[e2e] Bandwidth, was: Re: Free Internet & IPv6
detlef.bosau at web.de
Mon Dec 24 15:57:27 PST 2012
Am 19.09.2012 22:07, schrieb Daniel Havey:
> I wonder why the bandwidth is unused in the first place? Is it being wasted because it is of little value? What bandwidth are we talking about? I guess that bandwidth as a commodity would have a time and place. Bandwidth on what router and when?
Please allow me to refer to this old discussion.
Particularly as I have seen the following paper:
"Modeling Computer Networks for Emulation" by Herrscher, Leonhardi,
Rothermel, PDPTA'02. Which is a bit older too.
Let my quote some sentences from this paper.
> The simplest dynamic model is the table model.
> Instead of a static value, a table is provided that
> consists of tuples with triggers and actions. Each
> action can be a parameter change or another
> dynamic model. Tables are especially suited to
> replay parameters gathered from measurements.
> For example, the changing bandwidth of a mobile
> 802.11b node can be easily modeled that way (see
> fig. 3). The triggers can be either time values or
> packet counts. In most cases, a time triggered
> table will be appropriate.
Some of you know from private converstions, that I've struggled with
mobile networks for quite some years now.
The by far most important lesson is, that the term "bandwidth" is
frequently used in a questionable manner by CS guys (as I'm myself).
With sometimes more than questionable results.
It took me some time to say the following, because some colleagues
strongly advised me not to do so, because no one would take me seriously.
Be it that way or not, I would like to return to that topic.
Particularly when wireless networks are concerned, CS guys cooperate
closely with CE and EE guys and so, to my understanding, it is
inevitable to use well defined terms that way as they _ARE_ defined and
to avoid "redefinitions" by all means. (During my education, I got in
touch with civil engineers.
And for those, the first of all were the definitions of terms, the
second were the standards, particularly the DIN in germany, and in the
third place came the ten commandments, the rest was of minor importance.)
So, to my understanding (and I find this, e.g., in Shannon-Hartley's
Theorem) the term bandwidth denotes the frequency range of a signal or a
No more, no less.
Consequence: The sentences quoted above don't really make sense: A
wireless signal's "bandwidth" in 802.11 networks does not change when a
station moves around. When it comes to a mobile station in 802.11b, the
signal may be influenced e.g. by
- multipath interference
- free space loss
The text above refers to fee space loss, which is not necessarily the
most important influence. Particularly when mobility is considered,
multipath interference is extremely important, which makes a replay from
logged values highly questionable. (From signal theory we now, that it's
not a trivial task to replay a formerly logged motion scenario. Please
keep in mind that minimum and maximum points of multipath interference
can be quite closely, e.g. in the magnitude of several centimetres in
GSM or WLAN.)
In addition, up to know we only talked about the SNR or C/I ratio.
We did not consider scheduling issues.
I will not attempt to model a wireless interface in this post. (To the
best of my knowledge, this is a tough issue and although I'm looking for
convincing models for years now, I did not really find some. At least
none which can be used for the purpose of simulation or emulation. There
are models which can be used for these purposes and there are models
which refer to reality. Unfortunately, I've never seen a model which
matched both criteria.)
Back to 802.11: When I consider the situation in my appartment, I can
well ignore this whole SNR stuff. What bothers me most are collisions.
At the very moment, I "see" 6 (in words: six) 802.11 nets which I could
join, if I had the appropriate key. Hence, these networks inevitably
overlap. And so, collisions cannot be avoided. So, when I would move
from the living room to the kitchen, this would hardly influence the
throughput from my WLAN base station to my notebook. It would have
certainly have more influence, when some of my neighbours would turn off
their computers and I would see less collisions than now ;-)
Please note: The "bandwidth" does not change - neither when I move my
computer nor when neighbours turn off their computers.
There are changes
- to the SNR as seen by the signal's receiver (mobile station or base
station) and to
- the number of collisions, in other words: to the network load.
Both will results in changes of the wireless channel's throughput as
perceived by upper layers and - finally - by the application and the user.
However, I think the term "bandwidth" must not be used here. We should
make a proper distinction between bandwidth and throughput.
With particular respect to packet switching, I even would be careful
with "throughput", because the some observations indicate that packet
delivery times in wireless networks are not stationary in some
scenarios. In that cases, we should focus on delivery times - the term
"bandwidth" surely does not make sense here.
With particular respect to Daniel's question in the quoted post, I think
it's always a good idea to properly identify the resource of interest.
Typically the shared/common resource. Be it "bandwidth" (in FDM), be it
"sending time" (in TDM), be it "power" (in CDMA).
Personally, I'm a bit frustrated by a certain confusion of terms here.
This is hardly a problem when reading articles. In most cases, I can
well understand what authors want to say. However, when it comes to a
technical conversation, I often met a Babylonian confusion - which made
it hardly possible to communicate.
And which is particularly annoying when it comes to modelling. As it can
bee seen from the quoted text fragment. In quite some typical 802.11
implementations, applications, scenarios "free space loss" is simply a
non issue. And so it hardly makes sense to waste much programming effort
to model free space loss, while collisions (where the shared resource is
"sending time") are the much more important issue, of course amongst
Of course, this always depends on the scenario. But from my first hand
experience, I see quite some 802.11 networks in offices or apartments or
hot spot scenarios. I rarely get in touch with battlefield scenarios or
some isolated WLAN cells somewhere in the desert ;-)
So, basically, I'm not quite sure whether we should develop a certain
awareness for the correct terminology.
Do you agree here?
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