Bridge range calculation utility

Cisco makes it easy to calculate bridge distances by using the Cisco distance calculations spreadsheet that is available from the Cisco Web site. The user must follow several basic steps:

  • Select the product line being used.
  • Next select the proper antenna for both sites. For other non-Cisco antennas, enter the gain in dBi. The concept of the units dBd was formed to capture the gain of an antenna relative to a dipole antenna. A dipole is considered the standard basic horizontal antenna, and comparisons to it seemed to be more meaningful than comparisons to the isotropic radiator. If the antenna gain is provided in dBd, simply add 2.15 to the number to convert to dBi.
  • Then select the cable used on both sites. If using something other than standard Cisco antennas, enter in the length and cable loss per 30 meters (100 ft.) in the appropriate place. For Cisco cables this is 6.7dB /100 feet at 2.4Ghz. If a different cable is used, contact the cable vendor for this information.
  • Add any other losses due to splitters, connectors and so forth into the miscellaneous column.

Remember these are theoretical values, but they should provide a very good comfort level for proper operation. These values are for line of sight and provide a 10dB fade margin that provides assurance that the calculations will work.

To determine the bridging distance, the following items are considered:

  • Antenna gains are given in dBi, based upon a theoretical isotropic antenna, not dBd that is based upon a dipole antenna.
  • To convert from dBd to dBi add 2.15 to the dBd. As a result, 0dBd = 2.15dBi.
  • Cable lengths are a loss and are subtracted.

The antenna and radio parameters include cable losses at the receiver and transmitter sites, the antennas used at both sites, and the performance of the receiver and transmitter. Receiver gain changes with data rate. Always use the maximum data rate values needed by the customer.

Distances for these formulas are calculated in miles. For any given frequency, the atmosphere offers losses. This loss is a standard for any radio at that frequency. In this case, use the middle frequency of 2442 Mhz.

A sample radio and antenna parameters is calculated in Figure . This calculation uses a 20 dBm value for the transmitter power, 2 dBi to 13.5 dBi yagi antennas, and two cables of 6 m (20 ft) each. The values are entered into the formula to calculate maximum distance.

Lab Activity

Lab Exercise: Using the Bridge Range Calculation Utility

The student will learn how to use the Cisco Bridge Range Calculation utility to determine the following:

  • Bridge distances based on the type of access point
  • Antenna
  • Cables
  • Splitter
  • Other applicable wireless connectors used on the WLAN

Cisco Tools