IEEE 802.11, 802.11a, and 802.11b
In order for WLANs to be widely accepted, there needed to be an industry standard devised to ensure the compatibility and reliability among all manufacturers of the devices. The Institute of Electrical and Electronics Engineers (IEEE) has provided just that. The original standard IEEE 802.11 was defined as a standard in 1997 followed by IEEE 802.11a and IEEE 802.11b in September of 1999. The original standard operates at a radio frequency (RF) band that surrounds 2.4GHz and provides for data rates of 1Mbps and 2Mbps and a set of fundamental signaling methods and services. The IEEE 802.11a and IEEE 802.11b standards are defined at bands of 5.8GHz and 2.4GHz, respectively. The two additions also define new Physical (PHY) layers for data rates from 5Mbps, 11Mbps, to 54Mbps with IEEE 802.11a. These standards operate in what is known as the Industrial, Scientific, and Medical (ISM) frequency bands. The typical bands are 902-928MHz (26MHz available bandwidth), 2.4-2.4835 GHz (83.5 MHz available), and 5.725-5.850 GHz (125MHz available), with the latter allowing for IEEE 802.11a's higher data rate.
The standard defines the PHY and Media Access Control (MAC) layers for the wireless communication. A layer is simply a group of related functions that are separate from another layer of related functions. The layers in our wireless networking scenario can be best understood in the following analogy. Consider moving a book (representing a data packet) from a shelf on one side of the room to the desk on the other. Well, the MAC layer can be thought of as how one picks up the book and the PHY layer is how you walk across the room.
The PHY layer as defined by the standard includes two different types of radio frequency (RF) communication modulation schemes: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Both types were designed by the military for reliability, integrity, and security. Both types have their own unique way of transmitting data.
FHSS works by splitting the available frequency band into several channels. It uses a narrow band carrier wave that continuously changes in a 2-4 level Gaussian Frequency Shift Keying (GFSK) sequence. In other words, the frequency of transmission changes in a pseudorandom manner that is known by the sending and receiving nodes. This builds into the layer a decent bit of security. A hacker would generally not know the next frequency to switch to receive the entire signal. One advantage to FHSS is that it allows for multiple networks to coexist in the same physical space.