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control channel question

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Back in the days of tcxo repeaters the CC was usually the 4 lowest freqs in the system. Has that changed now with P25 repeaters? I've noticed that my xxxx (a rx only device that forum rules prevent me from mentioning here...) always shows the same freq for MECA's CC, and was wondering if the physical repeater changed but always kept the same freq.
 

N1BHH

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It all depends on what type system you are listening to. One system like EDACS has a rotating control channel. Motorola Type I, which are being upgraded to Type II, have rotating control channel, not necessary the lowest. Motorola Type II's usually have one primary and one secondary control channel. There are many variables.
 

wlmr

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Back in the days of tcxo repeaters the CC was usually the 4 lowest freqs in the system. Has that changed now with P25 repeaters? I've noticed that my xxxx (a rx only device that forum rules prevent me from mentioning here...) always shows the same freq for MECA's CC, and was wondering if the physical repeater changed but always kept the same freq.
I see two questions here, both asking about P25 systems.

With P25 sites, there is no rule about which frequencies HAVE to be the control channel(s). System design may decide that specific frequencies at a site are better to avoid intermod, or for frequency reuse possibilities, etc.

Rotating CCs also isn't done anymore with P25 systems. The radios are much better at not needing a "rest" after a 24 hour stint at being the control channel. The site stays with one radio/control channel frequency unless/until something happens with that radio. That will cause the system to switch to an alternate radio/control channel frequency. Whether it stays put on the new one or switches back automatically if the problem clears is set in the system.

I'm betting that you are seeing the same transmitter day-in/day-out and will only see a different transmitter & frequency come into use if there's something going on with the primary control channel. Using the same frequency amongst multiple transmitters so the frequency would stay the same with a different transmitter wouldn't avoid one possible reason for switching the control channel to begin with. That reason is an interferring signal on the control channel's frequency.

With older systems, and with systems that mix analog voice talkgroups with P25 digital talkgroups using a 3600 baud control channel, I don't know for sure. I think the system close to here that fits that type system still rolls the control channels every night.
 
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DiGiTaLD

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with systems that mix analog voice talkgroups with P25 digital talkgroups using a 3600 baud control channel, I don't know for sure. I think the system close to here that fits that type system still rolls the control channels every night.
The statewide SAFE-T system in Indiana seems to be pretty consistent with it's control channels, at least the site I usually monitor. I can think of maybe a dozen times I've ever seen the control channel go over to the alternate, and at those times it was either being worked on or in site trunking mode for some reason.
 

WayneH

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Back in the days of tcxo repeaters the CC was usually the 4 lowest freqs in the system.
The four lowest freqs? That can't be right as the old common practice was the opposite as the lowest freq had to be used for license announcement.

Nowadays it only exists out of old habit by Motorola, since Moto sets the systems up initially before shipping them to the customer. The newer controllers aren't bound by numbering restrictions that the older controllers used to have (Channels 1-4 could only be used for control and 5~28 were the only options for announcing a callsign on).
 

N4DES

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Typicially the CC selection is now based on the frequency layouts in the combiner/antenna systems and have nothing to do with the "actual frequency number".

As an example, in the 28 channel system I manage, 1 CC is assigned to one of the combiners and to one antenna. There are 4 antenna/combiner systems total with 7 channels assigned to each antenna.
Ss if one antenna or combiner fails only 1 cc is affected along with those other associated channels.

And what Wayne posted is correct on the FCC ID's. It is the "lowest frequency" for each call sign. This can get complicated in the larger systems because the FCC's system only allowed 6 TX sites per call sign so more than 1 ID is sent out on the lowest frequencies in the system.
 
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There are 4 antenna/combiner systems total
Whose combiner / multicoupler do you use?
I've been using dbSpectra 450 gear and have had pretty good luck, but have seen some intermod problems when using more than 12 rx ports on one antenna.

Question 2: Does P25 have a version of analog failsoft?
 

N4DES

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Cellwave combiners and TX/RX multicouplers/TTA's are in use.

I assume it has some find of failsoft that defaults to P25 failsoft. The newer systems don't support analog.
 
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wlmr

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The statewide SAFE-T system in Indiana seems to be pretty consistent with it's control channels, at least the site I usually monitor. I can think of maybe a dozen times I've ever seen the control channel go over to the alternate, and at those times it was either being worked on or in site trunking mode for some reason.
Motorola likes to set up the control channel priorities "out-of-the_box" with a 1,2,3,4 priority scheme. If something happens to the main control channel, as soon as that channel is back up in service it will take the control channel back from the priority 2 channel. The priority 3 and 4 channels only get to be the control channel if every channel with a lower number is unavailable.

Bet Indiana still has theirs set that way from your description.

There is nothing stopping the customer from re-assigning the order to be 1,1,3,4 or any other variation. If two control channels share a level 1, if one is reset, taken down for servicing, or quits for whatever reason, the other level 1 will take over and not give the control channel back until/unless it is made to by some event.

KS4VT said:
And what Wayne posted is correct on the FCC ID's. It is the "lowest frequency" for each call sign. This can get complicated in the larger systems because the FCC's system only allowed 6 TX sites per call sign so more than 1 ID is sent out on the lowest frequencies in the system.
Even that isn't completely accurate, at least in the FCC rules for public safety trunking systems. It says lowest frequency UNLESS the call signs are broadcast by the system in a decodable form. I've found the paragraphs multiple times in the rules but never remember the precise location.
 

N4DES

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Right from the FCC website....

Sec. 90.647 Station identification.

