Physical Layer

Definition of Physical Layer in The Network Encyclopedia.

What is the Physical Layer?

Physical Layer is the first Layer (or the PHY layer) of the Open Systems Interconnection reference model, also known as the OSI Model or the Seven Layer Model. The physical layer is the bottom layer of the seven-layer OSI networking architecture model.

Physical Layer is the first of seven layers of the OSI Model
Physical Layer is the first of seven layers of the OSI Model

It establishes the physical interface and mechanisms for placing a raw stream of bits onto the wire. It defines the voltage, current, modulation, bit synchronization, connection activation and deactivation, and various electrical characteristics for the transmission media (such as unshielded or shielded twisted-pair cabling, coaxial cabling, and fiber-optic cabling). Protocols at the PHY layer include IEEE 802.3, RS-232C, and X.21. Repeaters, transceivers, network interface cards (NICs), and cabling operate at the PHY level.

Physical Layer Functions

The main functions of the physical layer in the OSI Reference Model are:

  • Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).

  • Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.

  • Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!

  • Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.

Physical layer technologies are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.

The Physical Layer in the Internet Protocol

The Internet protocol suite, as 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 and interfaces, as this model does not concern itself directly with physical interfaces. Several RFCs mention a physical layer and data link layer, but that is in the context of IEEE protocols. RFC 1122 and 1123 do not mention any physical layer functionality or physical layer standards.

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