MDC1200 Repeater Access ?

Bob · Post by **Bob** » Thu Dec 23, 2004 7:13 am

I'm kicking around the idea of putting up a repeater, but I want it to be closed. The best way I can think of (right now anyway) is to use MDC1200 access. I'm not going to go as far as locking it down to the MDC ID level

Which controllers will support this? I haven't experimented with much beyond the CAT200 controllers.

RESCUE161 · Post by **RESCUE161** » Thu Dec 23, 2004 10:35 am

I'm curious as well.

Could this be used on a Spectra Securenet lash-up repeater? I've notice that any radio on the input frequency can key up the repeater that we're using. I'd like to cut that out. It's not really a problem now, but it could be in the future.

RKG · Post by **RKG** » Thu Dec 23, 2004 11:57 am

There are onlyl two ways I know to use MDC for repeater access.

If you have a Quantar, you can add the Station Access Module ("SAM"). The SAM fits where the second receiver would go, has its own DB9, and its own RSS. The SAM can be programmed to decode and vet MDC IDs and, if a valid one is detected, pass a variable to the Quantar; the Quantar, in turn, can be programmed to require a valid parameter from the SAM for qualifying incomming traffic for repeat.

Alternatively, you can acquire a Cimarron MDC decoder, which has auxiliary relays (and some useful preprogrammed routines) that can be used for controlling access to a repeater.

Post by **xmo** » Thu Dec 23, 2004 1:29 pm

SAM was introduced for the MSF5000. You can still get an MSF SAM board from Motorola. Interestingly enough - the Quantar SAM module contains an MSF SAM board!

Many Motorola radios can send a repeater access packet [RAC]. This can be programmed to a button press or to be sent with every PTT. The RAC packet is unique and distinct from PTT-ID.

SAM can respond to either RAC or PTT-ID to initiate specific actions. I have used this to set up repeater access to interoperability repeaters so they stay off the air [as required by NPSPAC rules] until activated by the dispatcher. MDC avoids the need for wireline repeater knockdown control.

With both a SAM and enhanced wildcard in a Quantar it's amazing what you can accomplish.

The STROKE repeater system uses Centracom II MDC decoder cards to recover the MDC signalling. These cards are equipped with custom firmware [the guys that built the STROKE system work at Motorola].

The decoded MDC ID and signalling is then passed to a custom written system control program called "Scrutenizer". The control program sets up user access and does other magic like displaying who is on the system.

I have played with this concept on a smaller scale using a Motorola BSC-II device as an MDC modem. I connected it to the serial port of an old PC. I connected the PC's second serial port to the serial port on a Link Communications RLC-3 repeater controller.

I wrote a simple program [in BASIC] to control the BSC-II, recover the MDC packets, and send commands to the RLC-3 based on the received MDC. With that arrangement you can disable the repeater, control auxillary devices, access the DVR mailboxes, etc.

If a person wanted to do this on the cheap - you could use a Maxtrac logic board as a decoder. You would get an 800 Maxtrac with a 16 pin logic board - cheap these days. You could rip out the existing RF board and feed the logic board from your repeater receiver.

Set the Maxtrac up as hi-signaling and enable one personality with MDC. Then set up a call list that the Maxtrac will use to alias one or more MDC ID's to a specific two-digit display number. The idea is to remove the 2 digit seven segment readout and use the driver IC to provide outputs. The specific "alias" programmed into the call list would be chosen to activate a particular display segment driver.

The decoded driver output would then be connected to an aux I/O input on your repeater controller. With a Link controller you could set up an input bit to activate a macro that would enable repeater access, etc.

With a perf board from Radio Shack, a handful of parts, and a little imagination - the sky's the limit.

ASTROMODAT · Post by **ASTROMODAT** » Thu Dec 23, 2004 7:07 pm

A great way to limit and strictly control access to a repeater is via a system that reads the "fingerprint" of the mobile or portable radio's PLL circuit (and it doesn't cost a $1,000 for a SAM card!). I know it sounds like a James Bond thing, but it is a real product. A Ham in my area has had such a sytem on 2 meters for about 10 years now. I believe there are ads for this sytem in QST magazine. I'm not sure what their webb address is, but I bet someone else on this board knows.

It has a digital buffer that looks at the unique ramp up signature of the radio's PLL VCO, and compares it to a known database of stored PLL VCO fingerprints. If it is a known "Bad Guy," he can't bring up the repeater. The default is that if you're not a Bad Guy, your radio will be allowed to key up the repeater. Of course, if the Bad Guy gets a different radio, you're screwed until he gets filthy mouthed, etc. and is locked out on that particular radio. The system is almost 100% bulletproof, with almost no falsing, and very rare instances of missed Bad Guys.

The only downside is that it is subject to aggressiveness by the repeater owner. The 2M machine in Seattle was run so strictly for a while that if a radio keyed up 3 times without ID'ing (e.g., kerchunking), the system owner tagged that radio as a Bad Guy. You had to turn yourself in to him with a sob story to get unlocked, or wait until you bought a new radio, if you ever wanted to get on his machiune again. I think things have dramatically loosened up in this regards in the last couple of yeasrs, as 95%+ of the Ham 2M and 440 users in Seatlle have gone away (e.g., switched their technical interests to PCs, the Internetet, video games, cell phones, etc.), so most surviving 2M and 440 machines are all but quiet now, anyway.

