[Namaste-dev] Re: Thoughts on ACP Interoperability

Timothy J. Salo salo at saloits.com
Sun Jul 13 21:14:39 PDT 2008

Bob McGwier wrote:
> I understand your feelings about P25 insofar as it uses closed, 
> protected, and/or patented vocoders.  I will not pretend to have read 
> the spec in depth but will pose the question all of this begs since the 
> rest of the thing seems at a glance to be an open standard.  Is there no 
> middle ground? That is,  what is to prevent us from implementing the CAI 
> sans the vocoder and passing the digital streams encapsulated in 
> “AMSAT-MAC”?  Is there something that prevents this?

If I understand what is being proposed, it doesn't seem to
make any technical sense.  As I understand it, the CAI is
the protocol used between a P25 transceiver and a P25 repeater.
If this is the case, then the utility of this non-standard
(proprietary?) CAI is not clear.  Yes, I think that the
repeater will be able to receive and repeat the stream of
non-standard CAI packets.  But what devices will be able to
receive, or more precisely decode, these packets?  I assume
that we aren't planning to modify a bunch of P25 radios to
use some non-standard codec.  And, I assume that we aren't
proposing a non-standard stovepipe system that can transmit
through a P25 repeater, but can't actually communicate with
any P25 devices.  But, perhaps I missed something.

> It would clearly 
> require control operators sitting at either end of the big pipe who can 
> hear the synthesized speech or see the content of file transfer to be 
> legal, as in sitting on a kill switch I think, without waivers.

This isn't entirely clear to me.  This topic is discussed
repeatedly within the packet community.  People seem to
fall into one of two camps: those who think that a live
person really needs to inspect every packet, and those
who think that other mechanisms may be adequate.  Stated
differently, it seems clear that there is no clear answer
to this issue that everyone agrees with.


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