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Physical Layer



The physical layer is level one in the seven level OSI model of computer networking. It performs services requested by the data link layer.

This level refers to network hardware, physical cabling or a wireless electromagnetic connection. It also deals with electrical specifications, collision control and other low-level functions.

The physical layer is the most basic network layer, providing only the means of transmitting raw bits. The shapes of the electrical connectors, what frequencies to broadcast at, and similar low-level things are specified here. An analogy of this layer in a physical mail network would be a specification for various kinds of paper and ink, for example.

The major functions and services performed by the physical layer are:
  • establishment and termination of a connection to a communications medium;
  • participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control;
  • conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.

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
  • performs character encoding, transmission, reception, and decoding
  • performs optional isolation functions.
  • Source: from Federal Standard 1037C

Examples

  • EIA standards: RS-232, RS-422, RS-423, RS-449, RS-485
  • ITU Recommendations: see ITU-T
  • DSL
  • ISDN
  • T1, E1
  • 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T, 1000BASE-T, 1000BASE-SX are all physical layer transports for Ethernet
  • electricity
  • radio

Hardware Examples

  • Repeater
  • Hub
  • Modem

Note: Physical layer Associated with transmission of unstructured bit streams over a physical link. Responsible for the mechanical, electrical and procedural characterstics that establish, maintain and deactivate the physical link.

Network Traffic
Traffic from Provider for ADSL Access

In: 35%
Out: 100%

Traffic from Provider for Cable Access

In: 4%
Out: 6%

 

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