Re: [Bloat] [e2e] bufferbloat paper

[Keith Winstein <keithw@mit.edu>]

Hello Michael,

On Wed, Jan 9, 2013 at 9:07 AM, Michael Richardson <mcr@sandelman.ca> wrote:
> Have you considered repeating your test with two phones?

Yes, we have tried up to four phones at the same time.

> Can the download on phone1 affect the latency seen by a second phone?

In our experience, a download on phone1 will not affect the unloaded
latency seen by phone2. The cell towers appear to use a per-UE
(per-phone) queue on uplink and downlink. (This is similar to what a
commodity cable modem user sees -- I don't get long delays just
because my neighbor is saturating his uplink or downlink and causing a
standing queue for himself.)

However, a download on phone1 can affect the average throughput seen
by phone2 when it is saturating its link, suggesting that the two
phones are contending for the same limited resource (timeslices and
OFDM resource blocks, or possibly just backhaul throughput).

> Obviously the phones should be located right next to each other, with
> some verification that they are actually associated to the same tower.

This is harder than we thought it would be -- the phones have a
tendency to wander around rapidly among cell IDs (sometimes switching
several times in a minute). We're not sure if the different cell IDs
really represent different towers (we doubt it) or maybe just
different LTE channels or logical channels. I understand in LTE it is
possible for multiple towers to cooperate to receive one packet, so
the story may be more complicated.

In practice it is possible to get four phones to "hold still" on the
same cell ID for five minutes to do a test, but it is a bit like
herding cats and requires some careful placement and luck.

Best regards,
Keith