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).)
