From owner-freebsd-wireless@FreeBSD.ORG Wed Dec 4 04:14:07 2013 Return-Path: Delivered-To: freebsd-wireless@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id DA45F1CD for ; Wed, 4 Dec 2013 04:14:07 +0000 (UTC) Received: from mail-qc0-x22b.google.com (mail-qc0-x22b.google.com [IPv6:2607:f8b0:400d:c01::22b]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by mx1.freebsd.org (Postfix) with ESMTPS id 8E61D1E51 for ; Wed, 4 Dec 2013 04:14:07 +0000 (UTC) Received: by mail-qc0-f171.google.com with SMTP id c9so1662817qcz.2 for ; Tue, 03 Dec 2013 20:14:06 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date:message-id:subject :from:to:cc:content-type:content-transfer-encoding; bh=yH+5QvhhqLDP092H9ERxRi2ZeKoKH1TbdQ5KTHzeOJI=; b=sLooLLH9f+ioDymtbXHBskFFYt9PD7y+CIgl33yp0HbwGIt79bGuSh3Rp3guTQNlC+ gP62Vq9vXN3/BTAVCIpM+0tSWLDiNhjFz/NXu5XeQQvyZ8uenz4GvQJj+nJDwCC+sa0l 97K4J2ntNqXAOjA+yPWQnQI//5ry4KeEwwSfpCKSy0EU+OoTQ4m7XD+OYdtyfPntf+D0 ryQI4H2hQQh8vBJh2undFLy/q3EZYJMK1RCUDFwpMKYcFewgojTbdmqVovZHDHDXYhl/ MhmsOg9e6iAdn6m5X+qwcVGj7HunGVzhMawTQEaC/Rp8y7z7LyVqrnTA49179Hv+lNdT p0wQ== MIME-Version: 1.0 X-Received: by 10.49.131.5 with SMTP id oi5mr90932408qeb.38.1386130446014; Tue, 03 Dec 2013 20:14:06 -0800 (PST) Sender: adrian.chadd@gmail.com Received: by 10.224.53.200 with HTTP; Tue, 3 Dec 2013 20:14:05 -0800 (PST) In-Reply-To: References: <46B1EFC5-1AA4-4E51-B199-D4CDACEEA0AC@netgate.com> Date: Tue, 3 Dec 2013 20:14:05 -0800 X-Google-Sender-Auth: pPU1ZT5YPX8Xl_wOvFO57e7aG6M Message-ID: Subject: Re: Channel interference scan and manual selection From: Adrian Chadd To: Jim Thompson 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 04:14:07 -0000 No, I get this bit. Totally. I had to deal with this with HT40 channels where .. the rejection isn't as great. Adjacent channel interference into the secondary channel just killed things. I'm right now more interested in making sure we can gather the data we need first. The channel selection bit is orthogonal. I completely agree with you. We're better off with completely overlapping channels where the competing access points have a chance of correctly detecting each others signals and backing off without raising the channel noise floor. There's also the HT20/HT40 coexistence stuff where we should actually disable doing HT40 on 2GHz channels if there's adjacent channel traffic. -adrian On 3 December 2013 19:59, Jim Thompson wrote: > 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 modu= lation. > 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 -6= 0dBm. > > > > 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 (d= B) of >>> the overlapping channels, e.g. for channel 6, the beacon RSSI of channe= ls 4, >>> 5, 6, 7, 8 are averaged. Normally channels 1, 6, and 11 are chosen, alt= hough >>> other channels may have less interference, in order to be a 'good neigh= bor'. >> >> 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