What does common air interface mean?

[N4JNW]

Was looking at Nashville's Trunked/Digital system. Noticed it says P25 Common Air Interface..

What does this mean? Does it mean they're digital, or does it mean they're trunked and digital both?

?? Confused ??

 

[BaLa]

not sure what it actually means, but it's P25 Digital

just looked at the DB page for Nashville/Davidson County System and it looks like most of everything worth listening to is P25/Digital.

 

[N4JNW]

Thanks!! Considering I'll be using a Pro-96, which is digital already, I guess it really dosen't matter. I was just curious more or less.

 

[fmon]

APCO-25 Common Air Interface Exclusive = 9600 baud digital
Analog and APCO-25 Common Air Interface = 3600 baud digital

 

[WayneH]

The CAI is the structure of all the bits used to pass various background information such as radio ID, NAC, etc. Think of CAI as the wheelbarrow and IMBE as the load of dirt.

It's the part of the standard that's completely explained in the documentation, and free to decode, yet the audio codec (IMBE) used to convey voice is proprietary.

 

[N_Jay]

APCO-25 Common Air Interface Exclusive = 9600 baud digital
Analog and APCO-25 Common Air Interface = 3600 baud digital

APCO-25 Common Air Interface Exclusive = 9600 BPS digital control channel
Analog and APCO-25 Common Air Interface = 3600 BPS digital control channel

Digital voice channels are 9600 BPS

 

[loumaag]

APCO-25 Common Air Interface Exclusive = 9600 baud digital
Analog and APCO-25 Common Air Interface = 3600 baud digital

Wrong!

APCO-25 Common Air Interface Exclusive = 9600 BPS digital control channel
Analog and APCO-25 Common Air Interface = 3600 BPS digital control channel

And Wrong!

"APCO-25 Common Air Interface" (Exclusive or otherwise) has NOTHING to do with the control channel. It only describes the type of modulation that is used on the voice channels. You will see that term used in our DB only on the System Voice line. Questions in regard to the control channel are dealt with on the System Type line. On the System Type line, if it says:

Project 25 (anything) = 9600 bps control channel
Motorola (anything) = 3600 bps control channel​

And just for the record, all control channels are digital; when someone is talking about a "digital system" they are referring to the type of voice modulation, not the control channel.

 

[16b]

As a reference to what Loumaag said, take the Ohio MARCS for example:
http://www.radioreference.com/modules.php?name=RR&sid=901

The system voice is listed as "APCO-25 Common Air Interface Exclusive." MARCS is not a P25 system, however; it is a Motorola Omnilink system (which means that the control channels run at 3600bps). The system voice is listed as such because there are no analog talkgroups on the system.

 

[loumaag]

I guess I should have provided an example myself when I posted previously...
Another example of an APCO-25 CAI Exclusive Motorola system is the South Dakota State TRS, which interestingly enough is a VHF Smartzone system. I might also add that there are a few examples of Motorola systems here in the DB that say they are Analog and Digital but actually have no analog traffic on them what-so-ever, just as there are some that say that and have no digital at all on them.

 

[fmon]

I might also add that there are a few examples of Motorola systems here in the DB that say they are Analog and Digital but actually have no analog traffic on them what-so-ever,

Five of these are in my area. Langley AFB and four in this Eastern Virginia Regional Communications System. Three systems are programmed in a 785D, Pro-96 and 2 Pro-2096's and scanned daily either base or mobile.

I wonder why these are listed as Analog and APCO-25 CAI instead of APCO-25 CAIE. There is also a Motorola II Smartnet in my area - Chesapeake Public Safety - listed as Analoge and APCO-25 CAI, but it does have both.

The trend I see is if my scanner indicates the CC is 36, it gets Analog and APCO-25 CAI listing in the database pages and CC 96 gets APCO-25 CAIE. Clearly, all but one of these 6 examples should be APCO-25 CAIE.

 

[loumaag]

Five of these are in my area. Langley AFB and four in this Eastern Virginia Regional Communications System. Three systems are programmed in a 785D, Pro-96 and 2 Pro-2096's and scanned daily either base or mobile.

I wonder why these are listed as Analog and APCO-25 CAI instead of APCO-25 CAIE. There is also a Motorola II Smartnet in my area - Chesapeake Public Safety - listed as Analoge and APCO-25 CAI, but it does have both.

The trend I see is if my scanner indicates the CC is 36, it gets Analog and APCO-25 CAI listing in the database pages and CC 96 gets APCO-25 CAIE. Clearly, all but one of these 6 examples should be APCO-25 CAIE.

Frank you make a good point. If you could submit those on the specific systems that you are sure no analog shows up, the regional admin could fix that. Thanks.

 

[fmon]

Will do Lou, thanks.

I thought N_Jay appropiatly modified my first post and Wayne's wheelbarrow metaphor provided food for thought, but your text file nails the OP question well.

I may upgrade the Wiki pages on the Pro's 92, 95, 96, 2067, 2096 and the BC 785D as you did with the 396. This idea may very will resolve many future questions on individual scanner capabilities, beyond what is available in the OM.

 

[loumaag]

I may upgrade the Wiki pages on the Pro's 92, 95, 96, 2067, 2096 and the BC 785D as you did with the 396. This idea may very will resolve many future questions on individual scanner capabilities, beyond what is available in the OM.

For those of you who may be confused as to this comment; Frank is actually referring to a different topic in a different forum. And the list & idea he refers to can be found attached to and explained in this post.

 

[studgeman]

Here is another defination of CAI.

CAI is the portion of the P25 Standards which deal with over the air modulation, frame organization and specifies the frame payload. All told it encompases several TIA standards which provide a commmon platform for P25 compliant radios to talk over the air to each other, this can be digital voice or data. To better understand this in context of the other P25 standards there are other standards which govern portions of a radio infrastructure which are not part of a subscriber radio transmitting and receiving, such as the intersystem interface, ISSI. The ISSI is again a suite of individual TIA standards.

 

[loumaag]

Here is another defination of CAI.

CAI is the portion of the P25 Standards which deal with over the air modulation, frame organization and specifies the frame payload. All told it encompases several TIA standards which provide a commmon platform for P25 compliant radios to talk over the air to each other, this can be digital voice or data. To better understand this in context of the other P25 standards there are other standards which govern portions of a radio infrastructure which are not part of a subscriber radio transmitting and receiving, such as the intersystem interface, ISSI. The ISSI is again a suite of individual TIA standards.

Yes, but in the context of monitoring and scanner use (the purpose of this site), the only thing important is voice communication type.

 

[SKEYGEN]

Unless you're one of those more interested in the technical details of a given system than the largely boring communications that take place on it.