From owner-freebsd-wireless@FreeBSD.ORG Wed Dec 4 04:22:31 2013 Return-Path: Delivered-To: freebsd-wireless@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id C9E2D27C for ; Wed, 4 Dec 2013 04:22:31 +0000 (UTC) Received: from mail-ie0-f171.google.com (mail-ie0-f171.google.com [209.85.223.171]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by mx1.freebsd.org (Postfix) with ESMTPS id 8AD3D1EB1 for ; Wed, 4 Dec 2013 04:22:31 +0000 (UTC) Received: by mail-ie0-f171.google.com with SMTP id ar20so25083594iec.2 for ; Tue, 03 Dec 2013 20:22:30 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:date :message-id:subject:from:to:cc:content-type :content-transfer-encoding; bh=tf/xT6vBS2TKECFhBgH1An6Z5e9EQE3n8ilZj2zySZc=; b=Nt4V9SFkTqb3XLpG1/K6tENsuJWHGK6rtJPt0D/Oj0ELcEwslwaox6a6/blj0Yto0N XLyOOUa1Mx8XWBIok2PV22PXXzOjq4euvut/90P9/YLG3vSlvElvVLSwg9yv1wWPlSn0 0fm49dBvyxrQQKZTmrM7LiyY/xQqJpieljt90Xiw+PJrXuba3/K3vJkniMCriPYEt9Nf 3h6srZg0FdBnAl4Bfl8us+cVySp5bANBiesARILk4uf4D2Bltflbtm7ql+idfW5rmmLr up0IY9Z6OiDPwd9lB8fwJlFbFqelSyQnK8g/E6Z8FpcY5rIeTYOlyKoLxStm9bFum/ja +F+A== X-Gm-Message-State: ALoCoQmoyZ5WU6lrtDGiuYbDp1XCZjaHWPB0V/AwnfvPJqR3lqsCGQEx0DI0uKuU5H5rQjH1C3+w MIME-Version: 1.0 X-Received: by 10.50.129.39 with SMTP id nt7mr5932792igb.13.1386129564686; Tue, 03 Dec 2013 19:59:24 -0800 (PST) Received: by 10.50.57.230 with HTTP; Tue, 3 Dec 2013 19:59:24 -0800 (PST) In-Reply-To: References: <46B1EFC5-1AA4-4E51-B199-D4CDACEEA0AC@netgate.com> Date: Tue, 3 Dec 2013 21:59:24 -0600 Message-ID: Subject: Re: Channel interference scan and manual selection From: Jim Thompson To: Adrian Chadd Content-Type: text/plain; charset=ISO-8859-7 Content-Transfer-Encoding: quoted-printable Cc: "freebsd-wireless@freebsd.org" X-BeenThere: freebsd-wireless@freebsd.org X-Mailman-Version: 2.1.17 Precedence: list List-Id: "Discussions of 802.11 stack, tools device driver development." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 04 Dec 2013 04:22:31 -0000 There is no "1, 6, 11" anymore. You're not listening. Back in the day, the adjacent (1 on 6, 6 on 1, 11 on 6, 6 on 11) channel rejection spec for 802.11b was 35dBm at 11Mbps modulation There never was an alternate (1 on 11, 11 on 1) channel rejection spec in the days of 802.11b. Most decent super-het designs back then used a SAW filter in the middle of the downconversion chain, and got around 41dBm of adjacent channel rejection. But we all moved to direct-conversion receivers ... because: cost. We all loves us some cheap 802.11 gear. The adjacent channel rejection spec for 802.11g (and 802.11a) is -1dBm @ 54Mbps modulation. The alternate channel rejection spec for 802.11g/a is 15dBm @ 54Mbps modula= tion. Just FYI, minimum ACR for 802.11g/a at 6Mbps is 16dB (alternate is 16dB more, for 32dB) If you're underwhelmed by the difference between 41dBm and -1dBm, then I can't help you. Free Space Path Loss =3D 20 log(4*p*r/=EB) dB, where r =3D distance between transmitter and receiver =EB =3D wavelength Path loss in the first meter @ 2.4GHz is 41dB. At 10m it's 60dB. Lets say you've got a garden-variety radio that puts up 32mW (15dBm) of tx power, and ignore antenna gain for now (so 0 dBi antennas on both radios). Old 802.11b (super-het receivers) world: 15dBm - 60dB - 41dB =3D -86dBm This is the in-channel 'noise power' of the adjacent channel radio. Notice that it is at least 15dB above the thermal noise floor. Translated: you've lowered your SINR. New 802.11g/a (direct conversion receivers) world: 15dBm - 60dB - -1 dBm =3D -44dBm. This is about 20dB higher than what is necessary to recover a 54Mbps receiver, but remember, it's the *noise power* of a radio operated on an adjacent channel. (Most 802.11 OFDM receivers are EVM-limited at 48Mbps and higher, but I digress.) Note as well that we're 57dB above the thermal noise floor, at 10m (33') Even if you back off to alternate channels (1 and 11), you're still at -60d= Bm. On Tue, Dec 3, 2013 at 9:40 PM, Adrian Chadd wrote: > On 3 December 2013 19:32, Tek Wiz wrote: >> Yes, the interference can be calculated by averaging the beacon RSSI (dB= ) of >> the overlapping channels, e.g. for channel 6, the beacon RSSI of channel= s 4, >> 5, 6, 7, 8 are averaged. Normally channels 1, 6, and 11 are chosen, alth= ough >> other channels may have less interference, in order to be a 'good neighb= or'. > > Sure. But we can also look at what kinds of frames we actually receive > during a sample window. > > What I'd also like to do is finally add receive power histogram > support. Ie, look at the RSSI of all the frames you receive, stick > them in buckets, respond to the power histogram action request frames, > use them for this.. that doesn't require any driver support. Just > net80211 work in the RX path. > > > > -adrian