From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-x22c.google.com (mail-wm0-x22c.google.com [IPv6:2a00:1450:400c:c09::22c]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id 561723B2A0 for ; Mon, 27 Jun 2016 18:12:09 -0400 (EDT) Received: by mail-wm0-x22c.google.com with SMTP id r190so23678975wmr.0 for ; Mon, 27 Jun 2016 15:12:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=broadcom.com; s=google; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc; bh=Ne3FnoUELwi3DzSxg9mDjmvt8Um6DH7cuR69qUASYaM=; b=Oamq2xExifI3nIecmsg1VaJKfrI6OL850Lc3Ey5IJjg8PXGxd+FYPfY88oQd6HU8DO ZJ21zZ7Bau2JtiWk1gHoM7EuhpGhd6a7TPPs6SryV7ocE2vbPD7wrOSSm0q6DuNyyp5n Wh9zTRIpRKae+L8GiQrRmkb9ikDQg+Jec0m8g= 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:from:date :message-id:subject:to:cc; bh=Ne3FnoUELwi3DzSxg9mDjmvt8Um6DH7cuR69qUASYaM=; b=hBhmQTxNzOxs5wJoswi0syHDYfIwXHBihJM85mzkHzW8poAdjEFH+CnmGRLUsKiVrT oD+tMpr01hb5eMbd3m3bI9rvEF7PmKru1rCNTjoq+ELn7LaN6e2EE5F99S6LgX0MBz04 oETBt3cmmOXPJe8pyUs5uTzxyzzNLvA94tAGtjGnN8XcpjgMBobxbV0yDMaubOXJ0C60 2S+C6TYAGZC6i7fGV16Jpg8Tn3TOeG1GY9eu2eSgDKp8UFywDQ62SC2Y2Bt0QoSzF/ae OnGl0B2e3SRAaqNC2amhsPcn94cXazLVJyG01zwfI0gR7X6srczWPd8Xy6a5mklH91jh riLw== X-Gm-Message-State: ALyK8tLv2eRvcbvZIcXtEzzrtCT+sZvOlMX0UpYTBK03VxOSwR6NTN1iCWKxo7V9TBRRB2L+DXv6BqfETZQTO/mT X-Received: by 10.28.145.68 with SMTP id t65mr11991006wmd.31.1467065528171; Mon, 27 Jun 2016 15:12:08 -0700 (PDT) MIME-Version: 1.0 Received: by 10.194.239.202 with HTTP; Mon, 27 Jun 2016 15:12:06 -0700 (PDT) In-Reply-To: References: <1466803464.927322699@mobile.rackspace.com> From: Bob McMahon Date: Mon, 27 Jun 2016 15:12:06 -0700 Message-ID: To: David Lang Cc: Dave Taht , make-wifi-fast@lists.bufferbloat.net, "cerowrt-devel@lists.bufferbloat.net" Content-Type: multipart/alternative; boundary=001a11469decabd67f053649cfe2 Subject: Re: [Make-wifi-fast] more well funded attempts showing market demandfor better wifi X-BeenThere: make-wifi-fast@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 27 Jun 2016 22:12:09 -0000 --001a11469decabd67f053649cfe2 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable While the 802.11 ack doesn't need to do collision avoidance, it does need to wait a SIFS, send a PHY header and its typically transmitted at lower PHY rate. My estimate is 40 us for that overhead. So yes, one would have to get rid of that too, e.g. assume a transmit without a collision succeeded - hopefully negating the need for the 802.11 ack. (It does seem the wired engineers have it much easier per the point/point, full duplex and wave guides.) Bob On Mon, Jun 27, 2016 at 2:09 PM, David Lang wrote: > On Mon, 27 Jun 2016, Bob McMahon wrote: > > packet size is smallest udp payload per a socket write() which in turn >> drives the smallest packet supported by "the wire." >> >> Here is a back of the envelope calculation giving ~100 microseconds per = BE >> access. >> >> # Overhead estimates (slot time is 9 us): >> # o DIFS 50 us or *AIFS (3 * 9 us) =3D 27 us >> # o *Backoff Slot * CWmin, 9 us * rand[0,xf] (avg) =3D 7 * 9=3D63 us >> # o 5G 20 us >> # o Multimode header 20 us >> # o PLCP (symbols) 2 * 4 us =3D 8 us >> # o *SIFS 16 us >> # o ACK 40 us >> > > isn't the ack a separate transmission by the other end of the connection? > (subject to all the same overhead) > > # >> # Even if there is no collision and the CW stays at the aCWmin, the >> average >> # backoff time incurred by CSMA/CA is aDIFS + aCWmin/2 * aSlotTime =3D 1= 6 =C2=B5s >> # +(2+7.5)*9 =C2=B5s =3D 101.5 =C2=B5s for OFDM PHY, while the data rate= with OFDM PHY >> # can reach 600 Mbps in 802.11n, leading to a transmission time of 20 = =C2=B5s >> # for a 1500 byte packet. >> > > well, are you talking a 64 byte packet or a 1500 byte packet? > > But this is a good example of why good aggregation is desirable. It > doesn't have > to add a lot of latency. you could send 6x as much data in 2x the time by > sending 9K per transmission instead of 1.5K per transmission (+100us/7.5K= ) > > if the aggregation is done lazily (send whatever's pending, don't wait fo= r > more data if you have an available transmit slot), this can be done with > virtually no impact on latency, you just have to set a reasonable maximum= , > and adjust it based on your transmission rate. > > The problem is that right now thing don't set a reasonable max, and they > do greedy aggregation (wait until you have a lot of data to send before y= ou > send anything) > > All devices in a BSSID would have to agree that the second radio is to be >> used for BSSID "carrier state" information and all energy will be source= d >> by the AP serving that BSSID. (A guess is doing this wouldn't improve t= he >> 100 us by enough to justify the cost and that a new MAC protocol is >> required. Just curious to what such a protocol and phy subsystem would >> look