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Cisco WT2700 Wireless Suite

Wireless Point-to-Point Troubleshooting FAQs and Checklist

Cisco - Wireless Point-to-Point Troubleshooting FAQ and Checklist

Document ID: 14241

Updated: Apr 27, 2006

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Introduction

This document provides questions and answers for wireless point-to-point troubleshooting.

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Q. What must I do or look for when my link does not work?

  • Symmetrical Signal Loss—Confirm that the receive signal level at each receiver end is close to the expected value. The value must not be lower than 4 dB below the original value. If the value is lower, especially if the value is lower by the same amount at both ends, you can suspect a problem with the antennas or cabling or connectors. Symmetrical signal loss can occur doe to misaligned antennas.

  • Moisture—Moisture can also damage antennas or connections. If not properly sealed at installation, moisture can condense inside antenna feedhorns and fill them within a few weeks! Moisture that gets into coax cabling is even more insidious and the damage that moisture causes is both invisible and severe. Most coax cable types have a foam-type internal dielectric, which can act like a sponge, soaking water into the coax for a significant portion of its length.

    Note: Do not merely cut off a few inches from the affected end and re-install the connector! If you find that moisture has ingressed into your coax cable, replace the entire run length.

  • Outdoor System Faults—Faults that develop within the outdoor parts of a system, for example the antennas and cables, manifest in a receive-signal level that is degraded by the same amount at both ends of a link. That is, the entire outdoor system acts bilaterally. Therefore, if you note a degraded receive-signal power measurement at the first end that you measure, do not assume that the fault is with components at that end; the fault can also be at the other end.

  • Asymmetrical Signal Loss—Finally, if the receive-signal level is low at one end but not the other, you can conclude that the outdoor system inherently functions properly. Such asymmetrical situations are commonly either the result of improper setup or configuration of the radio units (for example, TX power is set too low at one end), or of interference. Therefore, do not re-align antennas or take RF connections apart. If the setup is deemed to be correct and equipment failure is ruled out, interference is the likely issue.

Q. How can I check to determine if there is interference?

  • Check whether the problem is continuous or intermittent. Interference usually occurs intermittently, or else you would have noticed the interference when the link was first commissioned.

  • Check whether the receive-signal level is proper at both ends. This measurement reflects the receipt of both desired and undesired (interference) signals.

    An interference problem can newly manifest in a system with a history of reliable operation, yet the source of interference may have been there all along. This is possible, when some other problem affects the outdoor system (see above) and degrades the receive signal levels, and thus allows the interferer to cause a link quality problem. The interferer degrades the signal-to-noise ratio in this case, because the interferer is also just noise as far as your system is concerned. In such cases, the newly-degraded receive-signal level allows an already existing interferer to cause problems.

  • If the received-signal indication (RSSI) is correct and you suspect interference, measure the signal-to-interference+noise ratio (SINR) at each end of the system.

    You can plot and track this parameter (as well as RSS) as a function of time with the help of the radio histogram feature set. In this way, you can track these two parameters simultaneously and correlate them with observations of the degraded performance. If, for example, the RSS always remains steadily good, and SINR exhibits periods of abnormally poor readings, check the link performance (such as error rate) during those periods of poor SINR. If there is a correlation, the link experiences interference. When the RSSI is good at both ends, interference is intermittent.

  • Use a spectrum analyzer to diagnose interference. The spectrum analyzer shows an image of the frequency band, and shows interfering signals on or near your frequency. The toughest cases involve intermittent interferers because such interferers can appear infrequently and randomly. For these cases you must have a spectrum analyzer that can capture the images over time, so that you can verify infrequent and random interferers.

Q. What must I do when I confirm that I have an interference problem?

A. Locate the source of the interference. In order to do so, look around at both ends of your link. Do you see any other antennas or related equipment? If you identify other nearby radio equipment, try to learn:

  • Who operates it

  • Who owns it

  • What frequency or frequencies it operates on

  • With how much power does this other equipment transmit

  • What kind or type of antenna polarization is in use

See if you can contact the owner or operator of this other equipment to find out:

  • What are the hours of operation?

  • Will they work with you to confirm if their equipment is indeed the source of your problem? Diplomacy helps here.

When you have confirmed the type and location (and other details) of the interfering radio equipment, you can figure out a solution. Start with your antennas.

  • Is either one (or both) pointed at the other system antenna(s)?

  • Can you relocate and/or re-point one (or both) of your antennas to get the other system farther away from your path axis?

Change the polarization of your antennas to be opposite (crossed) to the polarization of the interfering system in order to solve the problem. You must first ensure that your systems antennas are cross-polarized to the other systems antennas, because this solution is so effective and requires the least cost and effort to implement.

If the problem persists, change the frequency of one of the systems (probably your own). Systems that are not on (or close to being on) the same frequency rarely interfere with each other. If the interferer is very high-powered (such as radar), interference can still occur because the powerful interferer overloads the receiver, and causes interference-like symptoms. Change of frequencies is usually effective.

Note: This solution is only appropriate in the case of systems that operate in unlicensed bands, where the use of a specific frequency is not necessary. This solution option is typically not available to licensed users, although such users are not likely to encounter interference that they can solve this way.

Sometimes, you need to change frequencies for both your system and the interfering system, in order to maximize the frequency separation.

In order to change the frequency of your system, swap the transmit and receive frequencies. This process requires removal and reinstallation of both of your duplexers, in order to turn them around to accommodate the now-reversed TX/RX frequency split. You do not need to make any other adjustment or re-alignment of the antennas. You can try this when you experience interference at only one end of your link (the most common situation), because the link end where the receiver was receiving interference now transmits on that frequency. In other words, only receivers can experience interference!

You must use the final tool to alleviate an interference problem only after you have unsuccessfully tried all the others. That tool is antenna gain. For antennas, higher gain is virtually synonymous with narrower beamwidth. When you change antennas to higher-gain, lower-beamwidth types your system becomes less able to see the interferer, and this step sometimes solves the problem.

Note: Usually only one end of a link experiences interference. Therefore, you only need to change the antenna at the link end where the receiver experiences the problem. Changing antennas is the most costly and time-consuming solution. Therefore, install antennas of adequate gain (narrow enough beamwidth).

If you decide to replace one or both antennas with higher gain units, verify that the TX power needs to be adjusted downward to ensure that the radiated TX power (EIRP) still complies with the applicable FCC rules.

Q. Can you give a short troubleshooting checklist for a point-to-point link?

Related Information

Updated: Apr 27, 2006
Document ID: 14241