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I know that ACARS and CPDLC are 2 different systems, so I can compare ACARS vs CPDLC, but can I do the same comparation between CPDLC vs VDL? Or a CPDLC it’s a VDL?

If CPDLC it is a VDL, ACARS it is a VDL also?

fooot
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Maximilian Kaden
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    @ymb1 In the context of aeronautical communication, VHF and HF often equates "analog voic transmission over the VHF.HF band", while VDL and HFDL are the support for digital in those radio bands (DataLinks). So ACARS and CPDLC does not use VHF nor HF. – Quentin Aug 23 '17 at 06:56
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    Also, not a duplicate of this particular question. – Quentin Aug 23 '17 at 07:44
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    The question is about confusion between the layers in the OSI model. VLD covers the lower two layers and a bit of layer 3. ACARS and CPDLC cover higher layers and work on top of various implementations of the lower layers. Voted to reopen. – DeltaLima Aug 23 '17 at 09:31

1 Answers1

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TL;DR : according to the most usual meanings of these acronyms :

  • ACARS and CPDLC are applications
  • VDL is a point-to-point communication technology

To give an everyday-world analogy, the relationship between VDL and CPDLC/ACARS is the same as the relationship between 3G/4G and the web browser/facewhatstime app of your smartphone.

More detailed answer

From a telecommunication point of view, we represent the different functions of a network by different layers. Ideally, one can replace the technology used to implement a given layer by any equivalent technology without impacting the other layers.

The VHF Data Link (VDL) is a technology used to implement the functions of the physical and datalink layers. It means that it is a technology designed to carry any digital data directly from one "device" (aircraft or ground station) to another (resp. ground station or aircraft), independently of its actual content. There are different versions of VDL, namely modes 1 to 4. The current deployments of VDL in europe by ANSPs use VDL mode 2.

CPDLC is an application (application layer). It is directly facing the user, and can use any layer below it (in particular, it can use VDL, HFDL or satcom as layers 1/2)

ACARS is ... a more ambiguous word. It predates the OSI model, and at first it described both the application (text-oriented transmission) and the point-to-point communication technology. Nowadays, pilots use the word ACARS to talk about the application (the printer in the cockpit), and that particular application can, as any application, use any underlying link layer (VDL, HFDL, satcom...). But the old meaning of "ACARS" to describe the original blob of "point-to-point technology + network + application" can still be found today, for example in the acronym POA (plain old ACARS). By the way, the point-to-point technology originally used by ACARS is also called VDL mode 1.

Quentin
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  • Many thanks for the reply!

    It helped me a lot, but if possible I would like to ask another question. If this is outside from the regulation, I apologize.

    My question is, if ACARS is VDL MODE 1, why is it still in use?

    From what I researched, ICAO removed the standards from VDL mode 1, in this way I interpret that VDL mode 1 is prohibited for aeronautical use.

    But I still see that ACARS is in use, so I still confuse about it.

    Thank you very much!

     – Maximilian Kaden Aug 29 '17 at 16:38
  • @MaximilianKaden Nowodays, when someone talks about ACARS, they (most likely) talk about the application (the printer that is aboard aircraft). This application can use any underlying data link : VDL(2-4), satcom... – Quentin Aug 29 '17 at 20:23
  • Humm, I asked that because I all the time read a lot of books saying that

    "VDL Mode 2 has a much higher and faster data capacity than the existing ACARS."

     – Maximilian Kaden Aug 31 '17 at 12:34
  • That confusion is quite common (see how your question was at first seen as a duplicate), even (especially) amongst people familiar with civil aviation. That's why I wrote that using the word ACARS is often ambiguous. In your books, they are certainly referring to the capacity of the original datalink used for ACARS, and they do not make a clear distinction between applications and the underlying data transmission technology. – Quentin Aug 31 '17 at 13:46
  • This confusion is especially common in civil aviation because, for a long time, a new system requiring communication meant systematically adding a new transmission device (with its antennas, allocated frequencies...). With new digital services, the distinction between the different OSI layers becomes necessary, hence my answer. – Quentin Aug 31 '17 at 13:51
  • And for the record, "VDL Mode 2 has a much higher and faster data capacity" : 31.5 Kbit/s. It is 10000 times slower than 4G. – Quentin Aug 31 '17 at 13:54
  • Humm, I'm still confused, I found in my EASA Books this statement

    "ACARS is gradually being replaced with VDL mode 2 which provides a digital link at a rate of 31.5 kbps, giving more than 10 times the capacity of ACARS. VDL mode 2 requires specific address for an aircraft and the unique mode S address will be used. This type of digital system is known as controller pilot datalink communications (CPDLC)"

     – Maximilian Kaden Sep 04 '17 at 17:15
  • This citation is a perfect illustration of what I was saying (the lack of distinction between the different OSI layers when civil aviation people are talking), and how confusing it is when you try to understand things in depth. The last sentence is misleading, and may indicate that the author does not make clearly the distinction between the application and the underlying layers. – Quentin Sep 05 '17 at 07:23
  • To repeat myself: CPDLC is an application that rely on a datalink. VDL2 is a datalink that can handle CPDLC, as well as other applications. – Quentin Sep 05 '17 at 07:25
  • Anyway, if you go by the EASA book, you will be as right as most of the people who work in civil aviation. I have met quite a few of them, and this confusion is very common (with the exception of network specialists). I am probably nitpicking, but I think that learning things correctly will help you. My conjecture is that datalink will be used in the future for other things than controller-pilot communications, so people will need to make that distinction sooner or later. If it is not too personal, could you tell me in which context you need this information ? – Quentin Sep 05 '17 at 07:37