P25 Trunked site NAC (AKA ACI)

[KevinC]

Is the site NAC in P25 trunked systems supposed to be the upper 8 bits of the system ID and the remaining 4 user defined? I know different manufacturers do it different ways, I was just wondering if it was defined in the specs how it should be derived.

 

[GTR8000]

Correct, that's how it should be done. I know MSI sticks to that spec, but I'd have to look at some Harris and Tait systems to see if they do as well.

 

[GTR8000]

Hmm so it appears that Harris does whatever they want. SysID 227 has Site NAC's of 47F for all three sites.

Maybe someone has the actual TIA spec, assuming there is one. I don't have access to those docs anymore, unfortunately.

 

[KevinC]

Correct.

So some Harris systems are out of spec?

 

[KevinC]

Hmm so it appears that Harris does whatever they want. SysID 227 has Site NAC's of 47F for all three sites.

Maybe someone has the actual TIA spec, assuming there is one. I don't have access to those docs anymore, unfortunately.

I found this in the Tiat Academy document...

Trunked System NAC Use This same ability to provide increased protection from undesired co-channel interference also applies to trunked systems. The Network Access Code is used for both trunked traffic and control channels. The NAC field contains a 12 bit value but in trunking there is a relationship between the NAC and the System ID.. ■ Eight of these bits derive from the upper 8 bits of the trunked system ID and are common to all sites throughout the entire system. For example, if the System ID is 3B5 in hexadecimal, all the NACs at all sites will begin with 3B. ■ The remaining four bits of the NAC (called the Access Code Index or ACI) are then configured on a per-site/per-simulcastsubsystem basis by the system installers to provide interference protection between sites.

Just wasn't sure if that's the spec or just how Tait and some others chose to implement it.

 

[GTR8000]

Sorry, I edited my post after you read it. Assuming it actually is in the TIA spec that it should be as you said, then yes some Harris systems are out of that spec. It might be a suggestion more than a mandatory spec, however it's obviously the reason why Validate NAC Against System ID doesn't work in MSI subscribers with most Harris systems.

 

[GTR8000]

For whatever it's worth, a local Tait P25 system follows the pattern. SysID 85A, NAC 85A.

 

[KevinC]

Sorry, I edited my post after you read it. Assuming it actually is in the TIA spec that it should be as you said, then yes some Harris systems are out of that spec. It might be a suggestion more than a mandatory spec, however it's obviously the reason why Validate NAC Against System ID doesn't work in MSI subscribers with most Harris systems.

I'm guessing it's NOT specified, otherwise MSI would have raised a stink. Just found it interesting Tait mentions ACI which is the exact term and way MSI does it.

 

[GTR8000]

You're probably right, it was probably more of a suggestion vs an actual spec/standard.

Harris does some weird stuff with their systems. I always get a kick out of so many of them having RFSS/Site ID's of 10-10, 20-20, etc. Some real oddballs in NYS.

 

[xmo]

The standard:

 

[KevinC]

The standard:

Thanks. So the way MSI does it is the "recommended" way.

 

[AM909]

Another counter example is system 14C. It appears that it's actually two networks; United States Department of Defense (14C) Trunking System, Nationwide, Multi-State has all sites* set to NAC 140 while US Marine Corps Installations West (14C) Trunking System, Various, Multi-State has all sites* set to NAC 14C.

* There are at least a few examples where the DB says otherwise. Were these confirmed OTA?: United States Department of Defense (14C) Site: Naval Air Station - Pensacola , US Marine Corps Installations West (14C) Site: Marine Corps Air Ground Combat Center - Creole through US Marine Corps Installations West (14C) Site: Marine Corps Air Ground Combat Center - Bearmat.

 

[GTR8000]

Another counter example is system 14C. It appears that it's actually two networks; United States Department of Defense (14C) Trunking System, Nationwide, Multi-State has all sites* set to NAC 140 while US Marine Corps Installations West (14C) Trunking System, Various, Multi-State has all sites* set to NAC 14C.

* There are at least a few examples where the DB says otherwise. Were these confirmed OTA?: United States Department of Defense (14C) Site: Naval Air Station - Pensacola , US Marine Corps Installations West (14C) Site: Marine Corps Air Ground Combat Center - Creole through US Marine Corps Installations West (14C) Site: Marine Corps Air Ground Combat Center - Bearmat.

