The Physical Layer

   Example: Manchester Encoding

A simple example of a Physical Layer protocol is the Manchester encoding technique used on copper-cabled local area networks (LANs). This is a synchronous protocol, because it is based on the "ticks" of an electronic timer. Each binary bit is allocated a fixed slice of time. At the middle of that time slice, if the voltage changes from low to high, this is interpreted as a binary 1. If the voltage drops from high to low at the middle of the bit time, this is a 0. The Manchester Encoding Diagram shows how a binary number is represented using this signaling technique. The device that receives this signal, such as a NIC, only pays attention to voltage changes that occur in the middle of a 1-bit time segment. Thus, to represent a series of 1s, the signal must first drop to low voltage at the beginning of a bit time, so it is ready to change from low to high again to represent another binary 1.

A variation of this method, called Differential Manchester encoding, avoids this skipping signal. It counts each voltage change at a clock tick, from high to low or low to high, as a binary 1. A binary 0 is represented by no voltage change at the clock tick. The Differential Manchester Encoding Diagram displays an example of this encoding scheme.