"dPMR very simular to NXND" ???

[Kristoff]

Hi all,


As I have been looking into dPMR lately, I found a statement on the wikipedia website of dPMR stateing that
"dPMR is very similar to NXDN protocol implementation by Kenwood and Icom; both now offer dual-standard equipment (July 2013)."

As there is support for NXDN/4800 in DSD, this looked like a good startingpoint to create a dPMR protocol analyser, so I started looking and the two protocols to check how well they match. I must say that -unless I am completely wrong- the results is not what I expected.


OK, both protocols use C4FM at 4800 bps and both uses 80 ms superframes (364 bits) with 3600 bps voice and the rest for control-messages and syncing, but this seams to be about the only simularity between the two protocols.


Appart of some differences which are still more or less "minor" (NXND uses 16 bit addresses, dPMR 24 bit addresses; the frame-sync pattern of NXDN is smaller), the main way control-messages are multiplexed into the superframes does look to be very different.

- dPMR is based on a fixed streamformat where every superframe (364 bits for 80 ms) contains a control-message frame (the CCH channel, 72 bits per 80 ms) , which has a fixed structure. So you know that bit "x" of the CCH frame means "y"

- NXDN has a much more generic concept of control-channels. It actually has two of them: the SACCH (Slow Associated Control Channel: 60 bits per superframe, present in every superframe) and FACCH (Fast Associated Control channal,: 144 bits that can replace two voice-frames in a process called "voice-channel stealing").
But, these channels are not directly mapped to any particular voice-channel. They should be concidered as -as their name implies- a "channel": a transparent pipe to send upper-layer control-messages down the stack.

One of the results of this differences is that dPMR has dedicated "traffic channel messages" in front of the stream (containing information like addressing, etc.), a function achieved by the FACCH in NXDN.



So, my conclussion would be that the statement on the wikipedia website (https://en.wikipedia.org/wiki/Digital_private_mobile_radio) that these two protocols are "very simular" is mainly based on the fact that you kenwood and icon now provide dual-standard radios and not on the protocol desciptions itself.


Has anybody delved into these two specifications before who can comment on this? (before I start changing the wikipedia page? :-) )



73
kristoff - ON1ARF

[Kristoff]

Hi all,


OK. I forgot to add this:

References:

dPMR (ETSI TS 102 658)
http://www.etsi.org/deliver/etsi_ts/102600_102699/102658/02.04.01_60/ts_102658v020401p.pdf
(or go to http://www.etsi.org/standards-search and search for TS 102 658)


NXDN:
https://www.nxdn-forum.com/download_file/
Look for document "NXDN-TS-1-A".


73
kristoff - ON1ARF

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