GSM TCH Congestion & Solutions
i
Contents
1 Overview........................................................................................................
1
1 Overview
Along with the development in telecommunication industry and introduction of
competitive mechanism, subscribe...
2 TCH occupation signaling & relevant
counters
2.1 TCH occupation signaling
MSC will send Assignment Request signaling to ...
GSM TCH Congestion & Solutions
4
formula
V3 (6.20)
(C900060020+C900060031+C900060043+C900060047)*100%
/(C900060019+C900060...
5
3 Causes of radio network congestion
1. Main causes for channel congestion are as follows:
2. High traffic density, whic...
7
4 Problem handling procedures
It’s suggested to locate the problems through checking radio parameters and equipment
hard...
GSM TCH Congestion & Solutions
Fig 4-1 Flow for handling TCH congestion
8
5 Common solutions to TCH congestion
Common solutions to TCH congestion comprise:
· Adopt traffic control in the congested...
GSM TCH Congestion & Solutions
Fig 5-1 Cell selection
Usually, priority of all cells should be set “Normal”, i.e. CBQ=0. I...
Chapter 5 Common solutions to TCH congestion
11
5.1.2 Cell reselection parameters
In accordance to GSM standards, when cel...
GSM TCH Congestion & Solutions
12
macro-micro handover. Traffic control in dual-band network can be reached through
these ...
Chapter 5 Common solutions to TCH congestion
13
5.1.4 Control of cell coverage
The main reason for some cells suffering fr...
GSM TCH Congestion & Solutions
1 Maximum output power – 2dB
2 Maximum output power – 4dB
3 Maximum output power – 6dB
4 Ma...
Chapter 5 Common solutions to TCH congestion
15
in the cell, whose calculation formula is shown bellow:
According to actua...
GSM TCH Congestion & Solutions
16
try to keep the drop of speech quality within the range acceptable to subscribers.
· HR ...
Chapter 5 Common solutions to TCH congestion
17
· Use of HR depends on terminal (MS) ability to support; currently, a cert...
GSM TCH Congestion & Solutions
Export each
cell’s traffic
(busy hour)
report of the
most recent
week
Calculate each
cell’s...
Chapter 5 Common solutions to TCH congestion
19
Conditions for cell split
Cell-split is aimed at macro-cells;
Macro-cells ...
21
6 Typical cases
6.1 High TCH congestion rate at an overseas BTS after site swap
Problem description:
TCH congestion rat...
GSM TCH Congestion & Solutions
Fig 6-1 Setting of ChanSelectPrio
Considering that about 10%-15% of MSs do not support HR T...
Chapter 6 Typical cases
23
date UserLabel
TCH
CONGESTION
KEY
TOTAL
CALLS KEY
TCH attempt
total
num(exclude
handover)
TCH
o...
GSM TCH Congestion & Solutions
24
Site77_bts1 26 24.89 28.9 737 25502008-4-12 21:00
- 22:00 Site93_bts2 25 23.72 20.89 507...
Chapter 6 Typical cases
25
Date UserLabel
TCH
available
TCH
traffic
TCH
congestion
rate
TCH
overflow
times
TCH call
attemp...
