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) = 27 us
# o *Backoff Slot * CWmin,  9 us * rand[0,xf] (avg) = 7 * 9=63 us
# o 5G 20 us
# o Multimode header 20 us
# o PLCP (symbols) 2 * 4 us = 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 = 16 µs
# +(2+7.5)*9 µs = 101.5 µs for OFDM PHY, while the data rate with OFDM PHY
# can reach 600 Mbps in 802.11n, leading to a transmission time of 20 µs
# 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 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 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 you 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.  (A guess is doing this wouldn't improve the
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 <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.  900Mb/sec is typically a peak-average throughput measurement
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 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?)  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.   (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 10K
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 limit
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.

  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