[e2e] Bandwidth, was: Re: Free Internet & IPv6

Detlef Bosau 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
- shading
- free space loss
- noise

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 

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?


Detlef Bosau
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