P25 ARMER Questions

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JaredDauer

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Jul 26, 2014
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New Ulm, Minnesota
I should start out by saying that I am definitely a novice.

I've noticed a few things and developed questions because of those observations.

1.) Is the ARMER system a giant blanket of coverage? As an example, if a radio in a talk group half way across the state attempts to reach other radios in the talk group, must those radios be within range of the same site, or does ARMER provide a means for that information to be relayed to other sites where the rest of the talk group is present.

2.) I noticed that some sites are assigned various frequencies. For example the site near my residence has seven assigned frequencies. One is the primary control channel, and another is the secondary control channel. What are the other five for? Are these seven listed frequencies the frequencies upon which communication is actually taking place, or are these frequencies just used for the sake of letting the radios know which frequencies to switch over to in order to communicate?

3.) I pulled the FCC license for my county, in which the ARMER site resides... and the frequencies in the FCC listing don't match the frequencies for the ARMER site on the Radio Reference database. 1800 units were assigned frequencies in 300 unit blocks, for a total of six frequency blocks, and then six more frequencies were assigned to six individual repeaters... are these repeaters considered to be the ARMER "site" or is the site a single location, and the repeaters are acting in another capacity.

4.) Do the frequencies assigned to the 1800 handsets split into 6 blocks of frequencies get used for actually transmitting voice data, or do these frequencies just get used to communicate to the site, and then the site tells the radio which frequency to transmit over?

I've been looking everywhere for a really good overview of the state ARMER system, but so far I am just left with more questions than answers.

I'd really appreciate any help someone can offer making sense of this system.


Thanks,

Jared.
 

Spitfire8520

Might be completely clueless! =)
Joined
Jun 29, 2009
Messages
1,890
Location
Colorado
I should start out by saying that I am definitely a novice.

I've noticed a few things and developed questions because of those observations.
It's good to have you aboard! Keep in mind that everyone was once a novice and had to work their way through learning all this technical stuff like you are doing now. I'll get started before jumping into radio systems by providing a few links to some of the existing explanations of trunked radio systems we have.

Trunking Basics - The RadioReference Wiki - As it says, the basics of trunking, a general overview of what is happening.
Trunked Radio Systems - The RadioReference Wiki - A collection of links to other resources regarding trunking.
http://forums.radioreference.com/ne...d-forum/279839-radio-system-basics-video.html - A video covering the basics of radio starting from the radio and leading to trunking systems. About 21 minutes long.

As for your question, I will try to explain it the best I can based on my understanding of trunked radio systems. I may not be able to fully explain my answer (or sometimes explain incorrectly), so feel free to ask follow up questions and take other responses into consideration.

1.) Is the ARMER system a giant blanket of coverage? As an example, if a radio in a talk group half way across the state attempts to reach other radios in the talk group, must those radios be within range of the same site, or does ARMER provide a means for that information to be relayed to other sites where the rest of the talk group is present.
Your observation is correct regarding the blanket coverage, but there are times where there is an exception due to system radio techs setting up radio denials and such. ARMER, as any other big networked system, is able to route the call through its infrastructure to any other site that has a radio affiliation (a radio listening to that talkgroup on that site). Radios do not need to be on the same site to hear the same talkgroup as the system will work figure out and network the calls to the necessary sites with radio affiliations. It is important to note that a radio needs to be affiliated to a talkgroup on that particular site to hear the radio traffic, especially if you are a scanner user. The system will try to preserve the frequencies that is has and will not transmit a talkgroup if no radios are affiliated.

Now the practice of switching to a talkgroup from halfway across the state would probably be frowned upon by system managers, but that's a different topic.

2.) I noticed that some sites are assigned various frequencies. For example the site near my residence has seven assigned frequencies. One is the primary control channel, and another is the secondary control channel. What are the other five for? Are these seven listed frequencies the frequencies upon which communication is actually taking place, or are these frequencies just used for the sake of letting the radios know which frequencies to switch over to in order to communicate?
The other five (and probably the alternate control channel as well) are used for the physical aspect of communicating. Wireless communications still requires a frequency to operate on in order to transmit and receive messages, and not everything can take place on a single control channel frequency. Basically what is happening is that all radios (and scanners) are listening to the control channel in order to receive information that it contains regarding what frequency it should switch to in the event someone presses the push-to-talk button on a radio. Once someone presses that button, the radio (and scanner) will effectively be told that there is a voice grant on a particular voice channel (one of the 6 frequencies listed in the database) and that they should switch to that channel order to hear the transmission.

