DMR Digital Mobile Radio Frequently Asked Questions

DMR logoQ. How does DMR coverage compare with analog?

The coverage of a DMR network is comparable to the coverage of a narrowband analog network. In many instances, the actual coverage of the DMR network is perceived to be better as the voice quality stays the same throughout the whole service area.

Q. Can you combine voice and data on one channel?

DMR supports both voice and data services and on a trunked network, channels can be dynamically allocated to the service that is required. With DMR, priority levels will ensure that traffic with the highest priority will get through.

Q. I have heard DMR suffers from ‘near/far’ issues, multipath and that the maximum coverage is 75km. Is this true?

The maximum coverage of TDMA systems is indeed influenced by ‘near/far’ issues. Almost all DMR vendors, including Tait, provide 0.5ppm frequency accuracy in the terminals so the theoretical limit will then be 150km. There are many other factors that impact on the coverage but the fact is that there are multiple DMR systems deployed with coverage of up to 100km. 

Q. Is the DMR standard finalized?

The DMR standard was first released in 2006. It is a live standard and is continuously enhanced to meet new market demands. The standard enhancements are being carried out by the DMR Association and where appropriate are fed back into the formal ETSI standard.

Q. What is the maximum DMR network size?

A Tait DMR network can support a maximum of 20 nodes and a maximum of 2,000 logical channels or 200 sites.

Q. Does the DMR standard support encryption?

The DMR standard is currently being updated to include DES and AES encryption, as well as RC4. This work is carried out by the DMR Association.

Q. Can DMR support GPS/AVL for large fleets?

The DMR standard supports AVL in the form of Short Data Messages (SDM). In a trunked network SDMs are transmitted over the control channel. To increase capacity, multiple control channels can be used or the messages can be transmitted using a traffic channel. This allows the support of a large number of GPS updates.

Q. What is the easiest way to migrate from analog to DMR?

There are various ways of migrating from analog to DMR. One method is deploying multi-mode terminals and changing over site equipment once all terminals on the site can support DMR. Tait terminals support three modes of operation; DMR, MPT and analog FM.
Alternatively, gateways can be deployed between DMR and analog technologies to provide a staged migration. These gateways can be deployed at the node site of the network or at the dispatch end. Various other scenarios are possible and for details please refer to the ‘Upgrading to Trunked DMR‘ white paper.

Q. Is there a certification plan to ensure DMR equipment from different manufacturers will interoperate?

The DMR Association has set up an Interoperability Process (IOP) that will certify terminals of one DMR vendor on networks of another DMR vendor. The IOP process has been well documented in the Association and tests that are carried out are verified by the Technical Working Group of the Association. When all is deemed in order, an IOP certificate is issued. Current valid IOP certificates can be found on the DMR Association website.

Tait DMR has IOP certification with other vendors of DMR Tier 3 equipment.

Q. Does the DMR standard include an interface for dispatchers and applications?

The DMR Association is defining the Application Interface Standard (AIS). This provides the interface for dispatchers, voice recorders and other third party equipment that can be linked to a radio network. It will also provide an interface for IP data applications. A draft of this standard is now available.

Q. When will location services be available?

The Tait DMR Tier 3 solution supports location services using the Short Data Messaging Service on the control channel. This increases the utilization of the control channel while it does not impact on traffic channel availability for voice calls. The interface to the AVL application will be based on the LIP standard, which will eventually be added to the DMR standard. The standard will be documented to allow integration with third-party applications. Clevest location services applications are supported.

Q. Why would an MPT customer upgrade to Trunked DMR?

DMR offers a number of significant advantages over MPT trunked solutions, some are listed below.

  • Improved spectral efficiency compared with MPT. 6.25kHz equivalent using DMR.
  • Easy upgrade from analogue MPT
  • Advanced control features
  • Superior audio quality
  • More data capabilities – Short data service and Packet data service
  • Higher reliability

Q. Why would a customer choose DMR Tier 3 over Tier 2?

  • Tier 2 is a conventional offering only
  • Trunking offers greater frequency utilisation
  • Trunking offers terminal authentication
  • Trunking provides talk group options

How does DMR compare to TETRA, TETRAPOL, dPMR, P25, NXDN and Opensky? Find out in our complimentary whitepaper Digital Radio Standards: Advantages and Disadvantages of Current Digital Radio Standards.megan-cta-images-white-papers3


Visit the Introduction to DMR page of the Tait Radio Academy for videos explaining the basics of DMR. You can also register for the Academy to keep track of your course progress and download free study guides.

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Comments

  1. Gary Helming says:

    What we and many local public safety folks have seen with digital radio, is that signal strength is not an issue with digital radios-linearity is the issue. Unfortunately, field strength TAP Bullington type programs can’t predict linearity. Thus, where one might expect good coverage due to a high signal strength, may have no coverage at all to linear distorison (mostly Fresnel Diffraction) issues. We have certainly seen this demonstrated by a pilot program digital system that we demonstrated, and I understand that most all local public safety outfits in this area have changed their P25 systems to narrowband analog due to this anomoly. If a path is linear then digital will give one a lower operational threshold than analog, but if it is not linear, the thing won’t work no matter how much signal there is.

    • Tait Communications says:

      Hi Gary,

      Thank you for this comment, we would agree that with digital communications there are effects beyond simply path loss that can effect the range and coverage. We believe the effect that you are talking about is delay spread which is caused by multi-path signal arrivals at a similar signal level closing the eye pattern on the digital modulation scheme. This is usually not a major issue within a short range of the base station as the time difference in arriving signals is small. However it can becomes an issue as you move further away from the site. Digital simulcast needs to be treated very carefully with respect to delay spread. Experience in this area suggests that significant site coverage modelling and the design of mitigation measure are required at an early stage. Can you confirm if this is likely to be what you are experiencing.

      • The issue we see occurs when there is any obstruction between the transmitter and receiver. Often that instruction can be quite small. This situation causes a pulse spread by spectral distortion. We have seen that lower data rates and narrower bandwidths lessen this issue but it is still there enough to make such a system unusable in our area. P25 users in this area have abandon the technology for that reason. We experimented with and abandon a pilot Nxdn system. Though the push to digital is on experience has shown analog to be the only feasible technology at this point.

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