Paging Analysis West

Nokia Networks Thailand Date: Prepar Prepared ed by:

4th August, 2002 Sam Sam Chang Chang

Paging Analysis for the

Western AIS Network

Sam Chang Nokia Networks Thailand

Nokia Networks Thailand Date: Prepared by:

4th September, 2002 Sam Chang

Table of Contents: 1. INTRODUCTION...............................................................................................................................................................3 2. BACKGROUND..................................................................................................................................................................3 3. PARAMETER RELATED TO THE PAGING CAPACITY IN THE BTS.................................................................3 4. CALCULATING THE BTS PAGING CAPACITY.......................................................................................................4 2 IMSIS..................................................................................................................................................................................... 5 5. COMBINED BCCH/SDCCH AND PAGING LOAD CALCULATION......................................................................6 6. EXISTING BTS CAPACITY LIMITATION..................................................................................................................8 7. CONCLUSION AND RECOMMENDATIONS............................................................................................................10

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Private and Confidential Nokia Networks Thailand Page 2 of 11



1. Introduction

This document describes the results of the Paging performance and analysis in Western AIS network of Nokia areas. Paging analysis was conducted to assess the paging success rate and paging load in all Nokia existing LAC area, the analysis also involved in BTS Paging capacity and paging load calculation. A good paging performance is essential to ensure all terminating calls can always reach the mobile subscribers. Busy hour paging statistics were collected between CW35, from 27 August 2002 till 02 September 2002 for both Nokia and HuaWei BTSs. However, due to lack of HW MTC counters, we can only estimate the number of MTC by total calls in the location areas. This report served to look into the possible causes and suggestion for improvements to the poor paging performance areas.

2. Background

When a Mobile Station (MS) is paged, a Paging message is sent from the MSC to each Base Station Controller (BSC) belonging to that MSC's service area, or to those BSCs serving at least one cell belonging to the LA where the MS is registered. For each Paging message received by the BSC, Paging Command messages have to be sent to all cells belonging to the LA where the target MS is registered. The number of cells in an LA ranges from a few tens up to perhaps one hundred cells, sometimes even more. This means that one incoming Paging message to the BSC leads to a considerably larger number of outgoing Paging Commands from the BSC. The BTSs have to broadcast all the incoming pages. The Paging Request messages are sent on the Paging Channel (PCH) on the Common Control CHannel (CCCH). Too large LAs may lead to a too high paging load in the BTS resulting in congestion and lost pages.

3. Parameter related to the Paging capacity in the BTS

After an MS tunes to the BCCH carrier and decodes the System Information, it performs an evaluation to which paging group it belongs, and hence, which particular paging block of the available blocks on the paging channel that is to be monitored. The operator can set the number of paging groups for each cell. 



A high number of paging groups means that the MSs have to wait for a longer time than the low number case before the right paging block arrives. This increases the time for paging. A low number of paging groups shortens the call setup time due to that the MS listens to the paging block more frequently. The drawback is that the MS power consumption is higher.

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Two main parameters determine the number of paging groups in a cell. The two parameters are NumberOfBlocksForAccessGrant, BS_AG_BLKS_RES (0 ... 7) and noOfMultiframesBetweenPaging, BS_PA_MFRMS (2 ... 9). •

•

NumberOfBlocksForAccessGrant (0 ... 7): This parameter determines how many of the paging blocks per multiframe that are reserved for the AGCH (Immediate Assignment). This parameter can be set to BS_AG_BLKS_RES = 0 (no blocks reserved) or BS_AG_BLKS_RES >= 1 ( block reserved). The number of paging blocks used for Access Grant depends on the traffic in the cell. Since Immediate Assignment has priority over paging in the Nokia GSM system S9, BS_AG_BLKS_RES should set to 0 (no blocks reserved) unless other is required. NumberOfMultiframesBetweenPaging(2..9): This parameter determines the multiframe period a nd defines the transmission interval of paging messages to the same paging group. For example, MFRMS=9 means that MSs belonging to a certain paging group is paged every 9 th multiframe. This means that the transmission interval of paging messages to paging groups is approximately 2.1 second (9 * 235.4 ms). A high value of BS_PA_MFRMS will lead to more paging groups in the cell.

