment, or may require a tool for assembly and removal, or may be a permanent electrical joint between two wires or devices....
s, the frequencies to broadcast on, the modulation
Modulation

In telecommunications, modulation is the process of varying a Periodic function waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and Pitch ....
 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.The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network network segment....
 into hardware-specific operations to effect 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 covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport....
 (LAN) or a metropolitan area network
Metropolitan area network

Metropolitan area networks, or MANs, are large computer networks usually spanning a city. They typically use wireless infrastructure or Optical fiber connections to link their sites....
 (MAN) using open systems interconnection
Open Systems Interconnection

The Open Systems Interconnection was an effort to standardize Computer network that was started in 1982 by the International Organization for Standardization , along with the ITU-T....
 (OSI) architecture, the physical signaling sublayer is the portion of the Physical Layer that:
  • interfaces with the medium access control sublayer (MAC) which is a part of the Data Link Layer
  • 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 language form of a natural language....
     encoding, transmission
    Transmission (telecommunications)

    In telecommunications, transmission 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 or wireless....
    , reception and decoding.
  • performs mandatory isolation functions.


Source: from Federal Standard 1037C
Federal Standard 1037C

Federal Standard 1037C, entitled Telecommunications: Glossary of Telecommunication Terms is a United States Federal Standard, issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, as amended....
.

List of Physical Layer 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 a Electrical conductor for joining electrical circuits together. The connection may be temporary, as for portable equipment, or may require a tool for assembly and removal, or may be a permanent electrical joint between two wires or devices....
      s and cable
      Cable

      A cable is a large fiber or metal rope, used for hauling, lifting, or towing, or an assembly of two or more insulated electrical conductors, laid up together as an assembly....
      s, for example maximum cable length
    • Electrical specification of transmission line
      Transmission line

      A transmission line is the material Transmission medium or structure that forms all or part of a Course from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission....
       signal level
      Signal level

      In a communications system, the signal level is the Signalling Power or intensity at a specified point and with respect to a specified reference level, e.g., 1 mW....
       and impedance
      Electrical impedance

      Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of Electrical resistance to AC circuits, describing not only the relative amplitudes of the voltage and Electric current, but also the relative Phase ....
    • Radio interface, including electromagnetic spectrum
      Electromagnetic spectrum

      The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
       frequency allocation
      Frequency allocation

      The radio frequency electromagnetic spectrum is an aspect of the physical world which, like land, water, and air, is subject to usage limitations. 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...
       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....
      , analog bandwidth, etc.
    • Specifications for IR over optical fiber
      Optical fiber

      An optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers....
       or a wireless IR communication link
  • Modulation
    Modulation

    In telecommunications, modulation is the process of varying a Periodic function waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and Pitch ....
  • Line coding
  • Bit synchronization in synchronous serial communication
  • Start-stop signalling and flow control
    Flow control

    In computer networking, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from over running a slow receiver....
     in asynchronous serial communication
  • Circuit switching
    Circuit switching

    In telecommunications, a circuit switching network is one that establishes a telecommunication circuit between Node and Terminal before the user may communicate, as if the nodes were physically connected with an electrical circuit....
  • Multiplexing
    Multiplexing

    In telecommunications and computer networks, multiplexing is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium....
    • Establishment and termination of circuit switched connections
  • Carrier sense and collision detection utilized by some level 2 multiple access protocols
  • Equalization
    Equalization

    Equalization, equalisation or EQ is the process of using passive or active electronic elements or digital algorithms for the purpose of altering the frequency response characteristics of a system....
     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 suit better to the communication channel by limiting the effective Bandwidth of the transmission....
     and other signal processing
    Signal processing

    Signal processing is the analysis, interpretation, and manipulation of signal . Signals of interest include: audio signal processing, , time-varying measurement values and sensor data, for example biological data such as electrocardiograms, control system signals, telecommunication transmission signals such as radio signals, and many others....
     of physical signals
  • Forward error correction
    Forward error correction

