Re: Radio Tower Sites Linked with 350 Bridges

wkelly · ‎03-08-2003

We are installing a mobile data system with IPMobileNet radios in police vehicles. These radios communicate at 400Mhz to base stations at 3 tower sites, sites A, B, and C. Site A is located about 1/2 mile from the PD office and serves as the relay between sites B and C and the PD, and between the PD and the local Sheriff's Office.

There is a Cisco Aironet 350 bridge located at each of the sites B and C. There is another 350 bridge located at site A that has 2 antennas connected to it, one pointed to site B and one pointed to site C. Also located at site A is a second 350 bridge that has one antenna connected to it that points to the PD, and also points to the SO, which are approximately 12 degrees apart. The bridges at site A are connected via a switch.

Our problem. We are experiencing poor network performance, lost packets, and application failures. I did some testing from site A and found the signal strength between site A and site B (approx 3 miles) to be min 98% and max 100%. I could not get connectivity to site B (approx 6 miles), although we have connected to that site in the past. Connectivity between site A and the PD was 57% avg, with min 52% and max 64%, and between Site A and the Sheriff's Office was min 13% and max 54%.

The company that is installing our mobile data system subcontracted the tower work and wireless links. I am new to wireless networking and radio communications, but something seems amiss to me in this setup. According to the Cisco documentation I read, the antenna connections for the 350 bridges are for diversity and should not be connected to antenna pointed to two separate locations. I was also wondering about the use of the single antenna to point to both the PD and the Sheriff's Office. Is our configuration wrong, or do our problems lie elsewhere?

I think we need to install an additional bridge at site A and connect the antenna from site B to the new bridge and the antenna from site C to the existing bridge. I also think we need to point the existing antenna on the other existing bridge directly at the PD, then install another bridge and another antenna and point this at the Sheriff's Office. Or am I way off base here?

kmarrero · ‎03-10-2003

You are correct, the antenna connections are for diversity. It would be best to get another bridge for each location like you state in your post. There could be some Radio Frequency (RF) interference in between the towers or at at site C itself. This could be caused by electrical lines, air conditioning units, other wireless devices, and by other antennas that may be on the same towers.

wkelly · ‎03-10-2003

Thanks for the confirmation on the diversity. Regarding the RF interference, the tower is pretty loaded, but most of the other traffic is in the 155 range. Would this still interfere?

Also, when checking the results of a carrier test, do I want to select and use the frequency with the lowest percentage carrier busy, the lowest noise level, or the frequency where these two are combined to give the lowest average. The documentation doesn't make that clear, at least not to me.

Thanks.