12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009
12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009
12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009
12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009
12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009
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12 gsmp&o b-en-gsm gsm tch congestion & solutions-word--201009

  1. 1. GSM TCH Congestion & Solutions
  2. 2. i Contents 1 Overview..................................................................................................................................................... 1 2 TCH occupation signaling & relevant counters ...................................................................................... 3 2.1 TCH occupation signaling................................................................................................................. 3 2.2 Definition of TCH congestion indicator............................................................................................ 3 3 Causes of radio network congestion ......................................................................................................... 5 4 Problem handling procedures................................................................................................................... 7 5 Common solutions to TCH congestion..................................................................................................... 9 5.1 Common methods for controlling traffic volume.............................................................................. 9 5.1.1 Cell selection parameters ....................................................................................................... 9 5.1.2 Cell reselection parameters .................................................................................................. 11 5.1.3 Handover based on layers .................................................................................................... 11 5.1.4 Control of cell coverage....................................................................................................... 13 5.2 Open HR ......................................................................................................................................... 14 5.2.1 Dynamic HR switching threshold ........................................................................................ 14 5.2.2 Suggestions on HR application ............................................................................................ 15 5.2.3 Some matters to be noted in HR application........................................................................ 16 5.3 Network expansion ......................................................................................................................... 17 5.3.1 Flow of network expansion.................................................................................................. 17 5.3.2 Principles of network expansion .......................................................................................... 18 6 Typical cases ............................................................................................................................................. 21 6.1 High TCH congestion rate at an overseas BTS after site swap....................................................... 21 6.2 Congestion due to traffic burst........................................................................................................ 23
  3. 3. 1 1 Overview Along with the development in telecommunication industry and introduction of competitive mechanism, subscribers' demand for high network quality is increasing, which has put the service quality of radio network at a more prominent position. Network quality is usually reflected in the indicators like congestion rate, call drop rate and call quality, etc.. Congestion often brings inconvenience to subscribers, thus it is the most complained problem. Besides, network congestion rate is also one important indicator to evaluate network operation situation. High congestion rate will affect indicators like call drop rate, handover success rate and call establishment rate, etc.. Therefore, currently it’s of great importance to reduce system congestion and improve network operation quality.
  4. 4. 2 TCH occupation signaling & relevant counters 2.1 TCH occupation signaling MSC will send Assignment Request signaling to BSC after it confirms MS’ application for TCH. Channel application and assignment are shown bellow: Fig 2-1 Flow of occupying TCH Upon receiving “Assignment Request” from MSC, BSC will search for suitable TCHs. If no usable TCHs are available, BSC will send a “Assignment Failure” message to MSC with the cause of no radio resource available. Refer to Fig 2-1 for details. 2.2 Definition of TCH congestion indicator Table 2-1 Definition of TCH congestion indicator KPI name TCH blocking rate Indicator definition TCH congestion times*100%/ TCH call attempts Counter V2 (2.97) (C11610-C11697)*100/(C11609-C11696) 3
  5. 5. GSM TCH Congestion & Solutions 4 formula V3 (6.20) (C900060020+C900060031+C900060043+C900060047)*100% /(C900060019+C900060030+C900060042+C900060046)
  6. 6. 5 3 Causes of radio network congestion 1. Main causes for channel congestion are as follows: 2. High traffic density, which even exceeds the designed capacity of BTS; 3. Equipment hardware problem, like lack of usable resources or channel congestion caused by unstable equipment performance; 4. Problems with adjacent cells; 5. Unreasonable LAC planning: if LAC boundary is set at high traffic areas or main transportation ways, where subscribers are in great number and in frequent movement, LAC renewal can be very frequent, which will form unreasonable calling modes and lower system capacity as well; 6. Unreasonable setting of radio parameters: such as delay of cell reselection, handover margin, level of outgoing handover trigger, etc., unreasonable setting of these parameters can result in Pingpong location renewal and Pingpong handover; 7. Burst of high traffic volume can happen in some areas (such as schools, playgrounds) with special traffic distribution modes, which exceeds the designed system capacity; 8. Too large coverage can cause isolated-island effect.
