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WiFi Security & RF Interference Colocation or Intermodulation StudyThursday TECHtionary.com TECH-Tip – WiFi Security & RF Interference Colocation or Intermodulation Study The animated TECH-Tip tutorial available at http://www.techtionary.com Summary Collocation/Intermodulation Studies are critical to maintaining WiFi performance, reducing interference and detecting rogue access points and security violations. Details Whenever a wireless, whether a transmitter or received radio frequency system is added to a site, a Collocation Study or Intermodulation Analysis should be performed to determine whether the new system will cause or suffer from harmful harmonic or other types of interference. An Intermodulation Study explores the various combinations of transmitter frequencies and harmonics generate "intermodulation products" that cause signal interference and errors in receiver frequencies. The study generally produces a list of potential problems where multiple transmit frequencies may interfere with a receiver in the same area. Receiver Intermodulation occurs when numerous signals enter a receiver and intermix (modulate) causing intermod products then appear at the receiver's demodulator input. For example, microwave oven frequencies interfere with Channel 11 WiFi-Wireless Fidelity signals. Refrigerators, car/truck ignition systems, electric motors can all generate radio frequency interference. Transmitter Intermodulation occurs when radio signals enter a transmitter via many different sources and causes. These new frequencies interact or mix and with the resulting intermod frequencies are re-radiated by the transmitting antenna. At two times the frequency is the 2nd Fundamental Harmonic. At three times the frequency is the 3rd Fundamental Harmonic. At four times the frequency is the 4th Fundamental Harmonic. Transmitter Harmonics are integer multiples (2, 3, 4...) of the transmitter frequency. Produced by non-linearity of the transmitter system from the RF power amplifier, harmonics are potential sources of interference to nearby receivers. CCK-Complementary Code Keying is the modulation format used in Wi-Fi-Wireless Fidelity 802.11b systems (802.11/a uses ODFM and 802.11/g uses both. CCK is a single carrier system. That is, all of the data is transmitted by modulating one single frequency or carrier. In 802.11/a OFDM-Orthogonal Frequency Division Multiplexing is used. OFDM is a multi-carrier or multi sub-carrier modulation format. The data is split into many closely spaced sub-carriers. The use of many different frequencies makes OFDM very reliable even in noisy and multi-path (see Multipath) situations. In addition, OFDM can support higher data rates than single carrier systems. That is, for data rates up to 11 MPBS-Mega Bits Per SEcond CCK is acceptable. OFDM can support data rates up to 54 MBPS or higher. A technical note, that 802.11g radios can use either CCK or OFDM. OFDM-Orthogonal Frequency Division Multiplexing is based on 52 subcarriers of which 4 are designated as Pilots. The use of a Pilot Subcarrier enhances coherent modulation on the data subcarriers and the use of advanced code keying or coding. The Center Subcarrier is not utilized to provide for improved performance. Increasing Subcarriers improves multi-path (interference from signal reflections "ghosts") and reduces the "guard interval" (the time interval between transmission) overhead. The cost for subcarriers is increased phase noise sensitivity and makes the granularity of the packet size/duration coarse. As suggested or rather exaggerated here, there are many different kinds of signal interference. Multi-path interference is multiple reflections or "ghosts." Hidden Node (station) occurs when some nodes (radios) cannot here one another and transmit at the same time. Because of this problem, RTS-Request To Send - CTS-Clear To Send is part of the 802.11 protocol standard. RTS/CTS have been used in modems for decades. RTS/CTS requires that each node must send a RTS message to the AP-Access Point or transmitting station and receive a CTS before transmission can begin. Because OFDM technology, the problem of "request to send" or permission before transmitting may be eliminated. FSK-Frequency Shift Keying varies the carrier (power level) signal's frequency and other variations are: MSK-Minimum Shift Keying or GMSK-Gaussian MSK where frequencies are shifted in time. PSK-Phase Shift Keying varies the carrier (power level) signal. Like with multiple shifts (changes) in frequency, there are many changes in phases. BPSK-Binary (two) PSK shifts 180 degrees (as shown here).DPSK-Differential PSK the phase is not shifted by some known factor but by the phase of the last symbol sent. PBCC-Packet Binary Convolutional Code modulation is a single carrier system. PBCC uses an 8-PSK. Complimentary Code Keying uses BPSK-QPSK. Both CCK and PBCC use a Preamble-Header (used to alert the receiving station for data) in the Payload. The maximum data rate for PBCC is 33 MBPS (OFDM is 54 MBPS). About TECHtionary.com – 303-444-6226 TECHtionary Corporation founded in 2001 and headquartered in Boulder, Colorado is the World's First and Largest Animated (rich media) Library/Magazine on Technology. Get the analysis and more than 2,630+ free tutorials on data, internet, wireless, VoIP-Voice over Internet Protocol (internet telephony), PBX systems, central office switching, protocols, telephony, telecommunications, networking, routing, power systems, broadband, WiFi-Wireless Fidelity and other technologies, TECHtionary.com provides "just enough – just-in-time" critical success information. TECHtionary produces web infomercials proven to "increase revenues, decrease customer support costs and increase customer satisfaction." Tom Cross CEO of TECHtionary is a member of the Technical Board of Advisors for the VOIP-Security Alliance. |