Physical Layer Details

Posted On 2008-04-08 by FortyPoundHead
Keywords: OSI Model
Tags: Networking Tutorial 
Views: 4453

The physical layer is level one in the seven-level OSI model of computer networking as well as in the five-layer TCP/IP reference model. It performs services requested by the data link layer.

The physical layer is the most basic network layer, providing only the means of transmitting raw bits rather than packets over a physical data link connecting network nodes. No packet headers nor trailers are consequently added to the data by the physical layer. The bit stream may be grouped into code words or symbols, and converted to a physical signal, which is transmitted over a physical transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes of the electrical connectors, which frequencies to broadcast on, what modulation scheme to use and similar low-level parameters are specified here. An analogy of this layer in a physical mail network would be the roads along which the vans carrying the mail drive.

Physical signaling sublayer
In a local area network (LAN) or a metropolitan area network (MAN) using open systems interconnection (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 encoding, transmission, reception and decoding

  • performs mandatory isolation functions.

List of physical layer services
The major functions and services performed by the physical layer are:
  • Bit-by-bit node-to-node delivery

  • Providing a standardized interface to physical transmission media, 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

  • Modulation

  • Line coding

  • Bit synchronization in synchronous serial communication

  • Start-stop signalling and flow control in asynchronous serial communication

  • Circuit mode multiplexing,[citation needed] as opposed to statistical multiplexing performed at the higher level

  • Establishment and termination of circuit switched connections

  • Carrier sense and collision detection utilized by some level 2 multiple access protocols

  • Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals

  • Forward error correction,[citation needed] for example bitwise convolutional coding

  • Bit-interleaving and other channel coding

The physical layer is also concerned with
  • 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

Physical Layer Examples
  • V.92 telephone network modems

  • IRDA physical layer

  • USB physical layer

  • Firewire

  • EIA RS-232, EIA-422, EIA-423, RS-449, RS-485

  • ITU Recommendations: see ITU-T

  • DSL

  • ISDN

  • T1 and other T-carrier links, and E1 and other E-carrier links

  • 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties of the Ethernet physical layer

  • Varieties of 802.11


  • GSM radio interface

  • Bluetooth physical layer

  • IEEE 802.11x Wi-Fi physical layers

  • Etherloop

About the Author

FortyPoundHead has posted a total of 1974 articles.

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