The relation between NumberOfBlocksForAccessGrant, NumberOfMultiframesBetweenPaging and the number of paging groups is: Example of the capacity of PCH

1.) Combined BCCH/SDCCH cells in AIS Southern Network, Nokia BTS: NumberOfBlocksForAccessGrant = 2 (default value for all cells in Southern region) NumberOfMultiframesBetweenPaging = 7 (default value for all cells in Southern region)

Number of paging groups = (3 - NumberOfBlocksForAccessGrant) * NumberOfMultiframesBetweenPaging = 7 Paging Groups 2.) Non-combined BCCH/SDCCH cells in AIS Southern Network, Nokia BTS: NumberOfBlocksForAccessGrant = 2 (default value for all cells in Southern region) NumberOfMultiframesBetweenPaging = 7 (default value for all cells in Southern region)

Number of paging groups = (9 - NumberOfBlocksForAccessGrant) * NumberOfMultiframesBetweenPaging = 49 Paging Groups

4. Calculating the BTS paging capacity

Due to SDCCH congestion, some cells have combined BCCH/SDCCH configuration. Changing BCCH/SDCCH to combined will lead to the reduce number of paging group per multiframe period. The total 76598558.doc




number of paging groups is 49 paging groups for the case of non-combined control channels. If combined control channels are used the number of paging groups will be equal to 7 paging groups only. The following configurations are possible in one CCCH or Paging Block: •

2 IMSIs

•

1 IMSI and 2 TMSIs 4 TMSIs 1 Immediate Assignment (Access Grant)

• •

For Combined BCCH in current AIS southern network: 3 Paging blocks per Multiframe (235 ms), NumberOfBlocksForAccessGrant = 2 •

The number of paging blocks per second (2 block used for AGCH -> 1 blocks for paging) = 1 block for paging / 0.235(per Multiframe) = 4.25 Paging blocks for paging per second

[1]

As an example, each paging block can fit 4 TMSI pages or 2 IMSI pages. Assuming 25 % second pages (IMSI), and no global pages, the number Paging Attempts per paging block is: 1 TMSI occupies ¼ of a paging block and 1 IMSI occupies ½ of a paging block. 4 pages (100%) = 75%(TMSI) + 25%(IMSI) =3*¼+1 *½ = 5/4 of a paging block Therefore, the estimated number of pages per paging bl ock is: 4/(5/4) = 3.2 pages/paging block [2] Hence, the allowed paging message per second in Combined BCCH/SDCCH cells will be: 4.25 * 3.2 = 13.6 pages / second [3]

Therefore, it is suggest that the parameter NumberOfBlocksForAccessGrant shall change from 2 to 0 since Access Grant Channel has priority over paging. This will directly increase the paging capacity and improve the paging success rate. The following shows the calculation of paging capacity if the mentioned reserved Access Grant parameter is change to 0.

Example: LAC =2733 (Data on 31/8, the highest AGCH attempts = 16881 in cell YPPT2) 76598558.doc




The number of paging blocks per second (3 blocks for paging, if Access Grant Block = 0): = 3 block for paging / 0.235(per Multiframe) [4] = 12.76 Paging blocks per second Highest AGCH access (SDCCH_SEIZ_ATT) in LAC 2733 on 31/8, Busy hour YPPT2

= 16881 SDCCH attempts (AGCH attempts) = 16881/3600 [5] = 4.67 AGCH per second = 4.67*0.235 = 1.1 paging blocks for AGCH per multiframe

If Access Grant Block is change from 2 to 0, then the following number of paging block will increase: •

The number of paging blocks per second (0 block used for AGCH -> 3 blocks for paging) = 3 block for paging / 0.235(per Multiframe) = 12.76 Paging blocks per second

[6]

Take into the consideration of total “Paging blocks per seconds” minus “AGCH per second” = 12.76 Paging blocks per second – 4.67 AGCH per second (CCCH to perform Access Grant) = 8.1 Paging blocks per second

[7]

So, the allowed paging message per second in Combined BCCH/SDCH cells (if Access Grant Block = 0) will be: 8.1 Paging blocks per second * 3.2 pages per paging block = 25.92 pages / second

[9]

Hence, by compare equation [3](reserved Access Grant Block =2), and equation [9] (reserved Access Grant Block = 0). The BTS paging capacity will then increase by > 190%.

