[DoCAN] Vehicle Diagnostic Communication Part 7 [ISO-TP 3]

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Introduction.
Extended addressing
11bit Mixed addressing
29bit Mixed addressing
Addressing Format Considerations
Conclusion

Introduction.

This article is a continuation of the description of the network layer addressing format for vehicle diagnostic communications.

Extended addressing

Here is an explanation of Extended addressing.
The structure is as follows.
Incidentally, there are both 11-bit ID (standard ID) and 29-bit (extended ID) patterns for CANID.

CANID	DataField
	1byte	2byte～
N_AI	N_TA	–

The addressing format described so far is closed within CANID, but Extended addressing also uses DataField.
As an image, it is an extension of normal addressing with the addition of N_TA information.

I have never seen this addressing format, so I can only speculate, but I think it will be used in the following circumstances.

Originally, the system was operated with Normal addressing.
The number of nodes increased and CANID started to run out.
N_AI was fixed and started handling with N_TA.

N_AI of Normal addressing is for the number of nodes, whereas Extended addressing has the advantage of only requiring the physical address and functional address group, not the number of nodes.

Normal addressing was meaningful for each CANID, so it is easy to exhaust it.
This kind of situation reminds me of IPv6.

11bit Mixed addressing

The explanation here is the 11-bit ID (standard ID) of Mixed addressing.

CANID	DataField
	1byte	2byte～
N_AI	N_AE	–

At first impression, it looks similar to Extended addressing, with N_TA simply replaced by N_AE, but the role of N_AE is more important.
N_AE corresponds to the network part in IP addressing, and is intended for communication across gateways.
There is no reason why N_TA of Extended addressing should not also straddle gateways, so it is possible that the result is the same, but it is important to remember that the motivation is different.

29bit Mixed addressing

Mixed addressing differs greatly between 11-bit ID (standard ID) and 29-bit ID (extended ID).
The structure of 29-bit mixed addressing is shown below.

CANID				DataField
24-28[bit]	16-23[bit]	8-15[bit]	0-7[bit]	1byte	2byte～
0x18	N_TAtype	N_TA	N_SA	N_AE	–

N_TAtype is
0xCE for a physical address.
0xCD for a functional address.

You may have noticed that it looks exactly like the Normal fixed addressing from the last issue.
In other words, it is a gateway-aware addressing format with normal fixed addressing.

This is the most flexible addressing format that can be used for a lot of ECUs.
However, it is not used very often because it requires a tight network design.
I have never seen this specification in use.

Addressing Format Considerations

There are various addressing formats, but which one is actually used most often?

As a rule of thumb, Normal Addressing and Normal fixed addressing are mostly used.
The reasons are as follows.

The predecessor of DoCAN is a communication format called KWP2000.
If the KWP2000 specification is followed as much as possible, Normal Addressing or Normal fixed addressing will be used.
Since not all ECUs need to support vehicle diagnostic communication, the increase in the number of ECUs is unlikely to be a direct reason for the depletion of N_AI and N_TA.

Keyword Protocol 2000, abbreviated KWP2000, is a communications protocol used for on-board vehicle diagnostics systems (OBD). This protocol covers the application layer in the OSI model of computer networking. The protocol is standardized by International Organization for Standardization as ISO 14230. KWP2000 also covers the session layer in the OSI model, in terms of starting, maintaining and terminating a communications session.
Wikipedia(https://en.wikipedia.org/wiki/Keyword_Protocol_2000)

You stated that “not all ECUs need to support vehicle diagnostic communications” but the ones that do, by law, are called emission systems.
Only ECUs that are related to emissions, such as engine control and transmission control, will be legally compliant.
Other than that, it is not required by law.

Therefore, it is almost impossible for them to be run out.
However, there is a reason why this is not the case.

As for diagnostic communication, there are also the purposes of checking operation after assembly at the factory and analyzing market defects.
If these are cost-effective, they are often implemented, so the reality is that it depends on the policy of the manufacturer.

In any case, it may eventually run out, and it is good to know.

Conclusion

Extended addressing is an extended version of Normal addressing with N_TA added.
Mixed addressing has 11bitID and 29bitID versions.
- The 11bitID (standard ID) version is based on Normal addressing and assumes gateway traversal.
- The 29bitID (extended ID) version is based on Normal fixed addressing and assumes gateway traversal.

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