The idea of a trunked radio system is that you are attempting to preserve radio frequency resources by what I usually describe as "visualizing radio channels (talkgroups)." Instead of dedicating a frequency to a channel that you likely aren't using round the clock, you add it to a pool of trunked radio frequencies (channels). The idea is that you can organize yourself by having a greater number of talkgroups and operate on the theory that most transmissions that do occur are short and not active 100% of the time. This means that you can effectively have more talkgroups to switch to while using less frequencies as they aren't infinite in quantity.

3.) I pulled the FCC license for my county, in which the ARMER site resides... and the frequencies in the FCC listing don't match the frequencies for the ARMER site on the Radio Reference database. 1800 units were assigned frequencies in 300 unit blocks, for a total of six frequency blocks, and then six more frequencies were assigned to six individual repeaters... are these repeaters considered to be the ARMER "site" or is the site a single location, and the repeaters are acting in another capacity.
Most trunked systems operate as a duplex repeater system, so they require an transmit (classed MO - lower 800 MHz) and a receive (classed FB2 - mid 800 MHz) frequency. For an 800 MHz system, the transmit offset is always 45 MHz lower than the receive frequency. The database generally only lists receive frequencies as that is what used to be important for scanner users (not so much with the P25 systems).

If you are looking at a site license, you probably want to be looking at the FB2 repeater licenses. I don't know what particular site you are speaking of, but you can be seeing a simulcast site depending if the exact same frequencies are operating at multiple locations. A simulcast site is effectively acting as a single radio site with multiple repeaters broadcasting the same information at roughly the same time. This is another method of preserving frequency resources in order to cover a larger geographic area with the same frequencies. If the site in acting in this capacity, then this would explain why one would see the same frequency at different sites. The other reason for this is that it is that some ARMER sites are licensed under the same license for ease of paperwork and organization. This would be indicated by seeing completely different frequencies being assigned to different locations.

4.) Do the frequencies assigned to the 1800 handsets split into 6 blocks of frequencies get used for actually transmitting voice data, or do these frequencies just get used to communicate to the site, and then the site tells the radio which frequency to transmit over?
As mentioned above, a repeater system requires 2 frequencies to operate on. If you look at the 45 MHz offset, you can actually see that these frequencies are paired with those licensed to a repeater to operate on. This also ties into question #2 where a frequency is required to transmit voice data. The second part of this is if you are the one pressing down the PTT button, your radio transmits that request to the radio site on what I believe is the frequency pair for the control channel. At this point the site + zone controller figures out an unoccupied frequency for your site (and other sites that someone else might be using) for which you and other can use to hear communications. The control channel will then tell your radio the lower 800 MHz frequency that you will transmit on as well as tell everyone else the necessary mid 800 MHz frequency in order to hear the transmission you are making. One this process is complete, you will hear the little push-to-talk tone telling you that you are free to talk. All this generally happens within milliseconds.

In the event that the system is particularly busy, all the frequencies assigned to that site may run out. In the event that this happens, then there are no more frequencies that can be used for radio transmissions. A site cannot use a frequency that it has not been programmed to, only those licensed (for example, the other 6 frequencies on the site near you). Once this happens, a user will actually be denied from talking on the radio through a harsh sounding busy tone. Generally the user is attempting to make a transmissions should keep the PTT button pressed down as the system will put your voice grant in a queue as it awaits for a frequency to be freed up for transmission.

I hope I was able to answer at least some of your questions. Remember, feel free to ask if you felt like one of my answers did properly answer your question!
 
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JaredDauer

Member
Joined
Jul 26, 2014
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Location
New Ulm, Minnesota
Spitfire. I wish I could just give you a hug. Seriously.


I do have another question. When I do an ID search, instead of ID scan, I pick up other talk groups which I have not provided the tags for. This is fine, but it creates some confusion with me...

How does a roaming radio know the frequency of a site which it doesn't normally communicate with? It seems like my scanner is occasionally picking up traffic from 3 or 4 counties away, from radios which probably don't normally use my local site for traffic. Is the ARMER site ONLY able to use those seven frequencies, or can it broadcast on other frequencies that other "non-normal" traffic might be using. And yet that still doesn't explain how these radios from counties away are able to use the local ARMER site unless their radios know the frequencies of our site!!

