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Decoding with lte-rrc.bcch.bch does not show masterInformationBlock node

asked 2018-11-12 21:59:28 +0000

mos gravatar image

Decoding with dissector lte-rrc.bcch.bch does not show the node named 'masterInformationBlock' from asn.1 as child of the 'message' node. I would expect that this node should be in the tree.

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answered 2018-11-13 06:32:40 +0000

Pascal Quantin gravatar image

updated 2018-11-13 06:34:26 +0000

MasterInformationBlock (see the capital letter at the beginning or the word) is not the name of a node, but the type of a structure referenced by the 'message' node:

BCCH-BCH-Message ::= SEQUENCE {
    message                 BCCH-BCH-MessageType
}

BCCH-BCH-MessageType ::=                        MasterInformationBlock

MasterInformationBlock ::=          SEQUENCE {
    (...)
}

The 'message' field could eventually have been named 'masterInformationBlock' but this is not how the ASN.1 description has been written. So it's perfectly normal not to have it displayed.

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The issue is that you are loosing the intermediate informational node representing

BCCH-BCH-MessageType ::= MasterInformationBlock

this could be changed by applying the rules for lterrc asn.1 names. So MasterInformationBlock would create masterInformationBlock.

If this information wouldn't be intended than the lterrc asn.1 definition would be

BCCH-BCH-MessageType ::= SEQUENCE { (...) }

like it is defined for

BCCH-DL-SCH-MessageType ::= CHOICE { (...) }

mos gravatar imagemos ( 2018-11-13 07:30:19 +0000 )edit

We are not losing any information, for BCCH-BCH there is a single message type (contrary to BCCH-DL-SCH, thus the CHOICE) and it is necessarily a MIB. BTW the other ASN.1 engines I played with also decode a BCCH-BCH message without displaying MasterInformationBLock (which again is not a field - or node as you name it - but a type). I don't think there is anything wrong with Wireshark dissection here, it simply a consequence of the way 3GPP wrote the description.

Pascal Quantin gravatar imagePascal Quantin ( 2018-11-13 14:34:23 +0000 )edit

For sure there is nothing wrong because there is no specification how to visualized the data. This is not a bug report. It's just about what helps the user most to understand quickly the output. I can only speculate why the intermediate type has been inserted in the ASN.1. The only reason I can think about is to tell explicit this is a MasterInformationBlock. I do not know which decoders you played with, from the two I checked the first shows as child of message node masterInformationBlock and the other _MasterInformationBlock.

mos gravatar imagemos ( 2018-11-19 11:33:43 +0000 )edit

I tested the one provided by OSS Novalka and ffasn1c on top of Wireshark one.

Pascal Quantin gravatar imagePascal Quantin ( 2018-11-19 13:09:42 +0000 )edit

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Asked: 2018-11-12 21:59:28 +0000

Seen: 77 times

Last updated: Nov 13