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GSM Network SDCCH Congestion &
Solutions
11gsmpo b-en-gsmgsmnetworksdcchcongestionsolutions-word-201009-130915134031-phpapp01
i
Contents
1 SD Seizure Signaling & Relevant Counters............................................................................................. 1
1.1 SD seizure and release flow.............................................................................................................. 1
1.2 Definition of SDCCH congestion indicator ...................................................................................... 2
2 Classification of SDCCH Congestion Causes .......................................................................................... 5
3 Procedure of Checking SD Congestion SD .............................................................................................. 7
4 Typical Cases of SD Congestion.............................................................................................................. 11
4.1 SD congestion due to LAPD delay caused by transmission fault ................................................... 11
4.2 SD congestion due to large quantity of LOC (caused by cross-LAC coverage)............................. 11
4.3 SD congestion due to strong interference ....................................................................................... 12
4.4 SD congestion due to large quantity of LOC caused by HLR cutover............................................ 14
4.5 SD congestion due to short message pager ..................................................................................... 16
11gsmpo b-en-gsmgsmnetworksdcchcongestionsolutions-word-201009-130915134031-phpapp01
1 SD Seizure Signaling & Relevant
Counters
1.1 SD seizure and release flow
Fig 1-1 SD Seizure and Release Flow
As shown in Fig 1-1, during LOC and the early stage of MOC and MTC process, MS
usually seizes SDCCH to exchange signaling. The signaling flow is as follows:
· After receiving Channel Required message, BSC applies for SD from database,
and the counter for SD seizure attempt times increases by 1;
· When database allocates SD, the number of successful SD seizure increases by
1;
· BSC sends SD activation message to BTS, then the number of SDCCH
allocation attempts increases by 1;
· BTS responds to BSC with a response message of SD activation, BSC increases
the number of successful SDCCH allocation by 1;
· BSC sends Immediate Assignment message to MS through BTS, BSC increases
1
GSM Network SDCCH Congestion & Solutions
2
the number of SDCCH assignment attempts by 1;
· MS report SABM frame, BTS reports Est Ind message to BSC, and BSC
increases the number of successful SDCCH assignment by 1.
1.2 Definition of SDCCH congestion indicator
Table 1-1 SDCCH Congestion Indicator Definition
KPI name SDCCH blocking rate
Definition Number of signaling channel blocking times*100%/ Number of signaling channel
call attempts
V2 (2.97) (C11625-C11626+C11697)*100%/(C11625+C11696)
Counter
formula
V3 (6.20)
(C900060005+C900060011+C900060039)*100%/(C900060003+C9
00060010+C900060038)
SD congestion means that upon receiving Channel Required message, BSC applies for
SD from database, while BSC has no SD for allocation (all SD resource are blocked or
occupied).
When BSC fails in applying for SD, it sends a message Immediate Assignment Reject
to MS, commanding MS to request for access after a period of time (set on T3122); and
BSC increases the counter for SDCCH congestion by 1. Message flow of Immediate
Assignment Reject can not solve SD overflow problem, but can help relieve and
smoothen it.
