Return-path: Received: from swan.laptop.org ([18.85.44.157]:37234 "EHLO swan.laptop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751541AbeCVVUc (ORCPT ); Thu, 22 Mar 2018 17:20:32 -0400 Date: Fri, 23 Mar 2018 08:20:26 +1100 From: James Cameron To: Tyler Gray Cc: linux-wireless@vger.kernel.org Subject: Re: iwlwifi intermittent beacon capture in monitor mode? Message-ID: <20180322212026.GE27398@us.netrek.org> (sfid-20180322_222035_887545_DECA62C9) References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Sender: linux-wireless-owner@vger.kernel.org List-ID: G'day Tyler, I've seen that kind of behaviour when there are multiple APs with the same beacon timing, and one or more APs are not backing off. In my case the beacons were colliding. The times without beacons followed a regular pattern; based on the variance in CPU oscillator clocks of the APs. Cooling or heating an AP changed the pattern. Behaviour also varied across cards; RF sensitivity of a batch of cards follows a statistical normal distribution, with a bit of warping caused by manufacturing test rejects. Have you access to a spectrum analyser? You might check what transmissions are happening at the same time, on or near 2.457 MHz. Can you exclude all other APs, e.g. by placing the devices inside a disconnected microwave oven? Can you monitor the current of the card with a digital storage oscilloscope? Can you watch the beacons with an RF probe and an oscilloscope? Simplest probe is a diode (axial, bandoleer) with leads cut for a multiple of 2.457 MHz held in oscilloscope probes within an inch or so of the card antenna. With both these last two tests, you may see dips corresponding to beacon transmissions. If they stop, you know you have a firmware or software problem. -- James Cameron http://quozl.netrek.org/