Example of a WLAN Radio Bridge

Econsult is an independent and private planning and consulting company. It works on innovative and effective concepts for other companies with at present 35 staff members.

Econsult's company concept is based upon professional qualification of their practically oriented employees and the successful realization of problem resolution.

Econsult was planning an extension of its office area. For these purposes half of a floor in another nearby office building was rented. avalaris accepted the bid to connect those two office premises network-wise.

A leased line would have been too slow and, for the required bandwidth, too expensive. Therefore the decision for a WLAN radio bridge was made easily. Besides, the buildings are just about 300 meters afar with existing intervisibility. In order to utilize the permitted maximal outdoor bandwidth, the deployment of the 802.11g standard for the bridge was chosen.

The connected office has access to the existing internal server environment and the internet connection via this bridge and transmits all the client backups to the backup server at the main location.

On average, transfer rates between 22 and 35 Mbit are achieved. This means, that backups from the notebooks of the colleagues situated in the new location can be done with about 1.2 GH in 15 minutes. Using the 802.11b standard, the same procedure lasts about one hour.

Technical Realization

At the request of the customer at the beginning two flagpole antennas were installed on the two sides of the buildings that are facing each other. Each of the antennas was connected with a LANCOM L-54ag access point. Antennas and cables were connected with the capability adjustment of the buildings by a lightning protection module. The access points (AP) were actuated in an exclusive bridging mode and a "closed network" was created by several other measures.

An average signal strength of about 200dB was achieved. Although this is near the lower limit, a stable connection in the 802.11g standard could be achieved.

The WLAN-bridge worked smoothly for about one month. In this period the telephone system was connected over a Kapsch DECT wireless connection. Although the Kapsch DECT connection works in the 2.4GHz area as well, it didn't seem to affect the WLAN-bridge.

After this month, however, severe problems with the WLAN-bridge occurred suddenly.

avalaris agreed to get to the bottom of the problem: Our WLAN-MRTG-monitoring came into operation. This program steadily monitors signal strength, error rates and noise/signal level over a long period. We found out, that an average signal strength of about 15 to 20 dB was available. This average however, was subject to a jitter of about 10 db, which caused the loss of the signal every now and then.

On top of the office building, where the new new office premises of Econsult are located, in the meantime new antennas of a mobile phone provider that works with WLAN hot spots, too, had been installed. And also other WLANs could be found with a simple channel scan.

In order to work with the broader bandwidth of the 802.11g standard despite big parasitic inductions and jitter the change to outdoor antennas of the type LANCOM O-30 Extender was chosen.

After first tests, we observed that the WLAN-bridge worked stable with about 35-40 db signal strength (noise about -90db, signal about -50db).

The jitter that probably occurred because of outside influences, are there in the same amount as before, but do not result in the short-time loss of the connection any more.

WLAN is a transmission technique based on radio waves – and they are, as one can see from this experience, quite sensitive to outside influences. Those can, however, be kept effectively under control by the deployment of adequate measures.

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