Trying to get some specific answers on public radio systems.

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waingro223

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Ive been doing some research online and can't seem to find the specific answers to some of my questions.
I'm trying to research and figure out exactly how the public safety systems, radios, repeaters all work together in harmony. I ask a lot of questions so I can have a better understanding of things. Im curious and would love someone to help me out. Ive been listening to scanner sense I was 10 and now I'm 25 and want to know the little details. Im not trying to be annoying, If someone has a link to explain this questions Id appreciate it a lot.

1. Whats the advantage over digital conventional system verses a analog conventional system? Encryption I'm guessing?

2. If the police use lets say six or eight repeater towers, do the repeater towers all repeat to each other to cover the whole radius of the area that's supposed to be covered?

3. How many watts does a police officers portable radio give off verses the one in the car? And does the portable transmit to his/her car or the nearest squad car in the same department and get re-amplified using the mobile radio inside the car and than send to the closest transmitter.

4. Is the input frequency on the portable the same as the one in the squad cars? I know the repeater is different.

5. What happens if two officers are on a Trunked system and are trying to talk at the same time to dispatch on the same channel, does the computer stop that and let the officer know to hold his/her traffic? Ive never heard two officers trying to talk on the same channel on my scanner, whether its trunked or conventional how does that work?

6. If there are only 5 system frequencies on a Motorola type 2 smartnet system and more than 5 people in different talk groups including public works, sanitation, etc are all trying to talk is that going to cause a problem being only 5 frequencies to use? If not how come?

7. In my county of Saint Charles Missouri the sheriff uses Digital Conventional. And theres only one dispatch channel. With the county being so big it seems like one channel is just not enough to help these officers out. How do they do it?
 

RKG

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Ive been doing some research online and can't seem to find the specific answers to some of my questions.
I'm trying to research and figure out exactly how the public safety systems, radios, repeaters all work together in harmony. I ask a lot of questions so I can have a better understanding of things. Im curious and would love someone to help me out. Ive been listening to scanner sense I was 10 and now I'm 25 and want to know the little details. Im not trying to be annoying, If someone has a link to explain this questions Id appreciate it a lot.

1. Whats the advantage over digital conventional system verses a analog conventional system? Encryption I'm guessing?

2. If the police use lets say six or eight repeater towers, do the repeater towers all repeat to each other to cover the whole radius of the area that's supposed to be covered?

3. How many watts does a police officers portable radio give off verses the one in the car? And does the portable transmit to his/her car or the nearest squad car in the same department and get re-amplified using the mobile radio inside the car and than send to the closest transmitter.

4. Is the input frequency on the portable the same as the one in the squad cars? I know the repeater is different.

5. What happens if two officers are on a Trunked system and are trying to talk at the same time to dispatch on the same channel, does the computer stop that and let the officer know to hold his/her traffic? Ive never heard two officers trying to talk on the same channel on my scanner, whether its trunked or conventional how does that work?

6. If there are only 5 system frequencies on a Motorola type 2 smartnet system and more than 5 people in different talk groups including public works, sanitation, etc are all trying to talk is that going to cause a problem being only 5 frequencies to use? If not how come?

7. In my county of Saint Charles Missouri the sheriff uses Digital Conventional. And theres only one dispatch channel. With the county being so big it seems like one channel is just not enough to help these officers out. How do they do it?
1) This question is too general for this medium. Some folks don't think there is any advantage to digital vs. analog. Motorola publishes a book called the "Astro System Planning Guide" which would give you some insight into the issues involved, as well as Motorola's case for its belief that digital offers some advantages.

2) Depends entirely on how the system is designed. However, it is almost always the case that the various "stations" (they may or may not be functioning as repeaters) are connected back to the dispatch point by some form of wire (we use the term "wireline"), which could be copper, leased phone line, fiber, or the like.

3)

A) Typical portable radios in public safety use are rated at 5-6 watts nominal, and work out to about 1-3 watts ERP. Mobile radios typically fall into the "mid power" class (25-50 watts) and "high power" class (90-100 watts). In repeated or voted systems designed for on-street portable coverage, most mobiles are mid power.

