P25 Operation

[magiclightman]

Hi all,

Quick question about P25 unit registration, when a radio registers, does it just register the Unit ID onto the system or does it register to each talk group?

 

[ecps92]

Conventional ? , Trunked ? or Vote-Scan ?

Hi all,

Quick question about P25 unit registration, when a radio registers, does it just register the Unit ID onto the system or does it register to each talk group?

 

[magiclightman]

Conventional ? , Trunked ? or Vote-Scan ?

P25 trunked system. More of a "how does P25 trunked system work" more than a scanner but I thought maybe some people could help from here too.

 

[cmjonesinc]

When the radio turns on, talks, or changes talkgroups it's id is sent to the system. If you have a scanner or sdr you can plug up to your computer, give unitrunker a try. That will help give you an understanding of what happens.

 

[doctordialtone]

how does P25 trunked system work

These three links might help:

Project 25 - Wikipedia

en.wikipedia.org

Common Trunking Programming Questions - The RadioReference Wiki

APCO Project 25 - The RadioReference Wiki

wiki.radioreference.com

 

[GTR8000]

The first thing that happens is that the radio requests registration on the system, i.e. "login". The radio broadcasts its Subscriber ID along with the SysID, the system verifies that it's a valid ID, and the radio is allowed to register.

The next thing that happens is that the radio requests affiliation with whichever talkgroup is selected. The system verifies that the talkgroup is valid for the system and (in the case of multi-site systems) at that particular site, and the radio is allowed to affiliate.

It's takes a fraction of a second for both steps to occur.

14:07:43.606 842 -AC 00 08 40 03 5B C6 03 5B C6 97 01 Unit Registration Response Sys 840 ID 035BC6 WUID 035BC6 accept
14:07:43.859 842 28 00 00 00 00 55 F1 03 5B C6 61 BA Group Affiliation Response I 035BC6 accept 55F1

 

[speedway_navigator]

It's takes a fraction of a second for both steps to occur.

Unless the system is overloaded due to a mass shooting at an airport baggage claim.
From the Ft Lauderdale after action report.

"At approximately 2pm, several hundred Broward county school bus drivers turned on their radios for duty (part of routine), which further taxed the already overwhelmed radio system.
This caused increased radio site trunking and loss of linked channels (several linked cities and resources were immediately disconnected from the event.)"

 

[GTR8000]

Unless the system is overloaded due to a mass shooting at an airport baggage claim.
From the Ft Lauderdale after action report.

Sure, except that incident took place on the antiquated 3600 baud SmartZone system, not a 9600 baud P25 system, the latter of which is the topic of this thread.

 

[Floridarailfanning]

Unless the system is overloaded due to a mass shooting at an airport baggage claim.
From the Ft Lauderdale after action report.

"At approximately 2pm, several hundred Broward county school bus drivers turned on their radios for duty (part of routine), which further taxed the already overwhelmed radio system.
This caused increased radio site trunking and loss of linked channels (several linked cities and resources were immediately disconnected from the event.)"

Just another merit for the argument of keeping schools, public works, trash pickup, etc. off of public safety trunking systems.

 

[GTR8000]

Sure, except that incident took place on the antiquated 3600 baud SmartZone system, not a 9600 baud P25 system, the latter of which is the topic of this thread.

PS - P25 control channels are actually 4800 baud, not 9600 as Motorola refers to them. 9600 bits per second is the correct unit of measurement. SmartZone systems are both 3600 baud and 3600 bits per second.

The point remains that a P25 control channel processes messages nearly 3x faster than a SmartZone control channel, making P25 signaling more efficient. Yes, you can still overwhelm a P25 system, however not as easily as you could with a SmartZone system.

 

[N4KVE]

Just another merit for the argument of keeping schools, public works, trash pickup, etc. off of public safety trunking systems.

The school bus system used to be separate from the BSO system. Wonder when that changed?

 

[chief21]

The school bus system used to be separate from the BSO system. Wonder when that changed?

1980-ish. When the county was planning the initial SmartZone system, they didn't have enough frequencies to purchase a full 28-channel system. The School Board, which was considering expanding their small system, agreed to assign their 5 (or 6) frequencies to the county, in exchange for use of the soon-to-be-constructed county system.