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LA- AND PAGING ANALYSIS FOR RADIO NETWORK INVESTIGATIONS
2(28) LVR/P-97:0001 Rev A 1997-04-07
1 IntroductionThe purpose of this document is to serve as an Ericsson internal guideline onLocation Area (LA)- and paging analysis for Radio Network Investigations.The objectives are to find out how to dimension LAs and how to optimise thepaging performance. The document should also serve as a "competence store"and shall be updated on regular basis by the unit e.g. after Radio NetworkInvestigations. Other LVR guidelines in this area are "Location areadimensioning guideline CME 20 R6" [3] and "SDCCH DimensioningGuideline" [4].
An LA is the area in which a normal page for a particular mobile, registered inthis LA, will be broadcasted. The upper bound for the size of an LA is theserving MSC’s service area. In general the LAs is smaller than that, mainly dueto the paging load that would result in such a large LA. A lower bound for thesize of an LA is set by the location updating load, causing increased demand forSDCCH resources.
2 Location Area Dimensioning
2.1 Paging capacity versus subscribersWhen designing LAs it is necessary to have some knowledge about the pagingcapacity requirement. The number of busy hour call attempts, the share of thetotal number of call attempts which are mobile terminated, the share of detachedmobiles, usage of SMS, etc. have to be estimated in order to calculate thepaging capacity needed. Thus the number of subscribers that a BSC is designedto support does not alone suffice to determine the paging capacity needed tosupport these subscribers.
2.2 ParametersIn Table 1 some of the most important BSC parameters for Location Updatingand Paging are given.
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Parameters Explanation MML
MFRMS Multiframes period RLDEC
AGBLK No. of reserved access grant blocks RLDEC
BCCHTYPE Type of BCCH RLDEC
T3212 [Deci hours] Time-out, MS periodic LU RLSBC
ATT Attach-detach allowed RLSBC
MAXRET Max. retransmission at access RLSBC
CRH [dB] Cell reselect hysteresis RLSSC
Table 1. BSC parameters related to paging and locationupdating.
Explanations and comments:
MFRMS sets the transmission interval of paging messages to paginggroups.
AGBLK sets the number of CCCH blocks in each multiframe that willbe reserved for access grants. Setting AGBLK to a value other than0 will reduce the page capability.
BCCHTYPE is either COMB, COMBC or NCOMB.
COMB = Combined; Indicates that the cell has a combined BCCH andSDCCH/4.
COMBC = Combined with CBCH; Indicates that the cell has a combinedBCCH and SDCCH/4 with a CBCH subchannel.
NCOMB = Not combined; Indicates that the cell does not have acombined BCCH and SDCCH/4.
If COMB is used the capacity of the CCCH will decrease three times,compared to the NCOMB case.
T3212 is the time between the periodic registration.
ATT sets if attach-detach is allowed. This should be YES.
MAXRET is the maximum number of times an MS tries to access thesystem (if the first time failed).
CRH is the hysteresis value used when the MS in idle mode crosses an LAborder. As default this parameter is set to 4. A higher setting might beadvantageous in areas with many LA borders and thus problems withmany Location Updatings.
In Table 2 MSC parameters and exchange properties relevant for paging andlocation updating are given.
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Parameters Explanation MML
BTDM [min] Base Time Duration,implicit detach
MGIDP
GTDM [min] Guard Time Duration MGIDP
TDD [days] Automatic de-registrationsupervision time
MGADP
PAGTIMEFRST1LA [s] Time supervision for thefirst page in one LA
MGEPP
PAGETIMEFRSTGLOB [s] Time supervision for thefirst global page
MGEPP
PAGEREP1LA Repeated paging in oneLA
MGEPP
PAGEREPGLOB Repeated global paging MGEPP
PAGTIMEREP1LA [s] Time supervision for therepeated page in one LA
MGEPP
PAGTIMEREPGLOB [s] Time supervision for therepeated global page
MGEPP
TMSIPAR TMSI master parameter MGEPP
TMSILAIMSC New TMSI at changingof LA
MGEPP
Table 2. MSC parameters and exchange properties.
Explanation and comments:
BTDM implicit detach supervision should be equal (or longer) thanT3212 in the BSC. If T3212 is increased, BTDM MUST also beincreased.
GTDM is an extra guard time in minutes before the subscriber is setto detached.
TDD sets the time (in days) that an inactive IMSI is stored in the VLRbefore it is removed.
PAGTIMEFRST1LA is the time supervision for the page response ofthe first page. The MS is paged in the LA with the first page if the LAIinformation exists in the VLR. The parameter is set according to thedefault values.
