Layer 1: The Physical Layer

The bottom layer is called the Physical Layer. This layer is responsible for the transmission of the bit stream. It accepts frames of data from Layer 2, the Data Link Layer, and transmits their structure and content, typically serially, one bit at a time. The Physical Layer is also responsible for the reception of incoming streams of data, one bit at a time. These streams are then passed on to the Data Link Layer for reframing.

The Physical Layer, quite literally, sees only 1s and 0s. It has no mechanism for determining the significance of the bits it transmits or receives. It is solely concerned with the physical characteristics of electrical and/or optical signaling techniques. This includes the voltage of the electrical current used to transport the signal (the line encoding and framing not to be confused with the data character coding and framing), the media type andvimpedance characteristics, and even the physical shape of the connector that is used tovterminate the media.

A common misperception is that OSI’s Layer 1 includes anything that either generates or carries the data communications signals. This is not true. It is a functional model only. The Physical Layer is limited to just the processes and mechanisms needed to place signals onto the transmission media, and to receive signals from that media. Its lower boundary is the physical connector that attaches to the transmission media. It does not include the transmission media, though the different LAN (Local Area Network) standards, such as 10BASET and 100BASET, do refer to the media type used.

Transmission media include any means of actually transporting signals generated by the OSI’s Layer 1 mechanisms. Some examples of transmission media are coaxial cabling, fiber-optic cabling, and twisted-pair wiring. The confusion seems to stem from the fact that the Physical Layer does provide specifications for the media’s performance. These are the performance characteristics that are required, and assumed to exist, by the processes and mechanisms defined in the Physical Layer.

Consequently, transmission media remain outside the scope of the Physical Layer and are sometimes referred to as Layer 0 of the OSI Reference Model.