Technical P25 questions

[centromere]

Hi. I have a few questions about P25:

1. Is the control channel *always* FDMA, regardless of whether the system is phase I or phase II? If so, in phase II systems, is TDMA used only for voice data on specific talk groups?

2. If I lookup my town on RadioReference and see a control channel of 123.4567 MHz, is that the frequency on which trunking messages are broadcast from the fixed site? Do mobile radios ever transmit *control data* on this frequency? If not, on what frequency do they transmit? Does it depend on whether it's phase I or II?

 

[nd5y]

Hi. I have a few questions about P25:

1. Is the control channel *always* FDMA, regardless of whether the system is phase I or phase II?

Yes. They are working on a way to have control channel data on one time slot and voice/data on another time slot, like DMR, but that will be sometime in the future.

If so, in phase II systems, is TDMA used only for voice data on specific talk groups?

The control channel data will always be FDMA. Voice channels can be either. OTAR and end user data sent on voice channnels can be either but should not be confused with control channel data.

2. If I lookup my town on RadioReference and see a control channel of 123.4567 MHz, is that the frequency on which trunking messages are broadcast from the fixed site?

Yes.

Do mobile radios ever transmit *control data* on this frequency?

No.

If not, on what frequency do they transmit? Does it depend on whether it's phase I or II?

They transmit on an input frequency, just like on voice channels. Phase I or II makes no difference, the input is always a different RF channel. Input frequencies are not listed in the RRDB for trunked systems. Some bands have standard offsets in the US and Canada.

 

[centromere]

They transmit on an input frequency, just like on voice channels. Phase I or II makes no difference, the input is always a different RF channel. Input frequencies are not listed in the RRDB for trunked systems. Some bands have standard offsets in the US and Canada.

1. Is the input frequency pre-programmed, or is it "discovered" when the radio is powered on?

2. If two users push their PTT buttons at exactly the same time, will they interfere with one another? If not, why?

 

[nd5y]

1. Is the input frequency pre-programmed, or is it "discovered" when the radio is powered on?

I assume all the control channel transmit and receive frequencies are preprogrammed but I have never done it so I don't know for sure.

2. If two users push their PTT buttons at exactly the same time, will they interfere with one another? If not, why?

I don't think it's possible for that to happen on a trunked system. They only transmit for a tiny fraction of a second and I think they have some type of carrier-sense multiple access with collision avoidance to keep more than one radio at a time from transmitting on the inbound control channel but I don't know that for a fact.

 

[nd5y]

You might find better answers in
P25 Training Guide at https://www.codanradio.com/library/guides/
and Understanding Advanced P25 Control Channel Functions - Aeroflex.com

 

[mikewazowski]

Thread moved as I have no idea what this has to do with Amateur Radio.

 

[centromere]

Thread moved as I have no idea what this has to do with Amateur Radio.

"Discuss use of digital voice technologies on the amateur radio bands. This is to include technologies such as VoIP, P25, DMR/TRBO, NXDN, D-STAR, etc."

 

[mikewazowski]

"Discuss use of digital voice technologies on the amateur radio bands. This is to include technologies such as VoIP, P25, DMR/TRBO, NXDN, D-STAR, etc."

and that's exactly why your thread was moved. Nothing amateur radio related about any of your questions. End of discussion.

 

[Thunderknight]

1. Is the input frequency pre-programmed, or is it "discovered" when the radio is powered on?

Yes, and no.
The radio will have a defined set of system channels preprogrammed. That is where is looks first. It is possible to have the subscriber unit scan for other (not programmed) frequencies of the same system. Whether that is enabled is a system by system choice.
Not sure about Motorola, but I believe Harris subscribers will first check the last site they were on to see if it's still there. It speeds up the registration time.
As to the input frequency, that detail comes from a bandplan that is broadcast on the control channel. That table tells subscribers the input split (offset), spacing, etc. the actual 123.4567 input frequency comes from adding or subtracting the offset from the control or working channel' frequency. E.g. 143.4567 control - 20MHz offset = 123.4567 input. Not sure if Motorola only uses what is programmed into the radio, but I think Harris radios will learn from that broadcast table and update themselves.

The links posted by nd5y are good resources for your questions.

 

[Thunderknight]

"Discuss use of digital voice technologies on the amateur radio bands. This is to include technologies such as VoIP, P25, DMR/TRBO, NXDN, D-STAR, etc."

The issue is, I think, p25 trunking is not used in ham radio (although that would be sooo cool!). P25 is used in amateur radio, but in conventional only mode which does not have control channels or TDMA. So your question is not a amateur radio one.