The physical layer is the first layer of OSI model.

The physical layer is in charge of the effective transmission of the electrical signals or optics between the interlocutors. Its service is typically limited to the emission and reception of a bit or a continuous bit train (in particular for the supports synchronous like fiberoptic). This layer is in charge of conversion between bits and electrical signals or optics. It is in practice always carried out by an electronic circuit specific.

The service of this layer is roughly defined by ISO 7498-1 and is specified by ISO 10022 and the X211 recommendation of CCITT.

In the case of the networks Ethernet, the data are transmitted on the physical layer by a PHYceiver.

Some encodings of layer 1

• Codings in baseband:
• NRZ
• Two-phase NRZI
• or Manchester
• differential Manchester
• simple Miller
• Bipolar or of a nature 2
• BHDn