Virginia FD Report: Digital Radios Extremely Vulnerable

[kyparamedic]

Digital is fine for fire ground ops.

The problem comes into play when they are using a trunking system. Why in the World would you want to run your FG ops through a repeater/trunking system when you're talking to fire team members that are a few feet away or to the Command Post that is right outside???

Back before digital, we NEVER ran our FG ops through a repeater - ever. It was always on a single frequency for transmit and receive.

Switch to a conventional (non-repeater) channel for FG ops. The Command Post can relay info on the TRS if the need arises.

I'll give you one reason - so that everything can be recorded by dispatch. Our fire dept. got a grant that put our whole county on a trunked system for interoperability. After 6 months, they went back to conventional VHF because they weren't satisfied with the in-building coverage. They wanted everything going through the repeater so that everyone was listening and so that it would be recorded and monitored by dispatch. They had some issues on a structure fire where the audio was garbled and unreadable on the TRS. Standard simplex channels were put in all the TRS portables for FG ops but they didn't want to use these as this defeated their whole purpose for the TRS, which was the emergency button. Very stupid in my opinion, but that's why I'm not on the FD. So now they no longer use the TRS and we have less interoperability than before when we were at least all on VHF. Now law enforcement and EMS are on trunked and FD is on VHF. Kind of ironic if you ask me. They still have to use simplex on some fire scenes because of coverage issues on VHF. Yet I'm able to go into that same building on my trunked radio and get out just fine.

 

[rescue161]

What's to stop dispatch from recording/monitoring a simplex channel? Dispatch did it that way long back before digital. It can still be done. People have become lazy in that it's easier to let the repeater/system make it easier for them and they're buying in to the idea that trunking is the BEST. It's not. It is very easy for an agency to record comms on a simplex frequency and lot cheaper than paying Motorola or M/A-Com to do it for them.

I'll give you one reason - so that everything can be recorded by dispatch. Our fire dept. got a grant that put our whole county on a trunked system for interoperability. After 6 months, they went back to conventional VHF because they weren't satisfied with the in-building coverage. They wanted everything going through the repeater so that everyone was listening and so that it would be recorded and monitored by dispatch. They had some issues on a structure fire where the audio was garbled and unreadable on the TRS. Standard simplex channels were put in all the TRS portables for FG ops but they didn't want to use these as this defeated their whole purpose for the TRS, which was the emergency button. Very stupid in my opinion, but that's why I'm not on the FD. So now they no longer use the TRS and we have less interoperability than before when we were at least all on VHF. Now law enforcement and EMS are on trunked and FD is on VHF. Kind of ironic if you ask me. They still have to use simplex on some fire scenes because of coverage issues on VHF. Yet I'm able to go into that same building on my trunked radio and get out just fine.

 

[richardc63]

Hi Scott,

I totally agree with you. The only exceptions is where the incident ground is so large that you cannot communicate back from a handheld to the appliance. For example, we have a shopping mall which is the largest in the Southern Hemisphere, 10 acres in area & 8 stories high. Using simplex from the appliance is impossible- so we use a simple analogue (could be digital in the future) PMR base on the roof. 10W and a dipole covers the entire centre to handheld grade coverage. Yes, it is a potential point of failure but in this case simplex will not work. Another one is rail tunnels... ours have trunking coverage in them through leaky coax but what happens when a train derails (or worse a bomb or fire)... ooops, you lose the leaky coax! This is where simple, highly mobile and robust incident ground systems are a must. But using the trunking system for FG ops- never.

Cheers,


Richard

Digital is fine for fire ground ops.

The problem comes into play when they are using a trunking system. Why in the World would you want to run your FG ops through a repeater/trunking system when you're talking to fire team members that are a few feet away or to the Command Post that is right outside???

Back before digital, we NEVER ran our FG ops through a repeater - ever. It was always on a single frequency for transmit and receive.

Switch to a conventional (non-repeater) channel for FG ops. The Command Post can relay info on the TRS if the need arises.

