The first layer is the physical layer. This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels, transmission frequencies, timing, etc. The physical layer provides an unstructured bit stream, which is the phycial basis for all the higher layers.
Devices at this layer includes: Multiplexer, Repeater, hub, cable, network card, unshielded twisted pairs (UTP), etc.
Examples are Ethernet, RS-232, T1, DSL, etc.
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