Physical layer

The Physical Layer is the first level in the seven-layer OSI model of computer networking. It translates communications requests from the Data Link Layer into hardware-specific operations to affect transmission or reception of electronic signals.

The Physical Layer is a fundamental layer upon which all higher level functions in a network are based. However, 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 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 of the electrical connectors, which frequencies to broadcast on, which modulation scheme to use and similar low-level parameters are specified here.

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.

Source: from Federal Standard 1037C

List of Physical Layer services

The major functions and services performed by the Physical Layer are:

• Bit-by-bit 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, 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, 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
• SONET/SDH
• GSM radio interface
• Bluetooth Physical Layer
• IEEE 802.11x Wi-Fi Physical Layers
• Etherloop

Hardware equipment (network node) examples

• Network adapter
• Repeater
• Network hub
• Modem

Relation to TCP/IP model

The TCP/IP model has no equivalent layer that deals exclusively with hardware-level specifications, as this model does not concern itself directly with physical interfaces. It assumes a functioning host operating system with a facility to transmit raw data blocks onto the local network. TCP/IP simply places all hardware specific components of the operating system, as well as interface firmware, etc., into the Link Layer. The TCP/IP model is not a top/down comprehensive design reference for general networks and networking hardware. It was formulated for the purpose of illustrating the design of the suite of internetworking methods used in the Internet Protocol Suite into logical group functions.

See also

• Ethernet physical layer
• line code

References

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