 

[kyparamedic]

What's to stop dispatch from recording/monitoring a simplex channel? Dispatch did it that way long back before digital. It can still be done. People have become lazy in that it's easier to let the repeater/system make it easier for them and they're buying in to the idea that trunking is the BEST. It's not. It is very easy for an agency to record comms on a simplex frequency and lot cheaper than paying Motorola or M/A-Com to do it for them.

They could, but depending on the incident location, the signal is unlikely to make it back to dispatch. That's why they use the repeater. A better solution would be to use a mobile voice logger that's installed in the incident commander's unit.

 

[DaveNF2G]

If the system is designed properly, then there should be no trouble getting a simplex signal back to the dispatcher. It is exactly the same as getting the signal on the repeater input back to the dispatcher. The dispatch center has to be connected to the receiver site rather than using the equivalent of a mobile radio. Then they can hear everything the main receiver hears.

 

[zerg901]

I thought that digital comms were being introduced in order to make more radio channels available. Does anyone know of any area that has made more channels available via the advent of digital comms? I think that Harry M said that LAPD came up with a bunch of new channels when they went digitial, but I have never worked thru their radio system to see how this was accomplished. 99% of the PDs in New Hamsphire went digital, but apparently very few extra channels have been added. The Atlanta area and the Dallas / Fort Worth area have lots of trunking, but I see no evidence that digital has allowed the introduction of multiple new channels there.

Boston FD has been running all of their comms on 4 UHF repeaters for about 25 years now. All comms are done via the repeaters - unless they get into a deep deep hole - then the units in the deep deep hole will switch to talkaround on the output of the repeater. The Boston FD dispatchers are always actively involved in fireground comms (since they can easily transmit and receive at the same time, in a quiet setting).

Peter Sz

 

[DaveNF2G]

Actually, it was refarming that made more channels available (changing the spacing between channels). Digital is supposed to make it possible to run true narrowband voice communications without adjacent channel interference.

 

[zerg901]

I think that channel spacing and channel width go hand in hand.

IIRC the VHF highband was originally spaced at .03 Mhz (155.01, 155.04, 155.07, etc). Then new channels were added at .015 Mhz (155.01, 155.025, 155.04, 155.055, 155.07, etc). Co-channel seperation was maybe 30 miles, and adjacent channel seperation was 6 miles IIRC. I know that with 20K0F3E emissions (regular analog FM) channels spaced at .015Mhz will overlap. (You can see that by just doing the math).

This report might shed some light on the situation -

http://www.ntia.doc.gov/osmhome/reports/2007/FreqAssignMethods_07_447.pdf

Peter Sz

 

[kyparamedic]

Are we talking about new frequencies that are available for use or new "channels" in the sense of talkgroups on TRS? If it's the latter, trunking alone has allowed us to have a lot more channels/TG's for everyday use that we couldn't have before when on VHF. However, we're actually taking up more of the radio spectrum than before because we operate both a VHF and 800 MHz system for redundancy.

 

[DaveNF2G]

Trunking has allowed manufacturer sales reps to inflate the number of "channels" available to a user unrealistically.

If you have a 6-channel trunked system with 5 talkgroups, all 5 users will be able to transmit at the same time. The sixth repeater is the control channel and cannot carry voice on most EDACS, Moto, or P25 systems.

If you have 250 talkgroups (or "channels"), then there still will be only enough real channel capacity for 5 users to transmit at the same time.

 

[rescue161]

But only having 5 "real" channels is unrealistic too unless each of those "channels" is being transmitted on 24/7 without any breaks. This as we know is not the case.

Trunking has allowed manufacturer sales reps to inflate the number of "channels" available to a user unrealistically.

If you have a 6-channel trunked system with 5 talkgroups, all 5 users will be able to transmit at the same time. The sixth repeater is the control channel and cannot carry voice on most EDACS, Moto, or P25 systems.

If you have 250 talkgroups (or "channels"), then there still will be only enough real channel capacity for 5 users to transmit at the same time.

 

[kyparamedic]

Trunking has allowed manufacturer sales reps to inflate the number of "channels" available to a user unrealistically.

If you have a 6-channel trunked system with 5 talkgroups, all 5 users will be able to transmit at the same time. The sixth repeater is the control channel and cannot carry voice on most EDACS, Moto, or P25 systems.

