Control, Alternate Questions on P25

[N9JIG]

On a true P25 system, such as the Michigan, Illinois or Colorado statewide systems, does each site have a limit of 3 available Control Channels, or could there be 4 like 3600 baud Moto systems? So far I have seen Sites with either 1 or 2 "Alternates" in addition to the current "Control" channel, but never with 3 Alternates.

Also, are the Alternates merely control channels that just are not in use at that time for Control? That is, if the Control Channel switches to an Alternate, would the original control channel then be labeled as an Alternate and the new Control Channel be labeled as such or would it remain as an Alternate, even though it is currently the active Control Channel?

[grshirley]

The P25 standards define the following 4 concepts when it comes to control channels:

1) primary control channel: periodically transmiting information about the site as well as information about secondary and adjacent control channels (as well as the required channel grants etc.)

2) secondary control channels: transmitting at the same site as the control channel. Can tranmsit the same information as the primary control channel. Used to allow extra handling/higher capacity

3) backup control channel: not transmitting. normally a traffic channel that can be turned into a control channel if necessary.

4) Alternate control channel - control channel at a nearby site.

So basically I think you are asking about secondary control channels. The standards allow any number of these to be defined as far as I'm aware. The Secondary Control Channel Broadcast message (sent on the primary control channel) can contain the definition of two of these at a time but there is no reason mroe than one of these messages couldn't be sent.

That's the standards, what different manufacturers allow is another question entirely.

A site is basically:

- 1 primary control channel
- 0 to X secondary control channels
- 0 to Y traffic channels
- 0 to Y backup control channels (which are the traffic channels until needed as an active control channel.

Hope that answers your question.

[wlmr]

Originally Posted by N9JIG

On a true P25 system, such as the Michigan, Illinois or Colorado statewide systems, does each site have a limit of 3 available Control Channels, or could there be 4 like 3600 baud Moto systems? So far I have seen Sites with either 1 or 2 "Alternates" in addition to the current "Control" channel, but never with 3 Alternates.

Yes, there can be a maximum of 4 control channels.

Originally Posted by N9JIG

Also, are the Alternates merely control channels that just are not in use at that time for Control? That is, if the Control Channel switches to an Alternate, would the original control channel then be labeled as an Alternate and the new Control Channel be labeled as such or would it remain as an Alternate, even though it is currently the active Control Channel?

Like a lot of questions about trunking, the answer is - it depends.
Motorola seems to like to ship the systems with the four control channels set with priorities from 1 through 4. A priority 1 control channel is the main one, if it fails, the #2 one takes over. If the problem was temporary, it goes back to the #1 channel. I'm guessing it would take something rather interesting to cause the system to have to use the #3 or even worse #4 control channels.
The setting can be changed so more than one channel are at priority 1. in that situation, if the active priority 1 channel experiences even a momentary "burp" the other priority 1 channel takes over and things stay that way until there's a reason it has to let go. Only then would the original control channel get the job back (assuming the system thinks it's ok to let it take over again).

[N9JIG]

The reason I ask is that I and others have traveled around Illinois and recorded data from most of the various StarCom21 sites, and have heard many sites from Michigan's and Colorado's P25 systems and I have never seen more that 2 Alternates reported along with a Control Channel. On some sites only a single Alternate is listed.

Is it up to the System Engineers to determine if 1, 2 or 3 Alternates are set up?

Would a program like Pro96Com always report all Alternates?

[wayne_h]

Originally Posted by N9JIG

Is it up to the System Engineers to determine if 1, 2 or 3 Alternates are set up?

Would a program like Pro96Com always report all Alternates?

Yes and yes. The system administrator's interface has the ability to select which channels at the site are to be alternate control channels. And standard hardware operating procedure is for a control channel to announce the alternates.