paging drop analysis

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1 © Nokia Siemens Networks WCDMA BTS PLM

Paging Drop Analysis

Sadok Ben ALI

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2 © Nokia Siemens Networks

Air Interface PCH Load

WCDMA BTS PLM

Paging Type1 messsages has been

incresed almost to the double due

to LAC fusion (More cells per LAC)

Number of droped pagging hasbeen incresed proportianly with the

paging attempts.

Paging Drop

LAC re-designeLAC re-designe

LAC re-designe

Daily CountersHourly Counters



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Air Interface PCH Load

WCDMA BTS PLM

LAC re-designe

After LAC re-designe the PCH load increase up to 8% which

is still under the recomded treshold which is 30%

This is should be anlyzed on the LAC based

RNC CHR

NO Congestion on

Air interface



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Paging Procedure - Paging Flow in RNC - Bottleneck

WCDMA BTS PLM

The first discarding option in RRC Master is due to processing load reason.

Is not the cause based on the counters (No paging drop due ICSU)

The second one in MAC-c happens during scheduling . The paging messages stored in buffer can

be rescheduled in the next paging occasion (SFN)Paging buffers are 1 each SFN up to the

maximum possible number related to DrxCycleLenght. Paging messages are discarded once the

buffer time out.

Is confirmed By counters

M1006C251: PAGING DROP LOW PRIORITY

When the L2 entity of RCN drops low priority paging message due to congestion

M1006C252: PAGING DROP HIGH PRIORITY

When the L2 entity of RNC drops high priority paging message due the congestion



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5 © Nokia Siemens Networks WCDMA BTS PLM

Paging ProcedureBuffering - Theory

M = int (window_size div DRXCycleLength)

Where window_size is an hidden parameter.

This maximum number of those RNC internal attempts

also allows to calculate the maximum

storage time of paging messages.

Using a typical value of window_size = 300 we have:DRXCycleLength=32 M=9;

DRXCycleLength=64 M=4;


DRXCycleLength=256 M=1.

Drxcycle length Numbe of paging occasion Number of buffer Duration of buffer ms M

32 127 127 2880 9

64 63 63 2560 4

128 31 31 2560 2

256 15 15 2560 1

Note: Using this coefficient M it is possible to calculate the overall

buffering time (e.g. with DRX cycle =320ms a M * DRX cycle = 9 *

320 ms = 2.88 sec of buffering time is possible; the relationship with

CN type1 repetition and in type2 RNC must be taken into count.



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Conclusion

• The bottleneck at the MAC-c entity, to avoid collision during the scheduling activity areduction of DrxCycleLength is recomnded:

• A smaller DrxCycle returns more occasion for UE and more buffering capacity asdescribed in in previous slides.

• The Congestion is not Hardware is on UMTS paging schedlunig procedure

WCDMA BTS PLM



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Annex - Paging Procedure

WCDMA BTS PLM



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8 © Nokia Siemens Networks NMI / Paging – 07/05/2009

Paging ProcedurePaging Occasion Calculation (1-3)

To be camped on the right PI a UE must perform the following procedure:

Select the appropriate Secondary-CCPCH

Identify the System Frame Number SFNIdentify the PI to be read

1. If multiple S-CCPCH are available the UE select one of the S-CCPCH according to the following:

S-CCPCH index = IMSI mod K

where K is the number of S-CCPCH used. If 2 S-CCPCH are used all UEs with even-numbered

IMSI will receive their paging on the first S-CCPCH and the UEs with odd-numbered IMSI willreceive the paging on the second S-CCPCH.

IMSI is given as sequence of digits of type Integer(0..9), IMSI in the formula above is interpreted as

a decimal integer number, where the first digit given in the sequence represents the highest order

digit.

2. The equation to identify the exact SFN to monitor is:

SFN = (IMSI div K) mod (DRXCycleLength) + n * DRXCycleLength

where n is an integer that varies from “0” to a maximum given by the DRXCycleLength.

Note: SFN in literature is known as „Paging Occasion‟



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The maximum value of the coefficient “n” of the previous formula comes from the

DRXCycleLength used as in the following: assuming, for example, a DRXCycleLength=64 frames

and considering that SFN range is from “0” to “4095” the maximum possible value of “n” is given by 4095/64=63

This means that the paging occasion never will be greater than 4095 and occurs one per DRXcycle.

RNC will use the SFN formula given to individuate the next paging occasion to be transferred to

UE. The MAC layer of RNC is responsible to schedule the PI and subsequent message inside the

appropriate SFN.

3. Once identified the frame the UE must read the right PI inside the frame.

PI = (IMSI div 8192) mod Np

where Np is the number of PI per frame.

In case of collision buffering procedure is performed.

The NSN current implementation gives an opportunity of 512 buffers to store the paging

messages (one for each paging occasion=SFN up to DRXcycle=5120ms!).



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When a new paging message arrives and the next paging occasion is already occupied, the

paging message is stored in the first free paging occasion belonging to the paging group.

The nearest paging buffer place (i.e. the nearest paging occasion) in the paging buffer can thenbe calculated (neglecting a typical delay of 2 frames to leave time for MCFTOR to schedule) as

in the following:

Nearest Paging Occasion = (SFN_not_free + k * DRXCycleLength)

where k = [1, 2,…M] should get the smallest possible integer value up to the place is found

It is possible to calculate for each DRXCycleLength the maximum of coefficient k (M) as in the

following:M = int (window_size div DRXCycleLength)

Where window_size is an hidden parameter.

This maximum number of those RNC internal attempts also allows to calculate the maximum

storage time of paging messages. Using a typical value of window_size = 300 we have:


DRXCycleLength=64 M=4;DRXCycleLength=128 M=2;

DRXCycleLength=256 M=1.

paging drop analysis

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