PAGETIMEFRSTGLOB is used instead of PAGTIMEFRST1LA if theLAI information does not exist in the VLR.
PAGEREP1LA decides how the second page is sent:
0 Paging in LA is not repeated1 Paging is repeated in LA with either TMSI
or IMSI2 Paging is repeated in LA with IMSI
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3 Paging is repeated as global paging withIMSI
PAGEREPGLOB defines how global paging is repeated according to:
0 Global paging is not repeated1 Global paging is repeated with IMSI
PAGTIMEREP1LA is the time supervision for the second page toLA. This is the timer used for the second page when PAGEREP1LAis set to 1 or 2.
PAGTIMEREPGLOB, the time supervision for the second page, if it isglobal.
TMSIPAR indicates if TMSI should be used or not:
0 TMSI is not allocated1 TMSI is allocated only on encrypted
connection2 TMSI is allocated
Setting this parameter to 0 means that TMSI is not used. The pagingcapacity will be decreased if TMSI is not used.
TMSILAIMSC states if a new TMSI shall be allocated at a change ofLAI within the MSC/VLR. Only applicable if TMSIPAR is not equal to0.
If TMSI is used it will be used at least in the first page. Then, depending onhow PAGEREP1LA is set the page is repeated with either TMSI or IMSI.However, there will always be some pages which are sent out globally in thefirst page. The reason for this is that information about the MS did not exist inthe VLR. Normally, this is due to that the MS was removed of the VLR, due tobeing inactive too long time, see parameter TDD above. At an incoming call,the HLR has information about the most recent location, i.e. VLR, where theMS was registered. Then, when the call is connected to the VLR a global pagewill be sent out due to that no information exist in the VLR about this particularMS. If the MS would have been registered in the VLR but not active, no pagewould have been sent out.
2.3 Important STS counters and formulas
2.3.1 General
Before any STS data can be collected a suitable measurement period should bedecided. The traffic level, on BSC level, can be measured by using the followingformula:
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T_TRAFF = TTRALACC / TNSCAN
TCH Traffic level
0.0100.0200.0300.0400.0500.0600.0700.0800.0900.0
1000.0
0000
-010
0
0200
-030
0
0400
-050
0
0600
-070
0
0800
-090
0
1000
-110
0
1200
-130
0
1400
-150
0
1600
-170
0
1800
-190
0
2000
-210
0
2300
-000
0
Time
Erl
ang
Erlang
Figure 1. Traffic level in Erlang for a BSC.
A measurement period with high traffic level can now be selected e.g. between15:00 and 19:00.
The next step is to measure the Locating Updating- and Paging performance.STS counters in MSC/VLR and in BSC will be used.
2.3.2 STS counters in MSC/VLR
Listed below are some very useful counters in STS in MSC/VLR.
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On Location Area Level: Objecttype = Location Area Level Statistics(LOCAREAST).
Events are counted per LA.
Counter-name Description
NLAPAG1LOTOT Number of first page attempts to a LA
NLAPAG2LOTOT Number of repeated page attempts to a LA
NLAPAG1RESUCC Number of page responses to first page to a LA
NLAPAG2RESUCC Number of page responses to repeated page toa LA
NLAPAGERR Number of unsuccessful page responses to a LA
NLALOCSSRFLT Number of location updating rejections due to theCSS restriction
NLALOCTOT Total number of location updating attempts
NLALOCSUCC Number of successful location updatings
Table 3. MSC counters on Location Area level.
Note. When a page attempt is made, a timer is set up and a page response isexpected. If the page response is received after the time supervision has elapsedor if the page response comes from an unidentified mobile subscriber, the pageresponse is counted as unsuccessful.
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On MSC/VLR Level: Objecttype = Update Location (UPDLOCAT).
Counter-name Description
NLOCNRGTOT Number of location updating attempts from nonregistered subscribers (IMSI attach, normallocation updating or periodic updating)
NLOCIMSERR Number of failed location updating attempts dueto unknown IMSI number series
NLOCREGSERR Number of rejected location updating attemptsdue to regional subscription
NLOCOLDTOT Number of location updating attempts for alreadyregistered subscribers
NLOCPERTOT Number of periodic location updating attempts foralready registered subscribers
NLOCATTTOT Number of IMSI attach messages received(already registered)
NLOCDETTOT Number of IMSI detach messages received
NLOCOLDSUCC Number of successful location updatings foralready registered subscribers
NLOCNRGSUCC Number of successful location updatings for nonregistered subscribers
NLOCIDTTOT Number of implicit detach events in MSC/VLR
NLOCNRRTOT Number of national roaming implicit detach eventsin MSC/VLR
Table 4. Location Update counters on MSC/VLR level.