  7. 7. 7 4 Problem handling procedures It’s suggested to locate the problems through checking radio parameters and equipment hardware. Handling procedures for TCH congestion are as follows: 1. Check if the problem cell and its adjacent cells operate normally, check TCH usability to locate the unstable equipment. If adjacent cells work abnormally, the problem cell will have to take part of their traffic besides its own load; 2. Check MS mobility to see if the TCH congestion is caused by excess incoming handovers. It it’s true, we can optimize handover parameters (increase HO Margin) to reduce number of handovers from adjacent cells to the congested cell, so as to ease the cell from congestion; 3. Check setting of radio parameters: such as delay of cell reselection, handover tolerance limit, level of outgoing handover trigger, etc., unreasonable setting of these parameters can result in Pingpong location renewal and Pingpong handover; 4. Through test of field strength, analyze if coverage is too large and if isolated-island effect exists. When isolated-island effect happens to one cell in an area, where predefined adjacent cells can not be detected, MS will constantly stay with the serving cell; and normal handovers can not be triggered, in spite of any changes on signals, and finally call drops will be resulted. To avoid this case, two methods can be adopted: (1) adjust the antenna of the isolated cell to eliminate the effect. However, due to the complexity in electric wave transmission, it takes several tests to abate the effect, and it’s really difficult to totally eliminate the effect due to high buildings. (2) define new adjacent cells for the isolated cell. The principle for defining related parameters is: handovers/LAC renewal from the isolated cell to normal cells has priority over the reversed ones. 5. Congestion due to high traffic density: check if the BTS capacity configuration reaches the max. If not, expand it with enough TRXs. General flow for handling TCH congestion is shown in Fig 4-1:
  8. 8. GSM TCH Congestion & Solutions Fig 4-1 Flow for handling TCH congestion 8
  9. 9. 5 Common solutions to TCH congestion Common solutions to TCH congestion comprise: · Adopt traffic control in the congested cell, so as to balance traffic load; · Open HR, increase system capacity; · Expand TRXs or split cells, so as to increase sites and increase system capacity. 5.1 Common methods for controlling traffic volume 1. control cell selection parameters; 2. control cell reselection parameters; 3. handovers based on layered cells; 4. control a cell’s real coverage. 5.1.1 Cell selection parameters C1 is applied as standard when MS is selecting cell. It will choose the cell with largest C1 value. According to GSM regulations: C1=(RXLEV- RXLEV_ACCESS_MIN) - Max(MS_TXPWR_CCH-P,0) RXLEV: level of MS receive signal; P: the max receive power of MS; ACCESS-MIN: the minimum receive level for MS access: MS-TXPWR-CCH: the allowed max transmitting power for MS access into BCCH; C1 reflects the condition of MS receive level (good/bad), whose value won’t be influenced by network deployment mode. 9
  10. 10. GSM TCH Congestion & Solutions Fig 5-1 Cell selection Usually, priority of all cells should be set “Normal”, i.e. CBQ=0. In some cases, like microcell application, dual-band network, multi-layer network, etc., operators may favorably want MS to access into certain type of cells, we can set priority of these cells as “Normal” and that of other cells as “Low”, or in some high traffic areas we can set cells’ priority as “Low” to reduce their load. CBQ has no influence on selection but cell reselection. CBQ and C2 should be used coordinately in optimization. In order to make dual-band cell phones access into 1800M system, we can set CBQ and CBA values to make a difference in priority of DCS1800 and GSM900 networks, so that 1800M network will be chosen preferably (cell’s priority won’t affect cell reselection). The relations among CBQ, CBA, cell selection priority and cell reselection condition are shown bellow: Table 5-1 Relations among CBQ, CBA, cell selection priority and cell reselection condition CellBarQualify CellBarAccess Cell selection priority Cell reselection condition 0 0 Normal Normal 0 1 Barred Barred 1 0 Low Normal 1 1 Low Normal In order to make MS choose 1800M network, we can set 1800M cell with Normal priority, its CBQ=0, CBA=0; set 900M cell with Low priority, its CBQ = 1, CBA = 0. Fig 5-2 Priority in cell selection 10
