P25 Phase II mixed mode TDMA/FDMA

[OCO]

I've been watching the TXWARN question for a while, because I suspect we'll eventually see the same issue (operation in a mixed-mode configuration of Phase I FDMA and Phase II TDMA) on the Michigan MPSCS system as it converts to Phase II in the future.

Paul Opitz posted a comment regarding the TXWARN system this morning on the Uniden forum that says in part " (Phase 2 systems use a different control channel datastream than Phase 1 systems). If it is not a true Phase 2 system (i.e. it is still using a Phase 1 control channel, which would make it an X2 TDMA system)". This got me to wondering - do the older radios that are still operating FDMA understand the Phase II Control Channel data stream or does a true Phase II system revert the CC datastream when a FDMA radio requests a channel assignment, or ??? Or maybe the question is - is TXWARN true Phase II or X2 TDMA?

 

[mikey60]

While some messages on the control channel are different, the control channel data stream is the same as Phase I. The voice channels on the other hand are different modulation and datarate than Phase I systems by my understanding.

Mike

 

[loumaag]

Thread moved by OP request.

Or maybe the question is - is TXWARN true Phase II or X2 TDMA?

The TxWARN P25 does indeed have real Phase 2 activity on it. The deciding factor is if any radio that is capable of only Phase 1 is affiliated on the TG, then the system keeps it at Phase 1. If all radios affiliated on the TG are Phase 2 capable then it goes to Phase 2 on the fly. Naturally the a great number of the radios in use are only capable of Phase 1, so the majority of the traffic is on Phase 1 currently.

 

[OCO]

Thread moved by OP request.

The TxWARN P25 does indeed have real Phase 2 activity on it. The deciding factor is if any radio that is capable of only Phase 1 is affiliated on the TG, then the system keeps it at Phase 1. If all radios affiliated on the TG are Phase 2 capable then it goes to Phase 2 on the fly. Naturally the a great number of the radios in use are only capable of Phase 1, so the majority of the traffic is on Phase 1 currently.

Thanks to all... looks like Uniden, GRE and the third party software guys are going to need to come up with a solution for in-transition systems. A good question would be what does a current scanner (other than the PSR800) do with a TDMA TG.. I think there are other threads dealing with the issue, I'm not sure it needs to be hashed out in the P25 forum..

 

[Jay911]

From my understanding of the P25 control channel standard, all channel grants, talkgroup affiliations, etc etc etc., are all transmitted identically regardless of the system type. For example, a Group Voice Channel Grant, even in its Explicit form, gives just the group address, the source (radio calling), and the transmit and receive LCNs (there's a few more things transmitted, but they're not relevant to this discussion). The only place that TDMA appears to be relevant is in the Identifier Update (the "band plan" declaration) message. There are four bits that replace (or enhance) the "bandwidth" in a non-TDMA IDEN_UP message, and specify the access type (FDMA or TDMA), bandwidth (6.25 or 12.5 kHz), slots (1, 2, or 4), and vocoder (half or full rate).

There are "Capabilities" that are transmitted from subscriber units (radios) upon registration/login (prior to affiliation). These include whether or not a radio is TDMA or FDMA capable. I would suspect that if a radio broadcasting FDMA (only) capability tried to register on a TDMA talkgroup, the system would send a rejection message.

 

[OCO]

Jay: from the P25 WIKI: "If a Phase I only capable radio affiliates with a Phase II talkgroup, these systems are configured to automatically use Phase I modulation on the frequency. "..
The real question is, what do the current generation of scanners that are P-25 Phase I capable do when a Phase II talk group is lit up and they see the TDMA encoding rather than FDMA.. Noise? At least there's a choice at that point on skipping that TG without losing the whole system.. BTW - I just looked at TXWARN's P-25 system and it now shows as an X2 system...I swear it was a Phase II this morning..At least the Texas HP1 crew should see it in the next DB release..

 

[mikey60]

Jay: from the P25 WIKI: "If a Phase I only capable radio affiliates with a Phase II talkgroup, these systems are configured to automatically use Phase I modulation on the frequency. "..
The real question is, what do the current generation of scanners that are P-25 Phase I capable do when a Phase II talk group is lit up and they see the TDMA encoding rather than FDMA.. Noise? At least there's a choice at that point on skipping that TG without losing the whole system.. BTW - I just looked at TXWARN's P-25 system and it now shows as an X2 system...I swear it was a Phase II this morning..At least the Texas HP1 crew should see it in the next DB release..

