CWNA考试常见RF术语

下面的术语解释的浅显易懂，非常适合CWNA认证学习。

 

- Wavelength o The actual length of an RF wave. The formula isn’t important for the test, but you should know that the two input values to the equation are the speed of light (300,000 kilometers per second) and the frequency of the radio wave - Frequency o How often one full cycle (called a Hertz (Hz)) is completed per second. A 2.4 GHz oscillates at 2.4 Billion cycles per second. o Frequency modulation (FM) uses slight changes in frequency to transmit data or voice - Amplitude o The height of the wave. Higher wave equals more power o Amplitude modulation (AM) uses slight changes in amplitude to transmit data or voice - Phase o Phase is the result of comparing two signals. One signal stream in itself can change phase. This can be used to transmit data. o Another comparison can be made of two separate signals. If the signals are out-of-phase and are of the same frequency, they will corrupt each other and reduce their amplitude at the receiver. If two signals are exactly opposite (180 degrees out-of-phase) the net result will be a null signal. Conversely, two instances of the same signal that are received in-phase at a receiver can improve signal strength. - SNR �C Signal-to-Noise Ratio o Difference between the noise floor (measured in dBm) and the received signal (also in dBm) - RSSI �C Received Signal Strength Indicator o The amount of signal received by a Wi-Fi device, typically displayed in dBm. o RSSI is loosely specified in 802.11 which means you will find great disparity between differing vendors. One vendors’ -77 dBm may be another vendors -73 dBm. - SOM - System Operating Margin o The difference between the signal needed for a bridge link, and the actual received signal, expressed as a positive number. For example, if your bridge needs -70 dBm to achieve 54 Mbps data rate and your actual signal is -60 dBm, you have a SOM of 10 dB (NOT 10 dBm). - Fade Margin o See SOM - Link Budget o Link budget is used to calculate the amount of signal needed between two bridge links. You will need to know what information is pertinent to calculating a link budget: Transmitter power (only of the transmitting side, not the receive side) Cable and connector losses (both transmit and receive side) Antenna gain (both transmitter and receiver antenna) Free Space Path Loss (FSPL) Receiver sensitivity Fade margin - Intentional Radiator o Measured signal including transmitter power and all sources of gain and loss up to, but not including, the antenna. - EIRP �C Equivalent Isotropically Radiated Power o Measured signal including the transmitter and all sources of gain and loss including the antenna. EIRP is the amount of power exiting the antenna. - Visual LOS (Line-of-Sight) o The ability to see the receiving antenna from the transmitting antenna. If you could stretch a string from one antenna to the other without encountering any obstructions, this is VLOS - Radio LOS (Line of Sight) o Radio signals are not a straight beam of energy. In outdoor implementations the energy is sent in an elliptical shape like an American football (See Fresnel zone). If you have RLOS, then the Fresnel zone is clear of obstructions - Fresnel Zone o Radio signal is not a straight beam of energy. The energy is sent in an elliptical shape like an American football. For any outdoor long distance link, it is required that 60% of the first Fresnel zone be unobstructed. The Fresnel zone size and shape is not determined by the antenna pattern or gain on each side (weird, I know). The only two factors that determine Fresnel zone size are frequency and distance between the transmitter and receiver. - Beamwidth o Measurement of signal shape emitted from an antenna; measured in degrees. For example, an omni-directional antenna has a horizontal beamwidth of 360 degrees. That same omni-directional antenna may have a vertical beamwidth of 90 degrees or less. The higher the gain of an antenna, the more focused the beamwidth. A higher gain omnidirectional antenna will have fewer degrees of vertical beamwidth. - Azimuth o A reference angle for the beamwidth of an antenna. An azimuth chart shows the horizontal beamwidth as if from a top-down view - Elevation o A reference angle for the beamwidth of an antenna. An elevation chart shows the vertical beamwidth as if from a side view - Passive Gain o Gain of signal achieved without supplying additional power. An antenna has passive gain, which is achieved by focusing the energy into a specific direction. - Isotropic Radiator o Theoretical antenna with a perfect spherical pattern. The “i” in dBi stands for Isotropic Radiator. See dBi for more details. - Polarization o Polarization refers to the shape of the signal as it exits the antenna. Vertical polarization is the most common. The waves go up and down like ripples on a pond. Horizontal polarization looks like a snake going through the desert �C side to side - Multipath o As a signal leaves the antenna, it often bounces off of multiple objects before arriving at the receiver. The receiver may receive multiple copies of the signal, which may arrive at different times due to the path of travel taken by the signal. This is called multipath. - Simple Antenna Diversity o Antenna diversity is used to mitigate the adverse effects of multipath. It uses two antennas and determines which antenna is receiving the best signal and uses it for that frame only. - Delay Spread o The delay between two separate received signals, usually due to multipath - Spatial Multiplexing o A new technology to Wi-Fi introduced with 802.11n. SM uses signal differences (such as multipath) that can occur in natural environments to its advantage to send multiple streams of data simultaneously, thus vastly improving throughput. Oddly enough, if an 802.11n system is deployed outside where there is much less multipath, throughput will be diminished. - Beam Forming o Introduced to Wi-Fi with 802.11n, beam forming is used to direct RF signals towards a desired area. There are many types of beam forming of which at the time of this writing were only implemented on a proprietary basis. - Maximal Ratio Combining (MRC) o Introduced to Wi-Fi with 802.11n. Where standard diversity can detect multiple reflected signals (multipath) and choose the best one, MRC can receive multiple copies of the same signal through different antennas and combine the copies into one great signal. This is one reason that 802.11n systems have superior receiver range than its predecessors. Even with legacy devices associated to an 11n AP, the AP’s receiver range is greatly improved.