The physical layer provides for physical connectivity between networked devices. Transmission and receipt of data from the physical medium (copper wire, fiber, radio frequencies, barbed wire, string etc.) is managed at this layer.

The physical layer receives data from the data link Layer, and transmits it to the wire. The physical layer controls the electrical and mechanical functions related to the transmission and receipt of a communications signal. It also manages the encoding and decoding of data contained within the modulated signal.

Note that for two devices to communicate, they must beconnected to the same type of physical medium (wiring). 802.3 Ethernet to 802.3 Ethernet, FDDI to FDDI, serial to serial etc. Two end stations using different protocols can only communicate through a multi-protocol bridge or a router.

The physical layer is responsible for two jobs:

1. Communication with the data link layer above it.
2. Fragmentation of data into frames
3. Reassembly of frames into data link Protocol Data Units.
4. Transmission and reciept of data.

Example Physical Protocols:

• CSMA/CD
• CSMA/CA