To provide a paper on designing and building a high performance, independent, secure, wireless network using easily obtainable hardware. The final design should be easy to construct, yet maintain all the goals at low cost. Network access will also be open to the public, or at least those in close proximity to our location. To also cover all technical aspects of wireless networking, including documenting little known facts, and illustrating the actual network setup and installation, or at least that's my idea.
Although some of the items in this how-to are Proxim hardware specific, the overall designs and ideas should help in the design of any and all types of wireless networks.
Also covered will be instructions for using MMDS antennas in 2.4 GHz wireless networks and several modifications for Proxim Symphony wireless network cards. Adaptation to wireless network cards other than the Symphony should be (is) trivial.
Some terms that will be used and their definitions :
- Decibel, a logarithmic unit of intensity used to indicated power lost or gained between two signals. Named after Alexander Graham Bell.
- Gain in decibels referenced to a standard half-wave dipole antenna. This is a more realistic reference to antenna gain.
- Gain in decibels referenced to an isotropic radiator. An isotropic radiator is a theoretical antenna with equal gain to all points on isotropic sphere. 2.15 dBi = 0 dBd
- Meaningless random large numbers generated by advertizing departments.
- Decibel referenced to 1 milliwatt into a 50 Ohm impedance (usually) 0 dBm = 1 mW
- Kilobytes per second, unit of data rate measurement, 1,000 bytes per second or 8,000 bits per second. Example: 30 kBps
- Kilohertz, unit of frequency measurement, 1,000 periods per second. Example: 455 kHz
- Kilobits per second, unit of data rate measurement, 1,000 bits per second. Example: 128 kbps
- Milliwatt, one thousandth (1/1000) of a Watt, used to indictate received or transmitted power.
- Direct Sequence Spread Spectrum, a RF carrier and pseudo-random pulse train are mixed to make a noise like wide-band signal.
- Effective Isotropic Radiated Power, actual power transmitted in the main lobe after taking in account all cable losses and antenna gain. Based on an isotropic antenna.
- Frequency Hopping Spread Spectrum, transmitting on one frequency for a certain time, then randomly jumping to another, and transmitting again.
- Frequency Shift Keying, modulating a transmitter by using data signals to shift the carrier frequency. Commonly used for digital communications.
- Gigahertz, unit of frequency measurement, 1,000,000,000 periods per second. Example: 2.4 GHz
- Hertz, the basic unit of frequency measurement, cycle per second. Example: 60 Hz
- Megahertz, unit of frequency measurement, 1,000,000 periods per second. Example: 147.075 MHz
- Multichannel Multipoint Distribution Service, wireless method of remotely sending conventional cable TV service to rural areas or over long distances using microwave radio frequencies.
- Megabits per second, unit of data rate measurement, 1,000,000 bits per second. Example: 1.544 Mbps
- Radio Frequency, electromagnetic radiation between 10 kHz and 300 GHz.
- The width of a signal on the radio spectrum. The greater the signal's bandwidth, the more frequency space it occupies, and the stronger the signal needs to be to overcome noise.
- Coaxial Cable
- A type of wire in which a center conductor is surrounded by a concentric outer conductor. Also called "coax".
- Fade Margin
- Fade margin is the difference, in dB, between the magnitude of the received signal at the receiver input and the minimum level of signal determined for reliable operation. Higher the fade margin, the more reliable the link will be. The exact amount of fade margin required depends on the desired reliability of the link, but a good rule-of-thumb is 20 to 30 dB. Fade margin is often referred to as "thermal" or "system operating margin".
- Front-To-Rear (Back) Ratio
- Antenna measurement that is determined from the peak power difference, in decibels, between the the main radiation lobe at 0° (front of an antenna) and the strongest rearward lobe (back of the antenna). Higher the ratio, the more directional the antenna is.
- Free Space Loss
- Attenuation, in dB, of a RF signal's power as it propagates through open space.
- Fresnel Zone
- The Fresnel (fre'-nel) zone is an elliptical region surrounding the line-of-sight path between transmitting and receive antennas. Must be obstruction free for a microwave radio link to work properly.
- The complex combination of resistance and reactance, measured in Ohms (50 typically). Impedance must be matched for maximum power transfer.
- Hypothetical, lossless electromagnetic wave radiating equally in all directions from a point source in free space. Used as a reference for antenna gain.
- When the transmit and receive antennas can physically see each other.
- When the RF signal arrives at the receiving antenna after bouncing through several paths. Significantly degrades the received signal power.
- Path Loss
- Free space loss of RF power plus any power loss due to link path obstructions, poor antenna height, and link distance.
- The polarity of a radio signal's electric field. Transmit and receive antennas must have the same polarity for maximum receive power.
- Radiation Fields
- There are three traditional radiation fields in free space as a result of an antenna radiating power.
