DMR is a "standard" (notice quotes), different manufacturers call it something different (Motorola TRBO, Harris Momentum, Icom IDAS). What's different about DMR from P25 is each manufacturer has A LOT of proprietary features they include on their DMR systems that are not compatible with other manufacturer's DMR radios. It happens in P25-land too, but P25 in general is much more standardized.
DMR is like Phase II P25 in that it is TDMA, where there are 2 "slots" per frequency, effectively doubling the resource capacity of a system as long as all the resources only demand TDMA. For comparison, Phase I P25 takes one full frequency for each resource (each transmission takes up one full channel, Phase II only takes up half).
There are many public safety agencies that utilize DMR systems for daily operations however I personally don't think they are robust enough to be used on a large scale when comparing to P25. With P25 you get a true standard. DMR is really pretty cool, there are SO many features for the price point it's unbelievable
Encryption: AES and DES:
Encryption is encryption. It's used on computers and radios, and they're the same. Only difference is for radios, it's used over the air. Basically, a radio will transmit it's information in an encrypted state, that is that only other radios with the same encryption key will be able to decode it. Keys can be manually loaded into a radio via software (Key Managers), or loaded into the radios over-the-air. Either way, they receive the same key. When radios receive their encryption keys over the air, only a pre-defined list of radios are able to receive the key, because they will be pre-loaded with a "Unique Key Encryption Key". Any radio that does not have the Unique Key Encryption Key cannot receive the actual encryption key over the air, so the security aspect of sending encryption keys over the air is "solved" there. Many newer P25 standard radios are capable of handling many encryption keys.
AES (Advanced Encryption Standard) is the new standard for encryption, replacing DES. You can google the difference between the two, but suffice to say that AES is more secure than DES.
What's the difference between Trunking and Conventional?
I see a lot of confusion on the forums about Trunking and Conventional radio systems. Here's a breif summary of the most typical types of radio systems that you'll encounter as a public safety professional:
SIMPLEX: A line of site form of communication between units. There are no repeaters, no systems, no nothin'. Everybody transmits and receives on the same frequency. Line of site because, without a repeater or other infrastructure, distances are limited due to power and hieght of the radios.
REPEATER: Explained in a post above me. Simplex and Repeater systems are considered "Conventional"
TRUNKING: There's a billion different types of trunking systems, so lets keep it simple. Depending on the amount of expected radio traffic (decided on by engineers and stuff), there is a block of frequencies all assigned to the trunking system. Let's say 10 for this example. At all times, ONE of those 10 channels is assigned as a "Control Channel". The control channel is always transmitting, 24/7/365. It transmits a continuous string of information in data bits. All of the radios on the trunking system silently listen to this control channel, waiting to see information that is relevant to itself. One of the things that the control channel transmits is information telling your radio which frequency to switch to to listen to the conversation you want to hear.
So lets say you're on "channel 1" on your channel selector knob on your radio. We'll call it "DISPATCH". The trunking system knows that your radio is on this channel, because your radio REGISTERS with the system whenever you turn it on or change the channel. Your radio is silently listening to the control channel, waiting for information that says "DISPATCH" talk group is active now, and it's going on channel 9. Your radio gathers that information from the control channel, switches itself to trunking channel 9, and listens to the conversation with no input from you at all. throughout the entire conversation, your radio is listening to the control channel, waiting for the next piece of information on where to continue listening to or responding to the conversation. Trunking systems can handle many more TALK GROUPS than there are channels, because the system will send the talk group's conversation to any open spot within the remaining 9 channels. Even if there are 20 talk groups conversations going on in the same time period, the system will try it's best to fit all of that in any open spot on any of those 9 channels. If you paid attention to one single channel in the trunking system, you'd see that it's passing traffic from a ton of different TALK GROUPS. Without going into detail about how it works, the system just looks for an open spot on any channel to pass traffic on, tells all the radios that need to know, and does that all the time for every PTT (push to talk) and every talk group. The only way you would fill up the system is if there are 9 talk groups being used all at the exact same time (actually transmitting), if someone on a 10th talk-group tried to key up, they would be Queued.
The 10-channel thing is just an example, trunking systems can be as small as a couple channels, up to almost an infinite amount of channels (not sure where you would build a 100 channel trunking site though, it would be massive)
