El nivel físico o capa física se refiere a las transformaciones que se hacen a la secuencia de bits para trasmitirlos de un lugar a otro. Siempre los bits se manejan dentro del PC como niveles eléctricos. Por ejemplo, puede decirse que en un punto o cable existe un 1 cuando hay presente un determinado nivel de voltaje y un cero cuando su nivel es de 0 voltios. Cuando se trasmiten los bits siempre se transforman en otro tipo de señales de tal manera que en el punto receptor puede recuperar la secuencia de bits originales.

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In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer. This layer may be implemented by a PHY. The physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture. The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here. Within the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to affect transmission or reception of electronic signals.

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