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Layer 1: Physical

A person pushing a large rock up a hill.  Source:  Microsoft.

The bottom layer of the OSI model is the physical layer. This layer transmits the unstructured raw data, i.e. bits, over the physical medium. The physical layer is totally hardware based and deals with all aspects of establishing and maintaining a physical link between communicating computers.

The physical layer defines how the cable is attached to the NIC. For example, it defines how many pins a connector has and the function of each. It also defines which transmission technique will be used to send data over the network cable.

This layer provides data encoding and bit synchronisation. The physical layer is responsible for transmitting bits (zeros and ones) from one computer to another and because different types of media physically transmit bits differently, the physical layer also defines the duration of each impulse and how each bit is translated into the appropriate electrical or optical impulse for the network cable.

Layer 1 PDU: Bit

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