When data is sent, a signal is transmitted from the transceiver. In order for the data to be transmitted, the signal must be manipulated so that the receiving station has a way of distinguishing 0s and 1s.
This method of manipulating a signal so that it can represent multiple pieces of data is known as a keying method. A keying method is what changes a signal into a carrier signal. It provides the signal with the ability to encode data so that it can be communicated or transported.
There are three types of keying methods that will be reviewed in the following: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), and Phase Shift Keying (PSK). These keying methods are also referred to as modulation techniques.
Keying methods use two different techniques to represent data: Current State With current state techniques, the current value (the current state) of the signal is used to distinguish between 0s and 1s.
The use of the word current in this context does not refer to current as in voltage but rather to current as in the present time. Current state techniques will designate a specific or current value to indicate a binary 0 and another value to indicate a binary 1.
At a specific point in time, it is the value of the signal that determines the binary value. For example, you can represent 0s and 1s using an ordinary door. Once a minute you can check to see if the door is open or closed.
If the door is open it represents a 0, and if the door is closed it represents a 1. The current state of the door, open or closed, is what determines 0s or 1s.
State Transition With state transition techniques, the change (or transition) of the signal is used to distinguish between 0s and 1s. State transition techniques may represent a 0 by a change in the phase of a wave at a specific time, whereas a 1 would be represented by no change in the phase of a wave at a specific time.
At a specific point in time, it is the presence of a change or the lack of presence of a change that determines the binary value. The section on Phase Shift Keying will provide examples of this in detail, but a door can be used again to provide a simple example.
Once a minute you check the door. In this case, if the door is moving (opening or closing), it represents a 0, and if the door is still (either open or closed), it represents a 1. In this example, the state of transition (moving or not moving) is what determines 0s or 1s.
Amplitude Shift Keying
Amplitude Shift Keying (ASK) varies the amplitude or height of the signal to represent the binary data. ASK is a current state technique, where one level of amplitude can represent a 0 bit and another level of amplitude can represent a 1 bit. Figure 1 shows how a wave can modulate an ASCII letter K using Amplitude Shift Keying.
The larger amplitude wave is interpreted as a binary 1, and the smaller amplitude wave is interpreted as a binary 0. This shifting of amplitude determines the data that is being transmitted.
The way the receiving station performs this task is to first divide the signal being received into periods of time known as symbol periods. The receiving station then samples or examines the wave during this symbol period to determine the amplitude of the wave.
Depending upon the value of the amplitude of the wave, the receiving station can determine the binary value.As you will learn later, wireless signals can be unpredictable and also subject to interference from many sources.
When noise or interference occurs, it usually affects the amplitude of a signal. Since a change in amplitude due to noise could cause the receiving station to misinterpret the value of the data, ASK has to be used cautiously.
Frequency Shift Keying
Frequency Shift Keying (FSK) varies the frequency of the signal to represent the binary data. FSK is a current state technique, where one frequency can represent a 0 bit and another frequency can represent a 1 bit (Figure below). This shifting of frequency determines the data that is being transmitted.
When the receiving station samples the signal during the symbol period, it determines the frequency of the wave, and depending upon the value of the frequency, the station can determine the binary value.
Figure 2 shows how a wave can modulate an ASCII letter K using Frequency Shift Keying. The faster frequency wave is interpreted as a binary 1, and the slower frequency wave is interpreted as a binary 0.
FSK is used in some of the earlier 802.11 standards. With the demand for faster communications, FSK techniques would require more expensive technology to support faster speeds, making it less practical.
Phase Shift Keying
Phase Shift Keying (PSK) varies the phase of the signal to represent the binary data. PSK is a state transition technique, where one phase can represent a 0 bit and another phase can represent a 1 bit. This shifting of phase determines the data that is being transmitted.
When the receiving station samples the signal during the symbol period, it determines the phase of the wave and the status of the bit. Figure 3 shows how a wave can modulate an ASCII letter K using Phase Shift Keying.
A phase change at the beginning of the symbol period is interpreted as a binary 1, and the lack of a phase change at the beginning of the symbol period is interpreted as a binary 0. PSK is used extensively in the 802.11 standards.
Typically, the receiving station samples the signal during the symbol period and compares the phase of the current sample with the previous sample and determines the difference.
This degree difference, or differential, is used to determine the bit value. More advanced versions of PSK can encode multiple bits per symbol. Instead of using two phases to represent the binary values, four phases can be used.
Each of the four phases is capable of representing two binary values (00, 01, 10, or 11) instead of one (0 or 1), thus shortening the transmission time. When more than two phases are used, this is referred to as Multiple Phase Shift Keying (MPSK).
Figure 4 shows how a wave can modulate an ASCII letter K using a Multiple Phase Shift Keying method. Four possible phase changes can be monitored, with each phase change now able to be interpreted as 2 bits of data instead of just 1. Notice that there are fewer symbol times in this drawing than there are in the drawing in Figure 3.
