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5G SIB8 (CMAS) [Expert Info (Error/Malformed): Number of pages should be <=15 (found 201)]

asked 2022-05-06 05:51:59 +0000

Norbie_E. gravatar image

Hello,

Im getting the Expert Info Warning when im analysing 5G SIB8 Messages, received from our own 5G gNodeB.

"[Expert Info (Error/Malformed): Number of pages should be <=15 (found 201)]"

Other Tools don't show Problems with decoding the same Messages.

It would also be good, if the Wireshark dissector would also show the "DataCodingScheme" information.

As I am not allowed to attach files, because i'm new here, i can' provide PCAP, but maybe Hex Stream of the SIB8 may be helpfull:

001904449194201164ca2a2d4a92598a879766c410b96fc3931e1e7d1054cbcfa2d9d871e8e5a71e8e80000000000000000000000000

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Put the capture file on a publicly accessible file sharing site and post a link here.

Jaap gravatar imageJaap ( 2022-05-06 08:18:04 +0000 )edit
Norbie_E. gravatar imageNorbie_E. ( 2022-05-06 09:46:31 +0000 )edit

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answered 2022-05-06 12:09:33 +0000

Pascal Quantin gravatar image

Hi Norbie,

the warning message fragment payload is invalid for a SIB8 with a single fragment, thus the error generated. As the dcs field is after the warning message fragment one, and as dissection is stopped due to the error, it is not printed. If you use a properly encoded SIB8 it will work.

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Hi Pascal,

I did the measurements and data recordings using an rooted Android mobile device.

First with an App I got from the Google Store (that is capable of showing the signaling) and then with my own Softwaretool on an PC with the same mobile connected via USB.

For my own software tooI I coded an dissector for the SIB 8 according to 3GPP 3GPP TS 38.331 version 16.7.0 Release 16 / ETSI TS 138 331 V16.7.0:

-- ASN1START -- TAG-SIB8-START SIB8 ::= SEQUENCE { messageIdentifier BIT STRING (SIZE (16)), serialNumber BIT STRING (SIZE (16)), warningMessageSegmentType ENUMERATED {notLastSegment, lastSegment}, warningMessageSegmentNumber INTEGER (0..63), warningMessageSegment OCTET STRING, dataCodingScheme OCTET STRING (SIZE (1)) OPTIONAL, -- Cond Segment1 warningAreaCoordinatesSegment OCTET STRING OPTIONAL, -- Need R lateNonCriticalExtension OCTET STRING OPTIONAL, ... } -- TAG-SIB8-STOP -- ASN1STOP

To Verify my decoding results im sending the SIB to Wireshark and compare the results. (Wireshark for me is THE Reference Tool).

I also ...(more)

Norbie_E. gravatar imageNorbie_E. ( 2022-05-06 20:21:12 +0000 )edit

Hello Pascal,

meanwhile i found out, what i was missing.

All the other tools, including my own, are not dissecting the warningMessageSegment any further and thus ignore the malformed content of this field.

I was wrong, thinking, this field just contains the plain Text, whereas this Text is indeed splittet into Pages and so the information about the page count and the length of the content of each page is also included in this field.

Nevertheless, Wireshark could decode the rest of the SIB 8, as the warningMessageSegment is transmitted as an OctetString and the length of the content is known.So the Expert Info could be shown for this Field, but the dissector could skip the decoding of it and could continue to decode the rest of the Message.

Kind regards

Norbert

Norbie_E. gravatar imageNorbie_E. ( 2022-05-07 02:20:07 +0000 )edit

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Asked: 2022-05-06 05:51:59 +0000

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Last updated: May 06