When AM is used for multiplexing digital data, it is known as amplitude shift keying (ASK). Other names include: on-off keying, continuous wave and interrupted continuous wave. An essential part of electronic communications and telecommunications is the ability to share the channel. This is true regardless of whether the channel is copper wire, optical fiber or free-space. If it's not shared then there can only ever be one person transmitting on it at a time. Think about the implications of this for a moment. Without the ability to share, there could only be one radio or TV station in each area. Only one mobile phone owner could use their phone in each cell at any one time. And there would only be the same number of phone calls between any two cities as the number of copper wires or optical fibers that connected them.

So sharing the channel is essential and there are several methods of doing so. One is called time division multiplexing (TDM) and involves giving the users exclusive access to the channel for short periods of time. On the face of it, this type of sharing might seem impractical. Imagine giving all mobile phone users in a cell just a minute or so to make their call then having to wait until their turn comes around again. However, TDM works well when the access time is extremely short (less than a second) and the rate of the sharing is fast. This allows multiple users to appear to have access all at the same time.

Notice that the ASK signal's upper and lower limits (the envelopes) are the same shape as the data stream (though the lower envelope is inverted). This is simultaneously an advantage and a disadvantage of ASK. Recovery of the data stream can be implemented using a simple envelope detector. However, noise on the channel can change the envelopes' shape enough for the receiver to interpret the logic levels incorrectly causing errors i.e. analog AM communications have the same problem and the errors are heard as a hiss, crackles and pops. Fig.1 shows what an ASK signal looks like time-coincident with the digital signal that has been used to generate it.

Here you'll examine the operation of an alternative method that involves using the digital signal to switch the carrier's connection to the channel on and off.