  11. 11. Chapter 5 Common solutions to TCH congestion 11 5.1.2 Cell reselection parameters In accordance to GSM standards, when cell selection is to be carried out, MS will order adjacent cells according to their C2 values and check which one fulfills the conditions for MS residing in the cell; if conditions are fulfilled, MS will reside in the cell. Cell reselection is based on its algorithm C2, which is shown bellow: · C2 = C1 + CRO – TO × H(PT – T), when PT ≠ 31, · C2 = C1 – CRO, when PT=31; CRO = CELL_RESELECT_OFFSET; TO = TEMPORARY_OFFSET; PT = PENALTY_TIME. According to C2 standard, in order to reduce cell reselection in dual-band network, we can set CRO of DCS1800 cell a large value to make C2 in DCS1800 larger than that in GSM 900, so as to keep MS residing in DCS1800 cells. During cell reselection, if we need some idle cells to share some traffic load with those with high traffic volume, we can increase their CRO; conversely, when some cells suffer from high congestion rate, we can set PT=31, reduce value of C2 in the serving cell, thus “push” away some traffic volume and reduce TCH load. We must note that CRO can not be set over 20dB. Example: Suppose an area is covered by two cells simultaneously (GSM900 cell and DCS1800 cell), and the two cells’ access priority is the same, CRO of DCS1800 cell=20, CRO of GSM900 cell=0, PT and TO of the two cells are 0, strength of MS receiving signal from GSM900 cell is -68dBm, that from DCS1800 is -78dBm, and their minimum access level is the same, -104dBm. Then C1(900)=-68-(-104)=36, C1(1800) =-78-(-104)=26. MS selects GSM900 cell when it’s powered on. After a while, in cell reselection, MS will resides in DCS1800 cell, because C2(900)=-68-(-104)+0-0=36, C2(1800)=-78-(-104)+20-0=46. 5.1.3 Handover based on layers From the perspective of multi-layered cells, effective traffic control and traffic balance can also be realized through planning layers and setting relevant parameters in dual-band network. Among the current ZTE system equipment, the layer-related and most commonly used handover algorithms comprise PBGT handover, traffic handover,
  12. 12. GSM TCH Congestion & Solutions 12 macro-micro handover. Traffic control in dual-band network can be reached through these handover algorithms, which are simply described as follows: · PBGT handover Through setting PBGTHoLayer and NCellLayer, we can control whether the handover can be carried out among undefined layer, same layer different frequency band, upper layer, and lower layer, thus we can reach flexible control over traffic distribution. For specific parameters, please refer to relevant technical guidebooks. · Traffic handover Through setting parameters: layer priority-TrafficHoLayrCtl (same layer, upper layer, lower layer), frequency band TrafficHoFreqCtl and NCellLayer, we can contol the layer and frequency band for target cell of traffic handover, and traffic distribution can be controlled flexibly as well. Settings for relevant parameters: Open traffic handover; Traffic handover threshold can be set70; Level threshold for traffic handover (TrafficLevThs)can be set 0dB; Frequency control value (TrafficHoFreqCtl) can be set 0. · Macro-micro handover Macro-micro handover is to handover the MS moving with slow speed from macro cell layer to micro cell layer. The micro cell mentioned here is just a concept in logic. In this example, DCS1800 cell can be regarded as micro cell, and the macro-micro handover can only be carried out to adjacent cells on lower layer. Relevant parameters: Set layer relations and set DCS1800 cell layer “Lower”; Open macro-micro handover function; Macro-micro handover threshold (MacroMicroHoThs) can be: -90~-80dBm Counter for Macro-micro handover threshold(MacroMicroHoN): 2~4
