Physical layer

The physical layer is the lowest layer in the OSI reference model. This layer gets data packets from the data-link layer above it, and converts the contents of these packets into a series of electrical signals that represent 0 and 1 values in a digital transmission. These signals are sent across a transmission medium to the physical layer at the receiving end. At the destination, the physical layer converts the electrical signals into a series of bit values. These values are grouped into packets and passed up to the data-link layer.

Transmission properties defined

The mechanical and electrical properties of the transmission medium are defined at this level. These include the following:

• The type of cable and connectors used: The cable may be coaxial, twistedpair, or fiber optic. The types of connectors depend on the type of cable.

• The pin assignments for the cable and connectors: Pin assignments depend on the type of cable and also on the network architecture being used.

• Format for the electrical signals: The encoding scheme used to signal 0 and 1 values in a digital transmission or particular values in an analog transmission depend on the network architecture being used. Most networks use digital signaling, and most use some form of Manchester encoding for the signal.

Note that this layer does not include the specifications of the actual medium used, but rather which medium should be used, and how. The specifications of, for example, Unshielded Twisted Pair as used by Ethernet is contained in specification EIA/TIA 568.

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