DMR Primer v1.0 Information

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Brian
Posts: 795
Joined: Thu Oct 21, 2004 8:54 pm

DMR Primer v1.0 Information

Postby Brian » Wed May 25, 2016 11:14 pm

Nice Information posted by Joe on the BCD536HP/436HP yahoogroup today.

______________
DMR Primer v1.0

DMR has at least six flavors: (with a seventh imminent)

1. Simplex - This type is RF-wise the same as conventional analog simplex. The only significant differences are that the mode is digital, and the carrier is not constant. What this means is that it typically cannot be "heard" on most scanners unless you open the squelch while monitoring the frequency. Likewise, they will (in the vast majority of
cases) not trigger a Close Call hit. These are both due to the rapid switching of the transmitter on-and-off. The transmitter only transmits half of the time, and the squelch action of most scanners will not react before the carrier is back off. fortunately, this is the only DMR mode (not counting inputs) that is so afflicted. The frequency, slot, and TGID used is programmed in the radio itself.

In theory, two simplex users can use the same frequency at the same time if the radios can hear each other for self-assigned timing. I have never confirmed this theory.

2. Duplex (Repeated Conventional) - This type is again like analog repeated except it is digital. Unlike simplex, the repeater outputs always transmit full-time - transmitting timing data for all users. This allows two users (one on each timeslot) at the same time. When two are using the repeater, that on slot 1 is instructed to transmit. While the repeater receives this data, the data from slot 2 is being forwarded out the repeater. Then the process reverses - user 2 transmits while the data from user 1 (that was stored) is transmitted out the repeater. The frequency, slot, and TGID used is programmed in the radio itself.

Conventional repeated can be easily heard on most any scanner in FM or NFM mode (technically, it is more narrow [7.6 kHz BW] than NFM [11.2 kHz BW], so that is the preferred mode to monitor). They can also be detected by Close Call. The input signal sounds like simplex and is like simplex in how it operates (no Close Call hits on the inputs unless you have two users within Close Call range using the system at the same time).

3. IPSC (Linked) Repeated - IPSC is Internet Protocol Site Connect. It is a simple linking of several conventional repeaters. Most ham networks use this mode (such as DMR-MARC). This mode operates exactly like Repeated Conventional except that there is a 2-second burst that is transmitted every 30 seconds. This is a distinguishing characteristic of IPSC systems. IPSC can reportedly have up to 15 sites, but obviously you can link networks together as the ham community has done to link many more sites worldwide. The frequency, slot, and TGID used is programmed in the radio itself.

4. Capacity Plus (AKA Cap+) - This is a trunked mode. The trunking logic exists through a databurst on the "rest channel". This databurst signals which frequency and slot will be used next. When a call is made, it will show up on this frequency and slot and the databurst will move to the next slot (usually on the same frequency UNLESS that is busy). If both slots are in use, it will move to the next LCN on the first available slot. If there are no "next" slots, it moves back to LCN1 slot A (if available).

A distinguishing characteristic of this system type is the rest channel burst which will usually occur every 3 to 10 seconds (my experience is usually 3) and lasts only 50 mS or so. It's usually not enough to open the squelch on a scanner, but will show on the S-Meter if you are sitting on the frequency in squelched mode. If in open squelch, you will hear the gap in the squelch noise.

There can be up to 8 LCNs on a Cap+ system each supporting two timeslots
(16 TGs can use the system at the same time). The color code is assigned PER LCN and need not be the same for all frequencies.

The TGID used is programmed in the radio itself. The frequencies and color codes are programmed in the trunk system information, and the LCN and Slot are controlled by the system.

Something I discovered recently is that if data is used on the same system, it will usually be assigned to the next LCN higher than the rest channel. This is likely to allow the databurst to continue uninterrupted. This can interfere with scanning a Capacity Plus system in conventional mode. The color code is still transmitted for the data LCN.

5. Linked Capacity Plus (AKA LCap+) - This is another trunked mode. I don't have much experience with this mode, but it is assumed it is a hybrid of Linked Conventional and Capacity Plus. Again, there is technically a 15-site limit.

6. Connect Plus (AKA Con+) - This mode is another trunked mode. It operates much like P25. You have a dedicated control channel (technically one slot on one LCN). This control channel is usually LCN 1 Slot A, but can be on any LCN and (assumed) on either slot. I've never heard it on any Slot B, but it might be possible. This control channel transmits continuously and sends LCN and Slot info for channel assignments to the user radios. There can be many linked sites in this system type and there are many multi-dozen site systems across the USA.
This is the only mode that has the continuous transmit (which is the bane of co-channel users everywhere). Some system admins try to use old paging channels for the control channel since they are often exclusive allocations within the same geographic area (unlike LMR channels). After all, they don't want interference with their control channels either.

Con+ can have up to 250 sites connected. Each site supports up to 15
repeaters. As one slot is used by the control channel, this allows up to
29 simultaneous TGs to use the system at the same time.

The TGID used is programmed in the radio itself. The frequencies and color codes are programmed in the trunk system information, and the LCN and Slot are controlled by the system.

7. Capacity Max. Not much is known about this mode yet, but it is Tier III compliant which means it should work across multiple vendors. This is available in single-site and multi-site variants. Linked Capacity Plus systems can be upgraded via software to this new flavor.

Encryption:
-----------
There is basic encryption (255 keys) and enhanced encryption (over 2 quadrillion keys - 40 bit). These may be employed on any flavor of DMR.

Also, encryption is typically not compatible between manufacturers.

Color Codes:
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All DMR frequencies use Color codes which are akin to CTCSS for DMR.
These color codes range in value from 0 to 15.

Slots:
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As mentioned above, DMR is a TDMA mode that has two voice (or data) paths per frequency. Each frequency has slot 1 and slot 2, and each can be used by different users. On conventional, they are independently assigned. For trunked modes, they are assigned by the controller and each TG can use any slot on the system.

TGID:
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TGIDs are essentially the same as any other modern trunking system.
TGIDs are used on every DMR flavor including conventional duplex and simplex.

______
Joe M.

http://dmrassociation.org
http://wiki.radioreference.com/index.php/DMR
https://en.wikipedia.org/wiki/Digital_mobile_radio

------------------------------------------------------------------------
Motorola has changed their names for two types of systems:
- Capacity Plus Single Site was formerly known as Capacity Plus.
- Capacity Plus Multi Site was formerly known as Linked Capacity Plus.

Justin's search tools:
Searchable database to assist those looking to find and scan Digital Mobile Radio (DMR) radio traffic in the area.

DMR Frequency Finder
http://justinpulliam.com/DMR/Search.html

NXDN Frequency Finder
http://www.justinpulliam.com/DMR/NXDNSearch.html

Conventional P25 Phase 1 Digital Frequency Finder
http://www.justinpulliam.com/DMR/P25Search.html

DMR Frequency Finder -- Quick Import for EZ Scan, Sentinel, ARC
http://www.justinpulliam.com/DMR/QuickImport.html

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