[Cerowrt-devel] [Make-wifi-fast] more well funded attempts showing market demandfor better wifi
David Lang
david at lang.hm
Sun Jun 26 20:00:55 EDT 2016
I don't think anyone is trying to do simultanious receive of different stations.
That is an incredibly difficult thing to do right.
MU-MIMO is aimed at haivng the AP transmit to multiple stations at the same
time. For the typical browser/streaming use, this traffic is FAR larger than the
traffic from the stations to the AP. As such, it is worth focusing on optimizing
this direction.
While an ideal network may resemble a wired network without guides, I don't
think it's a good idea to think about wifi networks that way.
The reality is that no matter how good you get, a wireless network is going to
have lots of things that are just not going to happen with wired networks.
1. drastic variations in signal strength.
On a wired network with a shared buss, the signal strength from all stations
on the network is going to be very close to the same (a difference of 2x would
be extreme)
On a wireless network, with 'normal' omnidirctional antennas, the signal drops
off with the square of the distance. So if you want to service clients from 1 ft
to 100 ft away, your signal strength varies by 1000 (4 orders of magnatude),
this is before you include effects of shielding, bounces, bad antenna alignment,
etc (which can add several more orders of magnatude of variation)
The receiver first normalized the strongest part of the signal to a constant
value, and then digitizes the result, (usually with a 12-14 bit AD converter).
Since 1000x is ~10 bits, the result of overlapping tranmissions can be one
signal at 14 bits, and another at <4 bits. This is why digital processing isn't
able to receive multiple stations at the same time.
2. 'hidden transmitters'
On modern wired networks, every link has exactly two stations on it, and both
can transmit at the same time.
On wireless networks, it's drastically different. You have an unknown number
of stations (which can come and go without notice).
Not every station can hear every other station. This means that they can't
avoid colliding with each other. In theory you can work around this by having
some central system coordinate all the clients (either by them being told when
to transmit, or by being given a schedule and having very precise clocks). But
in practice the central system doesn't know when the clients have something to
say and so in practice this doesn't work as well (except for special cases like
voice where there is a constant amount of data to transmit)
3. variable transmit rates and aggregation
Depending on how strong the signal is between two stations, you have different
limits to how fast you can transmit data. There are many different standard
modulations that you can use, but if you use one that's too fast for the signal
conditions, the receiver isn't going to be able to decode it. If you use one
that's too slow, you increase the probability that another station will step on
your signal, scrambling it as far as the receiver is concerned. We now have
stations on the network that can vary in speed by 100x, and are nearing 1000x
(1Mb/sec to 1Gb/sec)
Because there is so much variation in transmit rates, and older stations will
not be able to understand the newest rates, each transmission starts off with
some data being transmitted at the slowest available rate, telling any stations
that are listening that there is data being transmitted for X amount of time,
even if they can't tell what's going on as the data is being transmitted.
The combination of this header being transmitted inefficiently, and the fact
that stations are waiting for a clear window to transmit, means that when you do
get a chance to transmit, you should send more than one packet at a time. This
is something Linux is currently not doing well, qdiscs tend to round-robin
packets without regard to where they are headed. The current work being done
here with the queues is improving both throughput and latency by fixing this
problem.
You really need to think differently when dealing with wireless network. The
early wifi drivers tried to make them look just like a wired network, and we
have found that we just needed too much other stuff to be successful whith that
mindset.
The Analog/Radio side of things really is important, and can't just be
abstracted away.
David Lang
On Sun, 26 Jun 2016, Bob McMahon wrote:
> Is there a specific goal in mind? This seems an AP tx centric proposal,
> though I may not be fully understanding it. I'm also curious as why not
> scale in spatial domain vs the frequency domain, i.e. AP and STAs can also
> scale using MiMO. Why not just do that? So many phones today are 1x1, some
> 2x2 and few 3x3. Yet APs are moving to 4x4 and I think the standard
> supports 8x8. (I'm not sure the marginal transistor count increase per
> each approach.) On the AP tx side, MuMIMO is also there which I think is
> similar to the DAC proposal.
