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  • 1. HUAWEI BSC6000 Base Station SubsystemSignaling Analysis Guide 8 Ciphering 8 CipheringAbout This Chapter Ciphering is used to secure the information exchange between an MS and a BTS. The information consists of the signaling information and the subscriber information. The subscriber information consists of the subscriber data and the subscriber voice. 8.1 Ciphering Procedure Whether ciphering is used or not is determined by the radio resource management entity. The ciphering procedure is initiated by the network and is performed in the BTS. To cipher the user data, you must enter the corresponding parameters in the ciphering program. The ciphering key Kc is generated by the AUC, is stored in the MSC/VLR, and is sent to the BTS before ciphering. 8.2 Ciphering Mode Change The MSC sends a Ciphering Mode Change command, and the BTS performs the ciphering mode change. 8.3 Internal BSC Signaling Procedure of Ciphering This topic describes the internal BSC signaling procedure of ciphering. 8.4 Abnormal Ciphering Cases This topic describes the abnormal ciphering cases.Issue 01 (2007-09-05) Huawei Technologies Proprietary 8-1
  • 2. HUAWEI BSC6000 Base Station Subsystem8 Ciphering Signaling Analysis Guide8.1 Ciphering Procedure Whether ciphering is used or not is determined by the radio resource management entity. The ciphering procedure is initiated by the network and is performed in the BTS. To cipher the user data, you must enter the corresponding parameters in the ciphering program. The ciphering key Kc is generated by the AUC, is stored in the MSC/VLR, and is sent to the BTS before ciphering.Signaling Procedure Figure 8-1 shows the ciphering procedure. Figure 8-1 Ciphering procedure MS BTS BSC MSC Ciphering Mode Encryption Command(1) Ciphering Mode Mode Command(2) Command(3) SDCCH Ciphering Mode Command(4) SDCCH Ciphering Mode Command(5) The ciphering procedure is as follows: 1. The MSC sends the BSC a Ciphering Mode Command message. The Ciphering Mode Command message carries the required ciphering algorithms including A5/0, the ciphering key Kc, and whether the Ciphering Mode Complete message sent by the MS must carry the IMEI. 2. According to the ciphering algorithm in the Ciphering Mode Command message, the ciphering algorithm allowed by the BSC, and the ciphering algorithm supported by the MS, the BSC determines the algorithm to be used and notifies the BTS. 3. The BTS sends the Ciphering Mode Command message to notify the MS of the ciphering algorithm. 4. Upon receiving the Ciphering Mode Command message, the MS starts sending the ciphering mode and then responds the BSC with a Cipher Mode Complete message. 5. Upon receiving the Ciphering Mode Complete message from the MS, the BSC notifies the MSC of the ciphering completion.Procedure Description For details about the ciphering procedure, refer to the following: l A5 ciphering algorithm As specified in the GSM protocol, there are eight ciphering algorithms, A5/0–A5/7, in which A5/0 indicates Not Ciphered. The ciphering mode setting procedure is initiated by8-2 Huawei Technologies Proprietary Issue 01 (2007-09-05)
  • 3. HUAWEI BSC6000 Base Station SubsystemSignaling Analysis Guide 8 Ciphering the network. The required ciphering algorithm is specified in the Ciphering Information element of the Ciphering Mode Command message. l Ciphering algorithm selection When an MS initiates a call, it sends a CM Service Request message that carries classmark 2, classmark 3, and the ciphering algorithm supported by the MS. When the ECSC is set to 1, classmark 3 is carried in the CM Service Request message. When the ECSC is set to 0, classmark 3 can only be carried in the CM Service Request message through classmark change or update. Therefore, set the ECSC to 1 when ciphering is used. According to the ciphering algorithm in the Cipher Mode Command message, the ciphering algorithm allowed by the BSC, and the ciphering algorithm in the CM Service Request message, the BSC determines the algorithm to be used. In a reverse order, the BSC selects an applicable algorithm from the intersection of the early-mentioned three algorithms. The priority of the algorithms is A5/7 > A5/6 > A5/5 > A5/4 > A5/4 > A5/3 > A5/2 > A5/1 > A5/0. l Ciphering during handover The Handover Request message carries an Ciphering Information element. The Ciphering Information element