From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp113.iad3a.emailsrvr.com (smtp113.iad3a.emailsrvr.com [173.203.187.113]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by huchra.bufferbloat.net (Postfix) with ESMTPS id E23CE21F112 for ; Wed, 18 Dec 2013 07:19:31 -0800 (PST) Received: from localhost (localhost.localdomain [127.0.0.1]) by smtp7.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id 96E9940093; Wed, 18 Dec 2013 10:19:30 -0500 (EST) X-Virus-Scanned: OK Received: from app8.wa-webapps.iad3a (relay.iad3a.rsapps.net [172.27.255.110]) by smtp7.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id 72E6B400EB; Wed, 18 Dec 2013 10:19:30 -0500 (EST) Received: from reed.com (localhost.localdomain [127.0.0.1]) by app8.wa-webapps.iad3a (Postfix) with ESMTP id 62968280042; Wed, 18 Dec 2013 10:19:30 -0500 (EST) Received: by apps.rackspace.com (Authenticated sender: dpreed@reed.com, from: dpreed@reed.com) with HTTP; Wed, 18 Dec 2013 10:19:30 -0500 (EST) Date: Wed, 18 Dec 2013 10:19:30 -0500 (EST) From: dpreed@reed.com To: "Stephen Hemminger" MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_20131218101930000000_68186" Importance: Normal X-Priority: 3 (Normal) X-Type: html In-Reply-To: <20131217154345.0e91b65f@nehalam.linuxnetplumber.net> References: <52AF797E.6030600@imap.cc> <18972.1387302855@sandelman.ca> <1387319157.48330794@apps.rackspace.com> <20131217154345.0e91b65f@nehalam.linuxnetplumber.net> Message-ID: <1387379970.401720581@apps.rackspace.com> X-Mailer: webmail7.0 Cc: "cerowrt-devel@lists.bufferbloat.net" Subject: Re: [Cerowrt-devel] =?utf-8?q?treating_2=2E4ghz_as_-legacy=3F?= X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 18 Dec 2013 15:19:32 -0000 ------=_20131218101930000000_68186 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =0AYes - there are significant differences in the physical design of access= points that may affect 5 GHz and 2.4 GHz differently. There are also modu= lation differences, and there may actually be scheduling/protocol differenc= es.=0A =0AAll of these affect connectivity far more than center-frequency w= ill.=0A =0A1) Antennas. One of the most obvious problems is antenna "apert= ure". That is a measure of the effective 2-D area of the antenna on the re= ceiving side. A dipole antenna (the cheapest kind, but not the only kind u= sed in access points) is "tuned" by making its length a specific fraction o= f the wavelength. Thus a 5 GHz antenna of the dipole type has 1/4 the aper= ture of a dipole antenna for 2.4 GHz. This means that the 5 GHz antenna o= f the same design can access only 1/4 of the radiated energy at 5 GHz. But= that's entirely due to antenna size. If you hold the antenna size consta= nt (which means using a design that is inherently twice as big as a dipole)= , you will find that range dramatically increases. You can demonstrate th= is with parabolic reflecting *receive* antennas at the two frequencies. (th= e aperture can be kept constant by using the same dish diameter). If you = look at the antenna elements for 5 and 2.4 in an access pony, you will prob= ably see, if you understand the circuitry, that the 5 GHz antenna has a sma= ller aperture.=0A =0AThe other problem is antenna directionality for the tr= ansmit and receive antennas. Indeed almost all AP antennas have flattened = doughnut radiation patterns in free-space. Worse, however, is that indoor= s, the antenna patterns are shaped by reflectors and absorbers so that the = energy is highly variable, and highly dependent on wavelength in the patter= n. So 5 GHz and 2.4 GHz signals received at any particular point have high= ly variable relative energies. In one place the 5 GHz signal might be 10x= the energy of a 2.4 GHz signal from the same AP, and in another, 1/10th. T= he point here is that a "controlled experiment" that starts at a point wher= e 2.4 GHz works OK might find weak 5 GHz, but moving 1 foot to the side wil= l cause 2.4 to be unworkable, whereas 5 works fine. Distances of 1 foot c= ompletely change the situation in a diffusive propagation environment.=0A = =0AFix: get the AP designers to hire smarter antenna designers. Even big c= ompanies don't understand the antenna issue - remember the Apple iPhone des= ign with the antenna that did not work if you held the phone at the bottom,= but worked fine if you held it at the top? Commercial APs are generally m= ade of the cheapest parts, using the cheapest designs, in the antenna area.