Three magic wishes...
Peter N. Glaskowsky
png at ideaphile.com
Wed Apr 29 13:46:16 PDT 2015
> Date: Wed, 29 Apr 2015 06:24:56 -1000
> From: Jeff Francis? <jfrancis at gmail.com>
> To: All things digital and fun <seatcp at wetnet.net>
> Subject: Re: Three magic wishes...
> 1. Can't scale, can it? Assume for the moment that 1% of hams do APRS,
> and further assume that 10% of that 1% are on the air at any given time
> (obviously, I don't know that these are accurate, but even if they're off
> by an order of magnitude, it still can't work). You're looking at a
> potential of 1000x the current APRS activity at certain times of the day
> (granted, it'll never be quite that high, but we're talking order of
> magnitude numbers). In the programming world (as well as the networking
> world), we call it an N^2 problem. Resource usage goes up as the square of
> the number of participants.
I’m not sure what principle you’re referring to here, and I’m not familiar with any networking situation where utilization goes up as the square of participants. However, in the traditional definition of the “network effect” the cost of a network is proportional to the number of participants N, while the value of the network is proportional to N^2, which is a very good thing.
Of course that was just a conceptual definition. In practical terms, congestion can follow different rules. In shared-media implementations where collisions lead to retries (like APRS), there will be increasing utilization to some characteristic point followed by congestion-induced decay, but that decay isn’t an N^2 type of function.
But APRS isn’t the only protocol in the world, and there’s no reason to stick with it if some other protocol will give us better scalability.
> Doubling the hams requires four times the
> spectrum (assuming you want them all to communicate and not be segregated
> by frequency). Gatewaying the data from each frequency to each other
> frequency is it's own N^2 problem. It would take up most of the 2M/70CM
> bands just to send all of the traffic, and that's assuming you don't
> gateway the data from one frequency to another (ie, most of the hams
> couldn't communicate with most of the others without know which freq they
> were on and switching first). Which still wouldn't work, because everybody
> would try to switch to the frequency most likely to contain other people
> and overwhelm it. I'm not seeing value here, unless part of the magic is
> for everybody to keep their stations turned off until they magically sense
> the need you have to communicate with them and turn it on just in time to
> get your message, then off again.
> 2. What would doing this get us that we don't have already? We have
> automated stations, we have a nearly infinite supply of modes that work
> under all sorts of different conditions, we have more than enough spectrum
> for current usage and needs. About the only thing we can't do that some
> people would like to try (at least the only thing I can think of) is the
> very wide digital modes, but those disperse the energy so broadly that you
> have to have a 1kw amp to use them effectively (not to mention are subject
> to huge interference problems). Even in places where they're legal,
> there's not much use of them (like Pactor 4, for example).
There are technical solutions like spread-spectrum modulation schemes (CDMA, for example) that allow efficient channel sharing without collisions. And when there are more stations participating in an RF network, the solution is to reduce power, not increase it, thereby improving spatial reuse. The remarkable market success of “very wide digital modes” like LTE at very low power levels is proof of this. The same technology could be applied to amateur radio in a peer-to-peer manner rather than in the base station/mobile device structure of the cellphone network— still with cells, but with dynamic distributed management protocols instead of a single network operating authority.
In real-world terms, these solutions would be extraordinarily effective for us. A metropolitan-area digital ham network using, say, 10 MHz of the 70 cm band could carry an aggregate bandwidth of gigabits per second, not the few megabits that a single high-power radio could transmit— or the nearly-zero throughput that would be achieved in an overloaded network of many such transmitters.
Frankly I think it would be a vastly superior use of the 70cm band to devote it entirely to digital cellular communication. It could be every bit as functional for us as the cellphone network is for the general population, but without any dependencies on infrastructure we don’t control or vulnerabilities to single points of failure. It would be ready to go in the event of local emergencies, trivial to recreate at a new location if needed, and readily interoperable with other systems through fairly straightforward gateway devices.
One of my concerns here is that if we don’t do this, someone will take this spectrum away from us so THEY can do it. In the AWS-3 auction, the FCC sold 65 MHz of spectrum for over $41 billion. Our 30 MHz of the 70 cm band— which is intrinsically MORE valuable because it offers better structure penetration— is therefore worth more than $20 billion, and what are we using it for? Almost nothing of any practical value. The value of the spectrum will only increase over time, and we’re not increasing our use of it. I believe it’s inevitable that it will eventually be repurposed.
So basically I don’t agree that there’s an N^2 problem here, that wide digital modes are a problem, or that we can’t get hugely valuable capabilities that we don’t have now.
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