Physical Layer

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The Physical Layer is the first and lowest layer in the seven-layer OSI model of computer networking.

The Physical Layer consists of 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.

The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical link 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. The Physical Layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation 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 into hardware-specific operations to effect transmission or reception of electronic signals.

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.^[1]

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 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 switching
• Multiplexing
  • 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^[2] for example bitwise convolutional coding
• Bit-interleaving and other channel coding

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

Physical Layer examples

• Telephone network modems- V.92
• IRDA Physical Layer
• USB Physical Layer
• EIA RS-232, EIA-422, EIA-423, RS-449, RS-485
• Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX and other varieties
• Varieties of 802.11Wi-Fi Physical Layers
• DSL
• ISDN
• T1 and other T-carrier links, and E1 and other E-carrier links
• SONET/SDH
• GSM Um radio interface physical layer
• Bluetooth Physical Layer
• ITU Recommendations: see ITU-T
• Firewire
• TransferJet Physical Layer
• Etherloop
• ARINC 818 Avionics Digital Video Bus
• G.hn/G.9960 Physical Layer

Hardware equipment (network node) examples

• Network adapter
• Repeater
• Network hub
• Modem
• Fiber Media Converter

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).^[3] and simply absorbs all hardware specific components of the operating system and interface firmware into the Link Layer 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 to achieve internetworking between disparate local area networks.

See also

• Clock recovery
• Ethernet physical layer
• Data transmission
• Digital communication
• Digital modulation
• Line code
• Pulse shaping
• Bit synchronization

References

External links

• http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/phy.html
• http://www.tcpipguide.com/free/t_PhysicalLayerLayer1.htm