= And you buy them and use them with no understanding of how antennas actua= lly work. Caveat emptor. And get your antennas evaluated by folks who und= erstand microwave antennas in densely complex propagation environments, not= outdoor free-space.=0A =0A(and don't put your AP in the attic and expect a= good signal near the ground.... or in the basement. Physics will make sur= e that the signal is zero at any ground, so being closer to the ground than= the antenna weakens the signal a lot!)=0A =0A2) Modulation and digitizatio= n. Indoor environments are multipath-rich. OFDM, because it reduces the= symbol rate, doesn't mind multipath as much as does DSSS. But it does re= quire a wider band and equalization across the band, in order to work well.= The problem with 802.11 as a protocol is that the receiver has only a mi= crosecond or so to determine how to equalize the signal from a transmitter,= and to apply that equalization. Since the AP is constantly receiving pac= kets from multiple sources, with a high dynamic range, the radios may or ma= y not succeed in equalizing enough. The more bits/sample received, and th= e more variable the analog gain in the front-end can be adapted, the better= the signal can be digitized. Receiver designs are highly variable, and th= ere is no particularly good standard for adjusting the power of transmitter= s to minimize the dynamic range of signals at the receiver end of a packet = transmission. This can be quite different in 5 GHz and 2.4 GHz due to the = type of modulation used in the beacon packets sent by APs. Since the endp= oints are made by a different designers the PHY layer standards are require= d to do the job of making the whole system work. Advanced modulation and d= igitization systems at 5 GHz are potentially better, but may in fact be far= more incompatible with each other. I've seen some terrible design choices= .=0A =0A3) Software/Protocol. The most problematic software issue I know = of is the idea of using RSSI as if it were meaningful for adaptation of rat= es, etc. The rate achieved is the best measure of channel capacity, not si= gnal strength! You can get remarkably good performance at lower signal st= rengths, and poor performance at higher signal strengths - because performa= nce is only weakly affected by signal strength. Even in the Shannon capac= ity law, inside the log term, the key constraint is the ratio between S+N a= nd N. But that is then reduced by the "log" you take. Far more important= is the bandwidth/rate. The larger the bandwidth used to transmit the sam= e rate, the better the performance. This has nothing to do with RSSI. At= 5 GHz one could use larger bandwidths and lower the signal rate when there= is local noise.=0A =0AOne of the biggest issues at 5 GHz is that due to mu= ltipath the "hidden terminal" issue gets worse - and this is a specific iss= ue related to "listen-before-talk" protocols. There are much better ways = to deal with hidden terminals than using RSSI to adapt signal strengths or = rates on any pairwise link.=0A =0AFix: if we could, we should redesign larg= e parts of the 802.11 PHY and packet modulation protocol based on physical = properties of the indoor environments where it is use.=0A =0ASummary:=0A = =0AThere *are* differences between 5 GHz and 2.4 GHz. But they are not due= to how far the signals propagate indoors. First order: make sure aperture= and antenna patterns are proper, since they are different on the two bands= - that is the main reason that the urban legend continues to survive.=0A = =0AEncourage vendors to fix 5 GHz aspects of their products. They should h= ave no excuse, but they scapegoat propagation of the physical energy. That= 's a lie.=0A =0A =0A=0A=0AOn Tuesday, December 17, 2013 6:43pm, "Stephen He= mminger" said:=0A=0A=0A=0A> I concur with Jim.= =0A> =0A> My observation is that in our house, upstairs the 5Ghz AP has low= signal strength=0A> reported by the devices, and poor bandwidth.=0A> =0A> = Could it be that the radiation pattern of the antenna in WDR3800 laying=0A>= horizontally=0A> is different for each band. Maybe the 5Ghz band is more o= f a squashed donut?=0A> =0A> ------=_20131218101930000000_68186 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Yes - ther= e are significant differences in the physical design of access points that = may affect 5 GHz and 2.4 GHz differently.  There are also modulation d= ifferences, and there may actually be scheduling/protocol differences.

