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Re: TDMA - Reading the mail

on 12/21/00 6:27 AM, Bruce Bostwick at lihan@ccwf.cc.utexas.edu wrote:

> And as for stations not being "well behaved", what happens when people who
> want to "get there first" start tweaking the protocol on their own
> computers to their own advantage and start stepping on other time slots?

That's correct!  Additionally difficult is how to assign the slots.  I
suppose the satellite would do that, but how when there are hundreds of
stations calling at once?

In the cellular world, when you turn on your telephone, it sends out a
signal that pages the nearest cell site.  It then locks on your phone and
tells it what time slot and frequency to sit on.  Then as you drive around,
that information is continually updated from sector to sector within the
cell and cell to cell over a broader range.  If you have a TDMA phone, you
can hear it interfere with your FM radio sometimes as it communicates with
the cell site (you'll hear a chich, chich, chich in your FM radio signal).
The phone is under complete control by the base station and ultimately the
switch.  In a terrestrial cellular system, this works and works very well
(sorry Qualcomm - I am a TDMA fan after hearing and using both).  You have a
tightly controlled system.  You also have a fair amount of bandwidth (12.5
MHz per operator at 800 MHz in the US) where you can allocate your channels.
Additionally, the site design is set up so that there is very limited
coverage.  Antennas have downtilt on them to prevent their high gain from
making the site coverage too large (BIG problem if it is too much coverage).
Specific channels are assigned to each site and within each site to each
sector.  Multiple sites that are not adjacent to each other will use the
same or have overlapping channels (frequency re-use).  This way you can
accommodate the potentially thousands of users at one time in your 12.5 MHz
allocated bandwidth.

However, in a satellite system, you have none of this.  First of all, would
we even have 12 or 15 MHz of spectrum available to us in even the microwave
bands?  Perhaps yes.  But now the problem is that you've effectively got a
cell site that covers half the world!  You have no frequency re-use and
you've not got a "controlled" telephone situation like at cellular.  So once
you change frequency channels then how do you get assigned a time slot?
Would we all have a common paging frequency where we would page the bird and
get assigned a slot?  What if multiple people try to get a slot at the same
time?  How does that get sorted out?  And then you have the ham's ability to
hack and change things as noted above.....

While the concepts sound great and would work very well in a system with few
users, if it would become popular, you would have many, many users and the
"simple" aspect of it begins to break down.



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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