BCD436HP Multi-Site following

[linuz]

Hello,

I am a little confused with Project25 Phase II multi-site scanning on my Uniden BCD436HP.

I follow a fire department on the P25 Phase II system (TxWARN) so I mostly just monitor their dispatch channel and nothing else. I understand that their Motorola radios can be connected to different sites on the P25 system depending on their location. And it seems that even if 2 of their fire department radios are on 2 different sites, they can still talk to each other without any problem as if they know what sites each radio is connected to.

For my Uniden, is the only way to find transmissions from this fire department is to scan each and every site hoping that the scan hits on one while there is traffic going through it? It seems that I end up missing most transmissions because my scanner me be scanning the wrong site at the time of the transmission so it never picks it up. Is the uniden not able to just know what site the radios are transmitting on and follow them?

Or am I completely wrong on how all of this works?

Thank you!

 

[ScannerSK]

The scanner simply checks activity on the towers (programmed into the scanner) to see if those towers are carrying talkgroups enabled in the scanner. It's typically best to have the towers closest to the activity you are trying to monitor programmed into the scanner as they will most likely carry the activity.

As units (police, fire, etc.) drive around their radios are set to automatically connect/affiliate and disconnect/unaffiliate with different towers based upon changing parameters set in the radio. There is no way for the scanner to follow which sites or towers different units are switching to. Sometime a radio simply loses the signal from the tower and looks for another one to connect to.

Shawn

 

[jonwienke]

The choice of which talkgroups are allowed on which transmitter towers (AKA system sites) is up to the system administrator, and depends on the needs of the talkgroup users.

Uniden scanners use Location Control to decide which sites to scan. It takes about 2 seconds per site to process data from the control channel to see which transmissions are in progress, what talkgroups are involved, and which voice channels are being used. If an active transmission is detected that matches the conditions you have set (Service type is turned ON, the Department associated with the talkgroup is within your listening range, etc.), the scanner will play the traffic, and then wait on that site for the DELAY setting for replies. If no reply transmissions are heard, then the scanner will move on to the next site to see if it has traffic.

If you try to scan too many sites, you will miss traffic. Generally you want to scan no more than the nearest 4-5 sites on a trunked system.

 

[ofd8001]

It sounds like you are referring to a linked situation. That's where a user roams from his home area to elsewhere. The system will link the "elsewhere" site with the "home" site so the user remains in communication with folks "back at the ranch".

The communications are carried on both sites. So as long as you continue to monitor that "home" site, you'll still hear what that user has to say.

 

[linuz]

The scanner simply checks activity on the towers (programmed into the scanner) to see if those towers are carrying talkgroups enabled in the scanner. It's typically best to have the towers closest to the activity you are trying to monitor programmed into the scanner as they will most likely carry the activity.

As units (police, fire, etc.) drive around their radios are set to automatically connect/affiliate and disconnect/unaffiliate with different towers based upon changing parameters set in the radio. There is no way for the scanner to follow which sites or towers different units are switching to. Sometime a radio simply loses the signal from the tower and looks for another one to connect to.

Shawn

So the actual motorola radios seem to be able to talk to each other even if they are using different sites (towers) because they are far away from each other. My question is, how do all the radios here the same traffic even if some may be on different sites? Is this the "Linked situation" that ofd8001 is referring to?

 

[buddrousa]

The tower sites are linked together with T1 DATA lines or MICROWAVE LINKS

 

[ScannerSK]

So the actual motorola radios seem to be able to talk to each other even if they are using different sites (towers) because they are far away from each other. My question is, how do all the radios here the same traffic even if some may be on different sites? Is this the "Linked situation" that ofd8001 is referring to?

The sites can be linked together per system as required. Once a radio affiliates with a specific site for a given talkgroup then that site begins to carry (transmit/receive) the radio traffic for that talkgroup. So, the talkgroups a person will receive on their scanner per site is always changing as different radios connect/disconnect to the various towers.

Shawn

 

[ofd8001]

The concept is similar to cell phones. You can be in New York talking on your cell phone to a friend in Los Angeles on his cell phone. The "magic" or "brains" of the cell phone network link the New York and Los Angeles towers so that the conversation occurs. Cellular telephone networks and wide area radio networks are pretty sophisticated systems with computers (called controllers) dealing with a lot of things to achieve communications.

From a scanner listener's perspective, it's academic. Scanners monitor and listen to trunked systems different from how system radios work.

System radios have a control channel which cause the radio to remain in constant contact with the system. A lot of information is passed back and forth on that control channel. Scanners do not have that "back and forth" communication so their business is conducted a little differently.

Probably oversimplifying this a lot, but here is what is going on when your scanner is monitoring a multi-site trunked system: You have a list of talkgroups that you would like to monitor programmed in your scanner, the "scan list", (assuming you are ID Scanning). You also have a list of sites programmed in your scanner which include possible control channel frequencies for each site.

When the scanner is doing "its thing", it "listens" to the control channel for the first site programmed for about a second or two. The scanner will note which talkgroups have on-going transmissions (called channel grants). The scanner then compares that information to your scan list. If there is a match, the scanner goes to the voice channel where the conversation occurs (that info is part of the control channel).

If there are not matches found, the scanner "moves on" to the next site and listens to its control channel, just like described above.

The scanner keeps moving on and on until it goes through all the sites you have programmed. Then it repeats itself all over.

(If the scanner doesn't detect an active control channel for a site, such as because you are too far away, less time is spent on that site before it moves on to another site.)

 

[I_am_Alpha1]

Confused Yet?

I've seen similar questions and it can be confusing. Let me go over how we got here...not intending to insult anyone's intelligence, just presenting an oversimplified explanation for those that are new...we all were once and what is a second nature to some is greek to others.

