We know that 802.11a can be faster than 802.11b. However, the disadvantages of 802.11a are cost, compatibility, and signal strength.
Because of the already widespread use of 802.11b, most companies and homes do not need to upgrade to 802.11a. Although the benefits of 802.11a are clear, they do not outweigh the considerable cost of upgrading an 802.11b WLAN. In addition, 802.11a does not work with 802.11b. This means a company would have to scrap everything and start from scratch.
In addition, because of a higher (shorter) frequency, 802.11a signals die out much faster than 802.11b. To illustrate this, consider what you hear first when someone drives toward you with a loud stereo system in her car. You will almost always hear (or feel) the approaching bass before the treble. This is because the bass is at a lower frequency. Likewise, a WNIC will pick up a lower frequency wireless signal at longer ranges than a higher frequency signal.
In fact, the military uses this principle to communicate with its submarines far out in the ocean. Using ultra -low sonic waves, the government can send messages to a sub in hiding. Because the frequency is so low, the sound travels thousands of miles through water. Although this is also dependent on the physical qualities of water, it illustrates just how far a low frequency signal can carry.