Re: [sarex] PMS usage suggestions?

[Margaret Leber <maggie@xxxxxxxxxxxx>]

On Sunday 03 March 2002 23:15, Scott Avery wrote in "Re: [sarex] PMS 
usage suggestions?":

> Yes, APRS is a good way of using minimal bandwtih.
> A lot can be said in one line of data, and quick to send...

Folks, we need to bear in mind some things:

AX.25 protocol is based on X.25, and they are both fundamentally 
connection-oriented protocols. 

X.25 was developed for use as a way to packet-switch though virtual 
circuits connections over hard-wired channels. AX-25 replaced the 
hardwires with RF channels, but was still intended primarily for 
*fixed* stations. As soon as we started dealing with *moving*, mobile 
stations, the connection-oriented AX-25 protocol became unwieldy and 
was soon replaced with the connectionless UI frames carrying messages 
which were soon formatted using a new UI-based protocol we now call 
APRS. 

Imagine what a PITA it would be to maintain a useful 
connection-oriented session with a fixed station from a mobile station 
on a highway moving at only one mile per minute! How much information 
the operator (driver) would need to absorb about who was in range, and 
the decisions about who to connect to at any given time and when to 
break the connection. And after all that was done, how long would the 
connection be useful before the two stations lost contact? (Cell phones 
do this sort of thing all the time, of course, but they're using 
protocols built many decades later.) 

Now multiply that problem by LEO orbital speeds.The expanded range that 
a few hundred miles of altitude give doesn't do enough to solve that 
problem. We need a hang time like AO-40 gives to have a "connection" 
(read "QSO") last more than a minute or two. The ISS simply isn't that 
kind of bird...it's much more like UO-14 and AO-27. Look how different 
the operating styles are between UO-14 and AO-40. 

That's not so much because one is one channel of FM and the other is a 
chunk of bandwidth for SSB/CW, but much more because UO-14 is only in 
your sky for 5-10 minutes, and AO-40 is there for many *hours* at a 
time.     

No connection between a terrestrial staion and the ISS TNC can last 
more than a few minutes. The fact that the TNC--still largely an 
off-the-shelf device--*can* do AX.25 connections, and *can* serve as a 
store-and-forward message switch, doesn't mean it *should* be used that 
way. 

In fact, I'd argue that it *shouldn't* be used that way at *least* 
until there's somebody on orbit who has the time and inclination to 
read these messages, or even post some of their own. For now, 
digipeating UI frames is what the TNC does well. Once ARISS has a 
dedicated console up there, all kinds of new, additional "intelligent" 
things can be done using the TNC in KISS mode...and I think *none* of 
them are likely to be done well with AX.25 protocol connections as we 
know them today, or the kind of operation that the PMS programming was 
designed for.        

We have a tool that's so much more than just a hammer. Let's not define 
our goal as "driving nails".

 73 de Maggie K3XS 

-- 
-----/___.   _)   Margaret Stephanie Leber    / "The art of progress  /
----/(, /|  /| http://voicenet.com/~maggie   / consists of preserving/
---/   / | / |  _   _   _    `  _AOPA 925383/ order amid change and /
--/ ) /  |/  |_(_(_(_/_(_/__(__(/_  FN20hd / change amid order."   /
-/ (_/   '  K3XS  .-/ .-/    ARRL 39280   /___ --A.N.Whitehead ___/
/____ICQ 7161096_(_/_(_/__AMSAT 32844____/ <maggie@voicenet.com>

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