Digital Systems Clarification

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kandrey89

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OK, so I'm new, and I've listened to over 24 hours of broadcast from my local PublicSafety Service.
They are on APCO25 system.

What I'm unclear about is how the system works.

So I know there's a control channel. There are also different channels. How does the scanner allow you to hear everything from the system? I also know that sometimes units are asked to respond on a specific channel, do I hear that channel if I'm on the control channel? How do the units switch between channels, is it preprogrammed channel selection on the unit's radio or something, or how does it work?

In theory, for some reason I think that I'm supposed to hear all the traffic, but in reality I'd think that if the scanner is picking up all the traffic there'd be overlap between different channels meanwhile the units would be able to operate on those channels without being stepped on by other channels, hence thats why they are on a diff channel in the first place. But then what is the purpose of the control channel if you can't monitor all other channels. Doesn't the control channel (based on trunking systems) basically tells your radio to move to another channel? Do the units do it manually, or the dispatch moves them over to another channel? Would that cause confusion if it wasn't manual? So what's the purpose of the primary/control channel? Is it just a naming convention in APCO25 for central channels where dispatches are made?
 

kandrey89

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Yeah, I understand that, but that means that the scanner arbitrarily chooses which talkgroups to listen to right after you finish listening to a conversation and the scanner goes back to the control channel. This also means that there maybe be other 100 conversations going on in other talkgroups. But how come it feels like I hear all of the dispatcher's calls when I listen to the SFPD/FD stream found on this site? I didn't say I hear all of the conversations because they obviously move to other "respond on A3" channel and etc, but I do hear all the conversations from the dispatch to many different units including when multiple units step on each other while communicating with dispatch.

That's what I'm unclear about now.

Thanks for the link.
 

fineshot1

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How does the scanner allow you to hear everything from the system?
It may not depending on how busy the system is and how you have the scanner programed. If there are multiple conversations going on at the same time your scanner will only stop on one of them at a time.

I also know that sometimes units are asked to respond on a specific channel, do I hear that channel if I'm on the control channel? How do the units switch between channels, is it preprogramed channel selection on the unit's radio or something, or how does it work?
If your scanner has a talkgroup id(thats what the trs channels are called) in its scan list and this
talkgroup goes busy with a conversation the over head data on the control channel reflects that
and your scanner will switch to that talkgroup and follow it to the voice channel it gets assigned to.
Yes - talkgroups get programed into the users portable or mobile and appear to the user as a channel
and they simply switch to that on their radio.

In theory, for some reason I think that I'm supposed to hear all the traffic, but in reality I'd think that if the scanner is picking up all the traffic there'd be overlap between different channels meanwhile the units would be able to operate on those channels without being stepped on by other channels, hence thats why they are on a diff channel in the first place. But then what is the purpose of the control channel if you can't monitor all other channels. Doesn't the control channel (based on trunking systems) basically tells your radio to move to another channel? Do the units do it manually, or the dispatch moves them over to another channel? Would that cause confusion if it wasn't manual? So what's the purpose of the primary/control channel? Is it just a naming convention in APCO25 for central channels where dispatches are made?
Your confusing the data and voice parts of the system. There are usually 3 or 4 channels that are
assigned to be a both voice and control(data) channel and the rest are assigned as voice only. When your radio is sitting idle it is always listening to the control channel(data). If another member of your group keys his mic to talk his radio sends data(on the inbound control chan) to the system and the system assigns the conversation(on the outbound control chan) to a voice channel and all other units automaticely tune to the assigned voice channel and the squelch opens up and you hear the unit calling. On the idle radio the process of moving
from the control to voice chans is automatic and is part of the cpu logic circuits in the radio. If a dispatcher
tells a specific user to move to another channel that is not automatic - the user must turn his rotary switch to the correct channel or talkgroup. At this time his radio re-affiliates on the new talkgroup and the system knows his radio is logged on to that talkgroup and is tracked to each talkgroup he switches to on his rotary switch selection. NOTE: Each control channel has an inbound and outbound freq.