- Near-field, also called the reactive near-field region, is the region that is closest to the transmitting antenna and for which the reactive field dominates over the radiative fields.
- Fresnel zone, also called the radiating near-field, is that region between the reactive near-field and the far-field regions and is the region in which the radiation fields dominate and where the angular field distribution depends on distance from the transmitting antenna.
- Far-field, or Rayleigh distance, is the region where the radiation pattern is independent of distance from the transmitting antenna.
- Receiver Sensitivity
- The minimum required RF signal power received to meet are certain performance level. This level is usually referred to as the Bit Error Rate (BER). Example: -80 dBm or 0.000000010 mW
- When your bunghole friend won't answer the radio, send the cops to his work to tell him to turn on the HT so in all your pinkness, you don't buy the wrong computer hardware, again. Also people who transmit in mono.
- A directional antenna made up of one driven element and one, or more, parasitic elements.
Data Conversion Examples
1.0 Watt = 1000.0 mW = +30.0 dBm
10 Watts transmitted - 5 Watts lost in cable = 3 dB lost
-85 dBm for 1x10e-6 BER = 1 bit in a million error at a -85 dBm received power level
Useful Perl Equations
# Milliwatts to dBm
$dBm = 10 * (log10 $mW);
# dBm to milliwatts
$mW = 10 ** ($dBm / 10);
# Attenuation/gain in dB given RF power input and output
$dB = 10 * (log10 ($power_output / $power_input));
# dB to power ratio
$power_ratio = 10 ** ($db / 10);
# Free space loss, frequency in MHz and distance in kilometers (isotropic)
$free_space_loss = 32.4 + (20 * (log10 $freq)) + (20 * (log10 $distance));
# Calculating quarter wavelength, in centimeters, given frequency, in MHz
$quater_wavelength = 7132.32 / $freq;
# Fresnel zone boundary
# You must satisfy 60% clearance of the first Fresnel zone ($nth = 0.6)
# $fn Nth Fresnel zone boundary in meters
# $nth The Nth numerical value (0.6, 1, 2, etc)
# $d1 Distance from one end of path to reflection/obstruction in meters
# $dis Total path distance in meters
# $freq Frequency in MHz
$fn = 17.31 * sqrt (($nth * $d1 * ($dis - $d1)) / ($freq * $dis));
# Attenuation, in dB, for 30.5 meters (100 ft) of LMR-400 (frequency in MHz)
$loss = (0.12229 * sqrt $freq) + (0.00026 * $freq);
# Received power level and EIRP
# $pwr Output power of transmitter (in dBm)
# $tx_cab Total transmitter cable attenuation (in dB)
# $rx_cab Total receiver cable attenuation (in dB)
# $tx_ant Total transmitting antenna gain (in dBi)
# $rx_ant Total receiving antenna gain (in dBi)
# $rx_pwr Received power level at receiver (in dBm)
# $EIRP Effective Isotropic Radiated Power (in dBm)
$rx_pwr = $pwr - $tx_cab + $tx_ant - $free_space_loss + $rx_ant - $rx_cab;
$EIRP = $pwr - $tx_cab + $tx_ant;
# Parabolic dish antenna gain and beamwidth, in dB
# $dia Reflector diameter in centimeters
# $wave Wavelength of operating frequency in centimeters
# $illum Illumination factor. typically 0.6
# $freq Frequency in GHz
# $theta 3 dB beamwidth in degrees
$gain = (20 * (log10 ($dia / $wave))) + ((10 * (log10 $illum)) + 9.938);
$theta = 70 / (($dia / 30.48) * $freq);
# Distance and antenna height calculations
# All antenna heights are measured above ground level (AGL), in meters
# $radio_horizon Distance to radio horizon, in km
# $distance_max Maximum space wave communications distance, in km
# $rx_height Receiver antenna height
# $tx_height Transmitter antenna height
$radio_horizon = 4 * (sqrt $antenna_height);
$antenna_height = ($radio_horizon / 4) ** 2;
$distance_max = (4 * sqrt $tx_height) + (4 * sqrt $rx_height);
$rx_height = (($distance_max - (4 * sqrt $tx_height)) / 4) ** 2;
$tx_height = (($distance_max - (4 * sqrt $rx_height)) / 4) ** 2;
# Great circle distance calculator using decimal degrees
# $lat1 Latitude of site A
# $lon1 Longitude of site A
# $lat2 Latitude of site B
# $lon2 Longitude of site B
# $radius Radius of the earth in km (6367)
# $distance Distance between the two points in km
$dlon = $lon2 - $lon1;
$dlat = $lat2 - $lat1;
$a = ((sin(deg2rad $dlat / 2)) ** 2) + ((cos(deg2rad $lat1) * cos(deg2rad $lat2)) * ((sin(deg2rad $dlon / 2)) ** 2));
$distance = (2 * atan2(sqrt $a, sqrt(1 - $a))) * $radius;