The Physical Layer

The Physical layer is the lowest layer of the OSI Model. It provides the resources to transport the bits; that make up a data link layer frame across the network media. This layer accepts a complete frame from the data link layer and encodes it as a series of signals that transmitted the local media. (see figure 3.1) The encoded bits contain a frame received by either an end device or an intermediate device.

physical layer

This layer also deals with the physical connection to the network and with transmission and reception of signals. This layer also defines electrical and physical details represented as 0 or a 1. It’s also decided when the data transmitted or not and how the data would be synchronized.

The process that data travel from a source node to a destination node is following:

  • The user data segmented by the transport layer placed into packets by the network layer and further encapsulated into frames by the data link layer. Data link layer sent these frames to the physical layer.
  • The physical layer encodes the frames and creates the electrical, optical; or radio wave signals that represent the bits(0 and 1) in each frame.
  • These signals then transmitted to the media.
  • The destination node physical layer retrieves these signals from the media; restores them to their bit representations, and passes the bits up to the data link layer as a complete frame(see figure 3.1).
  • Line configuration: – This layer connects devices with the medium;  Point to Point configuration and Multipoint configuration.
  • Transmission Modes: –  Physical Layer also defines the direction of transmission between devices ( Simplex, Half Duplex, Full Duplex).


Many different types of media can be used for the physical layer. For example, telephone twisted pair, coax cable, shielded copper cable; and fiber optics are the main types used for LANs. Different transmission techniques generally categorized as baseband, or broadband transmission may apply to each of these media types.

There are three basic types of media. The physical layer produces the representation and groupings of bits for each type of media as:

To enable physical layer interoperability, all aspects of these functions governed by standards organizations.


Upper layer Protocol: – Protocols and operation of the upper OSI layers totally performed in software which designed by software engineers and computer scientists. IETF (Internet Engineering Task Force ) is an organization which defined the services and protocol for TCP/IP suite.

The physical layer consists of electronic circuitry, media, and connectors. Therefore, it is suitable that the standards governing this hardware are defined by the relevant electrical and communications engineering organizations.

There are many different international and national organizations, regulatory government organizations, and private companies involved in establishing and maintaining physical layer standards. For example, the physical layer hardware, media, encoding, and signaling standards defined and governed by the following:-

  • International Organization for Standardization (ISO)
  • International Telecommunication Union (ITU)
  • American National Standards Institute (ANSI)
  • Institute of Electrical and Electronics Engineers (IEEE)
  • Telecommunications Industry Association/Electronic Industries Association (TIA/EIA)
  • National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)
  • Canadian Standards Association (CSA)
  • European Committee for Electrotechnical Standardization(CENELEC)