Winston Lawrence

KD2WLL - Amateur Radio Notes

Making DMR connections

        
DMR connection sequence

DMR is an acronym for Digital Mobile Radio and as indicated by the name, it is a digital mode that maintains some connection to its analog radio roots. When you access a DMR repeater you must have the frequency, the color code, and the time slot matching those of the repeater. When you press the PTT key on your transceiver (01)  the repeater will send back an acknowledgement (02)  that you are connected and are okay to transmit.  If you are not okay to transmit you may hear a negative tone, or nothing from the repeater.

Frequency
The DMR repeater operates on an assigned frequency (simplex) or frequency pair (duplex) in the same way that analog repeaters work. The repeater has an uplink (receive) frequency and a downlink (transmit) frequency. In simplex operation the uplink and downlink frequencies are the same.

Color Code
The DMR repeater also has an assigned color code. The color code ranges from zero (0) to fifteen (15). The color code is analagous to the Continuous Tone Coded Squelch System (CTCSS) tones used for analog radio operations. The Color Code (or CTCSS) is a continuous tone that is transmitted along with the signal. The repeater listens for and responds only when the the color code sent by the radio matches the color code configured on the repeater. The reault is that two repeaters that are operating on one or both of the same frequencies may operate simultaneously without interfering with each other as long as they are using different color codes. When a radio makes a connection to the repeater and has both the frequency and color code correct (03) . Again, If you are not okay to transmit you may hear a negative connection tone sent by the repeater, or hear nothing from the repeater.

Time Slot
The main differentiator betwwen DMR and other digital (or analog) modes is the use of Time Division Multiple Access or TDMA. The repeater bandwidth is split into two time slots. The time slots are short enough that communications appear to be continuous to users of the repeater but it allows two conversations to take place at the same time. In order to connect to a group (talkgroup) or private call, you must know the correct timeslot (04)  on the repeater (or hotspot) that you are using. If there is a mismatch in the timeslot you will not hear anything. Note: there are a number of DMR networks including DMR-MARC the first and largest for some time, DMR+, Brandmeister (currently the largest), TGIF, RockyMountain Ham, Colorado Digital, DMR-FL, Bronx TRBO and lots more..

Connectivity
The connections (04) and (05)  between the repeaters and network can be made, either, over the internet or via a radio connection. If the intermediary is a hotspot then the connection to the network is via IP and the internet. The repeater may also reject a transmission (PTT) request from the radio if there is network congestion, or if another talkgroup (remember more than one timeslot) is transmitting, in fact the timeslot that you are using may be blocked if the other timeslot has been allocated priority.

'73 and clear.

'73 KD2WLL