Table 1-2 Timer T3122
Protection period of access attempt(T3122,s)
Value range 0 ~ 255
Unit s
Default value 10
Ad

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  • 1. GSM Network SDCCH Congestion & Solutions
  • 3. i Contents 1 SD Seizure Signaling & Relevant Counters............................................................................................. 1 1.1 SD seizure and release flow.............................................................................................................. 1 1.2 Definition of SDCCH congestion indicator ...................................................................................... 2 2 Classification of SDCCH Congestion Causes .......................................................................................... 5 3 Procedure of Checking SD Congestion SD .............................................................................................. 7 4 Typical Cases of SD Congestion.............................................................................................................. 11 4.1 SD congestion due to LAPD delay caused by transmission fault ................................................... 11 4.2 SD congestion due to large quantity of LOC (caused by cross-LAC coverage)............................. 11 4.3 SD congestion due to strong interference ....................................................................................... 12 4.4 SD congestion due to large quantity of LOC caused by HLR cutover............................................ 14 4.5 SD congestion due to short message pager ..................................................................................... 16
  • 5. 1 SD Seizure Signaling & Relevant Counters 1.1 SD seizure and release flow Fig 1-1 SD Seizure and Release Flow As shown in Fig 1-1, during LOC and the early stage of MOC and MTC process, MS usually seizes SDCCH to exchange signaling. The signaling flow is as follows: · After receiving Channel Required message, BSC applies for SD from database, and the counter for SD seizure attempt times increases by 1; · When database allocates SD, the number of successful SD seizure increases by 1; · BSC sends SD activation message to BTS, then the number of SDCCH allocation attempts increases by 1; · BTS responds to BSC with a response message of SD activation, BSC increases the number of successful SDCCH allocation by 1; · BSC sends Immediate Assignment message to MS through BTS, BSC increases 1
  • 6. GSM Network SDCCH Congestion & Solutions 2 the number of SDCCH assignment attempts by 1; · MS report SABM frame, BTS reports Est Ind message to BSC, and BSC increases the number of successful SDCCH assignment by 1. 1.2 Definition of SDCCH congestion indicator Table 1-1 SDCCH Congestion Indicator Definition KPI name SDCCH blocking rate Definition Number of signaling channel blocking times*100%/ Number of signaling channel call attempts V2 (2.97) (C11625-C11626+C11697)*100%/(C11625+C11696) Counter formula V3 (6.20) (C900060005+C900060011+C900060039)*100%/(C900060003+C9 00060010+C900060038) SD congestion means that upon receiving Channel Required message, BSC applies for SD from database, while BSC has no SD for allocation (all SD resource are blocked or occupied). When BSC fails in applying for SD, it sends a message Immediate Assignment Reject to MS, commanding MS to request for access after a period of time (set on T3122); and BSC increases the counter for SDCCH congestion by 1. Message flow of Immediate Assignment Reject can not solve SD overflow problem, but can help relieve and smoothen it. Table 1-2 Timer T3122 Protection period of access attempt(T3122,s) Value range 0 ~ 255 Unit s Default value 10
  • 7. Chapter 1 SD Seizure Signaling & Relevant Counters 3 Description If network has no appropriate channel to allocate to MS after it has received channel request message from MS, it sends Immediate Assignment Reject to MS. In order to avoid radio channel congestion which may be caused by continual channel request from MS, timer parameter T3122 (the so called Wait Indication message) is included in Immediate Assignment Reject message. After it receives Immediate Assignment Reject message, MS can not start a new call until T3122 expires. This timer is one of the system control parameters, which is sent to MS in the Immediate Assignment Reject. Recommended normal value for T3122 is 10~15s, and 15~25s in areas with dense traffic.
  • 9. 5 2 Classification of SDCCH Congestion Causes From the aspect of singling flow, there are two main types of causes of SD congestion. Too many Channel Required exceeds network capacity and all SDCCHs are occupied. · Too-many Channel Required means the cell is busy, while few SDCCH are configured, which results in frequent occupancy of SDCCH and overflow. · When checking SD congestion, what shall be considered is if there is possibility of traffic increase. Some operations at OMCR (such as HLR configuration or re-planning of LAC) may lead to traffic increase in network. Too long occupancy period of SDCCH due to non-in-time ending of signaling flow; · If signaling flow doesn’t end in time, which means channel activation/release period is too long due to some reason (say, transmission fault), it will lead to long period of SDCCH occupancy and reduce SDCCH resource, and eventually result in SDCCH overflow. · Too-long channel activation/release period causes MS to repeat Channel Request again and again (the number of repetition is decided by system parameter MaxRetrans), and to occupy SDCCH repeatedly, which worsens SDCCH overflow.
  • 11. 7 3 Procedure of Checking SD Congestion SD 1. Read performance report, check if all cells or some cells under the BTS are congested; It rarely happens that all cells under the BTS are congested. If it happens, the causes are usually related to terrestrial transmission and parameter configuration. 2. Read performance report, check if channel allocation failure (i.e. channel activation expires or fails) happens during SD congestion; Note: Too large LAPD flow will lead to LAPD transmission delay, which causes timer expiration before channel activation is completed. This kind of timer expiration shall be differentiated from that caused by transmission fault. 3. Check radio access measurement, analyze the access reason of Channel Request which causes SD congestion, count the number of Channel Request attempts and success times due to different reasons, and compare indicators with those in normal period.. The access cause of Channel Request falls into 4 types: · MOC (Mobile-Originating Call); · MTC (Mobile-Terminating Call); · LOC (Location Update); · Other reasons (call re-establishment) Generally, the number of LOC attempts takes 35% of the total Channel Request times The percentage may increases to 70%, if the site is located at LAC boundary. Number of MOC Channel Request and that of MTC Channel Request are almost equal, which are related to local subscribers’ call conditions; Other reasons rarely happen in normal conditions.