B) Back in the days when VHF simplex ruled, it was not uncommon to employ "vehicular repeaters" (we tended to use the term "Rat Pack"); an officer would have a UHF portable that would transmit to the Rat Pack in the cruiser, which would then repeat his audio over the cruiser's high power VHF mobile. Likewise, the Rat Pack might be programmed to repeat VHF audio over UHF to the officer's portable (or he might just rely on the cruiser's loudspeaker to hear cruiser radio audio). In all events, the Rat Pack portable had to be on a different band than the cruiser radio. Today, these types of systems are becoming rare.

4) In a duplex system (could be repeated or not), "input" frequency refers to the frequency that the field units (we use the term "subscribers") transmit on and the infrastructure receivers receive on; output frequency is the frequency that the infrastructure transmitters transmit on and the subscribers receive on. However configured, inputs and outputs are the same for mobiles and portables.

5) Depends entirely on the type of trunked system and how it is configured. On a SmartNet system, the system operator can configure so that talking over (we call it "barge in") is permitted or not. In general, the capacity for "barge in" is widely (but not universally) considered important in a public safety application.

6) Again, it depends in part on how the system is configured. In some systems, if all four of the voice channels on a five channel system are in use (one channel is dedicated to data), the fifth guy gets a busy signal when he transmits. If prioritization is configured, and if the fifth guy has a higher priority than one of the four who are already talking, he will knock off that guy (who will get a busy signal in the middle of his conversation).

7) Again, way too general a question. In theory, a conventional Astro 25 system employing the digital equivalent of voting and simulcast transmitters could cover an area of any size; in reality, the bigger the coverage area (and, usually, the larger the number of users on the air at one time) the greater the likelihood of system overloading. I don't know anything about the particular system you've referred to, but a dollar to a donut says that it was carefully engineered by qualified system designers after the design basis was agreed upon.
 
D

DaveNF2G

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I don't know anything about the particular system you've referred to, but a dollar to a donut says that it was carefully engineered by qualified system designers after the design basis was agreed upon.
And then chopped back to a more affordable configuration by bean counters. :D
 

zz0468

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Some interesting questions. You're going to get some conflicting answers, but take that to mean that there's often more than one way to do something right.

1. Whats the advantage over digital conventional system verses a analog conventional system? Encryption I'm guessing?
Enhanced features and the ability to encrypt. There is also a perception by some that audio quality is better in marginal conditions.

2. If the police use lets say six or eight repeater towers, do the repeater towers all repeat to each other to cover the whole radius of the area that's supposed to be covered?
This depends on specific system designs. A large city or a region of a large county or state may have one system simulcast over 6 or 8 towers. In other systems, each tower may represent a separate system, and other means are used to carry "inter-tower" traffic.

3. How many watts does a police officers portable radio give off verses the one in the car? And does the portable transmit to his/her car or the nearest squad car in the same department and get re-amplified using the mobile radio inside the car and than send to the closest transmitter.
Typical is 5 watts in an HT, give or take a few watts. Mobiles can run as high as 110 watts,. depending on make, model, and frequency band. Oh, and specific system requirements.

4. Is the input frequency on the portable the same as the one in the squad cars? I know the repeater is different.
Yes.

5. What happens if two officers are on a Trunked system and are trying to talk at the same time to dispatch on the same channel, does the computer stop that and let the officer know to hold his/her traffic? Ive never heard two officers trying to talk on the same channel on my scanner, whether its trunked or conventional how does that work?
Usually one will get the talk-permit tone, and the other will get "bonked". It's less than perfect, and I've seen cases where two units get a talk permit tone and cover each other.

6. If there are only 5 system frequencies on a Motorola type 2 smartnet system and more than 5 people in different talk groups including public works, sanitation, etc are all trying to talk is that going to cause a problem being only 5 frequencies to use? If not how come?
Yes, it's a problem. The last guy in get's queued. He has to wait. The problem is mitigated by proper system design. You want to build enough channel capacity to allow for minimal queuing during busy traffic times, but not go overkill, because that's expensive. System engineers are well versed in determining traffic loading, and how many channels are required.