>
> I'm far from a PHY & DSP expert, but I think the simultaneous AP receive is
> the most difficult issue per the power variations, aka SNR. Each of the
> mobile device energies is affected per inverse square law (unless some form
> of wave guide is used.) Hence wi-fi use of time domain slots prior to
> transmit (no longer simultaneous in time.) Particularly needed for devices
> that communicate with one another. Unfortunately, "collocated"
> energy/information sources must honor this TDM even when not communication
> with one another per tragedy of the commons. (Agreed there is no such
> thing as a collision so let's redefine it to mean that the receiver is
> unable to receive per RF energy "confusion", still a tragedy of the
> commons. I don't know what drives the limits to DSP decode that could
> minimize or eliminate this tragedy.)
>
> At the end of the day, would the ideal network would resemble a wired
> ethernet (including ethernet switch fabric) without the waveguides (or
> wires/fibers)? From that perspective, here are some thoughts to the goals
>
> o) TX op/access to transmit driven to zero. (Collision avoidance isn't
> nearly as good as instantaneous collision detect in this context, though
> "collision" should replaced with "confusion")
> o) RX confusion detection time propagated to stop offending TX(s) driven
> to zero
> o) Support of different encodings (e..g phy rates) pushes towards virtual
> output queueing prior (queuing at the transmitter)
> o) Power per bit xfered towards zero or driven per cost & energy density
> of batteries. Note: Atomic batteries not allowed per humans being involved.
> o) Transistor count (chip cost) per bit moved driven by Moore's law and
> those economics
> o) Reduce "collision/confusion" domain (less STAs per AP) ideally to zero
>
> Just some thoughts off the top of my head. Please do comment and correct.
> Thanks in advance for the discussion.
>
> Bob
>
>
>
>
> On Fri, Jun 24, 2016 at 2:24 PM, <dpreed at reed.com> wrote:
>
>> Without custom silicon, doing what I was talking about would involve non
>> standard MAC power management, which would require all devices to agree.
>>
>> David Lang's explanation was the essence of what I meant. the transmission
>> from access point on multiple channels is just digital addition if the DACs
>> have enough bits per sample. to make sure that the signals to the AP are
>> equalized, just transmit at a power that makes that approximately true...
>> which means a power amp with at most 30 dB of dynamic gain setting. typical
>> dynamic path attenuation range (strongest to weakest ratio) among stations
>> served by an AP is < 20 dB from my past experiments on well
>> operating-installtions, but 25 can be seen in reflection heavy
>> environments.-----Original Message-----
>> From: "David Lang" <david at lang.hm>
>> Sent: Fri, Jun 24, 2016 at 1:19 am
>> To: "Bob McMahon" <bob.mcmahon at broadcom.com>
>> Cc: "Bob McMahon" <bob.mcmahon at broadcom.com>,
>> make-wifi-fast at lists.bufferbloat.net, "cerowrt-devel at lists.bufferbloat.net"
>> <cerowrt-devel at lists.bufferbloat.net>
>> Subject: Re: [Make-wifi-fast] more well funded attempts showing market
>> demandfor better wifi
>>
>> well, with the kickstarter, I think they are selling a bill of goods.
>>
>> Just using the DFS channels and aggregating them as supported by N and AC
>> standards would do wonders (as long as others near you don't do the same)
>>
>> David Lang
>>
>> On Thu, 23 Jun 2016, Bob McMahon wrote:
>>
>>> Date: Thu, 23 Jun 2016 20:01:22 -0700
>>> From: Bob McMahon
>>> To: David Lang
>>> Cc: dpreed at reed.com, make-wifi-fast at lists.bufferbloat.net,
>>> "cerowrt-devel at lists.bufferbloat.net"
>>>
>>> Subject: Re: [Make-wifi-fast] more well funded attempts showing market
>> demand
>>> for better wifi
>>>
>>> Thanks for the clarification. Though now I'm confused about how all the
>>> channels would be used simultaneously with an AP only solution (which is
>> my
>>> understanding of the kickstarter campaign.)
>>>
>>> Bob
>>>
>>> On Thu, Jun 23, 2016 at 7:14 PM, David Lang wrote:
>>>
>>>> I think he is meaning when one unit is talking to one AP the signal
>> levels
>>>> across multiple channels will be similar. Which is probably fairly true.
