P25 TDMA Phase II Talk Around

[ASTROMODAT]

Can direct talk around (simplex) be implemented in P25 Phase II TDMA subscriber gear (e.g., APX 7500 or 7000)? My understanding is that P25 TDMA currently does NOT support talk around, but I am not at all sure. Any help will be much appreciated!

[tvsjr]

As with TRBO, TDMA requires time sync - thus infrastructure. Simplex needs to be configured as FDMA/Phase 1.

[trumpetman12]

I was under the impression that you can use TRBO in talkaround...but you lose the ability to have two conversations on the same channel (since there is no sync). I also have wondered the same thing about P25 TDMA in direct mode and if it were to behave the same way as TRBO.

I stopped by a shop a few months ago when they got TRBO units to play with and they said they were pleased with the simplex operation in digital, but I didn't see it first hand.

[tvsjr]

I was under the impression that you can use TRBO in talkaround...but you lose the ability to have two conversations on the same channel (since there is no sync). I also have wondered the same thing about P25 TDMA in direct mode and if it were to behave the same way as TRBO.

I stopped by a shop a few months ago when they got TRBO units to play with and they said they were pleased with the simplex operation in digital, but I didn't see it first hand.

Correct... both continue to use the same digital protocol, but TRBO and P25 both fall back to FDMA - for TRBO, there's no name for this... for P25, there is - Phase 1.

[ASTROMODAT]

Is it correct that Phase 2 TDMA radios will need to fall back to Phase 1 FDMA for operating in the simplex mode (talk around), and also when on a conventional system? Phase 2 TDMA is strictly trunked, and no such thing as simplex, or can a Phase 2 radio operate with talk around, as long as it combines 2 of the 6.25 kHz TDMA channels on a trunked system, and operates at 12.5 kHz, but still runs TDMA (as opposed to 12.5 kHz FDMA), or does it necessarily have to fall back to FDMA for these modes?

[Wowbagger]

The reason you cannot use TDMA in a talk-around mode is that there has to be a means for the radio to know when the time slots are. In a trunked system, that happens by the radio monitoring the control channel, which provides timing data (think of a conductor in a symphony). In a talk-around system, there is no conductor, thus no way for the radio to know when the time slots are. Sure, the radio could just transmit half the time, but without a way to coordinate with any radio that wishes to use the other half of the time, the two radios are just going to stomp all over each other. So, yes, in theory the radio could transmit half the time in talk around, but the other half of the time would be wasted. So why not just use the full time and keep things simpler?

Remember: there is no such thing as a 6.25kHz channel in TDMA. There is a 12.5kHz channel, that you use half the time. 2 slot TDMA gives you the equivalent of a 6.25kHz channel, because you use 12.5kHz half the time, and somebody else uses it the other half of the time. But if nobody else can use the channel that "other half" of the time because they don't know when that "other half" is, then one conversation takes up 12.5kHz.

This isn't a 2 hole outhouse - this is a one holer that you use half the time. But if you don't know when it's occupied, then only one person gets to use it.

[ASTROMODAT]

Thanks for the clarification, Dave! So, if 2 cops want to talk car-to-car, they will simply use a designated TG on the Phase 2 system. Got it. But, what happens if 2 cops are in a tactical situation, like inside of a building where there is poor (or no) repeater coverage, and they need to talk to each other? Or, a SWAT sort of environment inside of a warehouse, etc. Would they need to fall back to Phase 1 with FDMA for true direct simplex mode?

[tvsjr]

Thanks for the clarification, Dave! So, if 2 cops want to talk car-to-car, they will simply use a designated TG on the Phase 2 system. Got it. But, what happens if 2 cops are in a tactical situation, like inside of a building where there is poor (or no) repeater coverage, and they need to talk to each other? Or, a SWAT sort of environment inside of a warehouse, etc. Would they need to fall back to Phase 1 with FDMA for true direct simplex mode?

And here I thought I had already answered that question... not once, but twice.

[d119]

Thanks for the clarification, Dave! So, if 2 cops want to talk car-to-car, they will simply use a designated TG on the Phase 2 system. Got it. But, what happens if 2 cops are in a tactical situation, like inside of a building where there is poor (or no) repeater coverage, and they need to talk to each other? Or, a SWAT sort of environment inside of a warehouse, etc. Would they need to fall back to Phase 1 with FDMA for true direct simplex mode?

And here I thought I had already answered that question... not once, but twice.

You did. Don't forget who you're dealing with - in this case, the question must be answered multiple times in multiple iterations for the data to make it to the destination.

ASTROMODAT: Yes, that's correct. Phase 2 TDMA applies to situations using INFRASTRUCTURE only (read: trunking, with possibilities in the future for REPEATED (read: repeater) conventional). Simplex operation REQUIRES a fallback to Phase I FDMA due to the fact there is no infrastructure to handle the timing required to divide the channel into two slots.

[xmo]

Suggested reading on the subject:

TSB-102.BBAA
Project 25
Two-Slot TDMA Overview


Here is a relevant quote:

"2. Scope of the P25 Two-Slot TDMA Standard

This document describes the P25 Phase 2 Two-Slot TDMA Standard in a general way.
The P25 Phase 2 Two-Slot TDMA Standard is a suite of P25 standards documents. The
high level technical scope of the P25 Two-Slot TDMA Standard includes the following
major items:

• Interface Description of the Um2 interface designated for the P25 Phase 2 TDMA
Two-Slot CAI.
• TDMA CAI for Two-Slots operating at a gross bit rate of 12 kb/s with
Harmonized-Continuous Phase Modulation (H-CPM) for the inbound link and
Harmonized-Differential Quadratrure Phase Shift Keying (H-DQPSK)
modulation for the outbound link. The P25 Two-Slot TDMA CAI is divided into
a Physical layer (PHY) and a Media Access Control layer (MAC) to facilitate
TDMA operation.
• Dual Rate Vocoder at a gross bit rate of 7.2 kb/s for the full rate for use with
Phase 1 and 3.6 kb/s for the half rate for use with Phase 2.
• Trunking control signals using MAC Protocol Data Units (PDUs).
...

• Use of FDMA for Phase 1 trunking interoperability and direct mode (talkaround).
..."

[batdude]

not to digress.... but...

what modulation did the old next "directTalk" use...?

nextel = TDMA.... so just wondering.


doug

[Jim1348]

What a coincidence that you mention Direct Talk. My family and I are planning on a cruise this year. I was just searching recently about using my old Nextels on Direct Talk on the ship and I found our old post!

http://batboard.batlabs.com/viewtopic.p ... ip#p322013

[CTAMontrose]

not to digress.... but...

what modulation did the old next "directTalk" use...?

nextel = TDMA.... so just wondering.


doug

They used the same FHSS protocol as the 900MHz DTR radios, they just didnt share any hop patterns.

[Pj]

Whatever happened to just using a standard, plain old, analog or straight digital channel in the radio?

Thought I know radio's and the sales guys are becoming more complicated, but since when did we really need to overcomplicate switching to "Channel 2"?

Just sayin

[ASTROMODAT]

I thought Nextel Direct went thru the cell site even in direct mode, so it still was still using TDMA infrastructure.

[resqguy911]

I thought Nextel Direct went thru the cell site even in direct mode, so it still was still using TDMA infrastructure.

"direct connect" and "direct talk" are not the same.

one does not simply PTT into Mordor when the cell sites are offline.