(a) Conventional systems of communication shall be identified in
accordance with existing regulations governing such matters.
(b) Trunked systems of communication, except as noted in paragraph
(c) of this section, shall be identified through the use of an automatic
device which transmits the call sign of the base station facility at 30
minute intervals. Such station identification shall be made on the
lowest frequency in the base station trunk group assigned the licensee.
Should this frequency be in use at the time station identification is
required, such identification may be made at the termination of the
communication in progress on this frequency. Identification may be made
by voice or International Morse Code. When the call sign is transmitted
in International Morse Code, it must be at a rate of between 15 to 20
words per minute and by means of tone modulation of the transmitter, the
tone frequency being between 800 and 1000 hertz.
 

wlmr

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You were only one section away, look at (c) below. I had forgotten it only applies in the 800MHz band. While googling for this info, I also found petitions to the FCC to apply this rule in the 700MHz band.

Sec. 90.647 Station identification.

(a) Conventional systems of communication shall be identified in
accordance with existing regulations governing such matters.

(b) Trunked systems of communication, except as noted in paragraph
(c) of this section, shall be identified through the use of an automatic
device which transmits the call sign of the base station facility at 30
minute intervals. Such station identification shall be made on the
lowest frequency in the base station trunk group assigned the licensee.
Should this frequency be in use at the time station identification is
required, such identification may be made at the termination of the
communication in progress on this frequency. Identification may be made
by voice or International Morse Code. When the call sign is transmitted
in International Morse Code, it must be at a rate of between 15 to 20
words per minute and by means of tone modulation of the transmitter, the
tone frequency being between 800 and 1000 hertz.

(c) Stations operating in either the 806-824/851-869 MHz or 896-901/
935-940 MHz bands that are licensed on an exclusive basis, and normally
employ digital signals for the transmission of data, text, control
codes, or digitized voice may also be identified by digital transmission
of the call sign. A licensee that identifies its station in this manner
must provide the Commission, upon its request, information sufficient to
decode the digital transmission and ascertain the call sign transmitted.
 
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N4DES

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I doubt that you will find many agencies utilizing the last subsection as there are too many variables with all the different technologies out there and they aren't technicially savvy to show the commission how it ID's should they be asked.

CWID is simple and cheap.
 

wlmr

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I doubt that you will find many agencies utilizing the last subsection as there are too many variables with all the different technologies out there and they aren't technicially savvy to show the commission how it ID's should they be asked.

CWID is simple and cheap.
Agreed. However I am looking at a P25 trunked simulcast site right now using PRO96COM and the frequency that has been IDing via CWID isn't the lowest frequency, 2nd highest at the site actually.

The fact that the callsign can also be decoded with a $500 scanner hooked up to a laptop running free (PRO96COM) software listening to the control channel pretty much means at least one agency read the rules.
 

N4DES

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Don't look at the frequency allocation at the site. Look at the FCC license. If it isn't the lowest frequency as stated on the license for that particular call sign then it doesn't conform to Part 90.647.
 

wlmr

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Don't look at the frequency allocation at the site. Look at the FCC license. If it isn't the lowest frequency as stated on the license for that particular call sign then it doesn't conform to Part 90.647.
It's IDing the call sign nowhere near the lowest freq, either at the site on on the FCC license's list of frequencies for the call sign.

Are you saying you don't believe they can follow section (c) at all? As I read it, section (b) goes on at great length about using the lowest frequency, but only after saying right at the top "except as noted in paragraph (c)".

The system is an exclusively licensed 800MHz P25 system, completely digital (absolutely NO analog) as far as the control, signalling, and voice goes, and the call sign is broadcast on the control channel as part of the digital signal. I can't think of a system that wouldn't qualify more completely to use the paragraph (c) exception.

We're each interpreting this section differently. What would your opinion be of what type of system would qualify for the paragraph (c) exception to having to broadcast on the lowest frequency?
 

N4DES

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It's IDing the call sign nowhere near the lowest freq, either at the site on on the FCC license's list of frequencies for the call sign.

Are you saying you don't believe they can follow section (c) at all? As I read it, section (b) goes on at great length about using the lowest frequency, but only after saying right at the top "except as noted in paragraph (c)".

The system is an exclusively licensed 800MHz P25 system, completely digital (absolutely NO analog) as far as the control, signalling, and voice goes, and the call sign is broadcast on the control channel as part of the digital signal. I can't think of a system that wouldn't qualify more completely to use the paragraph (c) exception.

We're each interpreting this section differently. What would your opinion be of what type of system would qualify for the paragraph (c) exception to having to broadcast on the lowest frequency?
No I'm not saying that they can't utilize subsection (c) in its written form. The issue is, if I understand you correctly, they are ID'ing under both sections (b) and (c), but they are ID'ing incorecctly under sub-section (b) by not using the "lowest frequency" on the license. Even though they ID under section (c) it does not cancel out the fact that they are ID'ing under section (b) incorrectly.

IMO They would need to turn off the analog CWID to fully conform to the FCC rule 90.647. As of right now they are not ID'ing correctly and are violating the subsection as it takes no special equipment to monitor their CWID.
 

wlmr

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No I'm not saying that they can't utilize subsection (c) in its written form. The issue is, if I understand you correctly, they are ID'ing under both sections (b) and (c), but they are ID'ing incorecctly under sub-section (b) by not using the "lowest frequency" on the license. Even though they ID under section (c) it does not cancel out the fact that they are ID'ing under section (b) incorrectly.

IMO They would need to turn off the analog CWID to fully conform to the FCC rule 90.647. As of right now they are not ID'ing correctly and are violating the subsection as it takes no special equipment to monitor their CWID.
Sounds like we each read it differently. Guess that's why we haven't gotten rid of all the lawyers yet. Your interpretation might be right but I'm not convinced, just like you're not convinced by my arguments. I doubt we'll ever find out who was right but it's been fun debating the issue!

At least the OP got part the question answered about rotating -vs- not rotating control channels!

Have a good one! :)
 
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