physical layer

The physical layer is level one in the seven-level OSI model of computer networking as well as in the five-layer TCP/IP reference model. It performs services requested by the data link layer. The physical layer is the most basic network layer, providing only the means of transmitting raw bits rather than packets over a physical data link connecting network nodes. No packet headers nor trailers are consequently added to the data by the physical layer. The bit stream may be grouped into code words or symbols, and converted to a physical signal, which is transmitted over a physical transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes of the electrical connectors, which frequencies to broadcast on, what modulation scheme to use and similar low-level parameters are specified here. An analogy of this layer in a physical mail network would be the roads along which the vans carrying the mail drive.
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See Open Systems Interconnection--Reference Model.

<networking> Layer one, the lowest layer, in the OSI seven layer model, concerning electrical and mechanical connections to the network. The physical layer is used by the data link layer. Example physical layer protocols are CSMA/CD, token ring and bus.
(1994-12-23)