Unless they understand the Phase II TDMA_IDEN_UP message, they ignore the grant message, since they don't have a valid table for that identifier.

The Phase II TDMA_IDEN_UP message is different than the X2_TDMA_IDEN_UP message (uses a different Op Code).

Mike

 

[OCO]

Unless they understand the Phase II TDMA_IDEN_UP message, they ignore the grant message, since they don't have a valid table for that identifier.

The Phase II TDMA_IDEN_UP message is different than the X2_TDMA_IDEN_UP message (uses a different Op Code).

Mike

But would still respond to a Phase I FDMA grant, if the system was running mixed mode, as during a transition to Phase II? Unless I'm missing something, it sounds like programming a Phase II system into Phase I capable scanner should work, with the scanner just ignoring the TDMA...(unless the scanner hangs on the active TDMA channel anyway). I apologize for my lack of knowledge of the protocol - my experience runs more to Burroughs Poll Select between mainframes..

 

[loumaag]

What I think you are missing is this; if only Phase 2 radios are on a TG, then the controller only issues Phase 2 channel grants. As Mike points out, these channel assignments are different than those sent for Phase 1 (or X2 TDMA for that matter). Scanners don't recognize the channel grants and therefore ignore them, the PSR 800 does recognize X2 TDMA channel grants and responds to them. This is the objection that all Uniden HP-1 users had, Uniden (for no good reason) would initially not include anything that wasn't just Phase 1 in the HP-1 database, even though the radio, when forced to monitor an X2 capable system, would just ignore the channel grant. As I understand it GRE is working on a firmware upgrade to the PSR 800 to recognize Phase 2 channel grants but I guess lack of actual systems to test on is a problem.

If a Phase 1 radio affiliates on the TG that was all Phase 2, then the controller only issues Phase 1 type channel grants.

 

[Jay911]

What I think you are missing is this; if only Phase 2 radios are on a TG, then the controller only issues Phase 2 channel grants.

There's no "Phase 2 channel grant". Voice grants are completely ignorant of what type of talkgroup is in use - they just say "Radios on talkgroup X move to LCN Y (and optionally be prepared to transmit on LCN Z) for a message from subscriber unit ABC".

It's the IDEN_UP status message - which is broadcast constantly along with the SysID/WACN and RFSS/Site info etc., not on voice grants - that is the only thing which references phase 1/phase 2 (or FDMA/TDMA) operation.

 

[OCO]

And this is where it gets off topic for this area (and brand specific), but it sounds to me like Uniden could also export Phase II systems from RR, as they should be doing now with X2 systems since they've added that type to the programming software for the HP1, so that the FDMA portions can be monitored. I understand this gets sticky for them in getting the caveats to the users also ("you won't hear everything").

 

[mikey60]

The type of talkgroup (TDMA vs. FDMA) is totally dependent on the identifier table being used. The actual voice grant and voice grant update messages are still the standard format, only the identifier table number is different, and the channel number is handled based on the number of TDMA slots.

Pro96Com should be decoding the Phase II TDMA_IDEN_UP messages. A sample of the control channel data from that system that I've seen seemed to be decoded properly, although I can't test it directly.

 

[SCPD]

I can affirm what mikey60 and jay911 are saying.

The idea that TDMA is all-or-nothing is a bit of a misconception. All TDMA systems can do FDMA. In fact, all TDMA systems MUST have at least one FDMA channel - the control channel.

I see no reason for FDMA only capable scanners to not allow importing data on TDMA capable systems.

 

[MattSR]

Agreed there Unitrunker -

I'm loving this new acronym-fest. TDMA systems are, in fact, TIme multiplexed FDMA systems

Even in a pure TDMA system such as GSM, the modulation is still FDMA underneath.

 

[loumaag]

There's no "Phase 2 channel grant". Voice grants are completely ignorant of what type of talkgroup is in use - they just say "Radios on talkgroup X move to LCN Y (and optionally be prepared to transmit on LCN Z) for a message from subscriber unit ABC".

It's the IDEN_UP status message - which is broadcast constantly along with the SysID/WACN and RFSS/Site info etc., not on voice grants - that is the only thing which references phase 1/phase 2 (or FDMA/TDMA) operation.