  13. 13. Chapter 5 Common solutions to TCH congestion 13 5.1.4 Control of cell coverage The main reason for some cells suffering from congestion is unreasonable planning or non-standard installation work, which causes long coverage and large serving area to cells and makes the cells absorb too much traffic volume, thus cell congestion is inevitably formed. Common methods for locating cells with congestion due to over coverage are as follows: · Evaluate cell coverage through DT, analyze and find out if over coverage exists; · From TA distribution report at OMCR, get the distribution of the cell’s main traffic TA; combining planning data, analyze and find out that over coverage exists. There are two main methods for controlling cell coverage and eliminating over coverage problem. · Adjust antenna down-tilt and antenna height; As for antenna down-tilt, it’s 6-10°in dense urban area, 4-6°in urban area, 2-6 °in suburb, 0-4°in villages. When adjusting antenna down-tilt, we must take into consideration factors like the distance to neighboring cells, landforms. If it’s necessary, we can also use DT to get the down-tilt for best coverage. · Adjust TRX static output power Usually adjustment of TRX static output power can help achieve coverage control, but in order not to affect indoor coverage, it’s recommended that this method be applied only after adjustment in antenna fails to solve the problem completely. Note that power class of all TRXs in the cell must be adjusted to be unanimous during adjustment of TRX static output power, or UL-DL unbalance will be resulted. Currently, TRX static power class can be adjusted at OMCR. Its 7 classes are listed in Table 5-2: Table 5-2 Static power class static power class Actual maximum output power static RF power step Pn 0 Maximum output power
  14. 14. GSM TCH Congestion & Solutions 1 Maximum output power – 2dB 2 Maximum output power – 4dB 3 Maximum output power – 6dB 4 Maximum output power – 8dB 5 Maximum output power – 10dB 6 Maximum output power – 12dB 5.2 Open HR As for TCH/FS (Full rate Speech) or TCH/EFS(Enhanced Full rate Speech), 24 frames among the 26 are used to carry speech data, 1 frame (the 13th frame) used for transmitting channel associated signaling SACCH (Slow Associated Control Channel), and another 1 frame (the 26th frame) is idle frame. When the system adopts TCH/HS(Half rate Speech), the multi-frame structure of air interface won’t change. The odd frame is assigned to a MS and the even frame is assigned to another one, the original 13th frame is the first MS’ SACCH, the original 26th frame (idle frame) is the second MS’ SACCH. In this way, the channel, which could carry one TCH/FS or TCH/EFS channel before, can carry two TCH/HS channels now, thus the channel capacity is doubled. The relation between frame structures of FR channel and HR channel is shown bellow: Fig 5-3 Relation between frame structures of FR channel and HR channel 5.2.1 Dynamic HR switching threshold Dynamic HR exchanging threshold is that for FR mode switching to HR mode. After the threshold is set, system will make judgment according to it, corresponding switching process will be triggered if conditions are fulfilled. In this way, the percentage of HR channel in the system is in a dynamic status. Switching threshold is used in cells. The parameter represents the percentage of TCH 14
  15. 15. Chapter 5 Common solutions to TCH congestion 15 in the cell, whose calculation formula is shown bellow: According to actual network operation, default threshold recommended by ZTE is 70%. In the process of dynamic HR application, actual traffic volume can be obtained from performance statistics, and number of TCH needed can be obtained from ERLB. HR switching threshold can be obtained from calculation. 5.2.2 Suggestions on HR application Application of HR function enables fast expansion of existing network and relieves the pressure on radio network frequency band and capacity and solves traffic congestion. As for specific application scenarios, such as regular network expansion and dealing with urgent burst of traffic, different strategies should be adopted. Some suggestions on HR application are listed bellow: · HR application in area with burst traffic HR is most effective in dealing with burst of traffic in some areas, such as stadiums or sports fields, campus, and big assembly or meetings, etc.. The outstanding feature of traffic in these areas is that traffic is busy in periods and usually comes in a burst, for example, the traffic increases during matches in stadiums and intervals between lessons, which will have impact on network. When traffic volume is low, TCH is in FR; when burst of