Misleading ETSI standards

12. April 2016

As part of the so-called C-ITS Release 1 set of standards, ETSI published

Now, what are the problems?

Clause 5.3.3 of EN 302 637-2 claims:

At the originating ITS-S, the CA basic service shall provide the CAM embedded in a Facilitylayer Service Data Unit (FL-SDU) together with protocol control information (PCI) according to ETSI EN 302 636-5- [i.7] to the ITS Networking & Transport Layer.

In this one sentence there are at least three bugs:

  1. FL-SDUs are data exchanged via the NF Service Access Point (NF-SAP). Specifications of SAPs always are informative in standardization, thus the keyword shall is prohibited. The sentence could read "... CA basic service provides the CAM embedded in a Facilitylayer Service Data Unit (FL-SDU) together with protocol control information (PCI)." The reference to BTP has to be deleted. A specification of PCI has to be added.
  2. The reference [i.7] is an informative reference that cannot be used in a normative (shall) statement.
  3. [i.7] points to ETSI EN 302 636-5-1. Thus the reference number is written wrongly (-1 is missing).

As the CAM standard mentions "protocol control information (PCI) in the normative statement with reference to the BTP standard, EN 302 636-5-1 should provide at least information on how to use PCI. The reader will not find such a specification. The reader will not find anything on PCI except that PCI exists. The reader will find in clause 6:

A BTP packet shall be comprised of the protocol headers and the payload as depicted in figure 3

BTP figure 3

So far so good. The "Payload" field contains the CAM. That a BTP packet has a BTP header also is according to the text book. However the three other headers in figure 3 are not part of the BTP packet. This is a severe editorial bug. In no way the reader can learn what PCIs are, and how these are to be used. The figure implies that BTP (and thus CAM) can only be used in combination with GeoNetworking and an access technology that uses LLC headers and MAC headers.

The wrongly suggested statement, that CAM has to use BTP/GeoNetworking/ITS-G5, is fully in line with the "thinking" of some stakeholders. Because of this widely spread wrong suggestion, this article is needed to clarify the situation.

That was the bad part of the story that hopefully will be fixed by ETSI TC ITS as soon as possible.

The good part of the story is:

ETSI TC ITS confirmed twice (in the meetings in January and April 2016) that these problems are not implemented by intention and thus can be fixed in the next revision of these standards. ETSI TC ITS WG1 (responsible for EN 302 637-2) especially confirmed that the Cooperative Awareness Message shall be technology-agnostic, i.e. can be transmitted using any kind of communications protocol stack.

Thus, dear reader, you can be relaxed and use CAM as you like it to do. There is no normative requirement for using the GeoNetworking / Basic Transport Protocol stack!

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