like assuming collision avoidance could be replaced with collision >> detect.) >> > > if the second radio is on a separate band, you have the problem that > propogation > isn't going to be the same, so it's very possible to be able to talk to > the AP > on the 'normal' channel, but not on the 'coordination' channel. > > I'm also not sure what good it would do, once a transmission has been > stepped > on, it will need to be re-sent (I guess you would be able to re-send > immediatly) > > > David Lang > > Bob >> >> >> >> On Mon, Jun 27, 2016 at 1:09 PM, David Lang wrote: >> >> On Mon, 27 Jun 2016, Bob McMahon wrote: >>> >>> The ~10K is coming from empirical measurements where all aggregation >>> >>>> technologies are disabled, i.e. only one small IP packet per medium >>>> arbitration/access and where there is only one transmitter and one >>>> receiver. 900Mb/sec is typically a peak-average throughput measuremen= t >>>> where max (or near max) aggregation occurs, amortizing the access >>>> overhead >>>> across multiple packets. >>>> >>>> >>> so 10K is minimum size packets being transmitted?or around 200 >>> transmissions/sec (plus 200 ack transmissions/sec)? >>> >>> Yes, devices can be hidden from each other but not from the AP (hence t= he >>> >>>> use of RTS/CTS per hidden node mitigation.) Isn't it the AP's view of >>>> the >>>> "carrier state" that matters (at least in infrastructure mode?) If >>>> that's >>>> the case, what about a different band (and different radio) such that >>>> the >>>> strong signal carrying the data could be separated from the the BSSID'= s >>>> "carrier/energy state" signal? >>>> >>>> >>> how do you solve the interference problem on this other band/radio? Whe= n >>> you have other APs in the area operating, you will have the same proble= m >>> there. >>> >>> David Lang >>> >>> >>> Bob >>> >>>> >>>> On Mon, Jun 27, 2016 at 12:40 PM, David Lang wrote: >>>> >>>> On Mon, 27 Jun 2016, Bob McMahon wrote: >>>> >>>>> >>>>> Hi All, >>>>> >>>>> >>>>>> This is a very interesting thread - thanks to all for taking the tim= e >>>>>> to >>>>>> respond. (Personally, I now have better understanding of the >>>>>> difficulties >>>>>> associated with a PHY subsystem that supports a wide 1GHz.) >>>>>> >>>>>> Not to derail the current discussion, but I am curious to ideas on >>>>>> addressing the overhead associated with media access per collision >>>>>> avoidance. This overhead seems to be limiting transmits to about 10= K >>>>>> per >>>>>> second (even when a link has no competition for access.) >>>>>> >>>>>> >>>>>> I'm not sure where you're getting 10K/second from. We do need to lim= it >>>>> the >>>>> amount of data transmitted in one session to give other stations a >>>>> chance >>>>> to talk, but if the AP replies immediatly to ack the traffic, and the >>>>> airwaves are idle, you can transmit again pretty quickly. >>>>> >>>>> people using -ac equipment with a single station are getting 900Mb/se= c >>>>> today. >>>>> >>>>> Is there a way, >>>>> >>>>> maybe using another dedicated radio, to achieve near instantaneous >>>>>> collision detect (where the CD is driven by the receiver state) such >>>>>> that >>>>>> mobile devices can sample RF energy to get theses states and state >>>>>> changes >>>>>> much more quickly? >>>>>> >>>>>> >>>>>> This gets back to the same problems (hidden transmitter , and the >>>>> simultanious reception of wildly different signal strengths) >>>>> >>>>> When you are sending, you will hear yourself as a VERY strong signal, >>>>> trying to hear if someone else is transmitting at the same time is >>>>> almost >>>>> impossible (100 ft to 1 ft is 4 orders of magnatude, 1 ft to 1 inch i= s >>>>> another 2 orders of magnatude) >>>>> >>>>> And it's very possible that the station that you are colliding with >>>>> isn't >>>>> one you can hear at all. >>>>> >>>>> Any AP is going to have a better antenna than any phone. (sometimes >>>>> several orders of magnatude better), so even if you were located at t= he >>>>> same place as the AP, the AP is going to hear signals that you don't. >>>>> >>>>> Then consider the case where you and the other station are on opposit= e >>>>> sides of the AP at max range. >>>>> >>>>> and then add cases where there is a wall between you and the other >>>>> station, but the AP can hear both of you. >>>>> >>>>> David Lang >>>>> >>>>> >>>>> >>>> --001a11469decabd67f053649cfe2 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
While the 802.11 ack doesn't need to do collision avoi= dance, it does need to wait a SIFS, send a PHY header and its typically tra= nsmitted at lower PHY rate. =C2=A0 My estimate is 40 us for that overhead.= =C2=A0 So yes, one would have to get rid of that too, e.g. assume a transmi= t without a collision succeeded - hopefully negating the need for the 802.1= 1 ack.