>>>>
>>>>
>>>> David Lang
>>>>
>>>> On Thu, 23 Jun 2016, Bob McMahon wrote:
>>>>
>>>> Curious, where does the "in a LAN setup, the variability in [receive]
>>>>> signal strength is likely small enough" assertion come? Any specific
>>>>> power numbers here? We test with many combinations of "signal strength
>>>>> variability" (e.g. deltas range from 0 dBm - 50 dBm) and per different
>>>>> channel conditions. This includes power variability within the spatial
>>>>> streams' MiMO transmission. It would be helpful to have some physics
>>>>> combined with engineering to produce some pragmatic limits to this.
>>>>>
>>>>> Also, mobile devices have a goal of reducing power in order to be
>>>>> efficient
>>>>> with their battery (vs a goal to balance power such that an AP can
>>>>> receive simultaneously.) Power per bit usually trumps most other
>> design
>>>>> goals. There market for battery powered wi-fi devices drives a
>>>>> semi-conductor mfg's revenue so my information come with that bias.
>>>>>
>>>>> Bob
>>>>>
>>>>> On Thu, Jun 23, 2016 at 1:48 PM, wrote:
>>>>>
>>>>> The actual issues of transmitting on multiple channels at the same time
>>>>>> are quite minor if you do the work in the digital domain (pre-DAC).
>> You
>>>>>> just need a higher sampling rate in the DAC and add the two signals
>>>>>> together (and use a wideband filter that covers all the channels).
>> No RF
>>>>>> problem.
>>>>>>
>>>>>> Receiving multiple transmissions in different channels is pretty much
>> the
>>>>>> same problem - just digitize (ADC) a wider bandwidth and separate in
>> the
>>>>>> digital domain. the only real issue on receive is equalization - if
>> you
>>>>>> receive two different signals at different receive signal strengths,
>> the
>>>>>> lower strength signal won't get as much dynamic range in its samples.
>>>>>>
>>>>>> But in a LAN setup, the variability in signal strength is likely small
>>>>>> enough that you can cover that with more ADC bits (or have the MAC
>>>>>> protocol
>>>>>> manage the station transmit power so that signals received at the AP
>> are
>>>>>> nearly the same power.
>>>>>>
>>>>>> Equalization at transmit works very well when there is a central AP
>> (as
>>>>>> in
>>>>>> cellular or normal WiFi systems).
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Thursday, June 23, 2016 4:28pm, "Bob McMahon" >>>
>> bob.mcmahon at broadcom.com>
>>>>>> said:
>>>>>>
>>>>>> _______________________________________________
>>>>>>> Make-wifi-fast mailing list
>>>>>>> Make-wifi-fast at lists.bufferbloat.net
>>>>>>> https://lists.bufferbloat.net/listinfo/make-wifi-fast
>>>>>>> An AP per room/area, reducing the tx power (beacon range) has been my
>>>>>>> approach and has scaled very well. It does require some wires to
>> each
>>>>>>>
>>>>>> AP
>>>>>>
>>>>>>> but I find that paying an electrician to run some quality wiring to
>>>>>>>
>>>>>> things
>>>>>>
>>>>>>> that are to remain stationary has been well worth the cost.
>>>>>>>
>>>>>>> just my $0.02,
>>>>>>> Bob
>>>>>>>
>>>>>>> On Thu, Jun 23, 2016 at 1:10 PM, David Lang wrote:
>>>>>>>
>>>>>>> Well, just using the 5GHz DFS channels in 80MHz or 160 MHz wide
>> chunks
>>>>>>>> would be a huge improvement, not many people are using them (yet),
>> and
>>>>>>>>
>>>>>>> the
>>>>>>
>>>>>>> wide channels let you get a lot of data out at once. If everything is
>>>>>>>> within a good range of the AP, this would work pretty well. If you
>> end
>>>>>>>>
>>>>>>> up
>>>>>>
>>>>>>> needing multiple APs, or you have many stations, I expect that you
>> will
>>>>>>>>
>>>>>>> be
>>>>>>
>>>>>>> better off with more APs at lower power, each using different
>> channels.
