RADIO FREQUENCY
A basic understanding of the fundamental concepts and theories in RF electronics is an invaluable asset to the wireless hacker. At the end of the day, if your hardware isn't working, then no amount of clever software is going to help.Radio Frequency ( RF) Terminology
To start off, let's talk about what is meant by radio frequency or RF. Technically speaking, RF refers to any signal between the frequencies of 3 Hz and 300 GHz. More practically, however, RF refers to signals from about 3 MHz up to 300 GHz. Signals that fall into this range of frequencies are capable of traveling through space in the form of electromagnetic waves. The distance over which these signals can travel depends on factors such as the signal's frequency and atmospheric conditions.
Communications Systems
The history of wireless communications goes back to the late 1800s when a German physicist by the name of Heinrich Rudolf Hertz first discovered the existence of electromagnetic waves. This discovery marked the creation of the first radio. Although Hertz's radio was very primitive, it established that a signal could be generated at one location and detected at another location without the use of wires. Technology later advanced to the point where Morse code could be transmitted using radio waves rather than relying on telegraph wires. Wireless communication then progressed to the transmission of human voice and audio using radio waves, and today high-speed data communications over wireless links are used every day.
Components of a Communications System
Modern communication systems are generally constructed using the same set of fundamental components. Although the designs have changed over the years, the basic components are very similar to those used for the early radios. Pict 1 shows a block diagram illustrating these basic components as implemented in both a traditional analog (voice) communications system and a typical 802.11 wireless communications system (or radio). Both systems contain largely the same components for the RF front-end and transceiver. The primary differences are in the format of the incoming and outgoing data and the components that interface the data source to the transceiver as designated by the dashed boxes surrounding these components in the figure.
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| pict 1. Block diagram of basic communications system |
The data source is the component that is generating the information to be transmitted and received by the system. For an analog radio, this could be either a microphone in the case of a communications radio or music in the case of a broadcast radio station. For a digital communications system, the data source is any type of digital bitstream. Of particular interest for readers of this book is the bitstream originating from the bottom level of the TCP/IP stack where the raw data is passed to the physical link layer.
For digital radios, the next component of the radio is the baseband processing. This is where the digital bitstream coming from the data source is converted into an analog baseband signal through a process called modulation. (The process of modulation is not the same thing as a standard analog to digital conversion. There are various techniques for generating the modulated baseband signal that will be discussed later in the section on modulation.) Now that the data has been converted into an analog signal, the rest of the signal path becomes very similar to that used in more traditional radios. The baseband analog signal is typically the lowest frequency signal in the radio and is at too low a frequency for RF transmission, but this issue is taken care of in the transceiver.
