[e2e] What's wrong with this picture?
detlef.bosau at web.de
Thu Sep 10 09:23:37 PDT 2009
Dominik Kaspar wrote:
> A CDF illustration of the results is available here:
> 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...
I'm a bit curious about this discussion. I really thought, these things
were understood, and it's only me, who doen't know the literature.
A remarkable property of HSDPA is that service times may vary on an
_extremely_ large range.
This is due to variations in
- line coding,
- channel coding,
- transport block length, i.e. code usage and puncturing and
- MAC delay.
For quite a few days now, I'm thinking on whether this delay variation
may even affect the algorithms for TCP RTO calculation (refer to Edge's
paper and its assumptions).
I'm not surprised about a multimodal behaviour here. If, I would be
surprised to see _only_ two modes here.
To my knowledge, WLAN uses only two line codings (HSDPA and the like may
use three, is this correct? QPSK, 16 QAM and sometimes even 64 QAM?),
however, there is less variation in channel coding and puncturing etc.
I gathered some results from the EURANE project and related research
projects on http://www.detlef-bosau.de/index.php?select=symbols
in order to have an overview, at least for myself, about the delay
variation in HSDPA.
Please note, that even a transport block's "Payload" may vary from 176
to 21576 bits. And HSDPA may repeat a transport block up to three times.
So, without MAC and propagation latency, the HSDPA throughput (for
_code_ bits, not even for _information_ bits) may vary from 21576 bits /
2 ms = 10798000 bits/second downto 176 bits / 6 ms (eq. three sending
attemps) = 29333 bits/s. This is a factor of nearly 369.
And I neglected
- any propagation delay,
- the times used for ACKs/NAKs on L2,
- MAC delays.
If you think about up to 8 terminals in a cell, you may well see a
certain throughput (gross) (? I always mix up gross and net bit rate
here, I'm German, our chancelor always gets confused with gross and
net....) at one time, and a 2500 times larger one at another time.
And of course, you're likely to see anything in between. So, it's
basically somewhat astonishing to see _only_ two modes here....
Do we have more precise information about the scenario?
> 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 (220.127.116.11) 56(84) bytes of data.
>> 64 bytes from zermatt.csail.mit.edu (18.104.22.168): icmp_seq=1 ttl=44
>> time=209 ms
>> 64 bytes from zermatt.csail.mit.edu (22.214.171.124): icmp_seq=2 ttl=44
>> time=118 ms
>> 64 bytes from zermatt.csail.mit.edu (126.96.36.199): icmp_seq=3 ttl=44
>> time=166 ms
>> 64 bytes from zermatt.csail.mit.edu (188.8.131.52): icmp_seq=4 ttl=44
>> time=165 ms
>> 64 bytes from zermatt.csail.mit.edu (184.108.40.206): icmp_seq=5 ttl=44
>> time=224 ms
>> 64 bytes from zermatt.csail.mit.edu (220.127.116.11): icmp_seq=6 ttl=44
>> time=183 ms
>> 64 bytes from zermatt.csail.mit.edu (18.104.22.168): icmp_seq=7 ttl=44
>> time=224 ms
>> 64 bytes from zermatt.csail.mit.edu (22.214.171.124): icmp_seq=8 ttl=44
>> time=181 ms
>> 64 bytes from zermatt.csail.mit.edu (126.96.36.199): icmp_seq=9 ttl=44
>> time=220 ms
>> 64 bytes from zermatt.csail.mit.edu (188.8.131.52): icmp_seq=10 ttl=44
>> time=179 ms
>> 64 bytes from zermatt.csail.mit.edu (184.108.40.206): icmp_seq=11 ttl=44
>> time=219 ms
>> --- 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 (220.127.116.11), 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 18.104.22.168 (22.214.171.124) 207.416 ms 208.325 ms 69.630 ms
>> 6 cr84.cgcil.ip.att.net (126.96.36.199) 79.425 ms 89.227 ms 90.083 ms
>> 7 cr2.cgcil.ip.att.net (188.8.131.52) 98.679 ms 90.727 ms 91.576 ms
>> 8 ggr2.cgcil.ip.att.net (184.108.40.206) 72.728 ms 89.628 ms 88.825 ms
>> 9 220.127.116.11 (18.104.22.168) 89.787 ms 89.794 ms 80.918 ms
>> 10 ae-31-55.ebr1.Chicago1.Level3.net (22.214.171.124) 79.895 ms 70.927 ms
>> 78.817 ms
>> 11 ae-1-5.bar1.Boston1.Level3.net (126.96.36.199) 107.820 ms 156.892 ms
>> 140.711 ms
>> 12 ae-7-7.car1.Boston1.Level3.net (188.8.131.52) 139.638 ms 139.764 ms
>> 129.853 ms
>> 13 MASSACHUSET.car1.Boston1.Level3.net (184.108.40.206) 149.595 ms 154.366 ms
>> 152.225 ms
>> 14 B24-RTR-2-BACKBONE.MIT.EDU (220.127.116.11) 146.808 ms 129.801 ms 89.659
>> 15 MITNET.TRANTOR.CSAIL.MIT.EDU (18.104.22.168) 109.463 ms 118.818 ms 91.727
>> 16 trantor.kalgan.csail.mit.edu (22.214.171.124) 91.541 ms 88.768 ms
>> 85.837 ms
>> 17 zermatt.csail.mit.edu (126.96.36.199) 117.581 ms 116.564 ms 103.569 ms
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