specifies the required ciphering algorithm and the ciphering key. If one of the two A interfaces of the two BSSs is in PHASE I, because of the defects in the ETSI GSM PHASE I protocol (no ciphering mode setting information unit is included in the Handover Command message), the two BSSs interoperate only when they use the same ciphering algorithm such as A5/0 or A5/2. Otherwise, special processing (modifying the inter-BSC handover command) is required in the target MSC or the target BSC (or the serving MSC or the serving BSC). During the interconnection on the A interface in ciphering mode, to avoid unsuccessful handovers, you must know whether additional data configuration is required on the BSCs and MSCs from different manufacturers.8.2 Ciphering Mode Change The MSC sends a Ciphering Mode Change command, and the BTS performs the ciphering mode change. The BTS performs and manages ciphering. The BSC does no processing. If the ciphering mode is changed, 1. The BTS performs the sending configuration in old mode and the receiving configuration in new mode. 2. The MS is configured in a completely new mode, including transmission and reception. 3. The BTS is configured in a completely new mode.8.3 Internal BSC Signaling Procedure of Ciphering This topic describes the internal BSC signaling procedure of ciphering. The internal BSC signaling procedure of ciphering is as follows: 1. Upon receiving the Ciphering Mode Command message from the MSC, the BSC checks the classmarks of the MS and the settings of the Ciphering Algorithm in the Cell Attributes dialog box.Issue 01 (2007-09-05) Huawei Technologies Proprietary 8-3
  • 4. HUAWEI BSC6000 Base Station Subsystem8 Ciphering Signaling Analysis Guide 2. According to the ciphering algorithm in the Ciphering Mode Command message, the ciphering algorithm allowed by the BSC, and the ciphering algorithm in the CM Service Request message, the BSC determines the algorithm to be used. 3. In a reverse order, the BSC selects an applicable algorithm from the intersection of the early-mentioned three algorithms. The priority of the algorithms is A5/7 > A5/6 > A5/5 > A5/4 > A5/4 > A5/3 > A5/2 > A5/1 > A5/0. The determined ciphering mode is carried in the Cipher Mode Command message that is sent to the BTS.8.4 Abnormal Ciphering Cases This topic describes the abnormal ciphering cases. 8.4.1 BSS Sending a Cipher Mode Reject Message This topic describes the fault, probable causes, and handling suggestions. 8.4.2 MS Doing No Processing This topic describes the fault, probable causes, and handling suggestions.8.4.1 BSS Sending a Cipher Mode Reject Message This topic describes the fault, probable causes, and handling suggestions.Fault The BSS sends the MSC a Cipher Mode Reject message.Probable Causes The probable causes are as follows: 1. The BSS does not support the ciphering algorithm specified in the Ciphering Mode Command message. It sends the MSC a Cipher Mode Reject message with the cause value Ciphering Algorithm Not Supported. 2. If the BSS has initiated ciphering before the MSC requires changing the ciphering algorithm, the BSS sends the MSC a Cipher Mode Reject message.Handling Suggestions l Check whether the BSS supports the ciphering algorithm specified in the Ciphering Mode Command message. l Check whether the BSS has initiated ciphering.8.4.2 MS Doing No Processing This topic describes the fault, probable causes, and handling suggestions.Fault Upon receiving the Ciphering Mode Command message, the MS does no processing.Probable Causes The Ciphering Mode Command message is considered valid when:8-4 Huawei Technologies Proprietary Issue 01 (2007-09-05)
  • 5. HUAWEI BSC6000 Base Station SubsystemSignaling Analysis Guide 8 Ciphering l It carries an Algorithm Identifier and is received by the MS in No Ciphering mode. l It carries No Algorithm Identifier and is received by the MS in No Ciphering mode. l It carries No Algorithm Identifier and is received by the MS in Ciphering mode. The Ciphering Mode Command message received in other cases, for example, when it carries an Algorithm Identifier and is received in Ciphering mode, is considered erroneous. In this case, the MS sends an RR Status message with the cause value Protocol Error and does no further processing.Handling Suggestions Check the mode that the MS is in and the Ciphering Mode Command message.Issue 01 (2007-09-05) Huawei Technologies Proprietary 8-5