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General Scanning Discussion For general questions not specific to a model of scanner or general discussion of use of a scanner. Location specific posts should be directed to the regional forums listed below.

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Old 08-25-2008, 5:17 PM
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Default Mobile Data Terminals

Hi all,

I'm curious to know how police mobile data terminals generally work. From what I've gleened in the RRDB, MDTs are assigned a dedicated frequency, but if every MDT in a fleet is sharing the same channel, how do they not step on each other? Presuming that MDTs are "connected" all the time, how can more than one MDT be online at once? And even if MDTs were to "login" each time the officer ran a plate, how does the system prevent multiple MDTs from transmitting on the dedicated channel at the same time?

Just curious.

Dave
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Old 08-25-2008, 5:26 PM
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It depends what protocol you are talking about. With Mobitex, for example, the subscriber units transmit in bursts, and wait for a reply from the base. When multiple units key over each other, neither will get an acknowlegement, so they both wait a random amount of time and resend the same packet.

I dont know for sure, but I have a feeling that MDT4800 works in a similar way.
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Old 08-25-2008, 5:45 PM
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Very similar to how a computer network functions.
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Old 08-25-2008, 6:22 PM
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Thank you!
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Old 08-25-2008, 6:58 PM
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Quote:
Originally Posted by Dave_D View Post
Hi all,

I'm curious to know how police mobile data terminals generally work. From what I've gleened in the RRDB, MDTs are assigned a dedicated frequency, but if every MDT in a fleet is sharing the same channel, how do they not step on each other? Presuming that MDTs are "connected" all the time, how can more than one MDT be online at once? And even if MDTs were to "login" each time the officer ran a plate, how does the system prevent multiple MDTs from transmitting on the dedicated channel at the same time?

Just curious.

Dave
Think of it as like the internet on your home computer. You can have multiple windows open, multiple programs running, all being piped through one internet connection. Thousands of bits of information being funneled through and processed. Each command gets routed and returned to the correct location.

A lot of modern MDTs have two data functions and connections. A connection to the dispatch (CAD) system and a connection to criminal/records databases. CAD connections and security protocol is department/agency defined. However, most State Department of Justice offices have mandated that most (criminal) database systems are to be encrypted or secure within their standards.

You'll find that not all MDTs use typical radio networks to transmit/recieve data. Some MDTs utilize cellular connections (AirCards). In some cases a radio network will be used to send/recieve CAD data while a cellular connection is used for the criminal databases. These cellular aircard connections can also allow for internet connectivity which can give access to private intra-net connections.
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Old 08-25-2008, 8:05 PM
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Think of it as like the internet on your home computer. You can have multiple windows open, multiple programs running, all being piped through one internet connection. Thousands of bits of information being funneled through and processed. Each command gets routed and returned to the correct location.

A lot of modern MDTs have two data functions and connections. A connection to the dispatch (CAD) system and a connection to criminal/records databases. CAD connections and security protocol is department/agency defined. However, most State Department of Justice offices have mandated that most (criminal) database systems are to be encrypted or secure within their standards.

You'll find that not all MDTs use typical radio networks to transmit/recieve data. Some MDTs utilize cellular connections (AirCards). In some cases a radio network will be used to send/recieve CAD data while a cellular connection is used for the criminal databases. These cellular aircard connections can also allow for internet connectivity which can give access to private intra-net connections.
ohhhhhh! nice explaination!
MDT's always come up on here in every few times a month! the main reason is we all want to know how to read them
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Old 08-27-2008, 1:26 PM
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Believe it or not, my Google searches for Mobitex & MDC4800 came up with some surprisingly sophisticated instructions (with program code) for intercepting and decoding MDT transmissions. That's not my bag, for sure, but the point of the article worth sharing here is that the author claims that the encryption employed by MDTs is easily undone. And it would appear that he has proven it. That's pretty scary.

What I would consider doing, if it were worthwhile, is monitor for the presence of MDTs while driving. If they emitted a steady enough signal, you could pretty easily determine if you're approaching a speed trap. But this seems moot if law enforcement is using AirCards, etc. There'd be no way to isolate the MDT signals from every other cell phone in the vicinity. And, uh, nevermind that scanning cell phone frequencies is illegal....
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Old 08-28-2008, 7:43 PM
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This is where the challenge exists, finding different ways to display the data bursts as they get transmitted over the air.
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