Radio Bits

This section has been left until last because it is really not relevant to security at all. In fact, if your only interest is security, skip to the next section. However, a brief overview of the radio side seems relevant to a book focused on wireless LANs. We have seen that the MAC layer produces a frame of data that it desires to be transmitted over the radio waves. From the point of view of the radio, this is just a long stream of bits. It is the job of the radio to take the bits and generate a few electromagnetic waves that can be picked up somewhere else and converted back to the same bits. Simple, huh? Well, actually, no.

Currently (in other words, as of 2003), there are two frequency bands that are available for sending IEEE 802.11 data; these are referred to as the 2.4GHz band and the 5GHz band. Band allocation is a very complicated area because governments jealously guard and control the use of radio spectrum, especially after they discovered the value of spectrum auctions in the late 1990s. Different countries and regions of the world have different rules, and we could easily fill a book on this topic alone. Here we will limit ourselves to observing that these two bands exist and, at any point in time, your radio operates in one or the other.

Having determined the radio spectrum that is available, the designer needs to figure out how to convert the digital bits into a high-frequency analog signal that can be amplified into an antenna to generate electromagnetic waves. Converting from bits to analog is the same task that a regular telephone modem performs?and, in fact, the portion of the radio that converts bits into analog is called the modem. The radio can be considered as two bits. The first part contains the modem, sometimes called the baseband section, and the second part contains all the very high frequency electronics to drive the antenna, usually called the radio frequency (RF) section. RF design is very specialized, and we salute the designers and discuss no further. The MODEM deserves more of a look.

Remember that the object is to convert digital bits into analog signals. One of the simplest modem techniques is called frequency shift keying (FSK): Send one frequency for a 0 bit and another for a 1 bit. You could use such a scheme to send Morse code for example?if you only needed a few bits a second! Having invented our first simple scheme, now apply 50 years of research and stir in a large consignment of top-quality gray matter and you might arrive at the very sophisticated techniques used in today's wireless LANs, such as orthogonal frequency division multiplexing (OFDM) and convolutional coding.

According to natural laws, there is a limit to how much information can be sent in a given amount of radio bandwidth. Furthermore, as you increase the information rate toward the theoretical limit, you become more susceptible to corruption by random noise. The sophisticated mathematical techniques that have been applied to wireless LAN are designed to get the optimum balance between high data rate and range. Put the data rate too high and you are susceptible to noise?hence the range becomes too short. But use the right mathematical technique, and you can increase the data rate without sacrificing range.

Improvements in modem techniques (and some changes to the regulations) have resulted in successive versions of IEEE 802.11 offering higher speed. The original 1997 standard only provided 2Mbps in the 2.4GHz band. IEEE 802.11a allowed an immediate leap to 54Mps in the 5GHz band, partly due to better modem technology and partly due to more available spectrum. However, 802.11a implementation was not practical at the time the standard was completed and product didn't appear until 2002. In 1999, IEEE 802.11b increased the speed to 11Mbps in the 2.4GHz band and set the stage for rapid growth of the wireless LAN market. Recently IEEE 802.11g has increased speeds again in 2.4GHz by introducing more sophisticated modem techniques. Soon we can anticipate new versions in the 5GHz band that might push data rates up to 100 or 200Mbps.

Well, interesting as all this is, none of it is relevant to security. The same security techniques can apply whether you are using 100Mbps or going back to your Morse code transmitter.

    Part II: The Design of Wi-Fi Security