>>>>>>>>
>>>>>>>> David Lang
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Thu, 23 Jun 2016, Bob McMahon wrote:
>>>>>>>>
>>>>>>>> Date: Thu, 23 Jun 2016 12:55:19 -0700
>>>>>>>>
>>>>>>>>> From: Bob McMahon
>>>>>>>>> To: Dave Taht
>>>>>>>>> Cc: make-wifi-fast at lists.bufferbloat.net,
>>>>>>>>> "cerowrt-devel at lists.bufferbloat.net"
>>>>>>>>>
>>>>>>>>> Subject: Re: [Make-wifi-fast] more well funded attempts showing
>> market
>>>>>>>>> demand
>>>>>>>>> for better wifi
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> hmm, I'm skeptical. To use multiple carriers simultaneously is
>>>>>>>>>
>>>>>>>> difficult
>>>>>>
>>>>>>> per RF issues. Even if that is somehow resolved, to increase
>>>>>>>>>
>>>>>>>> throughput
>>>>>>
>>>>>>> usually requires some form of channel bonding, i.e. needed on both
>>>>>>>>>
>>>>>>>> sides,
>>>>>>
>>>>>>> and brings in issues with preserving frame ordering. If this is just
>>>>>>>>> channel hopping, that needs coordination between both sides (and
>> isn't
>>>>>>>>> simultaneous, possibly costing more than any potential gain.) An
>> AP
>>>>>>>>>
>>>>>>>> only
>>>>>>
>>>>>>> solution can use channel switch announcements (CSA) but there is a
>>>>>>>>>
>>>>>>>> cost to
>>>>>>
>>>>>>> those as well.
>>>>>>>>>
>>>>>>>>> I guess don't see any break though here and the marketing on the
>> site
>>>>>>>>> seems
>>>>>>>>> to indicate something beyond physics, at least the physics that I
>>>>>>>>> understand. Always willing to learn and be corrected if I'm
>>>>>>>>> misunderstanding things.
>>>>>>>>>
>>>>>>>>> Bob
>>>>>>>>>
>>>>>>>>> On Wed, Jun 22, 2016 at 10:18 AM, Dave Taht
>>>>>>>>>
>>>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>>> On Wed, Jun 22, 2016 at 10:03 AM, Dave Taht
>>>>>>>>>
>>>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>
>> https://www.kickstarter.com/projects/portalwifi/portal-turbocharged-wifi?ref=backerkit
>>>>>>
>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> "Portal is the first and only router specifically engineered to
>> cut
>>>>>>>>>>> through and avoid congestion, delivering consistent,
>>>>>>>>>>> high-performance
>>>>>>>>>>> WiFi with greater coverage throughout your home.
>>>>>>>>>>>
>>>>>>>>>>> Its proprietary spectrum turbocharger technology provides access
>> to
>>>>>>>>>>> 300% more of the radio airwaves than any other router, improving
>>>>>>>>>>> performance by as much as 300x, and range and coverage by as
>> much as
>>>>>>>>>>> 2x in crowded settings, such as city homes and multi-unit
>>>>>>>>>>> apartments"
>>>>>>>>>>>
>>>>>>>>>>> It sounds like they are promising working DFS support.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>> It's not clear what chipset they are using (they are claiming
>> wave2)
>>>>>>>>>> -
>>>>>>>>>> but they are at least publicly claiming to be using openwrt. So I
>>>>>>>>>> threw in enough to order one for september, just so I could
>> comment
>>>>>>>>>> on
>>>>>>>>>> their kickstarter page. :)
>>>>>>>>>>
>>>>>>>>>> I'd have loved to have got in earlier (early shipments are this
>> month
>>>>>>>>>> apparently), but those were sold out.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>
>> https://www.kickstarter.com/projects/portalwifi/portal-turbocharged-wifi/comments
>>>>>>
>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>>> Dave Täht
>>>>>>>>>>> Let's go make home routers and wifi faster! With better software!
>>>>>>>>>>> http://blog.cerowrt.org
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> Dave Täht
>>>>>>>>>> Let's go make home routers and wifi faster! With better software!
>>>>>>>>>> http://blog.cerowrt.org
>>>>>>>>>> _______________________________________________
>>>>>>>>>> Make-wifi-fast mailing list
>>>>>>>>>> Make-wifi-fast at lists.bufferbloat.net
>>>>>>>>>> https://lists.bufferbloat.net/listinfo/make-wifi-fast
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>> Make-wifi-fast mailing list
>>>>>>>> Make-wifi-fast at lists.bufferbloat.net
>>>>>>>> https://lists.bufferbloat.net/listinfo/make-wifi-fast
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>
>>
>>
>
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