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On MSC/VLR Level: Objecttype = Paging (PAGING).
Counter-name Description
NPAG1LOTOT Number of first page attempts to a location area
NPAG1GLTOT Number of first global page attempts
NPAG2LOTOT Number of repeated page attempts to a locationarea
NPAG2GLTOT Number of repeated global page attempts
NPAG1RESUCC Number of page responses to first page
NPAG2RESUCC Number of page responses to repeated page
NPAGERR Number of unsuccessful page responses (includesunexpected page responses)
Table 5. Paging counters on MSC/VLR level.
2.3.3 STS counters in BSC
The most useful BSC object types for the LA- and paging analysis are found inTable 6.
User-Formula related to STS Object Types
BSC LOAS
Random Access RANDOMACC
SDCCH CELLSDCCH, CELLCCHDR
TCH CELLTCH, CELLTCHDR
Table 6. Useful STS object types in the BSC.
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The most useful BSC counters for LA- and paging analysis are found in Table7.
Counter-name Object type Description
ACCLOAD LOAS Accumulated processor load inpercent
NSCAN LOAS Number of accumulations
TTCONGS CELLTCH TCH congestion time
TNSCAN CELLTCH Number of accumulations oftraffic level counter (TCH)
TTRALACC CELLTCH Traffic level accumulator.Accumulations of the trafficlevel counter
CCONGS CELLSDCCH SDCCH congestion counter.Stepped when a selection of anidle SDCCH fails
CME: CTRALACC,CMS: CTRALCNT
CELLSDCCH Traffic level accumulator .Accumulations of the trafficlevel counter (SDCCH).
CNUCHCNT CELLSDCCH Number of defined channels(SDCCH)
CTCONGS CELLSDCCH SDCCH congestion time
CNSCAN CELLSDCCH Number of accumulations ofSDCCH traffic level counter
CCONGSSUB CELLSDCCH Congestion counter for overlaidsubcell
CCALLS CELLSDCCH SDCCH call attempt counter.Stepped at every attempt toseize a SDCCH channel
RAACCFA RANDOMACC Failed random access
RAOTHER RANDOMACC Number of random accesses,All other cases
RAANPAG RANDOMACC Number of random accesses,answer to paging
CNROCNT RANDOMACC Number of random accesses
Table 7. STS counters in the BSC.
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A number of user formulas are defined in Turtle for producing the finalstatistics. Some useful formulas are:
P_TOT: Total page attempts (MSC level)
NPAG GLTOT NPAG LOTOT1 1+
To get Pages/s divide with the measurement period.
P_12_SUC-1: Successful First and Repeated Page Attempts ofTotal Number of First Page Attempts (MSC level)
NPAG RESUCC NPAG RESUCCNPAG GLTOT NPAG LOTOT
1 2
1 1100
++
⋅ [%]
P_1_SUC-1: Successful First Page Attempts of Total Number of First Page Attempts (MSC level)
NPAG RESUCCNPAG GLTOT NPAG LOTOT
1
1 1100
+⋅ [%]
PL_SUC-1: Successful First and Repeated Page Attempts of Total Number of First Page Attempts (LA level)
NLAPAG RESUCC NLAPAG RESUCCNLAPAG LOTOT
1 2
1100
+⋅ [%]
LU_TOT: Total location updatings (MSC level)
NLOCOLDTOT NLOCNRGTOT+
To get LU/s divide with the measurement period.