Hope that helps clear up the fog....

NOTE: There is actually a bit more to it than that but am trying to keep it simple.
 
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kandrey89

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Yes, this is much better, I'm almost there.

Now I just have to clear up the following confusion.

You say: when the person pushes to talk and is assigned to a voice channel, all the listeners are automatically sent to that voice channel to receive what the guy is saying. But what about a second guy who is also doing the same thing. What makes sure that the second guy is heard by the dispatch and that anyone is even listening to him in the first place? Since supposedly all the listeners were sent to the voice channel of the 1st guy.

EDIT: ALSO, how come then people still get stepped on with Trunking systems APCO25 like in San Francisco? Is it because there's not enough voice channels so the person ends up transmitting on an already used frequency?
 
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fineshot1

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Yes, this is much better, I'm almost there.

Now I just have to clear up the following confusion.

You say: when the person pushes to talk and is assigned to a voice channel, all the listeners are automatically sent to that voice channel to receive what the guy is saying. But what about a second guy who is also doing the same thing. What makes sure that the second guy is heard by the dispatch and that anyone is even listening to him in the first place? Since supposedly all the listeners were sent to the voice channel of the 1st guy.
That would be "All the listeners with the same talkgroup id(channel)" are sent to the same voice channel
so they can all hear the same conversation as everyone else with the same talkgroup id. This second
guy you are talking about would not be trying to talk due to him listening to another person talking at
this time. If you are referring to two people on the same talkgroup(channel) pushing the ptt switch on their radios at the same time thisis almost not possible as there is always a few milliseconds difference and
the system can differentiate between user a and b ptt time. If a 2nd user attempts attempts to step on
another person already talking he gets a reject bonk on his radio - in effect the system rejects his
attempt to get a channel grant and does not allow two people on the same talkgroup at one time to
transmit. If this 2nd guy is affiliated to a different talkgoup then he will be granted a voice channel as
long as this other talkgroup is idle.
 

kandrey89

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OK, so am I correct in assuming that there is 1 dispatcher on each talk group in reality/or fascimily?
And that a police unit 1 on talk group 1 talking to dispatcher 1 cannot directly hear or talk/tell police unit 2 on talk group 2 where he is at. So how does the announce work? How does dispatcher 1 tell the location of police unit 1's location to all other units on other talk groups?
 

fineshot1

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OK, so am I correct in assuming that there is 1 dispatcher on each talk group in reality/or fascimily?
And that a police unit 1 on talk group 1 talking to dispatcher 1 cannot directly hear or talk/tell police unit 2 on talk group 2 where he is at. So how does the announce work? How does dispatcher 1 tell the location of police unit 1's location to all other units on other talk groups?
Dispatchers are another story as they usually have access to many talkgroups represented each
as icons on a computer screen at the same time and listen to them all(or many) at the same time.
Yes - and they don't get paid enough for that either. :)

The announcement works due to the dispatcher being able to multiselect talkgroups on the
system console.
 

kandrey89

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That's exactly what I was thinking, thanks for confirming it. Obviously dispatchers have access to the computer system setup governing the talk groups, I wasn't sure if this was possible though, but it'd make sense that they should be able to talk to multiple talk groups at the same time in order to relay information between talk groups. That also explains for the messages like "Clear All Channels unless you have an Emergency" then followed by announcements like Hot Sheets.

Thank You for explaining.
 

fineshot1

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OK - no problem. Glad I could help.

Just think of a talkgroup as a "virtual channel" that can wind up on any of the trunk system
voice channels as the trunk system computer assigns them to a conversation when available.

Also - of course you can have many talkgroups in active conversation at the same time and
that is "one" of the main purposes of a trunk system which is to efficiently use radio spectrum.

Note: You can also have conventional freqs that are not part of the trunk system but may also
be accessable to the users provided by the agency whom ever that may be(ie: twp,county,state).
 
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