[e2e] What's wrong with this picture?

[Dominik Kaspar]

Hello David,

You mentioned the bimodal behaviour of your 3G connection. I recently
noticed the same thing but have not yet been able to explain why this
happens.

I also ran Ping tests over multiple days using an HSDPA modem (with
both the client and server located in Oslo, Norway). The experienced
RTTs were very stable over short periods of time, but sometimes they
averaged around 80ms, while at other times the average was at about
300ms.

A CDF illustration of the results is available here:
http://home.simula.no/~kaspar/static/cdf-hsdpa-rtt-00.png

What is the reason of these two modes? Is it caused by adaptive
modulation and coding on the physical layer? If so, why does it affect
the delay so much? I would only expect a reduced bandwidth, but not
much change in delay...

Greetings,
Dominik


On Tue, Sep 8, 2009 at 7:56 PM, David P. Reed<dpreed at reed.com> wrote:
> I should not have been so cute - I didn't really want to pick on the
> operator involved, because I suspect that other 3G operators around the
> world probably use the same equipment and same rough configuration.
>
> The ping and traceroute were from Chicago, using an ATT Mercury data modem,
> the same channel as the Apple iPhones use, but it's much easier to run test
> suites from my netbook.
>
> Here's the same test from another time of day, early Sunday morning, when
> things were working well.
>
> Note that I ran the test over the entire labor day weekend at intervals.
> The end-to-end ping time was bimodal.  Either it pegged at over 5000
> milliseconds, or happily sat at under 200 milliseconds.   Exactly what one
> would expect if TCP congestion control were disabled by overbuffering in a
> router preceding the bottleneck link shared by many users.
>
> ------------------------------
>
> $ ping lcs.mit.edu
> PING lcs.mit.edu (128.30.2.121) 56(84) bytes of data.
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=1 ttl=44
> time=209 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=2 ttl=44
> time=118 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=3 ttl=44
> time=166 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=4 ttl=44
> time=165 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=5 ttl=44
> time=224 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=6 ttl=44
> time=183 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=7 ttl=44
> time=224 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=8 ttl=44
> time=181 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=9 ttl=44
> time=220 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=10 ttl=44
> time=179 ms
> 64 bytes from zermatt.csail.mit.edu (128.30.2.121): icmp_seq=11 ttl=44
> time=219 ms
> ^C
> --- lcs.mit.edu ping statistics ---
> 11 packets transmitted, 11 received, 0% packet loss, time 10780ms
> rtt min/avg/max/mdev = 118.008/190.547/224.960/31.772 ms
> $ traceroute lcs.mit.edu
> traceroute to lcs.mit.edu (128.30.2.121), 30 hops max, 60 byte packets
>  1  * * *
>  2  172.26.248.2 (172.26.248.2)  178.725 ms  178.568 ms  179.500 ms
>  3  * * *
>  4  172.16.192.34 (172.16.192.34)  187.794 ms  187.677 ms  207.527 ms
>  5  12.88.7.205 (12.88.7.205)  207.416 ms  208.325 ms  69.630 ms
>  6  cr84.cgcil.ip.att.net (12.122.152.134)  79.425 ms  89.227 ms  90.083 ms
>  7  cr2.cgcil.ip.att.net (12.123.7.250)  98.679 ms  90.727 ms  91.576 ms
>  8  ggr2.cgcil.ip.att.net (12.122.132.137)  72.728 ms  89.628 ms  88.825 ms
>  9  192.205.33.186 (192.205.33.186)  89.787 ms  89.794 ms  80.918 ms
> 10  ae-31-55.ebr1.Chicago1.Level3.net (4.68.101.158)  79.895 ms  70.927 ms
>  78.817 ms
> 11  ae-1-5.bar1.Boston1.Level3.net (4.69.140.93)  107.820 ms  156.892 ms
>  140.711 ms
> 12  ae-7-7.car1.Boston1.Level3.net (4.69.132.241)  139.638 ms  139.764 ms
>  129.853 ms
> 13  MASSACHUSET.car1.Boston1.Level3.net (4.53.48.98)  149.595 ms  154.366 ms
>  152.225 ms
> 14  B24-RTR-2-BACKBONE.MIT.EDU (18.168.0.23)  146.808 ms  129.801 ms  89.659
> ms
> 15  MITNET.TRANTOR.CSAIL.MIT.EDU (18.4.7.65)  109.463 ms  118.818 ms  91.727
> ms
> 16  trantor.kalgan.csail.mit.edu (128.30.0.246)  91.541 ms  88.768 ms
>  85.837 ms
> 17  zermatt.csail.mit.edu (128.30.2.121)  117.581 ms  116.564 ms  103.569 ms
> $
>
>
>