P25 Phase II systems - always hybrid?

[BudTurpa]

Here on RR I notice that some P25 systems have talkgroups coded as "TDMA Capable Talkgroup". I get the gist that Phase II implementations are FDMA with the ability to do TDMA. Is that loosely correct?

- When talkgroups are a "TDMA Capable Talkgroup" when is TDMA used? Is it dynamic, FDMA at one moment and TDMA another moment, based on the ability of affiliated radios? Is it statically set by the admin, 100% FDMA today but could be switched to 100% TDMA in the future when all equipment is able?

- In a Phase II capable TG is "dispatch" always in FDMA to be compatible with any radio at any time?

- Do larger scale Phase II networks that are TDMA only (no possible FDMA functionality) exist?

I realize using a trunking scanner to monitor a system in vicinity could give insight to some of these questions, I am curious in a more general sense on how P25 is implemented.

Thanks!

 

[MTS2000des]

Motorola systems call this "dual dynamic mode" and depending on how the systems manager(s) provision a talkgroup(s), it can be DDM, TDMA only or FDMA only. In DDM operation, the "least common denominator" dictates whether a call assignment is FDMA or TDMA. All it takes in a single FDMA subscriber on any affiliated site to downgrade the talkgroup to FDMA, even if they are just listening. On our system, we monitor affiliation via a third party application that helps us find out where the legacy radios are to speed up replacement.

With DDM, a system operator can make migration to TDMA easy, and once all legacy equipment is retired, just go into Provisioning Manager and set the talkgroup to TDMA only if desired, or leave it DDM in case an old radio gets issued as a repair swap, interop, etc.

 

[GTR8000]

There are plenty of systems out there that have all talkgroups strapped for TDMA only, and never had the DDM option. The only FDMA on those systems is the control channel and/or FDMA data.

Don't be fooled by the generic RRDB explanation that T mode talkgroups are "TDMA Capable". That is merely an easy way to provide a catch-all description for these systems for the scanning/hobby community. A talkgroup flagged as T mode in the database may be either strapped or DDM, it all depends on how the system is configured.

 

[BudTurpa]

Motorola systems call this "dual dynamic mode" and depending on how the systems manager(s) provision a talkgroup(s), it can be DDM, TDMA only or FDMA only. In DDM operation, the "least common denominator" dictates whether a call assignment is FDMA or TDMA. All it takes in a single FDMA subscriber on any affiliated site to downgrade the talkgroup to FDMA, even if they are just listening. On our system, we monitor affiliation via a third party application that helps us find out where the legacy radios are to speed up replacement.

With DDM, a system operator can make migration to TDMA easy, and once all legacy equipment is retired, just go into Provisioning Manager and set the talkgroup to TDMA only if desired, or leave it DDM in case an old radio gets issued as a repair swap, interop, etc.

Thanks for this!

In your example, if the system is operating in DDM Phase II TDMA and a single radio that can not do TDMA affiliates, the system downgrades to FDMA. As soon as that single radio powers down (or changes systems it's afflicted to) does the entire system upgrade back to TDMA in that moment? Could it be that a system is bouncing between TDMA and FDMA all throughout a day given a couple FDMA-only radios coming on and off the system?

 

[BudTurpa]

From my understanding, amongst several benefits of a Phase II system strapped to TDMA, one is more efficient use of spectrum.

For example, if voice traffic density a Phase I system called for 20 frequencies allocated for voice. Once all the radios were migrated to TDMA capable and the system was strapped to Phase II TDMA, could the allocated voice channels be reduced to 10 frequencies? Does this ever happen in the real world?

If a manager left the system in DDM for an older repair swap radio to enter the system would this theoretically eliminate the spectrum efficiency opportunity for that system -- because the system could revert to FDMA and need all 20 voice channels back?

 

[Echo4Thirty]

its not the entire system, just that one affected talkgroup. So the rest of the TGs could remain in TDMA while the one is reverted to FDMA for the duration of the legacy radio being affiliated to the TG. That TG would also take the entire 12.5 kHz channel, and be unusable for a second conversation.

