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P25 Phase II mixed mode TDMA/FDMA

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grem467

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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:
In addition to the afforementioned OSP messages discussed:

X2 voice: 9600Bps, C4FM [8K10F7W] (non simulcast) or CQPSK [8K70D7W] CQPSK LSM Simulcast)
P2 voice: 12kbps, HDQPSK [9K80D7W]

It will be interesting to see if the 800 can handle the higher data rate and modulation.
 

DonS

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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.
From a CC perspective (and as far as a scanner is concerned), the only difference between Phase I, Phase II, and Mot X2 is in how the "tables are being broadcast".

On a P25 system, the tables are broadcast via the IDEN_UP message (Identifier Update). There are, at present, at least 3 different forms of IDEN_UP messages:
* Original, FDMA-only IDEN_UP
* Motorola X2 TDMA IDEN_UP
* Phase II TDMA IDEN_UP

The TXWARN system uses the first and third forms. It does not use the second (X2) form.

A given IDEN_UP message refers to a block of 4096 channels (the top 4 bits of a 16-bit channel number is the "identifier").

A "channel grant" contains a 16-bit channel number. The channel grant is only indirectly related to the voice modulation: via the type of IDEN_UP message that was sent for that block of channels.

When a receiver decodes [the generic] channel grant, it must look at the top 4 bits of the "channel number" (i.e. the "identifier") to determine whether it's Phase I FDMA, Phase II TDMA, Mot X2 TDMA, etc.

The PSR-800 won't "go to and decode a Phase 2 communication" only because it does not understand the Phase II TDMA IDEN_UP messages - those parts of the tables are left empty. When it receives a channel grant which refers to an identifier described in such an IDEN_UP message, it cannot calculate a voice frequency. Since it can't get and tune a VC, it ignores the channel grant. If a user was to manually program the tables for such a system, the PSR-800 would attempt to tune to the VC and deliver audio based on the [generic] channel grants. Of course, that wouldn't give the desired results.

Note that the above infers this: any scanner which depends solely upon IDEN_UP messages to populate its trunking tables need not care about "Phase I", "Phase II", or "X2" formats that it does not support. If it didn't understand the IDEN_UP message, it won't populate that part of the trunking tables. If that part of the trunking tables isn't populated, it can't calculate a VC. If it can't calculate a VC, it should ignore the channel grant.

(EDIT: The last paragraph above exactly describes the PSR-500 family. If you program one of those scanners for the TXWARN system and set that system to "P25 Auto", you'll only hear the FDMA traffic. This is only because the PSR-500 cannot populate its tables from the new IDEN_UP messages. The scanner tries to handle the channel grants that refer to Phase II TDMA channels, but it ends up with zero for the voice frequency and happily discards the grant - exactly as if the "talkgroup wasn't interesting" (not programmed).)
 
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SCPD

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People 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.
It's the same problem as talkgroups with part-time encryption. Hearing some traffic is better than none.
 

OCO

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It's the same problem as talkgroups with part-time encryption. Hearing some traffic is better than none.
Thank you for that!

When I asked the question initially (sorry Paul for dragging you into it) I was thinking of the large statewide systems that presumably will be in transition for quite some time. Until that transition is done, my non-TDMA radio is still useful on the system.

I'm going to save a shortcut back to this thread.. I refer to WayneH's old basic Trunking pages and the P25.com white papers quite often - this is a great addition for me.:)
 

grem467

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Don do you think the 800 can handle the differences in the voice channel on a Phase II TDMA call? specifically the data rate and HDQPSK? As im sure you know X2 TDMA voice and Phase II TDMA voice are not the same.
 

MattSR

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...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)?
Hi Rick,

The recent P25 specs show that this conversion can be done purely in the digital domain without any conversion to analog in the middle.

It would be interesting to see how well it works in practice!

Cheers,
Matt
 

SCPD

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The recent P25 specs show that this conversion can be done purely in the digital domain without any conversion to analog in the middle.
Ewww. I wasn't suggesting that. That would likely sound quite awful. I don't know how well full rate IMBE and half rate IMBE translate to one-another. I'm thinking something like JPEG to GIF (without projecting an image onto a wall and taking it's picture) or VSELP to our dear old friend USFS-1016 (aka CELP).
 

brownlab

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From a CC perspective (and as far as a scanner is concerned), the only difference between Phase I, Phase II, and Mot X2 is in how the "tables are being broadcast".

On a P25 system, the tables are broadcast via the IDEN_UP message (Identifier Update). There are, at present, at least 3 different forms of IDEN_UP messages:
* Original, FDMA-only IDEN_UP
* Motorola X2 TDMA IDEN_UP
* Phase II TDMA IDEN_UP

The TXWARN system uses the first and third forms. It does not use the second (X2) form.

A given IDEN_UP message refers to a block of 4096 channels (the top 4 bits of a 16-bit channel number is the "identifier").

A "channel grant" contains a 16-bit channel number. The channel grant is only indirectly related to the voice modulation: via the type of IDEN_UP message that was sent for that block of channels.

When a receiver decodes [the generic] channel grant, it must look at the top 4 bits of the "channel number" (i.e. the "identifier") to determine whether it's Phase I FDMA, Phase II TDMA, Mot X2 TDMA, etc.

The PSR-800 won't "go to and decode a Phase 2 communication" only because it does not understand the Phase II TDMA IDEN_UP messages - those parts of the tables are left empty. When it receives a channel grant which refers to an identifier described in such an IDEN_UP message, it cannot calculate a voice frequency. Since it can't get and tune a VC, it ignores the channel grant. If a user was to manually program the tables for such a system, the PSR-800 would attempt to tune to the VC and deliver audio based on the [generic] channel grants. Of course, that wouldn't give the desired results.

Note that the above infers this: any scanner which depends solely upon IDEN_UP messages to populate its trunking tables need not care about "Phase I", "Phase II", or "X2" formats that it does not support. If it didn't understand the IDEN_UP message, it won't populate that part of the trunking tables. If that part of the trunking tables isn't populated, it can't calculate a VC. If it can't calculate a VC, it should ignore the channel grant.

(EDIT: The last paragraph above exactly describes the PSR-500 family. If you program one of those scanners for the TXWARN system and set that system to "P25 Auto", you'll only hear the FDMA traffic. This is only because the PSR-500 cannot populate its tables from the new IDEN_UP messages. The scanner tries to handle the channel grants that refer to Phase II TDMA channels, but it ends up with zero for the voice frequency and happily discards the grant - exactly as if the "talkgroup wasn't interesting" (not programmed).)
http://forums.radioreference.com/pr...se-ii-mixed-mode-tdma-fdma-2.html#post1684821

So Don, do you feel that the 800 and 900 can be reprogramed to decode the voice channel of a true Phase II TDMA?
 
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