|The physical later is concerned with
transmitting raw bits over a communication channel. The design
issues have to do with making sure that when one side sends a
1 bit, it is received by the other side as a 1 bit, not as a 0
bit. Typical questions here ar e how many volts should be used
to represent a 1 and how many for a 0, how many microseconds a
bit lasts, whether transmission may proceed simultaneously in
both directions, how the initial connection is established and
how it is torn down when both sides are finished, and how many
pins the network connector has and what each pin is used for.
The design issues here deal largely with mechanical,
electrical, and procedural interfaces, and the physical
transmission medium, which lies below the physical layer.
Physical layer design can properly be considered to be within
the domain of the electrical engineer.
Example : The X.21 digital interface.