1. HUAWEI BSC6000 Base Station Subsystem
Signaling Analysis Guide 8 Ciphering
8 Ciphering
About 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.
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2. HUAWEI BSC6000 Base Station Subsystem
8 Ciphering Signaling Analysis Guide
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.
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 by
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3. HUAWEI BSC6000 Base Station Subsystem
Signaling 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.
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4. HUAWEI BSC6000 Base Station Subsystem
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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:
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5. HUAWEI BSC6000 Base Station Subsystem
Signaling 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.
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