Definition

physical layer

Contributor(s): Andrew Froehlich

Located at the lowest layer of the Open Systems Interconnection (OSI) communications model, the physical layer's function is to transport data using electrical, mechanical or procedural interfaces.

For example, the physical layer determines how to use electricity to place a stream of raw bits from Layer 2, the data link layer, onto the pins and across the wires of a twisted pair cable. From an optical standpoint, the physical layer converts a stream of 0s and 1s onto fiber using light as its physical medium. Lastly, the physical layer uses a wireless transmitter to convert these bits into radio waves for transport using radio waves.

The physical layer is usually a combination of software and hardware programming and may include electromechanical devices. Popular transport technology types include 1000BASE-T Ethernet, 1000BASE-SX Ethernet, T1, SONET/SDH, DSL and 802.11 physical layer variants.

Functions of the physical layer

The physical layer is responsible for sending computer bits from one device to another along the network. It does not understand the bits; rather, its role is determining how physical connections to the network are set up and how bits are represented into predictable signals as they are transmitted either electrically, optically or via radio waves.

To do this, the physical layer performs a variety of functions, including:

  • Defining bits: Determines how bits are converted from 0s and 1s to a signal.
  • Data rate: Determines how fast the data flows, in bits per second.
  • Synchronization: Ensures that sending and receiving devices are synchronized.
  • Transmission mode: Determines the direction of transmissions and whether those are simplex (one signal is transmitted in one direction), half-duplex (data goes in both directions, but not at the same time) and full-duplex (data is transmitted in both directions, simultaneously).
  • Interface: Determines how devices are connected to a transmission medium such as Ethernet or radio waves.

Physical layer devices

The physical layer covers a variety of devices and mediums, among them cabling, connectors, receivers, transceivers and repeaters.

OSI Physical layer illustrated
This was last updated in March 2018

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Because of all the various hardware and cabling technologies available, what new functionalities should be included in the physical layer?
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