Lesson 2 - 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.

Manchester Encoding

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.

Differential Manchester Encoding

   Activities

See the Activities and Extended Activities section in Unit 4 Lesson 2 in your textbook Introduction to Networking to test what you have learned so far.

[ Previous Section ] [ Unit Contents ] [ Next Lesson ]