If I understand the slide below correctly (source), CPDLC can be used either via ACARS or via ATN.
I know that, in the case of VDL Mode 2, there are specific ACARS frequencies. Which frequencies are used when CPDLC runs over ATN?
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Some additional information (and corrections)...
The slide is not totally correct (sorry for Airbus) as the same VDL-2 ground stations are used to transport both FANS-1/A CPDLC (A623) and ATN CPDLC over the AVLC protocol (VDL-M2). The two networks are interconnected... But depending on the media access scheme (Plain Old ACARS (POA - refered as VDL-A on the slide) or AOA (VDL2)) the modulation and the frequencies used differs. In Europe, VDL-M2 is used between 136.7 and 137 (ICAO).
As by 2020 ATN-B1 CPDLC above FL285 will be mandatory, datalink will only be performed on this band using VDL-M2 and no POA.
However, it is true to say that there are two CPDLC. The ICAO-standaridzed one (or compliant to the ICAO-ATN -- as used in Europe) and the non-ICAO compliant initially developed by Boing (as used in the US and oceanic -- also known as FANS-1/A). The two CPDLCs have very similar features and capabilities contrary to what have been told previously. The difference between the two is the end-to-end performance requirements and insurance of delivery (PM-CPDLC - PM=Protected Mode) available on ANT CPDLC and not for FANS-1/A CPDLC (there is no way to ensure a message have been successfully transmitted with FANS-1/A).
FANS-1/A CPDLC is transmitted via the ACARS network using Plain Old ACARS (POA - refered as VDL-A on the slide) or AOA (VDL2), while ATN CPDLC is only transmitted via VDL2 (SATCOM is being validated at the time of writing).
Link2000+ is the name of the development project of VDL2 (US and Europe).
ATN-B1 refers to the implementation of specific CPDLC services and/or messages standardized by ICAO. At the time of writing ATN-B1 is implemented in Europe and tests are being conducted for ATN-B2. ATN-B2 adds some new CPDLC messages but also the ADS-C which is the precursor for I-4D trajectory management. ATN-Bx refers to the ground implementation, and FANS-xx refers to the airborne implementation (FANS-2/B refers to the ATN-B1 airborne implementation, FANS-3/C to the ATN-B2 -- Don't try to find logic here ;-) )
Have a look in the book "Aeronautical Air-Ground Data Link Communications" for further information between the two (with some historical background).