Getting SU uplink frequencies from APCO P25 control channels

[panoboy]

Is it possible to obtain the SU uplink frequencies from control channel data in an APCO P25 system?

When looking at the control channel data I do not see anything that resembles a band plan.

Occasionally I see messages like this which mention an offset:

20230917 223803,PASSED,NAC:501 TSBK3 IDEN_UPDATE_TDMA ID:2 OFFSET:-45000000 SPACING:12500 BASE:832012500 TDMA BW:12500 TIMESLOT S:2 VOCODER:HALF_RATE

20230917 212807,PASSED,NAC:501 TSBK2 IDEN_UPDATE ID:1 OFFSET:30000000 SPACING:6250 BASE:742006250 FDMA BW:12500

There are only a few of these messages, not enough to cover the total amount of activity I am seeing.

Are the repeater input frequencies anywhere in the control channel data or are they only in the band plan stored in the SU's configuration?

Thanks.

 

[boatbod]

Is it possible to obtain the SU uplink frequencies from control channel data in an APCO P25 system?

When looking at the control channel data I do not see anything that resembles a band plan.

Occasionally I see messages like this which mention an offset:




There are only a few of these messages, not enough to cover the total amount of activity I am seeing.

Are the repeater input frequencies anywhere in the control channel data or are they only in the band plan stored in the SU's configuration?

Thanks.

The NET_STS_BCST includes either an abbreviated Channel ID or extended format Channel(T) & Channel(R) depending on the band. Other messages such as RFSS_STS_BCST similarly use the same parameters to convey expected uplink & downlink freqs. That said, the subscriber unit does have to have some basic band plan information programmed into it, otherwise it would never be able to find the RFSS in the first place.

 

[wgbecks]

Is it possible to obtain the SU uplink frequencies from control channel data in an APCO P25 system?
When looking at the control channel data I do not see anything that resembles a band plan.
Occasionally I see messages like this which mention an offset:

Actually, the information you posted (bolded) is the Band Plan information. The offset values instruct the SU how to compute the uplink
frequency that is either based upon the current control channel frequency or that of the indicated service channel frequency when following trunk traffic.

20230917 223803,PASSED,NAC:501 TSBK3 IDEN_UPDATE_TDMA ID:2 OFFSET:-45000000 SPACING:12500 BASE:832012500 TDMA BW:12500 TIMESLOT S:2 VOCODER:HALF_RATE

20230917 212807,PASSED,NAC:501 TSBK2 IDEN_UPDATE ID:1 OFFSET:30000000 SPACING:6250 BASE:742006250 FDMA BW:12500

The standard offset for 800 MHz systems is -45.000 MHz while on 700 MHz it is +30.000 MHz. You can then calculate the uplink channels
using posted site frequencies available from Radio Reference per the example below shown for Fayette County in Ohio having channel assignments in both the 700 and 800 bands.

IE: 769.95625 would have a corresponding uplink frequency of 799.95625 and 853.4000 would have an uplink of 808.4000.

I am not sure what your intention is in knowing the uplink frequencies as reception of the SU's will be largely dependent upon your
proximity to the unit. SU's transmitting on Phase-1 talkgroups will employ C4FM modulation while those transmitting on Phase-2 will
employ H-CPM that cannot be decoded by a digital scanner or any known SDR software applications.

 

[panoboy]

Actually, the information you posted (bolded) is the Band Plan information. The offset values instruct the SU how to compute the uplink
frequency that is either based upon the current control channel frequency or that of the indicated service channel frequency when following trunk traffic.

20230917 223803,PASSED,NAC:501 TSBK3 IDEN_UPDATE_TDMA ID:2 OFFSET:-45000000 SPACING:12500 BASE:832012500 TDMA BW:12500 TIMESLOT S:2 VOCODER:HALF_RATE

20230917 212807,PASSED,NAC:501 TSBK2 IDEN_UPDATE ID:1 OFFSET:30000000 SPACING:6250 BASE:742006250 FDMA BW:12500

The standard offset for 800 MHz systems is -45.000 MHz while on 700 MHz it is +30.000 MHz. You can then calculate the uplink channels
using posted site frequencies available from Radio Reference per the example below shown for Fayette County in Ohio having channel assignments in both the 700 and 800 bands.

IE: 769.95625 would have a corresponding uplink frequency of 799.95625 and 853.4000 would have an uplink of 808.4000.
View attachment 148646
I am not sure what your intention is in knowing the uplink frequencies as reception of the SU's will be largely dependent upon your
proximity to the unit. SU's transmitting on Phase-1 talkgroups will employ C4FM modulation while those transmitting on Phase-2 will
employ H-CPM that cannot be decoded by a digital scanner or any known SDR software applications.

Thank you very much for spoon feeding this to me.

Given the total amount of system traffic I am receiving, I assumed there would be more than the handful of unique OFFSET/BASE pairs I am seeing in the control channel data.

My intention in knowing the uplink frequencies is detecting proximate activity like Uniden close call. Demodulating the uplink transmissions would be nice but is not required for my application.

I really appreciate your detailed reply, this was very helpful. Thank you.