Thesis on impact of repeater on GSM Networks (Portuguese doc)
GSM BSS Communication Flow
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Forewo rd • Course Purpose: – Grasp the means of interface trace and the method of analyzing it to aid us for fault handling. – How to learn this course:
• Copy the interface trace result files. – Use the maintenance console’s interface trace review function to study,
• Attention: – It is strictly forbidden to use the interface tracing function when the system is busy or the system is under heavy duty.
After establishing the SCCP connection between BSC and MSC there might be authentication and encryption in an intact location update flow. – For Huawei MSC, authentication and encryption are all optional. And normally authentication is mandatory and encryption is optional in practical situation. – Huawei BSS system supports encryption algorithm A5/1 and A5/2.
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Identification: The ID in Est_IND message reported by the MS is TMSI. When VLR cannot identify the TMSI or TMSI authentication fails, MSC will start Identification flow. That means MSC will send transparent transfer message “Identity Request” to MS and MS will report “Identity Response” message which involves IMSI.
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NOTE 2 • To reallocate TMSI or not during location update is optional in MSC. • If MSC chooses “Reallocate TMSI during location update” to “No”, MS will not report “TMSI Reallocation Complete” message.
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NOTE3 • In this flow we have simplified the release flow. To know more detailed refer to the BSC Normal Release Flow.
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Abnormal Occasion • BSC receives “Location Updating Rejected” message from MSC. The possible reason is listed below: – MSC has not configured the CGI which has been configured in BSC. – Communication failed between MSC and VLR. – MS has not been registered in HLR.
BSC MSC Channel_Required Channel_Active Channel_Active_Ack IMMEDIATE ASSIGN COMMAND First SABM Establish_IND(CM Service Req) CR(Complete_L3_information) UA CC (NOTE 1) CM Service Accepted Setup Call Processing (NOTE 2) Assignment_Req Channel_Active Channel_Active_Ack ASSIGNMENT COMMAND First SABM Establish_IND UA ASSIGNMENT CMP Assignment_CMP Alerting Connect Connect Ack Conversation (NOTE 3) Disconnect Release Release Complete Clear_CMD (NOTE 4) Clear_CMP
Channel_req
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NOTE 1: Class Mark Update Flow 1. Authentication, encrytion, identification are optional. See Note 1 in Location Update Flow.
2. Class mark update flow: For dual-band MS, after the SCCP connection is established and before receiving message "CM Service Accepted" there will be a class mark update flow. CR message reports the information of class mark 2. And in the class mark update flow, MS will report class mark 3 information which is related to the dual-band capabilty.
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NOTE 2: Assignment Method • Very Early Assignment In “IMMEDIATE ASSIGN COMMAND ” ,TCH is directly assigned. When MSC sends “Assignment _Req” message, BSC will not assign new channel but perform “Mode_Modify” flow. After that, MS will report “Assignment _CMP” message to MSC.
• Early Assignment MSC sends “Assignment_Req” before “Alerting”.
• Late Assignment MSC sends “Assignment_Req” after ”Alerting”.
Very Early Assignment
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Abnormal Situation • Abnormal Occasion 1: MSC has sent “Assignment_Req” and BSC does not send “ASSIGNMENT CMD” but sends a “Assignment_Fail” message. Possible reasons: – A. BSC has not available TCH. – B. The CIC assigned by the MSC is not idle in BSC.
• Abnormal Occasion2: BSC has sent “ASSIGNMENT CMD message but MS sends “ASSIGNMENT FAIL”. Possible reason: – Downlink bit error rate is too high.
Measure Report from MS Channel_Active Channel_Active_Ack HANDOVER COMMAND Handover Access (NOTE 1)
PHYINFO PHYINFO First SABM UA
Handover_Detect
Establish_IND
HANDOVER COMPLETE Handover_Performed (NOTE 2)
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NOTE1:Asynchronous Handover and Synchronous Handover
• Intra-BSC asynchronous handover: – BTS1 and BTS2 do not belong to the same BTS. Thus the target BTS shall send PHY INFO message.
• Intra-BSC synchronous handover: – BTS1 and BTS2 belong to the same BTS. Actually they are two cells of one BTS. In such occasion there will not be PHY INFO message, After “Handover Access” message, MS will send SABM directly.
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NOTE2 • After reporting “Handover_Performed” message to MSC, BSC will start the local-end release flow for the previous channel (in BTS 2).
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Abnormal Occasion
• In case of intra-BSC asynchronous handover, when BTS1 has sent PHY INFO message to the maximum time but the MS still can’t access the new channel, BTS1 will send CONN FAIL IND message. Its causing value is “handover access failure”. When BSC receives this message, it will start local-end release flow to release the new channel in BTS1.
NOTE1 ： Two Occasions of Release • NSS originating release • MS originating release • In these two different occasions all the messages are the same except the direction of the first 3 messages.
Case Description • BSC’s ASSIGNMENT CMD message has been sent but the BTS sends several EST IND messages. Later MS sends ASSIGNMENT FAIL message and the causing value is Protocol error unspecified.
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Case Analysis • By analyzing the messages we can find that MS has reported several EST IND messages before ASSIGNMENT FAIL message. • When BTS receives a SABM frame it will report an EST IND message to BSC. So we know now that the MS must have sent many SABM frames to the BTS. • BTS can decode the SABM frame sent by MS but MS cannot decode the UA frame sent by BTS. And this causes that the BTS reports many EST IND messages.
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Case Analysis
Case realize:
In the lab we can simulate. Disconnect the TX antenna to make the downlink signal pure. Trace the A-bis signaling we can find that BTS sends several EST IND messages to BSC for the newly assigned TCH but can’t succeed. And then MS sends ASSIGNMENT FAIL via the
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previous SDCCH and the causing value is “Protocol Conclusion: –error In the practical field, when many ASSIGNMENT FAIL unspecified”. messages appear, the main reason might be the pure downlink receiving power level or receiving quality. – Connection between TRX and SCU (CDU).
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Appendix: Power Class Phase 2 MS output Power Class and the exact power relations. power
class
GSM900 Max peak power
DCS1800 Max peak power
Tolerance(dB) _normal
Tolerance(dB) _extreme
1
------
1W(30dBm)
+/-2
+/-2.5
0.25W(24dBm)
2
2.5
4W(36dBm)
2
2.5
2
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5W(37dBm)
4
2W(33dBm)
2
2.5
5
0.8W(29dBm)
2
2.5
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Phase 2 MS Power control level and the exact power relations.