traffic happens, it’s automatically switched from FR to HR, thus congestion will be relieved and the cost for network expansion will be saved for operators as well. · HR application at area with dense traffic High dense traffic often exists at dense urban areas, airports, train stations, and squares. Along with the fast development of cities and network subscribers, frequent network expansion will be needed at these areas. HR can be adopted to avoid frequent network adjustment and expansion. Before a new round of capacity expansion, we can appropriately keep dynamic or static HR open to deal with burst of traffic volume, and combine with long-term planning to provide operators with a more flexible expansion choice. Meanwhile, HR is also a network expansion scheme, when frequency resource is limited in dense urban area and the BTS type doesn’t allow expansion. Under this circumstance, please note that percentage of open HR shall not exceed 30%, and
  16. 16. GSM TCH Congestion & Solutions 16 try to keep the drop of speech quality within the range acceptable to subscribers. · HR application at areas with lower-end subscribers Considering network completion and their brand competitiveness on market, operators are willing to provide coverage at some areas with lower-end subscribers, like remote villages. While in most cases number of subscribers in these areas is very thin, and the ARPU is rather low, thus operators’ input-and-output ratio is very low. Since the lower-end subscribers' demand for speech quality is not high, only getting through is acceptable to them, so combining with some techniques for larger coverage, HR can be adopted to satisfy calling demand at large areas, so as to realize low-cost coverage. Both static and dynamic HR can be adopted. Besides, traffic burst can happen to lower-end areas too, under the circumstances like assemblies, migration of people, etc.. HR can be adopted to solve the problem. 5.2.3 Some matters to be noted in HR application HR function can quickly improve network capacity. While with a view to avoid influence on radio indicator and solve network congestion, the following matters shall be taken into consideration: · Interference in radio environment: · HR has no obvious harmful effect on network indicators, but when radio environment is bad and C/I is low, speech quality will drop more obviously than that of FR. Therefore, we shall try to avoid using HR when environment interference is strong; · Subscriber’s speech quality sensitivity · From the perspective of MOS grading, HR speech coding is inferior to enhanced FR speech coding. Its degree of distortion is higher when handling speech with rich frequency spectrum (eg. music). Therefore, we need to take careful consideration when using HR at areas of high value or with high demand on speech quality. Generally, the use of HR shall not exceed 30%. · Terminal(MS) ability to support
  17. 17. Chapter 5 Common solutions to TCH congestion 17 · Use of HR depends on terminal (MS) ability to support; currently, a certain percent of terminals don’t support HR services; according to statistical result, above 75% terminals (MS) in China support HR. It's suggested that the ratio of FR to HR channel assignment be controlled at 6:4 in commercial systems. Adjustment can be made in other countries and areas according to actual situations. 5.3 Network expansion Network expansion is based on traffic in busy hour, overflowed traffic in busy hour and evaluation of current site distribution density to make corresponding expansion plans. 5.3.1 Flow of network expansion Please refer toFig 5-4:
  18. 18. GSM TCH Congestion & Solutions Export each cell’s traffic (busy hour) report of the most recent week Calculate each cell’s overflowed traffic of busy hour (traffic lost due to congestion) Obtain the cell’s actual busy hour traffic volume Erl (actual) Look up in Erl B, obtain the cell’s theoretical busy hour traffic Erl (theoretical) After certain percent of HR is open, look up in Erl B and obtain the cell’s theoretical busy hour traffic, marked as Erl (theoretical, incl. HR) Erl(actual )> Erl(theoretical) No expansion need TRX(actual need)>max number of TRX allowed make TRX expansion (GSM900/1800), First make TRX expansion (GSM900/1800), total number of TRX shall not exceed the