    In telecommunication and information theory, forward error correction is a system of error control for data transmission, whereby the sender adds Redundancy to its messages, also known as an error correction code....
    , for example bitwise convolutional coding
  • Bit-interleaving and other channel coding


The Physical Layer is also concerned with
  • Bit rate
  • Point-to-point, multipoint
    Multipoint

    Multipoint may refer to:* Fuel_injection#Multi-point fuel injection, an injection scheme for metering fuel into an internal combustion engine...
     or point-to-multipoint
    Point-to-multipoint

    For general information for point-to-multipoint communication, refer to Link #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 multipoint connection, providing multiple paths from a single locat...
     line configuration
  • Physical network topology
    Topology

    Topology is a major area of mathematics that has emerged through the development of concepts from geometry and set theory, such as those of space, dimension, shape, transformation and others....
    , for example bus
    Bus network

    A bus network topology is a computer network in which a set of Client are connected via a shared communications line, called a computer bus. There are several common instances of the bus architecture, including one in the PC motherboard of most computers, and those in some versions of Ethernet networks....
    , 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 Network topology. In its simplest form, a star network consists of one central Network switch, Network hub or computer, which acts as a conduit to transmit messages....
  • Serial or parallel communication
  • Simplex
    Simplex communication

    Simplex communication is a name for a type of communication circuit. There are two definitions that have been used for the term. When one definition is used for "simplex", then the other definition is actually referred to as half duplex....
    , half duplex
    Duplex (telecommunications)

    A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions....
     or full duplex
    Duplex (telecommunications)

    A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions....
     transmission mode
  • Autonegotiation
    Autonegotiation

    Autonegotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed and duplex mode. In this process, the connected devices first share their capabilities as for these parameters and then choose the fastest transmission mode they both support....


Physical Layer examples

  • V.92 telephone network modems
  • IRDA
    Infrared Data Association

    The Infrared Data Association defines physical specifications communications protocol standardizations for the short-range exchange of data over infrared light, for uses such as personal area networks ....
     Physical Layer
  • USB Physical Layer
  • Firewire
    FireWire

    The IEEE 1394 interface is a serial communications 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....
  • EIA
    Electronic Industries Alliance

    The Electronic Industries Alliance is a trade organization composed as an alliance of trade associations for electronics manufacturers in the United States....
     RS-232
    RS-232

    In telecommunications, RS-232 is a standard for serial communications binary data signals connecting between a DTE and a DCE . It is commonly used in computer serial ports....
    , EIA-422
    EIA-422

    American National Standards Institute ANSI/TIA/EIA-422-B and its international equivalent ITU-T Recommendation , are technical Standardizations that specify the "electrical characteristics of the balanced voltage digital interface circuit"....
    , EIA-423
    RS-423

    RS-423 is a standard for serial communications, similar to RS-232. There is no common pinout for RS-423. The BBC Micro computer used a 5-pin DIN connector....
    , 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
  • ITU
    International Telecommunication Union

    The International Telecommunication Union is the second-oldest international organization still in existence , established to standardize and regulate international radio and telecommunications....
     Recommendations: see ITU-T
    ITU-T

    The Telecommunication Standardization Sector coordinates standards for telecommunications on behalf of the International Telecommunication Union and is based in Geneva, Switzerland....
  • DSL
    Digital Subscriber Line

    DSL or xDSL, is a family of technologies that provides digital data transmission over the wires of a local access network. DSL originally stood for digital subscriber loop, although in recent years, the term digital subscriber line has been widely adopted as a more marketing-friendly term for ADSL, which is the most popular...
  • ISDN
    Integrated Services Digital Network

    File:T-Concept-ISDN.jpgIntegrated Services Digital Network is a telephone system network. Prior to the ISDN, the phone system was viewed as a way to transport voice, with some special services available for data....
  • 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 multiplexing telecommunications carrier systems originally developed by Bell Labs and used in North America, Communications in Japan, and Communications in 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, worldwide standards have been created and deployed....
     links
  • 10BASE-T
    10BASE-T