= =0A

 

=0A

All of these affect connectivity far more than center-frequency will= .

=0A

 

=0A

1) Antennas.  One of the most obvious problems is antenna = "aperture".  That is a measure of the effective 2-D area of the antenn= a on the receiving side.  A dipole antenna (the cheapest kind, but not= the only kind used in access points) is "tuned" by making its length a spe= cific fraction of the wavelength.  Thus a 5 GHz antenna of the dipole = type has 1/4 the aperture of a dipole antenna for 2.4 GHz.   This mean= s that the 5 GHz antenna of the same design can access only 1/4 of the radi= ated energy at 5 GHz.  But that's entirely due to antenna size.  = If you hold the antenna size constant (which means using a design that is = inherently twice as big as a dipole), you will find that range dramatically= increases.   You can demonstrate this with parabolic reflecting *rece= ive* antennas at the two frequencies. (the aperture can be kept constant by= using the same dish diameter).   If you look at the antenna elements = for 5 and 2.4 in an access pony, you will probably see, if you understand t= he circuitry, that the 5 GHz antenna has a smaller aperture.

=0A

 

=0A

The = other problem is antenna directionality for the transmit and receive antenn= as.  Indeed almost all AP antennas have flattened doughnut radiation p= atterns in free-space.   Worse, however, is that indoors, the antenna = patterns are shaped by reflectors and absorbers so that the energy is highl= y variable, and highly dependent on wavelength in the pattern.  So 5 G= Hz and 2.4 GHz signals received at any particular point have highly variabl= e relative energies.   In one place the 5 GHz signal might be 10x the = energy of a 2.4 GHz signal from the same AP, and in another, 1/10th. The po= int here is that a "controlled experiment" that starts at a point where 2.4= GHz works OK might find weak 5 GHz, but moving 1 foot to the side will cau= se 2.4 to be unworkable, whereas 5 works fine.   Distances of 1 foot c= ompletely change the situation in a diffusive propagation environment.

= =0A

 

=0A

Fix: get the AP designers to hire smarter antenna designers.  E= ven big companies don't understand the antenna issue - remember the Apple i= Phone design with the antenna that did not work if you held the phone at th= e bottom, but worked fine if you held it at the top?  Commercial APs a= re generally made of the cheapest parts, using the cheapest designs, in the= antenna area.  And you buy them and use them with no understanding of= how antennas actually work.  Caveat emptor.  And get your antenn= as evaluated by folks who understand microwave antennas in densely complex = propagation environments, not outdoor free-space.

=0A

 

=0A

(and don't put = your AP in the attic and expect a good signal near the ground.... or in the= basement.  Physics will make sure that the signal is zero at any grou= nd, so being closer to the ground than the antenna weakens the signal a lot= !)

=0A

 

=0A

2) Modulation and digitization.   Indoor environments are= multipath-rich.   OFDM, because it reduces the symbol rate, doesn't m= ind multipath as much as does DSSS.   But it does require a wider band= and equalization across the band, in order to work well.  The problem= with 802.11 as a protocol is that the receiver has only a  microsecon= d or so to determine how to equalize the signal from a transmitter, and to = apply that equalization.   Since the AP is constantly receiving packet= s from multiple sources, with a high dynamic range, the radios may or may n= ot succeed in equalizing enough.   The more bits/sample received, and = the more variable the analog gain in the front-end can be adapted, the bett= er the signal can be digitized.  Receiver designs are highly variable,= and there is no particularly good standard for adjusting the power of tran= smitters to minimize the dynamic range of signals at the receiver end of a = packet transmission.  This can be quite different in 5 GHz and 2.4 GHz= due to the type of modulation used in the beacon packets sent by APs. &nbs= p; Since the endpoints are made by a different designers the PHY layer stan= dards are required to do the job of making the whole system work.  Adv= anced modulation and digitization systems at 5 GHz are potentially better, = but may in fact be far more incompatible with each other.  I've seen s= ome terrible design choices.

=0A

 <= /p>=0A

3) Software/Protocol.   The mos= t problematic software issue I know of is the idea of using RSSI as if it w= ere meaningful for adaptation of rates, etc.  The rate achieved is the= best measure of channel capacity, not signal strength!   You can get = remarkably good performance at lower signal strengths, and poor performance= at higher signal strengths - because performance is only weakly affected b= y signal strength.   Even in the Shannon capacity law, inside the log = term, the key constraint is the ratio between S+N and N.   But that is= then reduced by the "log" you take.  Far more important is the bandwi= dth/rate.   The larger the bandwidth used to transmit the same rate, t= he better the performance.   This has nothing to do with RSSI.  A= t 5 GHz one could use larger bandwidths and lower the signal rate when ther= e is local noise.

=0A

 

=0A

One of the biggest issues at 5 GHz is that due = to multipath the "hidden terminal" issue gets worse - and this is a specifi= c issue related to "listen-before-talk" protocols.   There are much be= tter ways to deal with hidden terminals than using RSSI to adapt signal str= engths or rates on any pairwise link.

=0A

 

=0A

Fix: if we could, we should= redesign large parts of the 802.11 PHY and packet modulation protocol base= d on physical properties of the indoor environments where it is use.

=0A=

 

=0A

Summary:

=0A

 

=0A

There *are* differences between 5 GHz and 2.4 GHz.=  But they are not due to how far the signals propagate indoors.  = ;First order: make sure aperture and antenna patterns are proper, since the= y are different on the two bands - that is the main reason that the urban l= egend continues to survive.

=0A

 =0A

Encourage vendors to fix 5 GHz aspect= s of their products.  They should have no excuse, but they scapegoat p= ropagation of the physical energy.  That's a lie.

=0A

 

=0A

 

= =0A=0A



On Tuesday, December 17, 2013 6:43p= m, "Stephen Hemminger" <stephen@networkplumber.org> said:

=

=0A
=0A

> I concur with Jim.
>
> My observation is that in our= house, upstairs the 5Ghz AP has low signal strength
> reported by = the devices, and poor bandwidth.
>
> Could it be that the = radiation pattern of the antenna in WDR3800 laying
> horizontally> is different for each band. Maybe the 5Ghz band is more of a squas= hed donut?
>
>

=0A
 ------=_20131218101930000000_68186--