(It does seem the wired engineers have it much easier per the= point/point, full duplex and wave guides.)

Bob

On Mon, Jun 27, 2016 at 2:09 PM,= David Lang <david@lang.hm> wrote:
On Mon, 27 Jun 2016, Bob McMahon wrote:

packet size is smallest udp payload per a socket write() which in turn
drives the smallest packet supported by "the wire."

Here is a back of the envelope calculation giving ~100 microseconds per BE<= br> access.

# Overhead estimates (slot time is 9 us):
# o DIFS 50 us or *AIFS (3 * 9 us) =3D 27 us
# o *Backoff Slot * CWmin,=C2=A0 9 us * rand[0,xf] (avg) =3D 7 * 9=3D63 us<= br> # o 5G 20 us
# o Multimode header 20 us
# o PLCP (symbols) 2 * 4 us =3D 8 us
# o *SIFS 16 us
# o ACK 40 us

 isn't the ack a separate transmission by the other end of the connectio= n? (subject to all the same overhead)

#
# Even if there is no collision and the CW stays at the aCWmin, the average=
# backoff time incurred by CSMA/CA is aDIFS + aCWmin/2 * aSlotTime =3D 16 = =C2=B5s
# +(2+7.5)*9 =C2=B5s =3D 101.5 =C2=B5s for OFDM PHY, while the data rate wi= th OFDM PHY
# can reach 600 Mbps in 802.11n, leading to a transmission time of 20 =C2= =B5s
# for a 1500 byte packet.

 well, are you talking a 64 byte packet or a 1500 byte packet?

But this is a good example of why good aggregation is desirable. It doesn&#= 39;t have
to add a lot of latency. you could send 6x as much data in 2x the time by sending 9K per transmission instead of 1.5K per transmission (+100us/7.5K)<= br>
if the aggregation is done lazily (send whatever's pending, don't w= ait for more data if you have an available transmit slot), this can be done= with virtually no impact on latency, you just have to set a reasonable max= imum, and adjust it based on your transmission rate.

