There must be channel steering/LCN assignments in there somewhere I would think, to tell the radio which timeslot & frequency it's supposed to be on.
You know, the more I look at Capacity Plus the more it seems like LTR standard, or its more advanced cousin (supporting radio ID, multi-site and ESN verification) "Passport." /\/\ has been making LTR standard and Passport radios for a long time, so this could be conceivable.
LTR standard is pretty primitive. It has no control channel. All traffic initially defaults to the home repeater, and the subaudibile outgoing signal word (which is only 40 bits) runs underneath specifying the current (goto) and free channels. If any other users try to key up another talkgroup during that time, their radio is redirected to a free channel by the OSW data on the home repeater. If a 3rd talkgroup keys up, they are further redirected to a different free channel (until all the free channels are used up). Passport is similar (no control channel) except it uses a longer 68-bit OSW, maps the goto and free channels using LCNs (which can be converted to the frequency using base/step/offset tables in the radio) instead of arbitrary repeater numbers (1-20), the ESN registration, site roaming and radio ID, etc.
There was another LTR variant called Multi-Net, which did use a dedicated control channel.
To make a long story short, with the substitution of digital data frames in lieu of subaudible data,
Capacity Plus seems awfully similar to a single site Passport system, and
Connect Plus seems similar to Multi-Net.
On my university's Capacity Plus system, only one frequency broadcasts a rapid data burst every two seconds (DMRDecode shows jitter and syncs with debug on, but decodes no data). This same frequency also carries voice traffic from multiple talk groups, and it is apparently the only frequency used during slow traffic periods. This leads me to believe it operates like a "Home repeater", much like LTR and Passport (Passport even uses a similar data burst every two seconds on its home and/or registering channels). Presumably when this home repeater frequency is busy, the other frequencies act as "Free channels" and begin carrying traffic for other talk groups. I would be really interested to ask someone with mototrbo CPS how the details of Capacity Plus are programmed. Are the repeater frequencies pre-programmed in the radios (like LTR, mapped to Ch 1-20 for example) or are the radios programmed with a base/offset/step table which converts from LCNs received over the air? Of course, it could be neither.
I had a lot of fun collaborating on these forums a couple years ago with others who were trying to reverse engineer all the possible Passport OSW formats, and with the systems at least functioning in a similar manner, those old threads in the control channel decoding sub-forum may be a good starting place. While the CSBK format is completely different, comparing with known passport or LTR frames could give some clues as to what possible allocations the bits could be given (goto LCN, free LCN, message type, etc.).
