On Thu, 07 Sep 2006 07:41:03 +0200, Mxsmanic wrote:

Greg Copeland writes:

The repeater initiates the call on your behalf. The repeater is queued
rather than the analog radio. Likewise, the reply goes to the repeater,
which then re-RXs ("repeats") as analog. For this to work, the analog and
digitial systems must have their own frequencies.

Is there a guarantee that transmissions will occur within a certain
period? Are these systems verified for safety-of-life applications?


You consider DoD, DoE, CIA, FBI, military, fire, and police to be
"safety-of-life applications?" If so, yes. Do keep in mind, these are
existing systems but do not specifically target aviation. I would imagine
aviation would require some adaptation or P25 or a P25-like standard.

Also, the concept of "emergency" call is also very useful. For example,
it places you at the top of the queue. Combine "emergency" with a GPS
source, plus data services, and now your squawking 7700, your GPS position
is sent with your PTT, and you now have priority with the controller.

Interesting.

This does bring to mind something else, though: If your channels are
so crowded that you need a system to queue messages and give priority
for emergencies, you need more channels. It's much safer to have
multiple channels that don't require queuing than it is to queue on a
single channel.


Yes. That's a function of scalability. It's up to the customer (FAA in
this case) to ensure enough talk groups exist to meet their often growing
needs. As it applies to aviation, talk groups would be ground, tower,
departure, arrival, etc...

Also, I don't want to be misleading. Queuing serves as a short term
solution for **peak** periods. In other words, queuing is only honored
for brief periods of time; usually in the 1-3 second range. The notion is
to allow for first-come, first-serve without forcing users to constantly
rekey their PTT if they didn't get their call grant. If a queue depth
greater than one becomes the norm, someone failed to properly scale the
system.

Also, how do you deal with analog users who have no queuing? They
will still walk over the simultaneous transmissions in digital and
analog.

Analog users would require an analog system to sit beside it or you would
require an analog/digital repeater. I must profess, I've never used the
analog P25 repeater. I'm in the P25 infrastructure development group at
my company so I don't use the analog stuff. As such, I must profess some
ignorance.


An anachronism? No worse off than they are today.

Actually they would be, since practices extended to digital users
would naturally tend to affect analog users, even though they don't
have the same advantages. This would put them at a safety risk.

How so? How is a current analog user "at risk"? It's not like it's
removing existing capabilities.


Until everyone
is converted, such features would simply be a perk to controllers; with
the potential to increase QoS for those that digitally participate.

Quality of service has to translate to increased safety in my book.
As I've said, if fancy queuing systems are required just to manage
traffic on the channel, then there are not enough channels, digital or
otherwise.

With analog, you don't have a queuing system...which translates directly
into walked on radio calls. That's a loss of service and wasted airtime.
"Fancy" queuing and resource allocation immediately translates into
increased QoS. In this case, that translates to increased safety. That's
just for starters, directly comparing analog to digital. By having an
analog to P25 repeater, on the digital side, you reap the same benefits.


Oh, most definiately not web browsing. TAFs, METARS, in route weather,
PIREPs, TFRs, ATIS, ASOS, TWEB, NAV IDs, etc...

As long as someone is still actually flying the plane. A beautiful
digital display of weather 300 nm ahead doesn't help if it distracts
you from the mountainside looming just ahead through the cockpit
window.

If you're trying to get weather while flying toward a mountain, you have
priority issues which conflict with safety. Somehow, I don't imagine data
services will be the probable cause of death.

Lastly, let me stress, I'm talking about existing services which does NOT
serve aviation; save only a few police helicopters and planes. And those
radios are used for unit to unit (person to person) and group calls
("party line")...not for aviation specific communication. As I said
above, I'm sure parts of P25 would need to be adapted to better serve the
needs of pilots. Nonetheless, the technology is both here and proven.


Greg