|  | 4.4 | Basics of Encoding Networking Signals
| 4.4.2 | Modulation and encoding

Encoding means converting 1s and 0s into something real and physical, such as:

- an electrical pulse on a wire
- a light pulse on an optical fiber
- a pulse of electromagnetic waves into space

Two methods of accomplishing this are TTL encoding and Manchester encoding.

TTL (transistor-transistor logic) encoding is the simplest. It is characterized by a high signal and a low signal (often +5 or +3.3 V for binary 1 and 0 V for binary 0). In optical fibers, binary 1 might be a bright LED or laser light, and binary 0 dark or no light. In wireless networks, binary 1 might mean a carrier wave is present, and binary 0 no carrier at all.

Manchester encoding is more complex, but is more immune to noise and is better at remaining synchronized. In Manchester encoding, the voltage on copper wire, the brightness of LED or laser light in optical fiber, or the power of an EM wave in wireless has the bits encoded as transitions. Observe that the Manchester encoding results in 1 being encoded as a low-to-high transition and 0 being encoded as a high-to-low transition. Because both 0s and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver.

Closely related to encoding is modulation, which specifically means taking a wave and changing, or modulating it so that it carries information. To give you an idea of what modulation is, examine three forms of modifying, or modulating, a carrier wave to encode bits:

- AM (amplitude modulation) - the modulation, or height, of a carrier sine wave is varied to carry the message
- FM (frequency modulation) - the frequency, or wiggly-ness, of the carrier wave is varied to carry the message
- PM (phase modulation) - the phase, or beginning and ending points of a given cycle, of the wave is varied to carry the message

Other more complex forms of modulation also exist. The Figure shows three ways binary data can be encoded onto a carrier wave by the process of modulation. Binary 11 (Note: read as one one, not eleven!) can be communicated on a wave by either AM (wave on/wave off), FM (wave wiggles a lot for 1s, a little for 0s), or PM (one type of phase change for 0s, another for 1s).

Messages can be encoded in a variety of ways:

1. As voltages on copper; Manchester and NRZI encoding are popular on copper-based networks.

2. As guided light; Manchester and 4B/5B encoding are popular on fiber based networks.

3. As radiated EM waves; a wide variety of encoding schemes (variations on AM, FM, and PM) are used on wireless networks.

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