In the beginning we had simplex radios...users transmitted and received using a single frequency. It worked, but was limited in range. The next evolution was to add repeaters. Now users transmitted on one frequency and their signal was boosted and relayed on another frequency. This allowed smaller radios to work as well as big ones. The signal's range was extended to cover a much larger area and now a cop on the east side of the county could talk directly to a cop of the west side of the county (with simplex the dispatcher would have to relay sometimes). These frequencies were referred to as channels...changing the channel on the radio changed the frequency. As municipalities continued to grow the number of available frequencies dwindled.

The next big step was trunking systems. Here's were it can get confusing. Now a municipality can share their few frequencies with a large group of users (really only limited by the hardware they install and how the system is used). With trunking, users do not have an assigned frequency (or channel) and everyone shares from a pool--one frequency will always be the control and be in charge of all radios (they do have alternates and can even rotate which is the control). Users are now assigned a talkgroup. When you keyup a trunking radio it sends a signal to the control radio with your talkgroup...basically saying I'm on talkgroup "X" and I want to talk to everyone else on talkgroup "X". The control radio then sends a signal to all radios on talkgroup "X" (including the person that keyed up) instructing the radios to go to an open frequency. While users on talkgroup "X" are chatting, users on talkgroup "Z" want to use their radio--no problem, the control radio directs all talkgroup "Z" radios to the next open frequency. Let's say that all the frequencies in the pool are being used, what happens when a user tries to keyup...well they get a busy signal and just try again. Lots of fancy stuff can be done with these systems...like dispatch can talk to a group or all talkgroups at once, GPS locations of users are available, even portable radio battery status can be checked (not the older systems, but the newer ones). The first trunking systems used analog signals. The new systems are digital and their are different formats of digital being used. P25 phase 1 and phase 2 have been adopted as the "standard" for public safety.

At the beginning, I mentioned repeaters and how they take one frequency, boost it, and transmit it on another frequency. With trunking the frequencies tend to be higher on the scale...most in the 700 and 800 MHz area limiting their range. Simulcast systems are used to provide larger area coverage. A county will have several towers connected to the same system--a radio transmission on one is broadcast on all. You may have three or ten--It varies with every system. A user's radio is smart enough to know which tower it should be affiliated (talk and listen to). Now a firetruck in the south part of the county using the south tower can talk to the fire chief in the north part of the county using the north tower. Because their are multiple towers broadcasting on the same frequency you can have an issue receiving...another story, search for simulcast and multi-path for more info.

Some trunking systems may have different frequencies for each tower (or site). Most states have what may be referred to as a state-wide system with sites spread across the state using the same system with the towers using different frequencies. These very large systems can operate just a bit different. Instead of a talkgroup being broadcast over all towers, now they are broadcast only in their "home" area. So if you are in the east part of the state you will not hear traffic from the west side. Their will be some exceptions. One advantage to this is that a user can travel the state and still be able to communicate with "home"--in which case you will hear them and their "home" traffic if they affiliate with the tower near you. An example would be a long distance ambulance transfer or prisoner transfer--they can contact their respective dispatcher. These systems are not limited to state use...the military is using them country-wide. There is a system in California that connects to Kansas and Texas. The individual military users don't normally hear the other state, but they have the capability if they wish.

I hope this helps new listeners. As I said, an oversimplified explanation...keep reading to build your knowledge base.

 

[linuz]

The concept is similar to cell phones. You can be in New York talking on your cell phone to a friend in Los Angeles on his cell phone. The "magic" or "brains" of the cell phone network link the New York and Los Angeles towers so that the conversation occurs. Cellular telephone networks and wide area radio networks are pretty sophisticated systems with computers (called controllers) dealing with a lot of things to achieve communications.

From a scanner listener's perspective, it's academic. Scanners monitor and listen to trunked systems different from how system radios work.

System radios have a control channel which cause the radio to remain in constant contact with the system. A lot of information is passed back and forth on that control channel. Scanners do not have that "back and forth" communication so their business is conducted a little differently.

Probably oversimplifying this a lot, but here is what is going on when your scanner is monitoring a multi-site trunked system: You have a list of talkgroups that you would like to monitor programmed in your scanner, the "scan list", (assuming you are ID Scanning). You also have a list of sites programmed in your scanner which include possible control channel frequencies for each site.

When the scanner is doing "its thing", it "listens" to the control channel for the first site programmed for about a second or two. The scanner will note which talkgroups have on-going transmissions (called channel grants). The scanner then compares that information to your scan list. If there is a match, the scanner goes to the voice channel where the conversation occurs (that info is part of the control channel).

If there are not matches found, the scanner "moves on" to the next site and listens to its control channel, just like described above.

The scanner keeps moving on and on until it goes through all the sites you have programmed. Then it repeats itself all over.

(If the scanner doesn't detect an active control channel for a site, such as because you are too far away, less time is spent on that site before it moves on to another site.)

Ah yes! This explanation helps a lot! Specifically this sentence: "Scanners do not have that "back and forth" communication so their business is conducted a little differently."

That pretty much answers my question why my scanner can't intelligently follow radio transmissions across towers like the radio's can.. the scanner does not ever communicate with the whole system, it only listens. Because of this, it has to continuously scan each site looking for the talkgroup instead of just knowing when and where the talkgroup is currently active.

Thank you!

 

[N0GTG]

Or, to put it another way, radios on the system tell the controller which sites need to carry the talkgroup. The system radio will actively communicate with the controller to get the TG on the site it needs.

Your scanner has to check the sites you have programmed in order to find a site that is carrying the talkgroup. It has to accept whatever the system controller does.