Our county has more than one ARMER site, are all handsets programmed to the multiple control channels of multiple ARMER sites?

Heres the example:

This is a link to the FCC license for the trunked system in my county (brown county MN):

ULS License - PubSafty/SpecEmer/PubSaftyNtlPlan,806-817/851-862MHz,Trunked License - WQUB993 - BROWN, COUNTY OF - Frequencies Summary

EXACTLY as you suggested there is a 45 MHZ offset between the MO (1800 handsets), and FB2 (6) repeaters.

HOWEVER, this is where my confusion comes in.

If you now go to the ARMER database, on the RR website, the frequencies listed for the site, are DIFFERENT than any of those frequencies specified in the FCC license. Which are:

Site location: New Ulm, frequencies, 851.48750, 851.67500, 851.98750, 852.42500, 852.78750c 853.10000a, 853.48750

HOWEVER, the frequencies listed in that FCC license DO come back for a site tower in sleepy eye, which is a city in our county, and another ARMER site.... if there is a radio in between these two sites, which are only separated by about 14 miles, and that radio makes a transmission, how do both sites, which operate on different frequencies, handle getting that transmission to other radios on the same talk group, which i presume will send that transmission over a single frequency? Yet neither tower shares a frequency????

SO I then searched the FCC database, by the control frequency that my scanner is picking up all local traffic on, and it came back with this license!!

ULS License - PubSafty/SpecEmer/PubSaftyNtlPlan,806-817/851-862MHz,Trunked License - WQKR226 - MINNESOTA, STATE OF - Frequencies Summary

Which is licensed to the state, and not to the county... and it is a different class of repeater.

Your explanation goes great lengths to help me understand this system, and I can't thank you enough for your help thus far. I think what ive described in this second post sums up the remainder of my questions.

I am really enjoying learning about this system. Its amazing how we have been able to utilize the fixed bandwidth we have.

Again, thank you so much... I wish there was a way I could more adequately thank you for your time.

-Jared
 

Spitfire8520

Might be completely clueless! =)
Joined
Jun 29, 2009
Messages
1,890
Location
Colorado
I do have another question. When I do an ID search, instead of ID scan, I pick up other talk groups which I have not provided the tags for. This is fine, but it creates some confusion with me...
An ID search basically sets your scanner to follow all traffic that a site may encounter and is provided as an option of hunting down talkgroups which may not have been logged in the database. It basically displays the talkgroup of whatever you are hearing and may be used to help figure out what kind of traffic a certain site might have or even as a quick way of saving talkgroups and tagging them later. Other scanners will call this feature a "wildcard" since it searches for everything. Most people will probably settle on using ID scan in order to hear traffic for what they are specifically targeting.

How does a roaming radio know the frequency of a site which it doesn't normally communicate with? It seems like my scanner is occasionally picking up traffic from 3 or 4 counties away, from radios which probably don't normally use my local site for traffic. Is the ARMER site ONLY able to use those seven frequencies, or can it broadcast on other frequencies that other "non-normal" traffic might be using. And yet that still doesn't explain how these radios from counties away are able to use the local ARMER site unless their radios know the frequencies of our site!!
I have several thoughts on how radios behave on a networked system, but I cannot say that any of them are 100% factual so take my statements with a grain of salt.

I know for a fact that the control channels constantly transmits an up-to-date list of neighboring sites and their control channel frequencies that is available for the radios to read. My first thought is that if the radio reached a point where it recognizes poor reception, it will then use the list and check through all the other neighboring sites until it finds one that has better reception and the roam onto that site.

I also know for a fact that all P25 systems transmit a both a WACN ID and a System ID. For ARMER, that ID is listed as WACN BEE07 and System ID 40F. My thought is that in the event that a radio turns on in a completely unknown location and is unable to find it's pre-programmed site, it will do a 800 MHz spectrum sweep until it locates a control channel matching that identifying information. Once it does so, it will attempt to query an affiliation and will be given an affiliation or denial based on how the system is setup, the radio system keys, and the talkgroup the radio is trying to access.

