[e2e] Some thoughts on WLAN etc., was: Re: RES: Why Buffering?
detlef.bosau at web.de
Sat Jul 4 10:04:29 PDT 2009
Hi to all.
In the last two weeks, I got some first hand experience with WLAN in
some educational setups, and perhaps it's interesting to share this
Perhaps the most astonishing experience for me was to see packet round
trip times from my PC to a neighbour, with about 5 meters distance,
about 80 (literally: eighty!) milliseconds.
I well remember that sometimes attendees ridiculed about a talk where
the speaker talked about RTT about 20 ms.
Of course, this huge RTT value was a rare, extreme value.
However, even a WLAN setup with about 10 people and, for educational
purposes, in three infrastructure based networks causes extreme
difficulties in communication. (In Germany, we have 13 WLAN channels
AFAIK, so actually we can only run three WLANs in parallel in a
classroom when the cells shall use disjoint frequency ranges.)
Of course, in some WWAN setups we pursue a better frequency setup and a
better antenna setup - but basically the problems are the same.
1. Obviously, depending on the setup we use, intercell interference may
cause extreme difficulties in wireless communication.
2. I strongly guess, that this holds true not only for intercell
interference but for intracell interference, interferences from external
sources and other problems as well.
3. I don't expect any realistic possibility to forecast causes for
interference for a "wireless channel".
4. Consequently, models which model a "wireless channel" as some memory
less, well behaved and easy going entity may be a bit questionable and
may be restricted for academic purpose, but do not really reflect the
Hence, some of my questions are:
- What is a wireless "channel"? What is a wireless "connection"?
I once was asked about the behaviour of some protocol in the presence of
"short time disconnections". What is a "short time disconnection"? Or,
the other way round, what is a connection all about in a wireless network?
In my opinion, there simply are no connections in a packet switching
network. Consequently, they are no disconnections in a packet switching
networks. (And as always, this rule is strongly confirmed by quite a
couple of exceptions. Exactly that's why it is a rule.)
I would prefer the question: How likely is a packet to reach the
receiver without corruption, once it has been sent?
To the best of my knowledge, even adaptation schemes for wireless
networks which make use of some kind of "pilot signal" only give a rough
estimation of the "channel's quality". However, even when a channel
with some well known AWGN in a lab will lead to a transport block
corruption rate of, say, 10 %, there is absolutely no guarantee that
this will hold in the real world.
So, I think we should abandon the idea of a really useful estimation of
something like a channel's block corruption ratio.
We may have some reasonable guess - but not really more.
Another question is:
- How do we deal with the aim of resource fairness?
In wireline networks, this is simple. In a wireline link, packet service
times vary about the same average and are the same for all users. So,
by Little's Law (and IMHO this is its correct place in this discussion)
we have, that a flow's share of throughput on a wireline link is
directly proportional to its share of space / resource consumption.
As an immediate consequence, the TCP congestion algorithm (see congavoid
paper) which targets at a fair distribution of ressources / spaces in
the first, leads to a fair distribution of rates as a side effect.
This hardly holds true in a network where Little's law does only apply
as a long term average and where quite a number of packets are not
successfully received, i.e a flow's throughput is not necessarily
proportional to the number of served packets because some packets are
sent once, others twice or more, and some packets are sent up to n
times, n depending on the setup, and there is no successful transmission
Hence, the algorithm as stated in the congavoid paper will be of little
use here, the same hold's true for the fairness consideration of
Kelly's paper "Charging and rate control for elastic traffic" from 1997.
From an end to end point of view, I would like to abandon terms like
"channel" and "connection" in a wireless environment. A connection could
only be defined as a logical association of a pair of peers.
The term "channel" may reflect a lab setup with some sender placed in
the one corner of the lab and a receiver in the opposite corner - and
some well behaved source of noise in between. For the real world, this
scenario is of limited use.
(Please note: I don't say: of no use. It's limited use - as long as we
deal with this model in a reasonable way.)
We can well use adaptation mechanisms to choose appropriate line coding
and channel coding schemes, however there's no way to forecast a flows
corruption ratio within narrow limits, neither is there a forecast for a
flow's achievable throughput or goodput.
O.k., that's just my experience from the last week. And I would like to
here some comments.
Particularly as this view is not reflected in the text books I know so far.
Detlef Bosau Galileistraße 30 70565 Stuttgart
phone: +49 711 5208031 mobile: +49 172 6819937 skype: detlef.bosau
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