Clear Channel Assessment and Details on Carrier Sense

Now that we've covered the preamble, you can begin to understand what the term carrier sense would mean in wireless.

The term clear channel assessment (CCA) represents how a radio determines if the air is clear or occupied. Informally, this is referred to as carrier sense. As mentioned previously, transmitters are required to listen before they transmit, to determine whether someone else is also speaking, and thus to help avoid collisions.

When listening, the receiver has a number of tools to help discover if a transmission is under way. The most basic concept is that of energy detection. A radio can figure out whether there is energy in the channel by using a power meter. This power meter is usually the one responsible for determining the power level, often stated as the Receive Signal Strength Indication (RSSI) of a real signal. When applied to an unoccupied channel, the power meter will detect the noise floor, often around −95dBm, depending on the environment. However, when a transmission is starting, the power meter will detect the signal being sent, and the power level measured will jump—let's say, to −70dBm for this example. That difference of 25dB can be used by the radio to clue in that it should attempt to turn on its modem and seek out the preamble. This allows the radio to have its modem off until real signals come by.

Energy detection can be used as a form of carrier sense to trigger the CCA. When done that way, non-802.11 noise that crosses a certain threshold, determined by the radio, will show up as an occupied channel for as long as the noise is present. This allows the radio to avoid transmitting into a channel at the same time as interference is present. In the 2.4GHz band, microwave ovens can often trigger the energy detection thresholds on radios, causing the radios to stop transmitting at that time.

On the other hand, energy detection for CCA has its limitations. If the noise coming in is something that would not interfere with the transmission, but does trip the energy detection threshold, then airtime is being wasted. Therefore, the carrier acquisition portion of CCA comes into play. Radios know to look for specific bit patterns in a transmission, such as the preamble. When they detect these bit patterns, they can assert CCA as well. Or, more importantly, when they detect some energy in the channel but cannot detect these bit patterns, they can conclude that there is no legitimate 802.11 signal and suppress CCA.