As for whether a site is allowed to use additional frequencies for "non-normal" traffic, the answer is quite simply no. This is what I was referring to when I stated "Now the practice of switching to a talkgroup from halfway across the state would probably be frowned upon by system managers..." The site only has so many frequencies it can work with and if someone does leave their normal area and leaves their radio on, they end up dragging that talkgroup to different towers that may not be specifically designed to handle the additional loads. If one works in a busy city and end up dragging the busy city's primary dispatch talkgroup to a rural area, there is potential for that site to run out of frequencies much more quickly than if it didn't have the city's dispatch on it. Some system managers will advocate for a independent travel talkgroup that does not have routine radio traffic on it in order to prevent overloading the system when you leave your primary zone of coverage.

Our county has more than one ARMER site, are all handsets programmed to the multiple control channels of multiple ARMER sites?
Once again, I'm not 100% clear on this matter. I believe that P25 radios have the option of being programmed with a number of sites that it will routinely check for and switch to depending on circumstances. I am not involved in the industry, so I have limited knowledge regarding the programming of these radios.

Heres the example:

This is a link to the FCC license for the trunked system in my county (brown county MN):

ULS License - PubSafty/SpecEmer/PubSaftyNtlPlan,806-817/851-862MHz,Trunked License - WQUB993 - BROWN, COUNTY OF - Frequencies Summary

EXACTLY as you suggested there is a 45 MHZ offset between the MO (1800 handsets), and FB2 (6) repeaters.

HOWEVER, this is where my confusion comes in.

If you now go to the ARMER database, on the RR website, the frequencies listed for the site, are DIFFERENT than any of those frequencies specified in the FCC license. Which are:

Site location: New Ulm, frequencies, 851.48750, 851.67500, 851.98750, 852.42500, 852.78750c 853.10000a, 853.48750

HOWEVER, the frequencies listed in that FCC license DO come back for a site tower in sleepy eye, which is a city in our county, and another ARMER site.... if there is a radio in between these two sites, which are only separated by about 14 miles, and that radio makes a transmission, how do both sites, which operate on different frequencies, handle getting that transmission to other radios on the same talk group, which i presume will send that transmission over a single frequency? Yet neither tower shares a frequency????

SO I then searched the FCC database, by the control frequency that my scanner is picking up all local traffic on, and it came back with this license!!

ULS License - PubSafty/SpecEmer/PubSaftyNtlPlan,806-817/851-862MHz,Trunked License - WQKR226 - MINNESOTA, STATE OF - Frequencies Summary

Which is licensed to the state, and not to the county... and it is a different class of repeater.
Alright, I think I understand some of your confusion in regarding to licensing. WQUB993 has a listed control point in New Ulm, however if you click on the not so obvious 'Locations' tab towards the top of the screen, you will see the physical location of the tower which is listed as Sleepy Eye. This means that the license is for the Sleepy Eye site located within Brown County. The New Ulm site which you are referring to is correctly listed back to the WQKR226 license that you have also found (location #6). As for the repeater differences, it's really dependent on who files the paperwork. The classes generally don't make too much of a difference, especially since they are all under the FB2 repeater class regardless. FB2C would probably be a more proper way of classifying a site, but it is not necessary. Large statewide systems can have multiple licenses filed by anyone, whether it is the state or county.

As for your question regarding the transmissions in between two sites, when a radio presses it's PTT key, the site controller that is receiving the request will report to the overall zone/system controller regarding the pending radio traffic. The system will then request a frequency from all the different site controllers that have a radio affiliated to that talkgroup to provide a frequency for use. You will then have the site you are using grant a frequency for you to use while the site someone else is using will simultaneously grant a completely different frequency that that other site as available for use. When you speak, the system will basically act as a repeater for your transmission to all necessary sites (which can range from a single site to dozens of sites) in order to provide service across a larger geographical area as opposed to older single repeater systems. Basically, a computer in the background does the work to figure out where the voice data needs to go and figures out what is needed to accomplish the task of getting a transmission of point A to point B.

Your explanation goes great lengths to help me understand this system, and I can't thank you enough for your help thus far. I think what ive described in this second post sums up the remainder of my questions.

I am really enjoying learning about this system. Its amazing how we have been able to utilize the fixed bandwidth we have.

Again, thank you so much... I wish there was a way I could more adequately thank you for your time.
No problem, all these answers are provided on a voluntary basis and I'm sure many other users would reply given the chance and if it weren't in the middle of a work week.
 
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