7. In my county of Saint Charles Missouri the sheriff uses Digital Conventional. And theres only one dispatch channel. With the county being so big it seems like one channel is just not enough to help these officers out. How do they do it?
Usually in cases like this, they get by because they have to. Law enforcement people are professionals, they want to do the job the best they can, and sometimes they have to do it with a less than ideal radio system. They learn to adapt, and hope for the day it will improve.
 

zz0468

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Encryption is not limited to digital. There is lots of encryption on analog systems.
This is very true. But encryption is one of the major selling points being used to justify new digital radio systems. Applying it to existing analog systems isn't as seamless or elegantly achieved as it is with a digital system.
 

mmckenna

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I'll take a crack at some of these, since this is what I do for a living...

Ive been doing some research online and can't seem to find the specific answers to some of my questions.
I'm trying to research and figure out exactly how the public safety systems, radios, repeaters all work together in harmony. I ask a lot of questions so I can have a better understanding of things. Im curious and would love someone to help me out. Ive been listening to scanner sense I was 10 and now I'm 25 and want to know the little details. Im not trying to be annoying, If someone has a link to explain this questions Id appreciate it a lot.

1. Whats the advantage over digital conventional system verses a analog conventional system? Encryption I'm guessing?
Others have pretty well covered this. One thing they didn't mention is occupied bandwidth. Since the amount of radio spectrum is limited, using it efficiently is encouraged. One way of fitting more radio systems into a limited amount of spectrum is to use as little amount of "space" as possible. A specific radio channel uses a certain amount of bandwidth. 25KHz is considered standard for FM. It's really easy to fit a human voice into 25KHz of bandwidth. As technology advanced, it became possible to fit voice onto 12.5KHz of bandwidth. Both 25KHz and 12.5KHz FM channels were done via analog audio. Easy and cheap to do. A few years back the FCC mandated that almost all radio users in the VHF High and UHF bands needed to stop using 25KHz channels and switch to 12.5KHz channels. This becomes a requirement on January 01, 2013. You will often see this referred to as "Narrow banding" (not to be confused with Rebanding). So, what does this have to do with the Analog vs. Digital question? Well, a couple of things. First is that while it's easy to do analog voice on a 12.5 KHz channel, people are anticipating that eventually the FCC will require users to stop using 12.5KHz channels and go to something narrower (6.25 is what the FCC says they will do next, but has not set a date). To go any narrower than 12.5KHz, you realistically need to use digital. You can easily fit digital voice traffic on to 6.25KHz of bandwidth, and two voice paths into 12.5KHz. So, it's an efficiency thing.
The other reason is that once you go to digital, it becomes easier to do much more with a radio. You can encrypt voices in digital easier than you can in analog. You can do text messages, send GPS coordinates along with voice, radio ID information, basically lots of different options.

So, since Digital is the way things will be going in the future, a lot of agencies and companies are choosing to make a bigger step now, rather than just taking the step to analog 12.5 KHz bandwidth.

Also, the Federal Governement has mandated that any Federal Grant money that is used to buy public safety radio equipment must be spent on P25 digital capable radios.

2. If the police use lets say six or eight repeater towers, do the repeater towers all repeat to each other to cover the whole radius of the area that's supposed to be covered?
Repeaters don't, They pick up a signal on one frequency, and transmit it back out on a second frequency with more power. Say they receive on frequency "A" and transmit on frequency "B". Since they receive and transmit on different frequencies, they would not "hear" each other transmitting.
Often what is done is that many tower sites around a specific area will all tie back via special phone lines, network, T-1, Fiber, or radio links to a central point. A device called a "voter" will take all the received signals from the receivers and select the best signal. That signal will then be sent back out to the transmitter (or transmitters). You can transmit the same signal from multiple locations, that is called Simulcast. It gets a bit tricky since each of the transmitters have to transmit the exact same signal at the exact same time or the receiving radios will hear and echo.

3. How many watts does a police officers portable radio give off verses the one in the car? And does the portable transmit to his/her car or the nearest squad car in the same department and get re-amplified using the mobile radio inside the car and than send to the closest transmitter.
Others have pretty much covered this. I'll add that it varies by frequency, usually. Most VHF Low portable radios transmit with about 6 watts of power on average. VHF High portables tend to be around 5 watts. UHF tend to be around 4, and 700 and 800MHz are usually 3 watts. Has to do with the efficiency of the RF amplifiers in the radios a different frequencies, etc.