    Ethernet over twisted pair refers to the use of a pair of copper cables, twisted around each other, for the physical layer of an Ethernet network ....
    , 10BASE2
    10BASE2

    10BASE2 is a variant of Ethernet that uses thin coaxial cable , terminated with BNC connectors. For many years this was the dominant 10 Mbit/s Ethernet standard, but due to the immense demand for high speed networking, the low cost of Category 5 cable Ethernet cable, and the popularity of 802.11 wireless networks, both 10BASE2 and...
    , 10BASE5
    10BASE5

    10BASE5 is the original "full spec" variant of Ethernet cable, using special cable similar to RG-8/U coaxial cable. This is a stiff, diameter cable with an impedance of 50 Ohm s , a solid center conductor, a foam insulating filler, a shielding braid, and an outer jacket....
    , 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties of the Ethernet physical layer
    Ethernet physical layer

    The Ethernet physical layer is the physical layer component of the Ethernet standard.The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several Magnitude s of speed....
  • Varieties of 802.11
  • SONET/SDH
  • 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 phone and the Base transceiver station ....
     physical layer
  • Bluetooth
    Bluetooth

    Bluetooth is a wireless protocol for exchanging data over short distances from fixed and mobile devices, creating personal area networks . It was originally conceived as a wireless alternative to RS232 data cables....
     Physical Layer
  • IEEE 802.11x Wi-Fi
    Wi-Fi

    Wi-Fi is a trademark of the Wi-Fi Alliance, founded in 1999 as Wireless Internet Compatibility Alliance , comprising more than 300 companies, whose products are certified by the Wi-Fi Alliance, based on the IEEE 802.11 standards ....
     Physical Layers
  • TransferJet
    TransferJet

    TransferJet is a new type of close proximity wireless transfer technology developed by Sony and 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 "next generation" Digital subscriber line 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

    Introduction ARINC 818 is a video interface and protocol standard developed for high bandwidth, low latency, uncompressed digital video transmission in Avionics systems....
     Avionics Digital Video Bus


Hardware equipment (network node) examples

  • Network adapter
  • Repeater
    Repeater

    A repeater is an Electronics 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 without degradation....
  • Network hub
  • Modem
    Modem

    Modem is a peripheral device that modulation an analog carrier wave Signal to encode digital information, and also demodulation such a carrier signal to decode the transmitted information....
  • Fiber Media Converter
    Fiber media converter

    Media converters play an important role in today's multi-protocol, mixed media Local Area Networks. For example, Network administrator can deploy media converters to integrate fiber optic cabling and active equipment into existing copper-based, structured cabling systems while achieving significant cost-savings....


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, as this model does not concern itself directly with physical interfaces. It specifies a functioning host operating system with a facility to transmit packets onto the local network via a local area network encapsulation method (e.g., RFC 1042). and simply absorbs all hardware specific components of the operating system and interface firmware into the Link Layer
Link layer

In computer networking, the Link Layer is the lowest layer in the Internet Protocol Suite, the networking architecture of the Internet . It is the group of methods or Communications protocol that only operate on a host's link....
 without detailed specifications. The TCP/IP model is not a top/down comprehensive design reference for general networks and networking hardware, but an architecural description of the suite of methods and requirements used in the Internet Protocol Suite
Internet protocol suite

The Internet Protocol Suite is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol and the Internet Protocol , which were the first two networking protocols defined in this standard....
 to achieve internetworking
Internetworking

Internetworking involves connecting two or more computer networks via gateway using a common routing technology. The result is called an internetwork ....
 between disparate local area networks.

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 standard.The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several Magnitude s of speed....
  • Data transmission
    Data transmission

    Data transmission is the physical transfer of data from point-to-point often represented as an electro-magnetic Signal over a point-to-point or point-to-multipoint communication channel....
  • Digital communication
  • Digital modulation
  • Line code
    Line code

    In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes. Line coding is often used for digital data transport....
  • 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 suit better to the communication channel by limiting the effective Bandwidth of the transmission....
  • Bit synchronization


External links