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TDMA w/ DCDM and Talk Groups on Simplex

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jonnyblanch

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Dec 25, 2017
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Hello all,

I apologize for the title; I hate acronyms but what can you do. I am a volunteer fire fighter, and I am investigating different possibilities with our radios. Bare with me because I am not going to get this across properly, but I am interested in knowing if the following is possible using Mototrbo XPR 7550e radios:

We would like to have everyone talking on the same channel, for example Fequency 1 Slot 1. If there is traffic on this channel and a talkgroup would like to start a new conversation, one user could start a talkgroup conversation on say Freq 1 Slot 2 and all other users in the talkgroup would start hearing this new conversation while the other continues on Slot 1. Once the conversation is completed, the talkgroup call ends and all users revert back to hearing each other on Freq 1 Slot 1.

To word this differently, here is the intention: If multiple crews arrive on a scene, say two fire engines and a tower, we would like them all to be on the same channel. However, if there is a discussion happening between Engine 1 and the Tower that does not involve Engine 2, we'd like to be able to have engine 2 switch to a different talkgroup (or channel?) automatically if one of the Engine 2 crew members starts a talkgroup call. We'd like to do this without having to interrupt the ongoing discussion to tell the Engine 2 crew to switch channels, and more importantly to switch back. Does this make sense? Maybe another possibility is to be able to interrupt someone outside of the talkgroup. Example: Engine 2 Member "talks over" an Engine 1 member, but only to the Engine 2 talkgroup. I understand dual channel direct mode, but I don't know if what my I'm describing is possible. Again, this is all simplex.

I feel like I have all of the pieces to the puzzle, but don't know where to put them. I can explain further why I'd like to do this, but I will stop here unless someone asks. I am not new to radios (Ham), but DMR is not in my comfort zone. I have done a lot a research but have no confident results. Any advice or even other suggestions would be appreciated.
 

masstech

Member
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Oct 22, 2017
Messages
128
> automatically

is unfortunately not how DCDM works. Additionally - when an Engine 2 member transmits, what determines if they are transmitting on the Engine 2 group call or the general group call?

If you don't require this being automatic, make two channels with the same frequency and different time slots, and you're done (electing one of the channels to be the preferred timing leader and the other as eligible, or both as eligible, of course).

I would not trust MOTOTRBO for fireground operations. Any technologically inclined whacker can figure out how to transmit the Radio Disable command - simplex too, no need for RAS if you're transmitting on the output - and disable radios that fire members rely on.

If I recall correctly as well, the USFA, after going over countless NIOSH reports, disrecommends trunked systems (and perhaps all digital mode as well) - I should look up the communications guide again and read over it some time.
 

kc8kek

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Oct 13, 2006
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65
Analog

Most best practices and white papers are still recommending FM analog, and this would satisfy the interop requirements, since DMR is not common in public safety. Since you're not using P25, I'm guessing you would have issues at incidents that grow in scope. At least consider the NPSPACs for fireground use at large incidents(?).

We are on a P25 trunked 700 system, but still go to analog FM for fireground. Part of the reason was to avoid the ineligibility issues with the first generation of vocoders. You might have a similar problem on DMR, I really can't say (saws running in the background, high noise environment, etc - FM might be better than DMR here).

Just my thoughts.
 

jonnyblanch

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Dec 25, 2017
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Thank You both for the info. To address both of your comments regarding analog, in addition to the county wide frequencies, our department has three frequencies for internal use. I was investigating making only one digital and leaving the other two analog for the same reasons you gave. I just figured if we had the digital radios we should try to utilize them, but maybe I'll abandon my efforts to go digital on one of the frequencies and we'll keep it all analog. Thanks.
 

masstech

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Oct 22, 2017
Messages
128
Being frequency-efficient isn't a bad thing - it's commendable and reduces costs.

If you only need 1 fireground channel, you can use the other 2 for 4 digital channels. If you need 2 fireground channels, use the third frequency for 2 digital channels.

You can use 1 channel for training, the other for administrative use, etc. Uses that do not have lives on the line.

If you need more frequencies, then just license up for some more.
 

mdsxfire

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Dec 24, 2005
Messages
944
Location
New Jersey
Most best practices and white papers are still recommending FM analog, and this would satisfy the interop requirements, since DMR is not common in public safety. Since you're not using P25, I'm guessing you would have issues at incidents that grow in scope. At least consider the NPSPACs for fireground use at large incidents(?).



We are on a P25 trunked 700 system, but still go to analog FM for fireground. Part of the reason was to avoid the ineligibility issues with the first generation of vocoders. You might have a similar problem on DMR, I really can't say (saws running in the background, high noise environment, etc - FM might be better than DMR here).



Just my thoughts.


We are moving our fire dispatch/ops along with all other town services to our county digital p25 system from an analog vhf repeater. I’m looking forward to the coverage as our current system even with voters(that always go down) has horrible portable coverage, but have wondered about how the digital handles talking while on air with background engine saw noise etc. All of our M/A depts are already on p25 systems and don’t seem to have any issues but time will tell I guess


Sent from my iPhone using Tapatalk
 
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