  • 12. GSM Network SDCCH Congestion & Solutions 8 Here are explanations for each type of cause: 1) If there are a big number of attempts due to other reasons, and all end in failure, the cause can be confirmed to be interference. 2) When the number of MOC attempts is big, and even exceeds the number of LOC, the reasons could be: · There is MS malicious pager operating in the network (which is commonly seen in Shenzhen and Quanzhou); · Interference exists. Judge the reasons through analyzing number of MOC success. Although operation of MS malicious pager causes great number of MOC attempts, number of MOC success increases accordingly; Interference won’t increase the number of MOC success. 3) If number of LOC increases abnormally, check if there are any changes on network parameters, such as re-planning of LAC or amendments of HLR and VLR. 4) Generally speaking, SD congestion caused by LOC won’t bring large number of assignment failures. If SD congestion is accompanied with large number of assignment failures, it’s very possible that the cell traffic volume is high or co-channel interference exists. Note: For BSCV2 system, basic measurement includes the number of attempts/success of MOC, MTC, LOC and other accesses. For iBSC system, there is a special radio access measurement, which needs to establish measurement task. In CS basic measurement, number of MOC/MTC attempts and number of MOC/MTC success are included, through which we can calculate the number of attempts/success of LOC access. 5) Make enquiries and find out if there are newly-setup sites, adjustments on LAC/HLR; 6) Check the performance report of the week when the problem appears, analyze if SD congestion exists for a long time during busy hours. If SD congestion is a long standing issue, and there’s no big fluctuation in the
  • 13. Chapter 3 Procedure of Checking SD Congestion SD 9 number of MOC, MTC, LOC attempt and success times, this means the cell is busy and its traffic volume is high, and expansion is needed.
  • 15. 4 Typical Cases of SD Congestion 4.1 SD congestion due to LAPD delay caused by transmission fault The performance report showed that during SD congestion (SDCCH occupancy failure counter), the number of SDCCH allocation failures was high. The signaling flow shows that BTS didn’t respond to the Channel Activation sent by BSC. Fig 4-1 No BTS Response to Channel Activation There was transmission warning during SD congestion; After BTS was reset, the problem disappeared; After transmission was adjusted, the problem was completely solved. 4.2 SD congestion due to large quantity of LOC (caused by cross-LAC coverage) Performance report of a site showed large amount of SD congestion in busy hours of each day. 11
  • 16. GSM Network SDCCH Congestion & Solutions From analysis of radio access measurement, it’s discovered that the SD congestion was caused by large quantity of LOC. When checking the site map, engineers found the site was located between two location areas (LAC), the cell belonged to LAC09523. A road, 1.5km from the site, was under the coverage of LAC09533. Through signaling tracing, we could see the large quantity of LOC under the cell was from LAC09533 to LAC09523. Fig 4-2 Signaling Showed Large Quantity of LOC under the Cell After DT on site, it’s discovered that the road should be covered by LAC09533, but the congested cell signal was very strong there, which means it stretched its coverage to the road, and resulted in frequent LOC on the road and SD congestion. 4.3 SD congestion due to strong interference On one night, large amount of SD congestion occurred at a BTS (2) in a town, which lasted for a long time. · Normal condition didn’t return even after reset of CMM and TRM; · Congestion disappeared after adjustment of frequency and BCC; 12
  • 17. Chapter 4 Typical Cases of SD Congestion 13 · Congestion phenomenon appeared again after frequency and BCC were changed back; · Congestion totally disappeared 30 minutes after adjustment on TA access threshold. Observed signaling, discovered the SD congestion was caused by large quantity of abnormal Channel Request, and all the Imm Assign generated from these Channel Request ended in failure. All abnormal Channel Request appeared once every four frames; all RA was 01; TA diminished from 63 to 0, then restarted from 63 after 815 frames; level value remained 63. Fig 4-3 Case III-Large Quantity of Channel Request Normal condition didn’t return even after the reset of CMM and TRM, which means the problem was irrelevant to BTS hardware and software; The problem disappeared after adjustment of frequency and BCC, but reoccurred when the frequency was changed back, this means the problem involved in frequency;
