Problem 2-3. GSM GSM has logical control and traffic channels. “Common Control C
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Question
Problem 2-3. GSM
GSM has logical control and traffic channels. “Common Control Channels” (CCCH) are used for signaling between a Base Station (BTS) and a mobile station (MS). “Broadcast Control Channel” (BCCH) is used to broadcast, for instance, information about the cell configuration and identity. Use wireshark on the following PcapNG capture to answer the questions:
(a) Find “CCCH System Information” BCCH messages. What is the MCC, MNC, and LAC of the BSC? What carrier frequencies does the BTS support (hint: find the ARFCNs). Explain where you found this information.
(b) Within the CCCH are “paging request” messages. Explain the purpose of these messages, and provide a TMSI observed in a paging request.
(c) Examine the raw hex bytes of the GSM messages, especially the end of the messages. What is the most frequently observed byte, and what is its significance? Explain your answer.
Explanation / Answer
GSM CCCH - System Information Type 5
L2 Pseudo Length
0100 10.. = L2 Pseudo Length value: 18
Protocol Discriminator: Radio Resources Management messages
.... 0110 = Protocol discriminator: Radio Resources Management messages (0x06)
0000 .... = Skip Indicator: No indication of selected PLMN (0)
Message Type: System Information Type 5
Neighbour Cell Description - BCCH Frequency List
..0. .... = EXT-IND: The information element carries the complete BA (0)
...0 .... = BA-IND: 0
10.. 100. = Format Identifier: 512 range (0x44)
Range 512 format
.... ...1 0110 1000 0... .... = ORIG-ARFCN: 720
.111 0000 01.. .... W(1): 449
..11 1111 00.. .... W(2): 252
..01 0001 01.. .... W(3): 69
..11 1100 1... .... W(4): 121
.000 0000 W(5): 0
0000 000. W(6): 0
.... ...0 0000 00.. W(7): 0
.... ..00 0000 .... W(8): 0
.... 0000 00.. .... W(9): 0
..00 0000 W(10): 0
0000 00.. W(11): 0
.... ..00 0000 .... W(12): 0
.... 0000 00.. .... W(13): 0
..00 0000 W(14): 0
0000 00.. W(15): 0
.... ..00 000. .... W(16): 0
...0 0000 W(17): 0
List of ARFCNs = 134 141 145 720 727
HEX : 49 06 1D 89 68 70 7F 11 7C 80 00 00 00 00 00 00 00 00 00
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