question about LTR Passport "trunkoff" frequencies

[mr_hankey]

this is directed to Eric C.

i have noticed that out of the 5 or so Passport sites i've uniquely id'd in my area, they all seem to share a "trunkoff" frequency - one that seems to burst faster than others and "repeats" traffic off other sites in a simulcast way...

for example, site 1034 has a very active trunkoff frequency of 463.6625, but it also simulcasts traffic to site 1037 on 463.7375.

i'm noticing similar behavior on site id 1036, which is frequency 462.1125...

so now we have sites 1034, 1036, and 1037 as reported by LTRTrunk, each with it's own trunkoff (or whatever you want to call it) frequency...this is making decoding of Passport systems in my area very difficult...

i have yet to determine more than 2 frequencies on any Passport site listed above - mostly due to the fact that NOT ALL Passport site frequencies "burst"! some are just "dead air".

my question is: can a Passport site have more than one trunkoff frequency, or does each site have a maximum of ONE trunkoff frequency?

i have spent all day decoding various Passport frequencies and to put things in perspective, have only revealed 2 frequencies for site 1034 as opposed to 10 frequencies for a Regular LTR previously undiscovered in my area...frustrating.

 

[EricCottrell]

Hello,

Passport seems to have a different system organization compared to Regular LTR. Each unit or group has a home site. This does not show up in LTRTrunk yet. LTRDump is still the tool to use to figure the system out. The following is referenced to LTRDump.

The group information is prefixed by the home site. If you had a group of 52-62125 then the home site is 52. In LTRTrunk this will show up as just 62125. In your case the site with sysid of 1034 in LTRTrunk would be the home site. The format of some of the decoded information will have S:52 G:62125 instead of 52-62125.

Each passport site seems to have at least 3 frequencies.
1. A "local" home channel for units that have this site as the home site.
2. A "roaming" home channel for units that have another site as the home site.
3. A "registering" channel that is used by roaming units to register with the site.

The local home channel is programmed into the radio. The roaming home channel is assigned when the unit registers with a site and seems to be the same for all units that register at that site. The registering channel is the frequency broadcast by neighboring sites in their "Neighbor" information.

I found in a 5 frequency site that the free information when a unit is on either the local home or roaming home channel will have the other 3 frequencies. It works like a regular ltr system with two home channels. The registering channel does not show up and appears dedicated to the task.

The "trunkoff" frequencies you are observing are the roaming home channels of each site. The sites are networked together via some sort of landline or microwave connection. You are hearing the same groups on multiple sites because radios are registered with each of the sites.

This system is interesting because the sites are close together. If you want to get a particular group then the best site to monitor is the home site otherwise you are depending on units of that group being registered with the site you are monitoring.

My suggested method to figure out sites.

Use LTRDump to look at the neighbor frames. If I was listening to 451.1375 in central CT I would pick a frame like this:
1c04a0a8a8c57c2 Passport Neighbor -> LCN 1111 c:1793 g:20-5397
This frame says that 451.1375 is site 20. 463.8875 = (450+(1111*0.0125)) is the registering frequency of a adjacent site.

Listening some more I get a CW ID.
016ca7fff80e423 Passport LCN 228 Free LCN 91 Active -> Group: 20-65535
016ca7fff84216a Passport LCN 1057 Free LCN 91 Active -> Group: 20-65535
016ca7fff80bf7e Passport LCN 191 Free LCN 91 Active -> Group: 20-65535
016ca7fff80c00e Passport LCN 192 Free LCN 91 Active -> Group: 20-65535
This also confirms it is site 20. 65535 is the CW ID group. The frequency is 451.1375 = (450+(91*0.0125)). I can also calculate the other 4 frequencies as 452.85, 463.2125, 452.3875, and 452.4000.

This channel is likely a roaming home channel because the site id of the groups are different than the site.
016d4056f30e4ad Passport LCN 228 Free LCN 91 Active -> MID: 40-2782
1c00a0a8a8c4677 Passport Neighbor -> LCN 1094 c:1792 g:20-5397
016d4792f842102 Passport LCN 1057 Free LCN 91 Active -> Group: 40-62047
016d4792f80bf16 Passport LCN 191 Free LCN 91 Active -> Group: 40-62047
016d4792f80c066 Passport LCN 192 Free LCN 91 Active -> Group: 40-62047
Unit 2782 with a home site of 40 is on talkgroup 62047 with a home site of 40.

If you are monitoring a registering channel then you need to look for Unknown Type 3 messages which are now known.
0ee4503f19ffae1 Passport (Unknown Type 3) LCN 2042 Free LCN 953 -> Group: 10-2019
Unit 2019 with a home site of 10 is being assigned a home frequency of 461.9125 = (450+(953*0.0125)). If you see 2042 for a LCN it is not a frequency but seems to indicate no frequency available. 2042 tends to show up in the free lcn field for registering frequencies.

Listening on 461.9125
0ee45794904931d Passport LCN 1171 Free LCN 953 Active -> Group: 10-62098
0ee45794904a702 Passport LCN 1191 Free LCN 953 Active -> Group: 10-62098
0ee45794900617d Passport LCN 97 Free LCN 953 Active -> Group: 10-62098
I now know that 464.6375, 464.8875, and 451.2125 are used.

I am working on added status messages to LTRTrunk so it will note registering and assignment of home channels.

I also need to address the home site display in LTRTrunk. This can open a can of worms because it increases the amount of data that needs to be stored. 10-62098 would be stored in 000agrp.txt and if you are not listening to site 000A you need to load that file and save any changes. So multiple group files may have to been handled.

73 Eric

 

[mr_hankey]

ok, this has helped tremendously.

now another question comes up:

monitoring 463.6625 with LTRTrunk, the LCN equates to 1093 Decimal, which becomes 450 + (1093 * .0125) = 463.6625

would it be wrong to convert the hex values generated by LTRTrunk in a sys.txt file for a Passport system to their frequency equivalents using the same formula [450 + (LCN * .0125)] ?

 

[EricCottrell]

Hello,

Convert the hex value to a decimal value and the formula will work.

A better method is to create a map file with the following line in it:
450.0125, 0.0125,1
You need to create one for each sysid. 1035map.txt, 1036map.txt, etc.
The advantage is any new frequency code will be converted to a real frequency.

Can you email me a 15 minute or so log file of an active frequency? One of the things I suspect is the network id and band are encoded into the neighbor information. I think that up to 8 networks can be joined together. It would be interesting to find such a system. It would be great if a Passport system not on the 450-470 band is found as well.
1c04a0a8a8c57c2 Passport Neighbor -> LCN 1111 c:1793 g:20-5397
5397 is 1515 in hex. This is a common number on the sites in CT. I suspect
it says both the site and the adjacent site are Network 1 on Band 5 (450 to 470).
If this is the case then I can modify LTRTrunk to use the network id as the sysid and then have site files (0001c0a.txt) like mtrunk. Also the group file would be expanded to include the site number and would be 0001grp.txt.

As you can see things are still in flux because I do not have a complete picture of how Passport works. As I learn stuff it will be put into the programs.

73 Eric

 

[mr_hankey]

Eric,

here is the ltrdump log that i ran last night on 463.6625