If you have 250 talkgroups (or "channels"), then there still will be only enough real channel capacity for 5 users to transmit at the same time.

Right, but on a conventional system, each separate channel would need its own frequency pair if it's going to be repeated. We have a TG to talk to the hospital on. We don't use it all that often so it wouldn't make a whole lot of sense to tie up 2 frequencies 24/7 just for the ocassional use. In the neighboring county, they have 6 hospitals. Obviously you wouldn't want to tie up 12 frequencies just to be able to talk to these hospitals. There are numerous other examples such as tactical channels, etc. that trunking makes a lot of sense for.

 

[zerg901]

Just to throw another angle at y'all.

If you are using a conventional repeater system, and someone nearby tells you that you are all broken up, you can switch to direct and reach the other party. This is typically not possible with a trunked system. (Unless the IC has a 2nd radio to listen on a dedicated simplex / direct / conventional freq).

There is a million angles to all of this radio communications stuff. Peter Sz

 

[richardc63]

Just to throw another angle at y'all.

If you are using a conventional repeater system, and someone nearby tells you that you are all broken up, you can switch to direct and reach the other party. This is typically not possible with a trunked system. (Unless the IC has a 2nd radio to listen on a dedicated simplex / direct / conventional freq).

There is a million angles to all of this radio communications stuff. Peter Sz

Peter,

Huh? So what? There are more things you can't do on a conventional system that you can do on a trunked system. Like multiple conversations, like private calls, and so on. That doesn't mean one is better than the other, but it always is a case of "horses for courses" and choosing the system that suits your needs. I use both and I like both- in the right place.

Cheers,


Richard

 

[richardc63]

But only having 5 "real" channels is unrealistic too unless each of those "channels" is being transmitted on 24/7 without any breaks. This as we know is not the case.

The trick with trunking is not to break your fleet down into ridiculously small talkgroups. I would question why any site with 5 channels would ever have 250 talkgroups affiliated to it. If I was the system manager I would be taking a look at the traffic stats to see whether some of these were warranted. We are a very very large organisation covering a very large area on a trunk system of 100+ sites and we have about 80 tgs of which about 4-5 are used daily and the rest used for special ops. Our smallest regular talkgroup would have 40-50 radios on it and we have stuff all contention between radios needing to transmit simultaneously.

Cheers,


Richard

 

[zerg901]

Richard said - huh so what

I say - it is a tremendous disadvantage if you cannot quickly switch to 'direct' to get out of a hole.

I know that trunked systems have some advantages. Some trunked systems regularly use direct and conventional for firescene ops (ie Cambridge MA FD).

Peter Sz

 

[Raccon]

I say - it is a tremendous disadvantage if you cannot quickly switch to 'direct' to get out of a hole.

Why not? If you (can) switch to direct is a matter of the radio, not the system. And there are trunked systems with radios that support direct communication.

 

[kyparamedic]

Why not? If you (can) switch to direct is a matter of the radio, not the system. And there are trunked systems with radios that support direct communication.

I was wondering about this. So it's possible to switch to a simplex mode on a trunked radio and talk to someone who's operating on a trunked channel? How does this work? We have a walkie-talkie mode on our trunked radios that is just a conventional, simplex set of channels but we cannot hear anyone operating on a trunked channel nor can they hear us, so it's rarely used.

 

[N1BHH]

Some systems have conventional frequencies connected with a specific talk group, VHF to 800 as an example. Some are even in the same band. That's called flexibility. Your firefighter needs to learn how to use the radio as part of his academy training and when he has trouble, they should know enough to switch the channel when needed.

 

[richardc63]

Some systems have conventional frequencies connected with a specific talk group, VHF to 800 as an example. Some are even in the same band. That's called flexibility. Your firefighter needs to learn how to use the radio as part of his academy training and when he has trouble, they should know enough to switch the channel when needed.

Clyde,

I think I understand what you are saying but I'm not sure how it works in practice (in a multsite trunking system). Are you saying that the conventional frequency used is one of the voice channels (base tx) on the trunking base? I can "get" that but in a multisite trunking system different frequency pairs are used at different sites (for obvious reasons) so how do you choose the conventional freq to match?

Cheers,


Richard