I will grant that the message is "supposed" (let's say reported) to be just the same, but the voice channel you are sent to is different; hence, a Phase 2 grant. However, there is a problem with that, the PSR-800 works on X2 TDMA "channel grants" and yet (to the best of our knowledge here in the Houston area) does not work with the Phase 2 "channel grants" or at least will not go to and decode a Phase 2 communication. So since we can plainly see that that the tables are being broadcast the same as other working X2 TDMA systems elsewhere in the country and a member here who happens to be in the Houston working on another P25 Ph2 system has pointed out that X2 and Phase 2 are not the same.

So in a simplistic way, the effective answer (by using channel grants) I was pointing out above was in response to your assumption (I would suspect that if a radio broadcasting FDMA (only) capability tried to register on a TDMA talkgroup, the system would send a rejection message.) is that if the TG is using Phase 2 (in other words no Phase 1 radios are affiliated with it) when a FDMA only radio affiliates the controller "downgrades" the TG to FDMA (Phase 1). This is of course seamless to the subscribers and if the PSR-800 would work like it does on X2, to the scanner user also.

 

[UPMan]

I'll add (since I was invoked in post 1) that I've since learned better. We are working to allow P2 systems to populate in the HP-1 (but of course it won't be able to follow any TDMA channel grants).

 

[grem467]

So in a simplistic way, the effective answer (by using channel grants) I was pointing out above was in response to your assumption (I would suspect that if a radio broadcasting FDMA (only) capability tried to register on a TDMA talkgroup, the system would send a rejection message.) is that if the TG is using Phase 2 (in other words no Phase 1 radios are affiliated with it) when a FDMA only radio affiliates the controller "downgrades" the TG to FDMA (Phase 1). This is of course seamless to the subscribers and if the PSR-800 would work like it does on X2, to the scanner user also.

If the TG is programmed for TDMA only and an FDMA subscriber tries to affiliate, he will get rejected
If the TG is programmed for DDM and an FDMA subscriber affiliates, the entire talkgroup will revert to FDMA for the duration that the FDMA radio is on the talkgroup.

This is per site as well, if all radios are TDMA capable on the site, the TG will remain in TDMA even if there is an FDMA only user on another site on the same TG.

DDM = Dynamic Dual Mode

It is up to the system manager to determine if he will allow FDMA radios on a TDMA capable talkgroup.

 

[loumaag]

If the TG is programmed for TDMA only and an FDMA subscriber tries to affiliate, he will get rejected
If the TG is programmed for DDM and an FDMA subscriber affiliates, the entire talkgroup will revert to FDMA for the duration that the FDMA radio is on the talkgroup.

This is per site as well, if all radios are TDMA capable on the site, the TG will remain in TDMA even if there is an FDMA only user on another site on the same TG.

DDM = Dynamic Dual Mode

It is up to the system manager to determine if he will allow FDMA radios on a TDMA capable talkgroup.

Thanks for the clarification on that. I thought that was the case on your second point (per site) but had not considered the first where the TG was to be TDMA only.

Um, you wouldn't want to speculate on why a PSR-800 can handle X2 but not Phase 2, would you? :wink:

 

[SCPD]

This is per site as well, if all radios are TDMA capable on the site, the TG will remain in TDMA even if there is an FDMA only user on another site on the same TG.

The P25 committee that wrote this part of the standard was keen on avoiding situations of transcoding the voice data to/from full vs. half rate IMBE. Perhaps the concerns in loss of fidelity were unwarranted. I'd like to know how well this works. Does audio degrade for either audience? Does it depend on who's talking (FDMA vs. TDMA user)?

 

[troymail]

I'll add (since I was invoked in post 1) that I've since learned better. We are working to allow P2 systems to populate in the HP-1 (but of course it won't be able to follow any TDMA channel grants).

I know this is what people are asking for but.... That'll be frustrating for some users.... hearing conversations one second and then nothing the next....

For example: for the Prince George Maryland system, most all activity is TDMA. But when a FDMA-only subscriber simply turns on a radio and it is on a selected talkgroup, you'll hear voice on THAT talkgroup. But as soon as that subscriber switches talkgroups, leaves the system or turns off his radio, the voice will go away again. Of course if several FDMA subscribers are active in an overlapping sense (time-wise) and/or a FDMA radio is active on a talkgroup for an extended period, scanner users will be able to possibly listen longer.

Poeple will need to understand clearly that this is what is happening - otherwise vendors (in this case Uniden) will start getting complaints that the radio is flaky.

Just my 2 cents. (not that anyone asked).