Question about Trunking

[instrumentphilip]

Qustion 1: I have a question about trunking. I am pretty sure I have the concept understood for the most part. Handheld radio sends request to the Control Channel and the control channel gives the radio / talkgroup a free Frequency from the pool of frequencies on that radio system. Then switches to another ( frequency) if a radio is not keyed up within a certain time.

What I have a question about is when the Let's say on a P25 Trunking System a Police Officer keys up his handheld radio on Talkgroup 310 ( Dispatch Talkgroup ) and his radio sends a request to Control. Now When he first keys up does the control channel / or just control have an input frequency? Like is that radio listening on the Control Frequency for commands but is transmitting to talk to the control frequency on another frequency / input frequency?


Question 2: When the radio on Talkgroup 310 Gets assigned a frequency how is that working? I mean I know there must be some sort of input frequency for the transmission of lets say a Portable Radio from a Police Officer to be re-transmitted / relayed over the transmit site at the Police Station so everyone can hear it. Because I know when they (Talkgroup ) get assigned that frequency from control channel they cannot be using simplex strictly on the assigned frequency.

Thanks,

Philip

 

[GrumpyGuard]

Let me see if I can break this down as you have a pretty good idea about trunking.

What I have a question about is when the Let's say on a P25 Trunking System a Police Officer keys up his handheld radio on Talkgroup 310 ( Dispatch Talkgroup ) and his radio sends a request to Control. Now When he first keys up does the control channel / or just control have an input frequency? Like is that radio listening on the Control Frequency for commands but is transmitting to talk to the control frequency on another frequency / input frequency?{/quote]

If you look at the frequency table for the system you are monitoring you will notice the control channel is listed in red and the alternate is listed in blue. When the radio is keyed the computer will assign it a frequency and the computer will tell all radio's on talkgroup 310 to open their squelch and allow them to hear the conversation. If the other person responds right away then they will stay on the same frequency. If the carrier drops when the person keys up the radio to answer they may be assigned a different frequency.

Question 2: When the radio on Talkgroup 310 Gets assigned a frequency how is that working? I mean I know there must be some sort of input frequency for the transmission of lets say a Portable Radio from a Police Officer to be re-transmitted / relayed over the transmit site at the Police Station so everyone can hear it. Because I know when they (Talkgroup ) get assigned that frequency from control channel they cannot be using simplex strictly on the assigned frequency.

This is handled by the computer and the control channel tells the radio what frequency to listen to and transmit on. Follow this link to Signal Harbor he has a wealth of information on this subject. You might look at the RadioReference wiki page about trunked systems also.

 

[exkalibur]

Question 1:

Like every frequency in a trunking system, each repeater has an input and an output. The subscriber unit (police radio) sends an initial request (called an ISW - Inbound Status Word) on the input to the control channel. The trunking controller then decides the appropriate response and sends its response (called an OSW - Outbound Status Word) on the output to the control channel, which the subscriber unit (and all those on the same talkgroup) respond to, such as switching to a voice channel for a voice call.




Question 2:

When a subscriber radio wants to initate a voice call, it sends its request as I wrote above. The controller sends an OSW basically saying "TG 310 switch to channel xx". On 800 and 900MHz systems, those channel numbers are hard-coded to reference a frequency. IE, Channel 360 on an 800MHz system is 859.9875, and is the same on every 800MHz radio around (re-banding not withstanding). On a VHF or UHF system, those channels must be assigned when being programmed. This is why you must input Base, Spacing and Offset when you program your scanner - it has to manually calculate the frequency.

