The very first layer of the OSI model is the physical one, which is in charge of the transmission of unprocessed data from a device, similar to an Ethernet port, and its transmission medium, such as an Ethernet cable. This layer defines all the characteristics that are related to the physical/material nature of the connection—size, shape, voltages of the connector, frequencies, and timings.
Another aspect defined by the physical layer is the direction of the transmission, which can be one of the following:
- Simplex: The communication is one way.
- Half duplex: The communication is two way, but the communication happens only in one direction at a time.
- Full duplex: Two way-communication, where both ends can communicate at the same time.
Many well-known technologies, including Bluetooth and Ethernet, include a definition of the physical layer they are using.