Wrong interpretation of the cbsp.rep_period field in epan/dissectors/packet-gsm_cbsp.c

Summary

Wrong interpretation of the cbsp.rep_period field in epan/dissectors/packet-gsm_cbsp.c

Steps to reproduce

Open capture that contains CBSP traffic. Observe field cbsp.rep_period.

What is the current bug behavior?

epan/dissectors/packet-gsm_cbsp.c interprets this field as a big-endian unsigned 16-bit integer.

This is not correct.

What is the expected correct behavior?

Section 8.2.8 of 3GPP TS 48.049 (https://www.3gpp.org/ftp/Specs/archive/48_series/48.049/48049-h00.zip) specifies the "Repetition Period" Information Element (IE) of the Cell Broadcast Service Protocol (CBSP) as follows:

|  8  |  7  |  6  |  5  |  4  |  3  |  2  |  1  |
+-----------------------------------------------+
|               Element identifier              | octet 1
+-----------------------+-----------------------+
|                  Repetition Period            | octet 2
+-----------------------+-----------------------+
|         Spare         |Repetition Period cont.| octet 3
+-----------------------+-----------------------+

It is a 12-bit field that occupies two octets, where bits 5-8 of the octet 3 ("Spare") do not belong to the representation of that quantity. They are always set to 0.

As for the correct byte order, section 8.2.1 states that:

The following conventions are assumed for the sequence of transmission of bits and bytes:

• Each bit position is marked as 1 to 8. Bit 1 is the least significant bit and is transmitted first;
• In an element octets are identified by numbers, octet 1 is transmitted first, then octet 2, etc. When a field extends over more than one octet, the order of bit values progressively decreases as the octet number increases. The least significant bit >of the field is represented by the lowest numbered bit of the highest numbered octet of the field.

A Repetition Period information element given as octets 06 01 00 is currently displayed as decimal 256.

The correct display should be decimal 16.

Sample capture file

220802_Huawei_2G_TC1.11_v2_frame_946.pcap

Build information

The issue occurs in the "master" branch.