5. Combined BCCH/SDCCH and Paging Load calculation

This section will further describe the existing Southern region number of BTS with and without BCCH/SDCCH configuration, and its BTS paging load situation in each Location Area. The following Table 5.1 and Figure 5.2 shows the number of combined BCCH/SDCCH BTS in each LA. Table 5.1: Number of Combined BCCH/SDCCH in each LAC Paging Load% Total BTS COMB NCOMB Comb BCCH/SDCCH (%) (Comb BCCH) 41 1 40 2.4 24.5

LAC BSC 1703 BSC1RBR

Area Ratchaburi

1713 BSC1KRI

Kanchanaburi

84

4

80

4.8

26.6

1723 BSC1SPB

Suphanburi

132

16

116

12.1

75.6

1733 BSC1NPT, BSC4NPT

Nakhonpathom

75

17

58

22.7

61.3

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1743 BSCSKN1

Samutsakhon

157

20

137

12.7

120.5

1753 BSC1SKM

Samusongkram

72

2

70

2.8

32.7

1763 BSCPBI1

Petchaburi

172

2

170

1.2

83.7

1773 BSC1PKN

Prachakirikhan

98

4

94

4.1

47.0

2703 BSC1SKM, BSC2RBR Ratchaburi

157

13

144

8.3

87.8

2713 BSC2KRI

Kanchanaburi

81

6

75

7.4

32.7

2723 BSC2SPB

Suphanburi

54

14

40

25.9

59.2

2733 BSC2NPT

Nakhonpathom

117

9

108

7.7

81.7

2743 BSCSKN1

Samutsakhon

36

5

31

13.9

22.5

2763 BSCCAM1

Cha Am city

63

1

62

1.6

20.4

2773 BSCCAM1

Cha Am city

67

1

66

1.5

26.6

3703 BSC3RBR

Ratchaburi

105

9

96

8.6

69.4

3713 BSC3KRI

Kanchanaburi

145

6

139

4.1

73.5

3733 BSC3NPT, BSC4NPT

Nakhonpathom

150

27

123

18.0

132.8

60

1

59

1.7

32.7

3773 BSC2PKN

Prachakirikhan

Some Location areas already exceeded 100% BTS Paging load. The calculation was make by collected the paging attempts measurements report 222 between 27 th of August to 2 nd of September. An example of the BTS Paging load calculation will elaborate as below:

Example LAC =3733 (Data from 27/8 to 2/9, the paging attempt was 65000 during the most busy hour) The number of paging attempts per second: = 65000 paging / 3600 [10] = 18.1 Paging per second

The allowed Paging per second for Combined BCCH/SDCCH BTS as in equation [3] = 13.6 pages / second. Therefore, the paging load in LAC 3733 for Combined BCCH/SDCCH with AGCH = 2 will be: = Sum (Number of Paging per second in LAC/Allowed Page per second) = (18.1/13.6) =1.3308

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= 133.1 % Paging Load ( Note: paging att. = 48960 reachs 100% paging load)

[11]

The following table and figure listed the paging load and capacity for all LAC in Southern region of Nokia area. Table 5.3: BTS Paging load No LAC 1 1703 2 1713 3 1723 4 1733 5 1743 6 1753 7 1763 8 1773 2703 9 10 2713 11 2723 12 2733 13 2743 14 2763 15 2773 16 3703 17 3713 18 3733 19 3773

Area Paging Load% Paging Successful Rate (%) BSC Paging Attempts Pages/Second (Comb BCCH) BSC1RBR Ratchaburi 12000 3.3 24.5 83.3 BSC1KRI Kanchanaburi 13000 3.6 26.6 126.9 BSC1SPB Suphanburi 37000 10.3 75.6 90.0 BSC1NPT, BSC4NPT Nakhonpathom 30000 8.3 61.3 76.7 BSCSKN1 Samutsakhon 59000 16.4 120.5 66.2 BSC1SKM Samusongkram 16000 4.4 32.7 77.4 BSCPBI1 Petchaburi 41000 11.4 83.7 79.6 BSC1PKN Prachakirikhan 23000 6.4 47.0 74.3 BSC1SKM, Ratchaburi 43000 11.9 87.8 81.4 BSC2RBR BSC2KRI Kanchanaburi 16000 4.4 32.7 69.3 BSC2SPB Suphanburi 29000 8.1 59.2 72.6 BSC2NPT Nakhonpathom 40000 11.1 81.7 82.5 BSCSKN1 Samutsakhon 11000 3.1 22.5 68.9 BSCCAM1 Cha Am city 10000 2.8 20.4 86.5 BSCCAM1 Cha Am city 13000 3.6 26.6 78.7 BSC3RBR Ratchaburi 34000 9.4 69.4 82.4 BSC3KRI Kanchanaburi 36000 10.0 73.5 79.5 BSC3NPT, BSC4NPT Nakhonpathom 65000 18.1 132.8 63.1 BSC2PKN Prachakirikhan 16000 4.4 32.7 66.7

