Friday, November 18, 2005

Mobile phone networks by P2P

About a year ago, I was at a conference (nothing to do with IT, actually) and jotted down some notes that I've just found.

There are two problems with mobile phones:
  • reception is not possible when too far from a mast
  • emittance is often high and suspected of causing brain damage or dysfunction
Both problems can be alleviated to at least some extent by having phones forward other phone's calls. In principle, in large cities, it should be possible to make within-city calls without ever using a mast, because there is a continuous and dense net of phones throughout the city that can pass on the call. There are other people who understand more about routing methods than I do, but I'll just take it as given that a call can reach its destination by this method. In fact, I heard a physics talk by Neil F. Johnson, in which he alluded to the optimal way for satellites to compete for transmission of a signal. I don't remember the details of his talk, and don't have time to look them up.

One of the main challenges in routing is how to ensure a) that the signal is passed on, and b) that it travels by a single route (i.e. does not fork). It is easier for now to think of a single data packet, such as a text message, travelling in one direction only (we'll pretend it doesn't send a "received" signal back; afaik this is a recent feature anyway).

One thing that helps is for a phone to be aware of other phones in its environment and haggle with these phones over how many milliseconds it should wait before passing on a signal it has just caught. This is necessary to prevent forking and overdubbing (again, this may not be a problem with current mobile phone data protocols, but I know nothing about mobile phone signal transmission).

It also needs to memorise the ID of all packets it has recently sent and not transmit them again when they come back to it (inevitable if all phones emit with the same signal strength). However, there may be other routing approaches that yield the same results.

A phone may also want to haggle for a larger delay if it has recently transmitted large amounts of data, to save battery life.

Further complications arise from phones that are switched off while data is in transit or while they are sitting on a computed route (it makes sense to first find the route and then send the data over it).

I'll also quickly admit that, as usual, some other people had the same idea at around the same time. It is a rather obvious idea, but it does surprise that it took so long... I'd like to learn how their protocol works. We've had walkie-talkies for so long, why did they ever think of putting base stations in?

tags:, , , , , , , , , , ,

No comments: