Questions about a Moto P25 trunked system

[rja1]

I monitor a moto P25 trunked system. I'm hearing some voice comms on alternate CC's. Is this common on a P25 trunked system? Do they ever use CC's (not alt CC's) for voice comms? When this occurs, the screen on my 996XT scanner shows "LNK". Any info would be very helpful.

Thanks,
Bob

 

[Outerdog]

Alternate control channels are routinely used for voice and data transmissions on most (probably all) P25 systems, yes.

 

[GTR8000]

This applies specifically to Motorola ASTRO 25 systems; your mileage may vary with respect to other manufacturer's P25 systems.

The alternate (backup) control channels are quite often used as traffic channels (voice and/or data). They are also available for use as a control channel should the active (primary) control channel fail.

Because they are allowed to act as the control channel, they are automatically given higher priority than the normal traffic channels. In other words, depending on the amount of resources at a given site (i.e. channels), the system will use the alternate control channels last for voice/data. Smaller sites that only have 3-4 channels will make constant use of the alternate control channels for traffic.

The active control channel cannot serve any other purpose other than to act as the site's control channel. Because it is an FDMA resource, the full bandwidth of the frequency is taken up by the carrier. Contrast this with DMR Con+ and TIII systems, which have a TDMA control channel, and can carry voice/data on Slot 2.

There are some types of single-frequency sites that can have something called "Voice/Data over Control" or something like that, which allows the single frequency to act as a control channel until a radio keys up, at which point it acts as a voice channel, then reverts to control when the transmission is completed. While the voice transmission is taking place, there is no control channel at the site.

There's a bit more to it all, technically speaking, but that's the overall gist of it.

 

[rja1]

It is a Motorola system. This answers a lot of questions I've had for a while.

Hey, GTR8000.......You show your location as BEE00.......Funny, that's the WACN of the trunked system I listen to.

Bob

 

[KevinC]

Hey, GTR8000.......You show your location as BEE00.......Funny, that's the WACN of the trunked system I listen to.

Bob

That's weird.

 

[rescue161]

On a Harris system any of the channels at a site can be the control channel, not just the primary and alternate.

 

[EWC_BDN]

If a system is sharing a channel with someone close by, they wouldn't want that channel to ever be a control. it could cause some more annoying disruptions vs if it was just a traffic channel. Like others have said. the site has one control channel at a time, and it doesn't switch unless there is a failure or the system has other rules for electing a control.
Some systems (like Nexedge)change control channel on a schedule. Every day the channel rolls over. This was done to even out the wear on the transmitters.

 

[rescue161]

Our old Motorola Type 2 UHF system rolled the control channel every day at right around midnight. It had 4 in rotation. Our new system stays on the same channel for control unless there is interference on that channel or there is a failure of hardware.

 

[rja1]

I also noticed that when an alt CC is used for voice traffic, I see "LNK" (Link) displayed on my 996XT. Also noticed the use of alt CC's appears to be random. I monitor this system with at least two scanners going at the same time, as I can receive several sites. Seems that even if the dedicated voice channel is not busy, they sometimes use the alt CC for voice.

Bob

 

[WA0CBW]

Alternate control channels have the lowest priority and would be assigned for voice only if all other voice channels are busy (Motorola systems).
BB

 

[GTR8000]

I also noticed that when an alt CC is used for voice traffic, I see "LNK" (Link) displayed on my 996XT. Also noticed the use of alt CC's appears to be random. I monitor this system with at least two scanners going at the same time, as I can receive several sites. Seems that even if the dedicated voice channel is not busy, they sometimes use the alt CC for voice.

Bob

You'll see LNK on any P25 frequency that is carrying voice traffic; this is not exclusive to just those channels designated as alt CC that happen to be carrying voice traffic. DAT for the active control channel, LNK when a channel is carrying voice traffic.

I'm going to assume that you're talking about the NYSEG system, in which case you have to keep in mind that nearly all of the sites have only 3 channels total; one active (primary) control, one alternate (backup) control, and maybe also one voice channel. With such limited RF resources per site, of course the alt CC is going to be treated as a normal voice channel. The "randomness" of the channel assignments is simply the system balancing the load across both voice capable channels, and is perfectly normal.

I'm curious if you ever discovered a third channel at the Putnam Valley site. You started a thread about it a few months ago, but never followed up.

 

[tunnelmot]

I'm on the outside looking in, but wouldn't you want your control channel to rotate between transmitters? IE, the frequency doesn't change, but the system tells the next transmitter in line to start TXing the CC on the same freq? Are the transmitters that rugged for 100% duty 24/7 CC duty?

Example on a 3 freq site:
Week 1
TX(repeater) 1=CC 866.xxx
TX(repeater) 2=VC
TX(repeater) 3=VC

Week 2
TX(repeater) 1=VC
TX(repeater) 2=CC 866.xxx
TX(repeater) 3=VC

RF trunking is extremely interesting to me for some reason.

 

[GTR8000]

I'm on the outside looking in, but wouldn't you want your control channel to rotate between transmitters? IE, the frequency doesn't change, but the system tells the next transmitter in line to start TXing the CC on the same freq? Are the transmitters that rugged for 100% duty 24/7 CC duty?

Modern transmitters like the Motorola GTR 8000 won't break a sweat running 24/7 @ 100% duty cycle. There are countless P25 control channel repeaters that have been on the air for years without a failure. If the control channel does happen to "roll" (i.e. switch to another repeater/frequency), it's generally only for a short span of time before it rolls back to the most preferred control channel.

As far as the idea of having the same control channel frequency rotate between repeaters, that's neither necessary nor ideal. It's not necessary because trunked subscriber radios have no issue with quickly switching frequencies to keep up with the site's control channel should it happen to roll to a different frequency. It's not ideal because factors other than transmitters failures cause control channels to roll, e.g. interference detected on the control channel's input. Rolling to the same frequency on a different transmitter is not going to mitigate that interference, however rolling to a different frequency likely will.

There is also the factor of wasteful duplication of resources, which would occur if you had two or more repeaters programmed with the same frequency. It may sound like a good idea for redundancy of the control channel, but having multiple repeaters with different control channel capable frequencies is just as robust, probably moreso do to the aforementioned interference scenario.

To reiterate, what I'm explaining is applicable to Motorola ASTRO 25 systems, as we are in the Motorola forum with the OP asking about Motorola branded P25 systems. What other manufacturers do with their systems, i.e. Harris designating all channels as control capable, is beyond the scope of this thread.

 

[WA0CBW]

Adding to the above the only time all sites would have the same frequencies is if the system is simulcast. In which ALL of the sites are identical and transmitting the same information at the same time.
BB

 

[rja1]

[QUOTE="GTR8000, post: 3088158, member: 737341"}
I'm going to assume that you're talking about the NYSEG system, in which case you have to keep in mind that nearly all of the sites have only 3 channels total; one active (primary) control, one alternate (backup) control, and maybe also one voice channel. With such limited RF resources per site, of course the alt CC is going to be treated as a normal voice channel. The "randomness" of the channel assignments is simply the system balancing the load across both voice capable channels, and is perfectly normal.

There are 38 sites, 35 have one primary CC, one alt CC and one voice channel. Two sites have one primary cc, one alt cc and two voice channels. The other site has one primary CC, two alt CC's and one voice channel.

I'm curious if you ever discovered a third channel at the Putnam Valley site. You started a thread about it a few months ago, but never followed up.[/QUOTE]

I'm right across the TSP from Putnam Valley. I've never found a dedicated voice channel for that site. Very little traffic from that site. They use the alt cc for voice traffic. Compared to what it used to be, the volume of traffic on the PV site is almost non existant.

NYSEG covers a rather large area in northern Westchester.....Yorktown, Somers, Katonah, Bedford, Lewisboro, South Salem, Goldens Bridge and Cross River. Most of the traffic for Westchester used to go through the PV site, but not for a while. Now, it looks like the Brewster site handles those areas. Why they don't have a site in the middle of the Westchester service area is a mystery to me. Much of the Westchester service area is 20-25 miles from the Brewster site.

Bob

 

[GTR8000]

I'm right across the TSP from Putnam Valley. I've never found a dedicated voice channel for that site. Very little traffic from that site. They use the alt cc for voice traffic. Compared to what it used to be, the volume of traffic on the PV site is almost non existant.

Try not to get too caught up in the terminology of "dedicated voice channel" or "alternate control channel". Any channel at a site has the ability to act as a traffic (voice/data) channel, with very few exceptions. The fact that a channel is provisioned in the system as a potential control channel really doesn't mean all that much when you've only got 2-3 channels per site. Believe me, it's a lot more common than you might think for voice/data traffic to be carried on a frequency that is also designated as a control channel. As long as the frequency is not acting as the active control channel, it's free to do whatever else the site/system requires of it.