It also works for the old crystal controlled radios. At the same time, in recent years, the manufacturer has markedly improved the software so that it can properly and reliably discern the incresasingly smaller differentials in PLL fingerprints as mass production (e.g., laser trimmed diodes, surface mount technologies, etc.) have made newer technology radios less and less different in terms of their unique fingerprints. This has been an ongoing battle for the vendor to stay ahead of this trend for a particular brand of any given model to be less and less distingusihable between production output samples as the fingerprints of modern radios get more tightly similar.

larry

jim · Post by **jim** » Fri Dec 24, 2004 3:05 pm

The problem is that if the system doesn't get a clean signal, you may not access it. Something as a simple component change to your radio may change the "fingerprint",also.

ASTROMODAT · Post by **ASTROMODAT** » Fri Dec 24, 2004 5:40 pm

1. If your signal is so bad as to not be clean enough for the RF fingerprint ID system to work, then you are maybe 1 dB, or less, above the repeater’s receiver squelch threshold level, in which case that crappy of a signal should not be “allowed” to bring up the repeater anyway. Most repeater owners tend to set their repeat threshold to at least 2+ dB above the squelch break point. He has measured the C/N requirement, with and without the fingerprint system, and it only involves less than 1 dB increased signal level, which is insignificant. From a practical standpoint, the typical squelch adjustment on the repeater is such that the fingerprint system will not be the determining factor in bringing up the repeater for any given input signal. The signal quality (as measured by its C/N ratio) will determine this, rendering the fingerprint system essentially irrelevant in affecting the minimum C/N required to “bring up” the repeater.

2. Today’s modern day typical Ham transceivers involve all flatpacks with surface mount technologies, using multi-layered circuit boards, and they involve extremely few (if any) discretes in the RF chain. The idea of a typical Ham opening up such a radio and repairing it (e.g., “replacing a component”) is a joke. The days of Micors and Heathkits have been over for many years now. Just doesn't happen anymore. For the one in 5,000+ cases where someone might attempt to replace a discrete component, it would merely mean that if this person had been previously marked as a Bad Guy, then he could now possibly bring up the repeater. If the discrete was not in the RF chain, it’s quite doubtful that it would markedly affect the radio’s RF fingerprint. He could change his RF fingerprint by borrowing someone else's radio, or otherwise using a different radio anyway. Regardless of this, the typical Bad Guy will soon get foul mouthed again anyway, in which case his changed component radio will be soon be locked out as a Bad Guy. In any event, I would surmise that folks who need to be marked as Bad Guys are probably not the type of person who typically would have the smarts, self discipline, nor knowledge, etc. to be able and competent to successfully replace a discrete in one of today's modern day technology radios.

larry

jim · Post by **jim** » Fri Dec 24, 2004 7:17 pm

...and if they CAN'T access it, they can still put RF on the input to keep anyone else form accessing it.

Then you track them down and smack them silly with an HT500 lowband antenna!

Post by **/\/\y 2 cents** » Fri Dec 24, 2004 7:52 pm

you can always use an i750 repeater controller which has MDC1200 access as an option. (sold a few and they are pretty MDC feature rich but are hell to set up.) Or you could try RAT tones..those have been used even by government agencies to deter unauthorized transmissions.

apco25 · Post by **apco25** » Sat Dec 25, 2004 5:59 am

I thought the whole point behind MDC or Single Tone RAT was to access rarely used repeaters or select specific repeaters in a coverage area.

SlimBob · Post by **SlimBob** » Tue Dec 28, 2004 10:32 pm

/\/\y 2 cents wrote:you can always use an i750 repeater controller which has MDC1200 access as an option. (sold a few and they are pretty MDC feature rich but are hell to set up.) Or you could try RAT tones..those have been used even by government agencies to deter unauthorized transmissions.

As in Nextel i750?

Post by **xmo** » Wed Dec 29, 2004 2:01 pm

"As in Nextel i750?"
________________

Radius i750R interconnect controller.

You can also set up MDC access with a RICK if you have a signaling capable radio [Max300 Hi Sig / GM300] as the repeater receiver.

Bob · Post by **Bob** » Wed Dec 29, 2004 2:46 pm

xmo wrote: You can also set up MDC access with a RICK if you have a signaling capable radio [Max300 Hi Sig / GM300] as the repeater receiver.

Do tell....
i750s are tough to find, and about $600 new. RICKs are plentiful. I've just never played with either.

I have a few signalling capable radios around - the plan was to use a maxtrac for the RX radio and a Maratrac or M400 for the TX radio.

Post by **xmo** » Wed Dec 29, 2004 2:51 pm

"Do tell...."
________

I had forgotten about that - but you can use MDC call alert for repeater set-up and knock-down with a RICK! It's all explained in the RICK instruction manual.

RESCUE161 · Post by **RESCUE161** » Wed Dec 29, 2004 3:00 pm

What about using the homemade type RICKs?

Post by **bradlington** » Sat Jan 22, 2005 11:17 am

Hi
My thought would be to use 5-tone as the way you could impliment it is basic to very advanced.
The advantage is one could allocate groups -emergency down to individual calls , so radios and repeater can be used in a diverse manor.
MDC-1200 is almost never used in our country but 5-tone is very well used amoungst utility and police services as well as my ham repeaters.
Regards

Brad

Post by **bradlington** » Sat Jan 22, 2005 11:19 am

I meant to add that freeware programs such as Wintone can be used to log all traffic with time/date stamps etc.

Brad

apco25 · Post by **apco25** » Sun Jan 23, 2005 1:44 pm

why not simply config the RX maxtrac to AND squelch requering a MDC-1200 ID packet to open it up. You will still have logging issues but the radio would stay muted and ignore non mdc signals.

RESCUE161 · Post by **RESCUE161** » Mon Jan 24, 2005 3:29 am

Couldn't the pirate just wait to talk during the squelch tail? Or could you set it to mute on AND? If it's set to AND, as soon as you stop talking, the receiver will go deaf, right?