Keying Methods

Keying methods allow data signals to be manipulated so that receiving stations can distinguish between 0s and 1s. These keying methods are also commonly called modulation techniques. There are two different general techniques that keying methods use to represent data:

• Current State: With this technique, the value of the signal at a given time determines the binary value. For instance, you could use a door to represent data by checking if it is open or closed at a specific moment in time (open is 0 and closed is 1).
• State Transition: Here, a change in the signal is used to distinguish between 0s and 1s. For example, a change in phase at a specific time could be a 0, while no change in phase would be a 1.

There are three types of keying methods:

Amplitude Shift Keying (ASK): ASK varies the amplitude of the signal to signify the binary data, designating a specific amplitude value for a 0 bit and another for a 1 bit. However, noise on the amplitude of the signal can make ASK less reliable.

Frequency Shift Keying (FSK): FSK works by varying the frequency of the signal to represent binary data. One specific frequency can represent a 0 bit, while a different frequency can represent a 1 bit. FSK is less susceptible to certain types of noise than ASK.

Phase Shift Keying (PSK): PSK changes the phase of the signal to denote the binary data. Similar to FSK, PSK designates a specific phase shift for a 0 bit and another for a 1 bit. PSK can be more resistant to noise and interference than the other two methods.