## 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](/uploads/7b311d76b806b0e5d336d2c5a9325c0c/220802_Huawei_2G_TC1.11_v2_frame_946.pcap) ## Build information The issue occurs in the "master" branch.