  • 18. GSM Network SDCCH Congestion & Solutions 14 Considering Channel Request rules, engineers confirmed there was an interference signal whose frequency was the same as the site’s, and the signal just contained all the training sequence of AB frame. The interference signal was periodical and it created periodical deviation to timeslot 0 window. Just because of this deviation, TX changed periodically. Besides, the interference signal just interfered timeslot0. Therefore, adjustment on TA access threshold could only relieve the problem, but couldn’t solve it completely. There was army garrisoning in the area, the interference signal was probably sent from the army. 4.4 SD congestion due to large quantity of LOC caused by HLR cutover Large amount of SD congestion occurred in a GSM network on Nov. 19th , 20th , 21st and 22nd , 0:30-1:00am. Performance report showed there was a large number of LOC Request during congestion. The measurement data of LOC at MSC showed that at 0:30am on Nov.20th , the number of inter-MSC LOC attempts jumped from 2000 to over 20000, and it returned to normal after half an hour; Table 4-1 Case IV-Dramatic Increase in LOC Number
  • 19. Chapter 4 Typical Cases of SD Congestion At 0:00am and 0:30am on Nov. 21st , the number of inter-MSC LOC attempts jumped from 3000 to over 17000 twice. Number of inter-MSC LOC success-UMTS Number of inter-MSC LOC attempts-GSM Number of inter-MSC LOC success-GSM 1677024752008-11-20 1:30 1787025612008-11-20 1:15 2506032152008-11-20 1:00 6695076322008-11-20 0:45 187970201972008-11-20 0:30 1634024322008-11-20 0:15 1987027752008-11-20 0:00 1812025612008-11-19 23:45 2011027732008-11-19 23:30 2081029662008-11-19 23:15 Time Table 4-2 Case IV- Dramatic Increase in LOC Number Number of inter-MSC LOC success-GSM Number of inter-MSC LOC attempts-GSM -GSM Number of inter-MSC LOC success-UMTSTime 2467 2532 4134 15562 4809 10138 7287 2657 2133 2311 0 0 0 0 0 0 0 0 0 0 3725 3790 5460 17118 6322 11784 8859 3954 3462 3674 2008-11-20 23:15 2008-11-20 23:30 2008-11-20 23:45 2008-11-21 0:00 2008-11-21 0:15 2008-11-21 0:30 2008-11-21 0:45 2008-11-21 1:00 2008-11-21 1:15 2008-11-21 1:30 At 1:00am on Nov. 22nd , the number of inter-MSC LOC attempts jumped from 6000 to over 16000, and it returned to normal on the early morning of 23rd and 24th . On the early morning of 25th , the number of LOC attempts was 15000. These LOC attempts matched closely with the time point of SD congestion and the 15
  • 20. GSM Network SDCCH Congestion & Solutions 16 cause of SD occupancy. There was no obvious change on the number of inter-MSC LOC attempts during SD congestion. During congestion period, the number of TMSI re-allocation also increased dramatically. On-site engineers reported that Nokia was cutting over Nortel HLR when SD congestion occurred. Therefore, we concluded that the cutover of HLR caused MS to make large quantity of inter-MSC LOC at midnight, resulting in SD congestion. If it’s BSS fault, then not only the number of inter-MSC LOC, but that of intra-MSC LOC will be affected. 4.5 SD congestion due to short message pager On Nov. 20th , SD congestion of different degrees occurred in 6 cells under one BTS. Performance report showed allocation failures of different levels in the cells. Transmission was confirmed with no problem after investigation. Checked performance data of radio access, discovered that the cause of large quantity of radio access was MS sending short message: For example, at 10:30, in cell3 the number of MOC Access Request reached 9959 within 15 minutes, while the number of successful access was 1070, and 1045 of the 1070 times were short message requests. By observing other daily data of the BTS, engineers found the traffic volume was not high. It’s concluded that the problem was caused by heavy transmission load. Mass-sending device of short message caused heavy transmission load, which led to LAPD delay, and then led to SD allocation overtime, wasted SD resource, and finally resulted in SD congestion.