As a side note, if a channel is only processing one TDMA call and the other timeslot is empty, technically that call is also taking the entire 12.5 bandwidth.

P25 repeaters always are taking the full 12.5 kHz bandwidth, Phase 2 just adds the ability to multiplex two simultaneous calls together in that bandwidth hense the 6.25e (equivalent) designation. DMR falls under this same scheme. TETRA as well, albeit with 4 slots in a 25 kHz bandwidth.

NXDN is true 6.25, but is still FDMA.

 

[GTR8000]

As Echo said, FDMA/TDMA/DDM is determined at the talkgroup level, generally speaking. When you provision a talkgroup, you decide which mode it will operate in. DDM is an optional feature, it's not standard, and so if you don't pay for that feature you can only choose between FDMA or TDMA, no dynamic switching between the two modes.

Systems that are going to use TDMA or DDM must also have the repeaters capable of TDMA mode, which is normally an added cost to provision that feature. Not all repeaters are capable of TDMA (Quantar being an example), they must have linear amplifiers in order to transmit in H-DQPSK/LSM modulation. Of course the tradeoff of paying a higher up front cost for all of the TDMA features is that you can essentially cut in half the amount of physical repeaters that are required, as each 12.5 kHz physical channel now functions as two traffic channels by using alternating time slots. There are other benefits of TDMA that aren't available with FDMA, in particular when it comes to signaling. The subscribers are listening to the alternate slot in between the 40ms bursts of transmitting, which means they can be preempted for emergency traffic or for system console transmissions. The radio is instructed to stop transmitting and will switch to receiving in that situation, something that isn't possible with FDMA as the radio is locked in transmit mode until the PTT is released by the user.

That's the simplified version of it, but as with most things related to these systems, things can be significantly more complicated under the hood. For example, ASTRO 25 systems give you the ability to provision TDMA per-site or even per-channel using partitioning. Most systems don't go that deep, they simply purchase the TDMA features and decide what mode each talkgroup is going to operate in. If all subscribers are TDMA capable, then chances are good that the system will never allow FDMA traffic, as it's a resource hog.

 

[BudTurpa]

As Echo said, FDMA/TDMA/DDM is determined at the talkgroup level, generally speaking. When you provision a talkgroup, you decide which mode it will operate in. DDM is an optional feature, it's not standard, and so if you don't pay for that feature you can only choose between FDMA or TDMA, no dynamic switching between the two modes.

Systems that are going to use TDMA or DDM must also have the repeaters capable of TDMA mode, which is normally an added cost to provision that feature. Not all repeaters are capable of TDMA (Quantar being an example), they must have linear amplifiers in order to transmit in H-DQPSK/LSM modulation. Of course the tradeoff of paying a higher up front cost for all of the TDMA features is that you can essentially cut in half the amount of physical repeaters that are required, as each 12.5 kHz physical channel now functions as two traffic channels by using alternating time slots. There are other benefits of TDMA that aren't available with FDMA, in particular when it comes to signaling. The subscribers are listening to the alternate slot in between the 40ms bursts of transmitting, which means they can be preempted for emergency traffic or for system console transmissions. The radio is instructed to stop transmitting and will switch to receiving in that situation, something that isn't possible with FDMA as the radio is locked in transmit mode until the PTT is released by the user.

That's the simplified version of it, but as with most things related to these systems, things can be significantly more complicated under the hood. For example, ASTRO 25 systems give you the ability to provision TDMA per-site or even per-channel using partitioning. Most systems don't go that deep, they simply purchase the TDMA features and decide what mode each talkgroup is going to operate in. If all subscribers are TDMA capable, then chances are good that the system will never allow FDMA traffic, as it's a resource hog.

GTR8000 and Echo - Thanks for all the info, great to learn about this stuff! I appreciate the time and help!

 

[GTR8000]

40ms bursts of transmitting

I need to correct myself...the slot timing is 30ms not 40ms.