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[aprssig] An amusing aside (Text Pagers)

Wes Johnston, AI4PX wes at ai4px.com
Thu Jan 28 22:46:10 UTC 2010


That's the thing.... Messages do NOT imply an internet connection.  My
friend here in town should be able to send a message to me from his d700 (or
other ).  Yes, certainly it would be really nice to have people 1/2 way
around the world send a message to my pager, but the first priority is
APRS's "come as you are" philosophy.  If we could come up with an algorithm
to convert my callsign to CAP code on the fly then if I showed up in your
town I could get messages.  Of course the falicy of my idea is that with the
pager being RX only, I have no way to notify a POCSAG message converter
station of my presence.  We certainly can't just send all messages that
appear on the APRS IS to POCSAG... we need to only send to stations which
may have a pager.

So we have 2 problems....
1)algorithmically deriving a CAP code from a callsign.  With 21 bits, we
can't even pack 7bit characters tight enough to represent callsigns.  What
if we used some XOR hash scheme?  Or other hashing method?  I'm open to any
idea.

2)Notifying the POCSAG message server that my pager is around.  Perhaps my
APRS station being heard in the area would serve to notify the POCSAG server
that I had a pager?  For example, my TX only tracker sends status packets
which indicate that I'm in the area.  The POCSAG server remembers me for a
few hours and gates any messages addressed to my tracker's callsign to the
pager.  Why not just have the tracker beacon my CAPCODE?  This solves
problem #1.  This concept of TX by tacker on 2m and RX messages on pager
( 2m or 70cm) closes the loop for deaf trackers!  It also works for the
kenwood and yeasu radios.

Wes
---
God help those who do not help themselves.


On Thu, Jan 28, 2010 at 17:27, Matti Aarnio <oh2mqk at sral.fi> wrote:

> On Thu, Jan 28, 2010 at 01:02:45PM -0500, Wes Johnston, AI4PX wrote:
> >
> > Can we reprogram the CAP code to something that is predictable and
> derived
> > from our callsigns?  I just checked the numeric pager on my hip and it
> seems
> > to have a 9 digit cap code.  What would be nice would be to have a 12
> digit
> > or more cap code.... if so, we could use pairs of digits to represent
> ascii
> > codes for our call letters.
>
> Maybe, but the CAP code may not be related on the real addressing mode
> of the pager.  POCSAG had 21 bit device address field, and commercially
> the pager numbers were mapped to these POCSAC RIC codes at "pager center"
> server.
>
> (The CAP code is FLEX's version of POCSAG's RIC, value space is perhaps
> different..)
>
> > A= ascii 65
> > I = ascii 73
> > 4 = 52
> > P = 80
> > X = 88
> >
> > So my cap code would be 00 65 73 52 80 88.  This is the same trick we use
> on
> > TCP mac addresses in the AMPR domain, right?
>
> You are looking for an algorithm that does this above mentioned translation
> algorithmically without centrally coordinated database.
>
> On the other hand, reliance on APRS-IS implies network internet
> connectivity,
> and on a possibility to periodically download a fresh copy of such mapping
> dataset.   Furthermore, there could be regional server supplying these
> callsign to address code mappings, and other necessary coordination
> services.
>
> What those services would be?  No idea, but if somebody can really supply
> pagers receiving on ham bands, I am pretty confident we can come up with
> datastream generation methods for them, and generate ideas on what those
> coordination services would be.
>
> If all that pager hardware has gone the way of all e-waste, there is not
> much incentive on creating programs using them, especially as at these
> here parts of the world, pagers died away several years ago.
>
> > This sounds to me like we'd need to do this on another frequency away
> > from the normal aprs ops.  I strongly suspect that it will be far easier
> > to find 70cm pagers than 2m alpha pagers.
>
> Definitely on different frequency.  70cm ISM frequencies are free of
> ham radio traffic for obvious reasons, there could be free freq slot
> there somewhere...   High placed powerful transmitters reach better the
> low placed receivers, than low placed weak transmitters reach the
> high-placed receivers.  Add on it the radio noise power that a low
> placed pager receives vs. high-placed digipeater receiver.
>
> > Wes
>
> 73 de Matti, OH2MQK
>
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