The radio then switches to the assigned voice channel and the voice call can proceed. There are three types of voice transmission types in a trunking system - Message Trunking, Transmission Trunking and Wireline PTT-ID. Message Trunking makes the voice channel work like a regular repeater. That is, until the call is terminated, the voice repeater is active and radios don't need to authenticate with the controller every time they want to transmit. This reduces load on the controller and gives faster channel access time. Downside is that it is possible to "double". Transmission Trunking only keeps the voice channel active for each transmission, meaning it'll get a new voice channel (usually) for each transmission. This allows for greater security as each PTT must be authenticated but causes greater load on the control channel. There is the inherrant possiblity that due to a bad location or what not, a particular subscriber radio won't be able to "raise" the control channel via ISW, so it won't get access to transmit. Not a good idea in a public safety system, or one where users are paying per transmission (IE, lost revenue for the system operator). Enter PTT-ID. It is basically message trunking, however ever PTT is sent to the controller. It doesn't require the controller to authenticate each transmission, but it does allow for logging of who is talking and when.

Once the voice call is complete, the system sends an End Code which tells all radios to go to the control channel, so the process can repeat.

 

[GrumpyGuard]

exkalibur, you did a much better job explaining trunking than I was able to do. Thanks.

 

[loumaag]

Philip:

I recommend that you read the Trunking Basics article, even though the above explanations were fine. However, this is not the forum for such questions, please note that this is the Commercial and Professional Radio area and it is assumed everyone here has this basic knowledge already.

 

[instrumentphilip]

Question 1:

Like every frequency in a trunking system, each repeater has an input and an output. The subscriber unit (police radio) sends an initial request (called an ISW - Inbound Status Word) on the input to the control channel. The trunking controller then decides the appropriate response and sends its response (called an OSW - Outbound Status Word) on the output to the control channel, which the subscriber unit (and all those on the same talkgroup) respond to, such as switching to a voice channel for a voice call.




Question 2:

When a subscriber radio wants to initate a voice call, it sends its request as I wrote above. The controller sends an OSW basically saying "TG 310 switch to channel xx". On 800 and 900MHz systems, those channel numbers are hard-coded to reference a frequency. IE, Channel 360 on an 800MHz system is 859.9875, and is the same on every 800MHz radio around (re-banding not withstanding). On a VHF or UHF system, those channels must be assigned when being programmed. This is why you must input Base, Spacing and Offset when you program your scanner - it has to manually calculate the frequency.

The radio then switches to the assigned voice channel and the voice call can proceed. There are three types of voice transmission types in a trunking system - Message Trunking, Transmission Trunking and Wireline PTT-ID. Message Trunking makes the voice channel work like a regular repeater. That is, until the call is terminated, the voice repeater is active and radios don't need to authenticate with the controller every time they want to transmit. This reduces load on the controller and gives faster channel access time. Downside is that it is possible to "double". Transmission Trunking only keeps the voice channel active for each transmission, meaning it'll get a new voice channel (usually) for each transmission. This allows for greater security as each PTT must be authenticated but causes greater load on the control channel. There is the inherrant possiblity that due to a bad location or what not, a particular subscriber radio won't be able to "raise" the control channel via ISW, so it won't get access to transmit. Not a good idea in a public safety system, or one where users are paying per transmission (IE, lost revenue for the system operator). Enter PTT-ID. It is basically message trunking, however ever PTT is sent to the controller. It doesn't require the controller to authenticate each transmission, but it does allow for logging of who is talking and when.

Once the voice call is complete, the system sends an End Code which tells all radios to go to the control channel, so the process can repeat.

Ahhhh Thank you very much That is a great explanation. Yeah shortly after posting this thread I saw the wiki on here about trunked systems but That response was a nice bit more information.

 

[instrumentphilip]

Philip:

I recommend that you read the Trunking Basics article, even though the above explanations were fine. However, this is not the forum for such questions, please note that this is the Commercial and Professional Radio area and it is assumed everyone here has this basic knowledge already.

ohhhh ouch. haha. Okay Gotcha. So where should I have posted this then Mr. Moderator?

 

[loumaag]

ohhhh ouch. haha. Okay Gotcha. So where should I have posted this then Mr. Moderator?

General Scanning.

 

[instrumentphilip]

Hmm.... That is interesting that is where I was going to post it first but thought that an Mod might kick me out of the forum for posting it there. I thought this was more specific.