Physical Layer Definition

The physical layer is the bottom layer in the seven layer OSI (open system interconnection) reference model.

This model was developed in 1977 in order to standardize and simplify definitions relating to computer networks. It divides the networking process into seven logical layers, starting at the physical layer and ascending to the application layer (which interfaces with application programs on computers). Services and protocols (i.e., agreed-upon formats) are specified for each layer, and each layer has unique responsibilities, including passing information to the layers above and below it.

The physical layer defines all physical and electrical specifications for devices used to interface to the network, including the shape and layout of pins in connectors, voltages, cable specifications and broadcast frequencies. It provides the means for transmitting raw bits, but it is not concerned with MAC addresses, IP addresses and packets; rather, these are dealt with by layers higher in the hierarchy.

The physical layer performs services requested by the data link layer, which is the layer directly above it. Its major functions and services are: (1) the establishment and termination of connections to a communications medium (e.g., twisted pair cable or optical fiber cable), (2) conversion between the representation of digital data in computers (or other network devices) and the corresponding signals transmitted over the communications medium and (3) participation in the efficient sharing among multiple devices of the communications medium through the use of flow control and collision resolution (i.e., recovery from simultaneous transmission by two or more devices).

Devices that operate at the physical layer include repeaters, hubs, network interface cards (NICs), cables and connectors. Repeaters are used to regenerate electrical signals that have attenuated (i.e., weakened) as a result of distance. A hub is a common connection point for twisted pair or optical fiber connecting devices in a local area network (LAN).

Examples of physical layer protocols are CSMA/CD (carrier sense multiple access/collision detection), DSL (digital subscriber line) and RS-232 (which is commonly used in computer serial ports).

Created September 29, 2005.
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