General discussion question on P25

[BinaryMode]

I was hearing dispatch talk to fire on the dispatch channel and then a page came though and what I was listening to prior vanished with the page taking precedence. So my question is: is it possible for more than one conversation on the same talkgroup in a P25 trunked system? I can perhaps see the utilization of more than one voice frequency for such a thing. This is a phase I system.

Thanks.

 

[CanesFan95]

No, there's only gonna be 1 conversation per talk group, regardless of Phase I or II. What you're describing sounds like you have a TG set to a higher priority and it became active and bumped you off the first TG. But when you say "page", what do you mean? And what receiver / scanner are you using?

 

[BinaryMode]

Fire dispatch page.

It seemed like the conversation I was hearing vanished and the page took precedence. "Seemed" being the operative word.

I thought perhaps this was possible because I seem to remember many years ago two users keyed up their radios at the same time and dispatch must have saw each RID and said there was double traffic or whatever.

 

[wlmr]

Dispatch consoles can be configured with supervisory takeover. Not sure if that's the correct term. One dispatchers transmission can be pre-empted by a higher level console.
What you're describing could be this.

 

[BinaryMode]

They do use a dedicated console for dispatches. I know because the same RID shows up.

 

[MTS2000des]

It's known as console priority, and is typical in dispatch environments for a console to be connected to a trunking system core and dispatch outgoing audio takes priority over an SU, but the SU audio still comes through the headset/select audio speaker even if the dispatch console user is talking (this is how you can transmit while a dispatcher is keyed up, no one else in the field will hear you, but the console user will- allowing for "officer safety" and for a dispatcher to "keep control" of the channel).

 

[GTR8000]

It's akin to a 4-wire repeater setup in the conventional world.

In the most basic terms, the system consoles are keying the repeater up directly on the output frequency for whichever traffic/voice channel is assigned to the active talkgroup, while the subscriber radio out in the field is still talking in on the repeater input frequency. Just like a 4-wire setup, the console audio preempts the repeated audio on the output frequency without actually "stepping" on the input frequency, so the subscriber radio can still be heard on the dispatch console. Often times if the dispatcher stops talking you can actually hear the subscriber radio's ambient audio coming out of the console speaker and getting back into the mix through the console mic.

Remember, all trunking really is are a bunch of repeaters that are networked together and controlled by a computer that tells each subscriber radio which repeater to use for whichever talkgroup those radios are active on/affiliated with. So whether it's an old analog system or a modern P25 Phase II system, it's really nothing more than a repeater with an input frequency and output frequency.

By the way, P25 Phase II (and other TDMA systems in general) has some features that are not available with non-TDMA systems, such as true subscriber preemption. While a subscriber radio is transmitting on one time slot of a frequency, it's also listening to the other time slot. It can do this because TDMA subscribers transmit in very quick pulses (power amplifier on/off/on/off/etc.) rather than a continuous, uninterrupted transmission. The system broadcasts signaling on both time slots even when they are both occupied with active voice transmissions, and the subscribers can receive those transmissions and act accordingly. It's a very handy feature, in particular the ability for the system to direct a subscriber to "stop transmitting" so it can receive an emergency transmission. Even if you have the PTT button held down, the radio will "bonk" and stop transmitting. Another neat feature is the ability of the system to vary the power output of the subscriber if "adaptive power" is enabled in the codeplug. So rather than operating at full power, if the subscriber has very good inbound signal, the system can direct it to back down to a mid-power setting. Good way to conserve battery runtime on portables, especially for systems that have very robust coverage.

 

[CanesFan95]

^ Great explanation.

 

[KevinC]

It's akin to a 4-wire repeater setup in the conventional world.

In the most basic terms, the system consoles are keying the repeater up directly on the output frequency for whichever traffic/voice channel is assigned to the active talkgroup, while the subscriber radio out in the field is still talking in on the repeater input frequency. Just like a 4-wire setup, the console audio preempts the repeated audio on the output frequency without actually "stepping" on the input frequency, so the subscriber radio can still be heard on the dispatch console. Often times if the dispatcher stops talking you can actually hear the subscriber radio's ambient audio coming out of the console speaker and getting back into the mix through the console mic.

Remember, all trunking really is are a bunch of repeaters that are networked together and controlled by a computer that tells each subscriber radio which repeater to use for whichever talkgroup those radios are active on/affiliated with. So whether it's an old analog system or a modern P25 Phase II system, it's really nothing more than a repeater with an input frequency and output frequency.

By the way, P25 Phase II (and other TDMA systems in general) has some features that are not available with non-TDMA systems, such as true subscriber preemption. While a subscriber radio is transmitting on one time slot of a frequency, it's also listening to the other time slot. It can do this because TDMA subscribers transmit in very quick pulses (power amplifier on/off/on/off/etc.) rather than a continuous, uninterrupted transmission. The system broadcasts signaling on both time slots even when they are both occupied with active voice transmissions, and the subscribers can receive those transmissions and act accordingly. It's a very handy feature, in particular the ability for the system to direct a subscriber to "stop transmitting" so it can receive an emergency transmission. Even if you have the PTT button held down, the radio will "bonk" and stop transmitting. Another neat feature is the ability of the system to vary the power output of the subscriber if "adaptive power" is enabled in the codeplug. So rather than operating at full power, if the subscriber has very good inbound signal, the system can direct it to back down to a mid-power setting. Good way to conserve battery runtime on portables, especially for systems that have very robust coverage.

Pop quiz (again). How often does the subscriber "look" back at the system while it's transmitting (at least in MSI subs anyway)?

 

[BinaryMode]

Thanks for the explanation. Good Info.

 

[speedway_navigator]

The adaptive power scheme came from the cell industry, did not know 2 way had adopted it.
I have heard uplink audio and console at the same time on phase 1 systems.

DMR TDMA uses 30 ms time slots with 2.5 ms in between slots for signaling and a delay for the transmitter PA (power amp) to stabilize.
I can't find that in my P-25 docs right now but if our beloved P25 master would quit snooping on drone video and pop in here I'm sure he will have the proper details.