An officers radio can work 3 different ways (usually):
1. They can transmit "simplex", which is transmitting and receiving on the same frequency. This usually only works over "line of site", not very good, but sometimes used because it doesn't rely on a repeater. Good for covering small areas, or in areas where there is no repeater coverage.
2. They can transmit directly to a repeater on a tower or mountain top.
3. Using a thing called a Vehicular repeater, they can transmit from the portable radio to their car, where a radio picks up the signal and then sends it to the higher powered mobile radio in the car. Same thing operates in reverse, when the mobile radio picks up a signal, it uses the vehicular repeater to transmit the signal to the portable radio. This is really useful to cover big areas as often a hand held portable radio won't have enough power to transmit back to the mountain or tower top repeater. California Highway Patrol does this.


4. Is the input frequency on the portable the same as the one in the squad cars? I know the repeater is different.
Depends on the way they are using it. If the system is simplex (radio to radio directly, no repeater) then everything is on the same frequency.
If they are using a repeater on a mountain or tower top, then the mobile and hand held radios will transmit on one frequency and receive on another.
If they are using a vehicular repeater system, then the portable will transmit on, say, frequency "A" and the vehicular repeater will receive on frequency A. The vehicular repeater will connect to the higher powered mobile radio. The mobile radio will transmit the signal out on frequency "C". When the mobile radio receives a signal, the mobile repeater will transmit that back to the portable. It can get more confusing depending on the rest of the system, but that's a basic overview.

5. What happens if two officers are on a Trunked system and are trying to talk at the same time to dispatch on the same channel, does the computer stop that and let the officer know to hold his/her traffic? Ive never heard two officers trying to talk on the same channel on my scanner, whether its trunked or conventional how does that work?
It can get pretty deep explaining exactly how a trunked radio system works, and there isn't enough space for me to get into it all here, but I'll give it a quick try:
A trunked system has what is called a "central controller". The central controller "directs traffic" for the system. They (usually) won't let two people transmit at the same time on the same "channel". The second person trying to transmit will get a tone letting them know they can't transmit yet, and then they will hear the conversation already in progress. There are ways to change this. Most systems have a system that allows prioritizing certain radios or channels. So, one radio could technically have a higher priority than another, and it would get priority. Conventional analog is a free for all, sometimes. It would be possible for two radios to transmit at the same time, however due to something called "capture effect", usually the receiver will lock onto the strongest signal. It is possible to program most modern radios with a function that won't let a radio transmit if there is already a signal being received, however. I don't usually use that on any of my public safety radio systems since in an life or death situation, you want someone to be able to get through.

6. If there are only 5 system frequencies on a Motorola type 2 smartnet system and more than 5 people in different talk groups including public works, sanitation, etc are all trying to talk is that going to cause a problem being only 5 frequencies to use? If not how come?
Yep, it will. It's actually a bit different than that. A 5 channel Moto system like that will always have one channel working as the "control" channel. This channel is running a slow speed data that handles the function of the trunking. It has to do with assigning radios to the available frequencies. So, a 5 channel system like that will only have 4 voice paths available. So, what happens when all 4 "dispatch" channels are in use? The next users will get a "busy", and it sounds basically like a telephone busy signal. The "5th" users will get put into a queue and when a channel is available, their radio will beep to let them know. Again, there is a priority system that lets certain radios or talk groups have priority over others. On the system I run, our PD and Fire have highest priority, and then the other no public safety talk groups have lower priority. Also, you can assign priory levels to specific radios.

7. In my county of Saint Charles Missouri the sheriff uses Digital Conventional. And theres only one dispatch channel. With the county being so big it seems like one channel is just not enough to help these officers out. How do they do it?
Discipline. Being brief on the radio and getting the messages across as clearly and quickly as possible is one of the skills they have to have. Often, they will also have other resources. Some departments may use cell phones for some stuff, or computer terminals in the cars. But, basically it the discipline that makes the difference.
 
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waingro223

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Thank's you guys!

Thanks for all the information that you all have just explained to me, you have made it more clear now and I have a better understanding of the subject and I'm stilling interested in learning more and more. If anybody knows any good links that have some good information and questions like I had I would greatly appreciate it. Ive found some decent websites that explain some stuff, but If anyone knows a good one with detailed information let me know.

