Wireless heisenbug

14.06.2009 13:18

It seems even hardware is not immune to heisenbugs.

A week ago my wireless access point in Logatec (an old version of SMC Barricade G) started behaving weirdly. A network scan showed the SSID broadcast, but association was not possible. I tried several different devices (among those my Eee 901 with Atheros card and the work laptop with an Intel IWL), but none was able to connect. iwconfig just showed not associated and an ever increasing Rx invalid nwid counter.

Interestingly, kismet found the network and showed constant traffic on it. It also warned of broadcast disassociate requests which it seemed to indicate a denial of service attack on the network. But I just couldn't believe that someone would keep such an attack going for that long in that part of the country.

The problem fixed itself after a couple of days. But since it started working just after I moved the access point around, I thought that maybe there's a bad connection somewhere. So I took it apart.

SMC Barricade g wireless router

The only two things that caught my attention were a loosened nut on the left antenna connector and plenty of dust on the RF part of the circuit.

I tightened the connector and vacuumed the dust and the access point has been working flawlessly since then.

I'm not sure I actually did anything. It's conceivable that the loosened nut did something to the grounding of the circuit or that there was something nasty in that dust that shorted a trace or changed its impedance enough, but I find that hard to believe.

SMC Barricade g, RF front-end

Here's a close-up of the RF front-end of the transceiver (minus the dust). The chip in the middle is the receive/transmit and diversity switch.

The microstrip line carries microwave frequencies, so the correct characteristic impedance is very important. There's no solder resist over he line to prevent it from affecting its characteristics and the upper and lower ground planes are well connected with a lot of vias.

Posted by Tomaž | Categories: Digital

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