max allowed. Complete expansion plan Is capacity need fulfilled? Add GSM1800 BTS Add GSM900 BTS Cell split No Yes Yes No Yes No Each cell’s busy hour traffic Total of traffic overflowed Average call time TRX available in the cell Busy hour overflowed traffic=total of traffic overflowed* average call time/ 3600 Actual busy hour traffic=busy hour traffic + busy hour overflowed traffic Look up in Erl B, get number of TRX needed when GOS=2% and Erl (actual) is fulfilled. Filter out each cell’s max traffic volume to be the base of expansion calculation. Calculate number of TRX needed for expansion (actual need) Erl(actual)> Erl(theoretical, incl. HR) Yes Use of HR shall be within 30%. Open HR for expansion No Fig 5-4 Flow of cell expansion 5.3.2 Principles of network expansion Principles for GSM900/1800 TRX expansion First we need to calculate and obtain GSM900/1800 frequency resource and the max configuration plan, which can be reached basing on frequency planning scheme. Compare the max configuration with that required by actual traffic need to see if the max traffic can be fulfilled. · When the actual configuration need is under the max configuration plan, we can consider carrying out expansion. · When the actual configuration need is beyond the plan, we can consider carrying out cell split or adding new sites. 18
  19. 19. Chapter 5 Common solutions to TCH congestion 19 Conditions for cell split Cell-split is aimed at macro-cells; Macro-cells of single frequency band; In the BTS, which the macro-cell belongs to, there is only one busy cell; the cell can be split; Pay attention to adjustment of antenna parameters during cell-split. Conditions for setting up new GSM900 BTS If the traffic need still can not be satisfied when the TRX is expanded to the max allowed, new BTSs need to be set up; The average distance between the BTS and those around >400m, and number of TRX configured in the BTSs around doesn’t reach the max allowed for GSM900, in this case, new GSM900 BTSs can be set up. Conditions for setting up GSM1800 BTS If the traffic need still can not be satisfied when the TRX is expanded to the max allowed, new BTSs need to be set up; The average distance between the BTS and those around >400m; and number of TRX configured in the BTSs around has reached the max allowed for GSM900, new GSM900 BTSs would make the frequency interference out of control. In this case, we can set up new GSM1800 BTSs, and make them co-site with those of GSM900 to absorb some traffic.
  20. 20. 21 6 Typical cases 6.1 High TCH congestion rate at an overseas BTS after site swap Problem description: TCH congestion rate at an overseas BTS was shown higher than usual after it's been swapped with ZTE equipment. Problem analysis: From the dynamic data management, we observed that all FR TCHs have been occupied, while a lot of HR TCHs were idle in 3 cells. After investigating TCH configuration in the 3 cells, we found that except the BCCH TRX all the other 3 TRXs in the cells were configured with HR TCH, while the BCCH TRX was just configured with 3 FR TCH. Therefore, the congestion probably occurred on FR TCH. Through signaling analysis, we found congestion just occurred on the assigned FR TCH. Basically, it was confirmed that the assignment failure was caused by congestion due to lack of FR TCH. From the recorded signaling, we didn’t find assignment failure of HR TCH. After checking the channel assignment parameters of MSC, BSC and cells, we found the system takes the first speech version assigned by MSC as default; after most TRXs were configured with HR TCH, the channel assignment priority in radio parameters has not been changed accordingly, which led to channel assignment according to the default, while there were only 3 FR TCH, thus TCH congestion was inevitably resulted. The primary cause of this problem is that certain percent to MSs do not support HR. Problem handling: Adjust “ChanSelectPrio” (channel selection priority), change the default “No Select” to “half Rate First” as shown in Fig 6-1:
