19
Oct
2008
12:50 PM

Tomato: When Diversity Doesn't Work

After the changes of last week things were still not quite right. The WET would still periodically fail, usually only for a few seconds, but sometimes for much longer. I began to suspect that this was an external problem; something in the house was interfering with the signal.

After a few days of monitoring the link, I could find no correlation between the intermittent failures and changes in the local environment. Back to the proverbial drawing board.

I decided to go right back to the beginning and check the set-up from scratch. Whilst removing and reseating the external antenna from one of the routers, I remembered something I’d read previously. These Buffalo WHR-HP-G54 routers actually have not one, but two antenna — the obvious external rubber duckie and an internal plate providing some level of diversity reception. This can be clearly seen on the FCC certification report.

The drops I was seeing could potentially be explained by the routers switching the active path from one antenna to the other. If both ends of the connection were acting in a similar way this could explain the symptoms I’d seen. Fortunately, the Tomato Firmware allows very specific control over which antenna is used for both transmission and reception:

Tomato Default Antenna Settings

The default setting is Auto which will attempt to use the antenna with the best signal for each task (which may be the same for both). However, in my particular set-up this seems to cause the router to periodically select the wrong antenna — causing the Wireless Pit Of Despair effect described previously.

A simple trial and error process revealed that the external antenna on a Buffalo WHR-HP-G54, when using Tomato Firmware at least, is antenna B. A simple change forced the use of the external antenna for both transmission and reception:

Tomato External Antenna Settings

You may also notice a couple of other settings above that are different from the defaults. Firstly, on this particular router, I have Enhanced RX Sensitivity disabled. The screen-shot is taken from the WET, which is located above a desk containing all manner of electronic devices. Enabling the Receive amplifier (controlled by the Enhanced RX Sensitivity setting) on this router causes the signal quality to deteriorate due to the local electrical noise. On the access point that the bridge connects to I have this setting enabled — test and see which gives a better result for your own location.

Secondly, I have the Transmit Power bumped up to 40 mW. I’m in the process of testing this, reducing it a few mW at a time to find what level is actually required.

So far, so good. No problems for a few days, and a stable, strong signal at all times.



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