The problem is that right now thing don't set a reasonable max, and the= y do greedy aggregation (wait until you have a lot of data to send before y= ou send anything)

All devices in a BSSID would have to agree that the second radio is to be used for BSSID "carrier state" information and all energy will be= sourced
by the AP serving that BSSID.=C2=A0 (A guess is doing this wouldn't imp= rove the
100 us by enough to justify the cost and that a new MAC protocol is
required.=C2=A0 Just curious to what such a protocol and phy subsystem woul= d
look like assuming collision avoidance could be replaced with collision
detect.)

 if the second radio is on a separate band, you have the problem that propog= ation
isn't going to be the same, so it's very possible to be able to tal= k to the AP
on the 'normal' channel, but not on the 'coordination' chan= nel.

I'm also not sure what good it would do, once a transmission has been s= tepped
on, it will need to be re-sent (I guess you would be able to re-send immedi= atly)


David Lang

Bob



On Mon, Jun 27, 2016 at 1:09 PM, David Lang <david@lang.hm> wrote:

On Mon, 27 Jun 2016, Bob McMahon wrote:

The ~10K is coming from empirical measurements where all aggregation
technologies are disabled, i.e. only one small IP packet per medium
arbitration/access and where there is only one transmitter and one
receiver.=C2=A0 900Mb/sec is typically a peak-average throughput measuremen= t
where max (or near max) aggregation occurs, amortizing the access overhead<= br> across multiple packets.


so 10K is minimum size packets being transmitted?or around 200
transmissions/sec (plus 200 ack transmissions/sec)?

Yes, devices can be hidden from each other but not from the AP (hence the
use of RTS/CTS per hidden node mitigation.) Isn't it the AP's view = of the
"carrier state" that matters (at least in infrastructure mode?)= =C2=A0 If that's
the case, what about a different band (and different radio) such that the strong signal carrying the data could be separated from the the BSSID's=
"carrier/energy state" signal?


how do you solve the interference problem on this other band/radio? When you have other APs in the area operating, you will have the same problem there.

David Lang


Bob

On Mon, Jun 27, 2016 at 12:40 PM, David Lang <david@lang.hm> wrote:

On Mon, 27 Jun 2016, Bob McMahon wrote:

Hi All,


This is a very interesting thread - thanks to all for taking the time to respond.=C2=A0 =C2=A0(Personally, I now have better understanding of the difficulties
associated with a PHY subsystem that supports a wide 1GHz.)

Not to derail the current discussion, but I am curious to ideas on
addressing the overhead associated with media access per collision
avoidance.=C2=A0 This overhead seems to be limiting transmits to about 10K<= br> per
second (even when a link has no competition for access.)


I'm not sure where you're getting 10K/second from. We do need to li= mit
the
amount of data transmitted in one session to give other stations a chance to talk, but if the AP replies immediatly to ack the traffic, and the
airwaves are idle, you can transmit again pretty quickly.

people using -ac equipment with a single station are getting 900Mb/sec
today.

=C2=A0 Is there a way,

maybe using another dedicated radio, to achieve near instantaneous
collision detect (where the CD is driven by the receiver state) such
that
mobile devices can sample RF energy to get theses states and state
changes
much more quickly?


This gets back to the same problems (hidden transmitter , and the
simultanious reception of wildly different signal strengths)

When you are sending, you will hear yourself as a VERY strong signal,
trying to hear if someone else is transmitting at the same time is almost impossible (100 ft to 1 ft is 4 orders of magnatude, 1 ft to 1 inch is
another 2 orders of magnatude)

And it's very possible that the station that you are colliding with isn= 't
one you can hear at all.

Any AP is going to have a better antenna than any phone. (sometimes
several orders of magnatude better), so even if you were located at the
same place as the AP, the AP is going to hear signals that you don't.
Then consider the case where you and the other station are on opposite
sides of the AP at max range.

and then add cases where there is a wall between you and the other
station, but the AP can hear both of you.

David Lang




--001a11469decabd67f053649cfe2--