  21. 21. GSM TCH Congestion & Solutions Fig 6-1 Setting of ChanSelectPrio Considering that about 10%-15% of MSs do not support HR TCH, increase number of FR TCH to 15% of total TCH. After parameter adjustment, congestion rate dropped obviously. Table 6-1 Related congestion indicators before parameter adjustment date UserLabel TCH CONGESTION KEY TOTAL CALLS KEY TCH attempt total num(exclude handover) TCH overflow total num(exclude handover) Site1_bts1 19.51 2553 3092 532 Site1_bts2 12.17 2011 2282 258 2007-7-31 14:00 - 18:00 Site1_bts3 0.49 418 425 1 2007-8-1 010:00 Site1_bts1 25.39 12994 17562 4511 22
  22. 22. Chapter 6 Typical cases 23 date UserLabel TCH CONGESTION KEY TOTAL CALLS KEY TCH attempt total num(exclude handover) TCH overflow total num(exclude handover) Site1_bts2 14.95 10844 12880 1970- 24:00 Site1_bts3 3.48 2866 3062 163 Site1_bts1 23.9 2831 3782 937 Site1_bts2 11.89 2292 2606 307 2007-8-1 10:00 - 14:00 Site1_bts3 0.63 510 523 4 Table 6-2 Related congestion indicators after parameter adjustment date UserLabel TCH CONGESTION KEY TOTAL CALLS KEY TCH attempt total num(exclude handover) TCH overflow total num(exclude handover) Site1_bts1 0.64 1272 1282 6 Site1_bts2 1.59 902 925 18 2007-8-2 12:00 - 13:00 Site1_bts3 0 174 177 0 6.2 Congestion due to traffic burst Problem description: Congestion rate in two cells under a certain BTS increased suddenly during 21:00~ 23:00 pm, and the rate reached 30%. Because evaluation period was during 21:00~ 22:00, these two cells had great influence on BSC congestion rate. From performance report, we could see that TCH usage rate was normal when congestion occurred, but number of call attempts and traffic volume were obviously increased and their increase was even doubled. Table 6-3 Cell congestion indicator Date UserLabel TCH available TCH traffic TCH congestion rate TCH overflow times TCH call attempts Site77_bts1 26 24.78 23.51 612 26032008-4-11 21:00 - 22:00 Site93_bts2 25 22.48 18.17 428 2355 Site77_bts1 26 24.27 23.39 589 25182008-4-12 21:00 - 22:00 Site93_bts2 25 23.14 17.95 407 2267
  23. 23. GSM TCH Congestion & Solutions 24 Site77_bts1 26 24.89 28.9 737 25502008-4-12 21:00 - 22:00 Site93_bts2 25 23.72 20.89 507 2426 Problem analysis: We checked hardware warning and TCH availability rate, but no problem was found. However, the report showed that traffic during this period increased obviously. After observation of a week, we found that the traffic increased regularly during this period. We doubted traffic burst happened in the area. From testing, we found that the two cells covered a high school. After school, traffic burst emerged apparently, it wasn't caused by reasons like abnormal calls. Problem handling: From planning software we found that the dormitory building area was mainly covered by the two cells, and other cells were a bit far from the school, so it’s difficult to reach traffic balance. We checked the two cells' configuration, which has already reached the max allowed and the TRXs could not be expanded. After checking we found HR in the two cells was off, so it’s suggested that HR be open for cell expansion. Through analysis of the cell’s actual traffic, we found that the traffic undertaken by the two cells has already reached 23~24ERL; considering the high congestion rate, we supposed the actual traffic could be even higher. It's stipulated in Erl B that 33 TCHs are needed to support traffic of 23~24ERL, while there were only 26 TCHs available, and 30% of HR needed to be open to satisfy traffic need. Besides, HR needed to be open since the congestion was caused by burst traffic. Opened dynamic HR in the two cells, and set the HR threshold as 70%. We checked indicators during the same period for two days thereafter, the congestion problem was found disappeared. Table 6-4 Congestion disappeared when HR was on Date UserLabel TCH available TCH traffic TCH congestion rate TCH overflow times TCH call attempts Site77_bts1 26 24.78 23.51 612 26032008-4-11 21:00 - 22:00 Site93_bts2 25 23.48 18.17 428 2355 Site77_bts1 26 24.27 23.39 589 25182008-4-12 21:00 - 22:00 Site93_bts2 25 23.14 17.95 407 2267
  24. 24. Chapter 6 Typical cases 25 Date UserLabel TCH available TCH traffic TCH congestion rate TCH overflow times TCH call attempts Site77_bts1 26 24.89 28.9 737 25502008-4-12 21:00 - 22:00 Site93_bts2 25 23.72 20.89 507 2426 Site77_bts1 37 29.73 0.44 13 28312008-4-14 21:00 - 22:00 Site93_bts2 35 28.42 0.26 7 2692 Site77_bts1 40 30.12 0.53 15 28812008-4-15 21:00 - 22:00 Site93_bts2 36 28.14 0.11 3 2655

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