6. Existing BTS Capacity limitation

The current paging load is probably close to the capacity limit and whenever there is a high paging load the page message might be deleted whenever the BTS buffer is full. This especially can be observed by execute the Network Doctor Report 186. One can be obviously monitored is the Paging Buffer Space counter. This Paging Buffer Space is sent by BTS in the CCH_Load_Ind message to a BSC every 30 s. A BSC sends current paging load as Paging_Buffer_Size to a statistical unit. The minimum value of this is recorded as counter 3018. If Min Paging Buffer (counter 3018) equals to zero, paging blocking has occurred. However, It is difficult to say when the problems start. Even if the counter 3018 does not yet show the 0 value, there may have been the situation in one or some of the buffers that the capacity has run out. From most of the Network Doctor report 186 between 18 th of August to 23 rd of August, it indicates that those BTS with Combined BCCH/SDCCH has low number of Paging Buffer Space. Some BTS even has only “1” for the paging buffer space and observed with high amount of paging delete message. This also means that the BTS has reached the limitation of the capacity constraint when AGCH is set to 2 in this case. The following show one of the Report 186 on 30 th of August, busy hour. The * mean that the BTS has problem with the paging delete message and limited paging buffer space problem.

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Nokia Networks Thailand 4th September, 2002 Sam Chang

Date: Prepared by:

Table 5.4: Example of BTS Capacity Traffic, Busy Hour BTS paging traffic between 20020827 and 20020902

CS pgn (Aif) PS pgn (Gb) CS pgn (Gb) BSC BCF name Pgn delet (BTS id) BTS name MTC --------------- ------------------ -----------BSCSKN1 WPTW 4637475 (145) WPTW1 2813 0 151611 23769 BSCSKN1 (85)

KLAI KLAI1

4637473 2812 0 151596 12181

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GSM buf free space ***** ave min ----49 18

49 17

GPRS buf free space ***** ave

GSM buff occup (%) **** ave max ----- ----121 9 66

121

9 66

GPRS buff occup (%) **** ave max ----4 29

Air-if occup (%) **** DRX pg DRX AG nDRX AG ------15 0 0

4 28

15 0 2




BSCSKN1 (82)

LSKN LSKN1

4637474 2812 0 151560 362

49 18

121

9 67

4 29

15 0 1

BSCSKN1 (84)

LSKN LSKN3

4637473 2812 0 151520 31846

49 18

121

9 66

4 29

15 0 1

BSCSKN1 (146)

WPTW WPTW2

4637473 2813 0 151488 37347

49 18

121

9 66

4 29

15 0 1

BSCSKN1 (87)

KLAI KLAI3

4637473 2813 0 151481 28048

49 19

121

9 66

4 28

15 0 0

BSCSKN1 (3)

SNTG SNTG3

4637474 2812 0 151371 19227

49 20

121

9 66

4 28

15 0 3

7. Conclusion and Recommendations

The current paging performance in Southern Region is relatively low. The reason of the problem is probably due to paging congestion in combined BCCH/SDCCH BTSs. The current paging load is probably close to the capacity limit and whenever there is a high paging load during busy hour period . This Paging analysis has provided the way of BTS Capacity calculation for Combine BCCH/SDCCH BTS. With the additional of Network Doctor 186 report, it is proven that lot of BTSs have suffer buffered in a queue problem , or could be worse as the queue is full, the incoming pages are "thrown away"/deleted. In order to avoid the above circumstances and improve paging successful rate, the below actions are recommended:

•

The parameter NumberOfBlocksForAccessGrant determines how many paging blocks per multiframe are reserved for Access Grant Channel (Immediate Assignment). Si nce Access Grant Channel has priority over paging, NumberOfBlocksForAccessGrant are recommended to change from the existing value of “2” to “0” in all cells in Southern region.

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•

Consider to re-configure cells with no SDCCH congestion from combined to non-combined BCCH/SDCCH configuration. This will increase 30% of the paging capacity. For those cells with high SDCCH/TCH congestion, it is suggest to change NumberOfBlocksForAccessGrant from 2 to 0 for cells with combined BCCH and from 7 to 2 for non-combined BCCH.

•

MSC parameter, Repaging attempts is set to 2 and Number of seach repetition is equal to 2. This means that the MSC will at least send 4 paging messages if the MS is not reachable. Experimental results from live networks show that more than 3 paging attempts are usually unnecessary. Suggest to change the Number of seach repetition from 2 to 1.

•

Always monitor signalling load and avoid any occurrence of signalling link congestion.

•

To avoid large LA size, the most important rule is not to exceed the maximum paging capacity of the BTS. LA split might need to consider if paging capacity exceeded BTS capacity limitation.

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Paging Analysis West

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