Wednesday, May 28, 2008

capacity of multihop networks...

We are interested in this as multihop networks may be built in an ad hoc way by consumers (same as P2P networks) and so the question is how much cpacity per user do you get as you scale up the number of users in a given area (or volume).

most useable radio frequencies mean we are not in near field (where there are really cool scaling results due to very fast fading) so as soon as you forward a signal more than one hop, you are causing interference with other nodes, and the question is
partly just geometric - increasing the number of nodes in a volume, with (say) a uniform random set of traffic between sources and destinations, the path length increases, so the number of hops that receive your signal (and therefore cannot receive someone else's signal) increases. However, the power needed to get the signal to a neighbour decreses (or the capacity increases at that shorter distance for the same power) - so there's a race between increasing capacity at each hop, and decreasign capacity because of the number of hops - the simplest answer is that the capacity of the system grows slower with N than the number of senders, so you get
a net that eventually has no capacity.

so there's a growing body of work on this topic - stemming, I guess, from the original work by Gupta/Kumar (1/srt(n ln (n)), then moving through the various modifications that allow for mobility (Grossglauser/Tse) (2 hop relaying has overall increasing capacity at expense of eventual diverging delay!), fading (various models, but typically still decreasing capacity with N), fading and mobility (has a worse lower bound than just mobility but better overall capacity in the system so growing with N), variable traffic demand (non uniform random traffic matrix, or multicast traffic).

All these are under the assumption of transmission on a "hop" being to one receiver, and all other senders to that receiver contributing noise, and all other receptions of that transmission being interference at other receivers. Of course, then one can add diversity (i.e. receive the signal from multiple diverse transmissions, and forward similarly) and there's constructive schemes for cooperative diversity (again due to Tse and others), which show you can increase the capacity again, including systems that have effectively fixed capacity per user no matter how many users, but require very good clock synch (well, perhaps similar to WCDMA).

so diversity is one trick - but I was wondering about _deliberate_ fading

can we take a simple 1D scheme with a set of nodes on a line at uniform (or random) distances from each other, and construct a transmission schedule where nodes relay in one direction in phase and nodes further "back" away from the forwarding path, re-relay out of phase. This is not quite the same as diversity - what we are trying to do is to build a "noise cancellor" from the previous hop to the current node. I have no idea if this is practical - but pretend the previous hop acts like a perfect radio mirror. we measure the distance between us and next hop, and previous hop and our next hop and any intermediate hops and we setup a reflection that is perfect multipath interference at the hops we dont want to receive our signal

Now re-do this in 2D (tricky) or 3D (trickier!)

problem (very bad) is that unlike diversity, this creates a massive growth in thermal (i.e. pure random) background...which will eventually swamp everything (except that it is also subject to some types of fading and path loss)...

Tuesday, May 27, 2008

E-mail Decrepitude: "You'll have to speak up, I'm getting old and a bit deaf"

I've noticed more and more that people are responding to e-mails without having read them properly - either this is the ever advancing attention span deficit disorder, or else people are so overwhealmed with the sheer quantity of e-mail (that is, people that still use this obsolete technology) that they don't have time to absorb what was originally written. This response is, of course, counter productive as it generates more email for the sender, and then for them.

I propose a simple rate limit per to: field implemented in all MTAs. In fact, there should probably be two.

Rate limit 1 is an overall limit to the rate one can generate messages from a given
O/R name (sorry, posh, X.400 old fashioned terminology for originator or from:).

Rate limit 2 is a mean+peak (i.e. leaky bucket) rate applied to To: fields from a given from: one can trade between rates so that the sum of means is no more than rate limit 1, but by going slower to some people, can go faster to others for "chatty" conversations. Perhaps a check on the length of the email could be based on typical reading/writing speeds to make sure that the "new text" in each message is a plausible product of the interval since the last message...

This is not intended as an anti-spam technology per se, just a help....

of course it could be implemented by recipients to some extent but only if they collude

Reference: Deaf Sentence (novel) by David Lodge (2008).
See also a recent article in the
Intl Herald Tribune of all places, which seems to make some sense

Saturday, May 17, 2008

RF propagation and cray's law- useful children's metaphors

so my kids helped me paint the kitchen and various other things around the house that were not entirely intended for a clean slate - the distribution of paint reminded me of the black magic that is radio propagation - it so often goes where you dont want it, and not all where you do.

in the procedss of having a nice clean redecorated kitchen, i was reminded of Cray's statement (or is it amdahl's) about optimisation - now we have a clean kitche, the bathroom (that looked ok before) looks decidedly shabby...

oh well....optimisation is the name of the game (or is it
always look on the bright side of the pipeline?)

Friday, May 16, 2008

Euroclue meter

traveling a bit in europe as I have been,
i have visited hi tech labs in many countries
and what is remarkable, is not that there is much
variation in smartness of researchers, or
even in facilities, but in how
aware of what is going on in the rest of the world
a given lab is.

There seems to be a very strong variation in clue - not just a perctange difference - literally orders of magnitude -
a few examples

thomson labs in paris, MPI SWS in Sarbrucken, Ericsson research in Kista in Sweden, all showed increibly up to date knowledge of who (whether in japan, australia, US or within europe) was doing what that was cool - to the extent that (for example) I gave a tak in one lab and mentioned a talk I had seen the day before in Microsoft research 1500km away, and they commented on it as one of them had got slides and worked on related stuff....oh, add to this TID (Telefonica research lab in Barcelona) - really amazing set of people given how short a time it has been up and running!

on the other hand, there are people that write papers that don't appear to be aware of work done 2-3 years before...

it does seem to co-rrelate somewhat with the organisation of (national, not EU) funding and whether there is good evaluation of how national work compares with internatonal well as (and this is just the histrical luck of the draw) whwther the local language is spoken in other parts of the world where one can recruit lots of cool PHD students ....

I don't want to list the negative experiences as I dont think that helps anyone:(

Saturday, May 10, 2008

robot vacuum cleaners - dalek free zone?

so its bad enough people want to sell us (homeopathic-like) devices to clean vacuums, but to enslave perfectly good robots and have them spend their whole lives cleaning a perfectly good spanking-clean vacuum seems adding virtual insult to virtual injury

mow that lawn, clean that window,
dust that broom, tote that shopping,
now, about greening the planet... ... ...

Friday, May 02, 2008

Percentage of humas that write books

lets say that there are about 20M books (18M in the British Library)
and about 10B people
if the population doubles every generation, then, since you Adam and Eve it,
there's 20B people

if eachauthor only ever writes 1 book, ten that is 1 in 1000 people that are auhors

if an author tends to write (say) 10 books, then it is one in 10,000 (sounds plausible)

how many books do you read (I have about 10,000 books I guess)...???

I am thinking of writing a new book about Kinky Dictators
Mao's Strap

would you give me an Advance?

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