I do have one other question though. The local police in my town use the Motorola Type 2 Trunked 800Mhz Smartnet, and earlier I was talking about there portable radios transmitting to there patrol cars to get re-amplified to make it to the repeater. My question is, If two officers are riding in the same car or another officer is closer to another one of his buddy's department car, would it use the same radio to re-amplify it?

I would also assume that the county would use all the repeater towers at once just to cover the whole area because they are all on the same frequency.

Thanks again.
 
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RKG

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Thanks for all the information that you all have just explained to me, you have made it more clear now and I have a better understanding of the subject and I'm stilling interested in learning more and more. If anybody knows any good links that have some good information and questions like I had I would greatly appreciate it. Ive found some decent websites that explain some stuff, but If anyone knows a good one with detailed information let me know.

I do have one other question though. The local police in my town use the Motorola Type 2 Trunked 800Mhz Smartnet, and earlier I was talking about there portable radios transmitting to there patrol cars to get re-amplified to make it to the repeater. My question is, If two officers are riding in the same car or another officer is closer to another one of his buddy's department car, would it use the same radio to re-amplify it?

I would also assume that the county would use all the repeater towers at once just to cover the whole area because they are all on the same frequency.

Thanks again.
In general, Rat Packs are not used on trunked systems. (As always, there are applications that might qualify this statement.) In any event, good Rat Pack systems had the ability to detect if another Rat Pack was nearby and active, in which case they would not activate.
 

mmckenna

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Ive found some decent websites that explain some stuff, but If anyone knows a good one with detailed information let me know.
This is first in a series of 10 that was done last year. It starts off pretty basic and then builds up from there. I had a couple of my non-radio savvy staff read these for a better understanding:

Part 1: Designing a land mobile radio system | LMR | Definitions - Urgent Communications article

I do have one other question though. The local police in my town use the Motorola Type 2 Trunked 800Mhz Smartnet, and earlier I was talking about there portable radios transmitting to there patrol cars to get re-amplified to make it to the repeater. My question is, If two officers are riding in the same car or another officer is closer to another one of his buddy's department car, would it use the same radio to re-amplify it?
There is a system built into those vehicular repeaters that negotiates which one will work when multiple units are on scene, so only one of them will be working. If more than one on scene were trying to do this, there would be interference issues.

I would also assume that the county would use all the repeater towers at once just to cover the whole area because they are all on the same frequency.

Thanks again.
Could be. Often when there are multiple sites on a trunked system like that, they are actually running a SmartZone system. It's just like a SmartNet, except that it is actually a bunch of systems all tied together. Gets really complicated to explain without visuals, so I'll leave it at that.

Glad we could help.
 

waingro223

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Smartzone?

I read on radio reference that the Saint Charles County Sheriffs was using a digital conventional system and not a trunking system. They have 3 input frequencies that they can use for there mobile inputs and one output frequency. I take it the 3 inputs are divided up amongst the force for some reason? I wonder why they would use 3 separate inputs for the mobiles if there conventional? But everywhere you go in the county they have the best coverage. I almost always have 5 bars on my BCD396XT wherever I go in the county and that's only when the key up. They don't send a constant signal. I can't say that for the Missouri Highway Patrol, they need some major upgrades and there coming soon. I assume the County uses all there repeaters at once when they broadcast. They will be going to a full trunking digital system in the next couple of years though. I believe its called the MOSWIN P-25 and its statewide for everyone. I wonder why the county is digital conventional and not just analog, they output on 155.4900MHz Narrow Band.

This in the link to the the Saint Charles County Sheriff in my area.
If you find out anything let me know I appreciate it sir.


St. Charles County, Missouri (MO) Scanner Frequencies and Radio Frequency Reference
 
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brscomm

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Rat Packs?

Who calls vehicular repeaters that? Motorola made two lines, one called PAC-PL and the other PAC-RT

These were the most common and resulted in the slang term "Pack Rats." In 35 years of LMR, I never heard them called Rat Packs. That was the term for a group of Hollywood actors like Frank Sinatra, Sammy Davis Jr and Dean Martin.
 
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