All of the sites of those 14C ASTRO 25 systems follow the TIA recommendations for site NAC. Not sure what example you're trying to show here; Kevin already noted that MSI ASTRO 25 systems follow the convention. What "counter example" are you referring to?

 

[Project25_MASTR]

You're probably right, it was probably more of a suggestion vs an actual spec/standard.

Harris does some weird stuff with their systems. I always get a kick out of so many of them having RFSS/Site ID's of 10-10, 20-20, etc. Some real oddballs in NYS.

To Harris guys, they think it is extremely weird that everyone else doesn't do that. Have you seen what Harris does when they reach that 256 site limit they impose with forcing the RFSS/Site ID to match? New WACN...

In all honesty, part of me wonders if that is a limitation in how the VIDA core operates but I don't have a ton of experience with that. I'll have to ask the new guy at the office who came from Harris.

My observation is MSI tends to use the first 8 bits of the system ID and the last 4 bits of the site ID. There are exceptions...but that tends to be what I've observed with most of the Astro 25 systems here in Texas. For the record, Validate NAC against SysID is something that can cause issues with Viking subs as well when setting them up for some systems.

 

[GTR8000]

It's pretty hard to set an ASTRO 25 NAC other than that which follows the SysID. Last time I was poking around in CSS, the only choices for a trunking GTR's NAC was 0-F selection based on the SysID that is already defined.

 

[KevinC]

To Harris guys, they think it is extremely weird that everyone else doesn't do that. Have you seen what Harris does when they reach that 256 site limit they impose with forcing the RFSS/Site ID to match? New WACN...

In all honesty, part of me wonders if that is a limitation in how the VIDA core operates but I don't have a ton of experience with that. I'll have to ask the new guy at the office who came from Harris.

My observation is MSI tends to use the first 8 bits of the system ID and the last 4 bits of the site ID. There are exceptions...but that tends to be what I've observed with most of the Astro 25 systems here in Texas. For the record, Validate NAC against SysID is something that can cause issues with Viking subs as well when setting them up for some systems.

MSI has to use the first 2 digits of the system ID and then a locally defined number. That’s hard coded in CSS and how the system was designed.

 

[GTR8000]

Yup, exactly.

Also, Harris being Harris.

 

[Echo4Thirty]

pulling from memory last time I spoke to my standards guy, he said the WACN is supposed to be the encoded system FCC callsiign and thus the "system ID" with the remaining simply being whatever NAC the manufacturer chooses for the sites. I never understood what you were supposed to do if the system had multiple callsigns.

The Harris guys used to tell us that Moto was out of spec since they used the "Region ID/NAC" as the system ID with every system having the same BEE00 "System ID" as they referred to it. In RPM they do refer to the xxx field as "sysid/region ID" and the xxxxx field as "network address (WACN)"

Other manufacturers (im looking at you EFJ) simply make up their own three digit "system ID" and WACN, often 00001. Their system keys also seem to utilize both fields so a system key for 123/BEE00 would not match 123/BEE01 for example. I will say its pretty handy having an authorized Motorola key for system ID 00A and it be able to be used on a huge majority of Harris trunked systems lol.

The one thing I do like about the way moto does it, is you can usually tell the age of system relative to other systems as they seem to increment the system ID logically as they create systems.

Maybe @KevinC can tell us why there are oddball WACNs that deviate from BEE00, such as GATRRS using BEE09. Was this at the customer request? What benefit would it have for them with Moto keys ignoring the WACN but other manufacturers having to spell it out anyway?

 

[KevinC]

Maybe @KevinC can tell us why there are oddball WACNs that deviate from BEE00, such as GATRRS using BEE09. Was this at the customer request? What benefit would it have for them with Moto keys ignoring the WACN but other manufacturers having to spell it out anyway?

I wouldn't have any idea.

 

[GTR8000]

I always figured that any deviation from BEE00 was a fat fingering that was never corrected, in particular BEE09. But, I could be wrong (and probably am), agencies have been know to do weird stuff.