IMBE Simplex vs Repeater

[Renamon]

Listening to LAPD in analog & digital concurrently (2 radios at the same time, heh too much free time), sometimes you will hear something that sounds like a preamble, and other times you will not - all on the same channel, and it is all voice traffic on the IMBE decoded side. I cannot recreate this sound on simplex, even if I turn the preamble up to 255. So, is there a difference in how IMBE is handled in a SIMPLEX operation over a CONVENTIONAL REPEATER operation?

[MattSR]

Can you explain what the preamble sounds like?

is it a short high pitched tone?

[Wowbagger]

On a repeater's voice channel, the sequence goes something like this:
System allocates voice channel.
Voice channel starts sending TERM WITH LC frames
System assigns radios to channel.
------
TX radio comes on voice channel, sends VOICE HEADER frame
Voice channel sends VOICE HEADER frame
TX radio starts sending LDU1/LDU2 frames (voice frames)
Voice channel sends LDU1/LDU2 frames
TX unkeys, sends TERM frames
------
Voice channel starts sending TERM WITH LC frames
After hang time, voice channel shuts down and is deallocated.

The bit between the lines happens on a simplex channel the bits outside the lines doesn't. It may be what you are hearing is the TERM WITH LC frames.

[Renamon]

That make sense, thank you.

[MattSR]

It may be what you are hearing is the TERM WITH LC frames.

At the start of a transmission on a conventional, non trunked channel...? That doesn't sound right... For trunking maybe but for conventional, naaah....

[Wowbagger]

It may be what you are hearing is the TERM WITH LC frames.

At the start of a transmission on a conventional, non trunked channel...? That doesn't sound right... For trunking maybe but for conventional, naaah....

He did say LAPD - IIRC they are running full APCO-25 trunking.

[akardam]

He did say LAPD - IIRC they are running full APCO-25 trunking.

LAPD has been on "Astro" conventional for many years, first VSELP, and now P25 IMBE, but not trunking.

[Wowbagger]

He did say LAPD - IIRC they are running full APCO-25 trunking.

LAPD has been on "Astro" conventional for many years, first VSELP, and now P25 IMBE, but not trunking.

So you are saying they don't have any control channels - they are running just a bit better than amateur radio repeaters?

[Renamon]

So you are saying they don't have any control channels - they are running just a bit better than amateur radio repeaters?

Yea, pretty much. They are not on IIRC - they are just running a mess of conventional P25 repeaters. They must have as many individual repeaters as IIRC has, lol.

[escomm]

He did say LAPD - IIRC they are running full APCO-25 trunking.

LAPD has been on "Astro" conventional for many years, first VSELP, and now P25 IMBE, but not trunking.

So you are saying they don't have any control channels - they are running just a bit better than amateur radio repeaters?

No, amateur repeaters get tuned once in awhile.

[motorola_otaku]

..sometimes you will hear something that sounds like a preamble, and other times you will not - all on the same channel, and it is all voice traffic on the IMBE decoded side.

Is what you're hearing a short little bit of digital data (brrrrrrp) followed by decoded voice? I've noticed that before but not on actual system radios - only on scanners. I chalked it up to "lol crummy scanner design" and didn't give it much thought after that.

[Renamon]

I am using an Astro Saber to moniter, and just an analog radio to hear the digital data itself, and i will hear sometimes something that sound like a preamble; it does not sound the same as the digital data for voice.

I will try to get a recording of it.

[MattSR]

....sound like a preamble; it does not sound the same as the digital data for voice..

I'm very curious. I know the P25 protocol very well, and what you are hearing could be one of a few things. If you can post a recording i'm sure we can figure it out

[Renamon]

Ok, here is the file...

http://simpleskye.com/p25.wav

There are 2 separate transmissions - both have the same "preamble", but both have different "post-ambles". I shortened the majority of the transmissions, so you didn't have to hear 30 seconds of p25 voice data.

-Jake

[Bill_G]

I hear that on a regular scanner when monitoring a P25 system. I always thought it was a form of EOT.

[MattSR]

So is your question regarding the tail at the end, or the preamble (which is the same)

..........

[Renamon]

Originally, just the preamble. Now I am curious about the end as well.

[MattSR]

The long buzzy tail is a string of TDU without link control frames. These are short 15ms frames that tell the mobile radios that the voice traffic is over - Yes you only hear them on repeaters, and they're configured on the Quantar under the "Drop out delay" field. Apart from telling the mobiles the transmission is over, they also keep all radios in sync so when one wants to key up again, theres no delay.

As for the premable, well, that always sounds the same on all of the transmissions I've heard (including the ones given above) so i'm not sure what you're referring to here..?

Cheers,
Matt

[Bill_G]

So, the runt at the end of the data is an EOT.

[MattSR]

Yep.

The preamble is just the HDU packet. This is a header that contains the NAC, and some encryption sync information. This is the short "brrt" heard at the start of the overs.

Cheers,
Matt