LU_SUC_TOT: Successful Location Update Attempts of Total Numberof Location Update Attempts (LA level)
NLALOCSUCCNLALOCTOT
⋅100 [%]
LU_R: Location Update Attempts from Already Registered Subscribers of Total Number of Location Update
Attempts (MSC level)
NLOCOLDTOTNLOCOLDTOT NLOCNRGTOT+
⋅100 [%]
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LU_SUC: Successful Location Update Attempts of Total Number ofLocation Update Attempts (MSC level)
NLOCOLDSUCC NLOCNRGSUCCNLOCOLDTOT NLOCNRGTOT
++
⋅100 [%]
LU_PERIOD: Periodic Location Update Attempts of Total Number ofLocation Update Attempts from Already RegisteredSubscribers (MSC level)
NLOCPERTOTNLOCOLDTOT
⋅100 [%]
LU_IMSI_AT: IMSI Attach Attempts of Total Number of LocationUpdate Attempts from Already Registered Subscribers(MSC level)
NLOCATTTOTNLOCOLDTOT
⋅100 [%]
S_TRAFF: Average SDCCH Traffic Level (cell level)
CTRALACCCNSCAN
[E]
S_CONG: SDCCH Congestion of Total Number of SDCCH SeizureAttempts (cell level)
CCONGS CCONGSSUBCCALLS
+⋅100 [%]
T_TRAFF: Average TCH Traffic Level (cell level)
TTRALACCTNSCAN
[E]
RA_TOT: Total Number of Random Access Attempts (BSC level)
CNROCNT RAACCFA+
RA_OTHER: Random Accesses with Cause “All Other Cases”, e. g.Location Updating, Detach, Attach, etc. of Total Number
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of Accepted Random Accesses (BSC level)
RAOTHERCNROCNT
⋅100 [%]
CP_LOAD: CP Load on Average (BSC or MSC level)
ACCLOADNSCAN
[%]
When the over all behavior is known a deeper analysis on cell level can be done.The cells with the highest rate of Random accesses due to Location Updatingshould be analysed (see example in Figure 2). SDCCH/TCH congestion timeshould be checked for those cells.
72
76
80
84
88
A B C D E F G H I JCell
[%]
0.0
5.0
10.0
15.0
20.0
25.0
Erl
ang
LOCATION UPDATING
TCH TRAFFIC LEVEL
Figure 2. Cells with the highest rate of Random Accesses dueto Location Updating and TCH traffic level.
2.4 Rural areasThe size of the LAs in rural areas, characterised by a low subscriber density, isnot very critical. The possible need for more SDCCH resources in the LAborder cells has a marginal effect on the system since in general capacity is nota scarce resource in rural areas. (Cellular systems in rural areas are most oftenlimited by coverage and not by capacity.)
LA borders should be drawn up outside villages and minor cities andunnecessary criss-crossing over high ways should be avoided.
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2.5 Medium size citiesIt is preferable to fit a medium sized city (less than 1 million people) into oneLA to reduce the location updating load. For major cities and detailedinformation, refer to chapter 2.6.
The LA borders should be drawn up in low subscriber density areas welloutside the city. LA borders crossing high ways should be avoided as much aspossible.
2.6 Major size citiesLAs in large cities (more than 1 million people) tend to be quite large, inparticular where the city is served by more than one BSC1. One reason to this isthat it is often difficult to find a good way to split a city into more than one LA.However, provided that it is possible to split a city in two or more LAs, withoutcreating overload situations in those cells that thereby becomes LA border cells,this is recommendable.
The upper bound of the size of an LA depends on the capacity of the equipmentused and the scenario. It is thus difficult to give a generally applicablerecommendation on the size of an LA. The best approach is to measure theactual paging load and to evaluate the performance.
In case a city is covered by more than one LA, the LA border should be drawnup in low density subscriber areas and it should not criss-cross over high ways.The SDCCH capacity of the LA border cells should be dimensioned to cater forthe expected location updating load.
1 If more than one BSC/LA the paging load of the LA is shared by all the involved BSCs. Hence the total paging capacity increases with the numberof BSCs at least at BSC level. The cells in the LA are split between the BSCs. The paging load in the BTSs is however not reduced by load sharingin the BSCs and it is thus important to look out for overload situations in the BTSs/paging channels.
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3 Paging
3.1 GeneralThe MSC knows in which location area the mobile is and sends a pagingmessage to the BSCs handling this location area. In Ericsson's GSM systemsthe BSC stores information about which cells belong to which location area.The MSC sends down the identity of the location area and the BSCs distributethe paging message to the concerned BTSs.
Since an LA normally is served by only a few BSCs2, each page is onlydistributed to a few BSCs. For each paging message, sent from the MSC, theBSCs have to generate paging commands to all cells in the LA. The number ofcells in an LA ranges from a few tens up to perhaps one hundred cells,sometimes even more. Hence an incoming page to a BSC gives rise to aconsiderably larger number of outgoing paging commands from the BSC. TheBSC is thus more likely than the MSC to be the unit limiting the paging rate.
Each paging command from a BSC is received by a BTS. Upon receipt of apaging command a BTS3 has to broadcast the page at least once. A BTS has tohandle all pages addressing mobiles in the LA, which might amount to aconsiderable load. Thus also the BTS or the paging channel are likely to set alimit on the overall paging capacity.
The second page, issued if the first page is not answered, could optionally belocal, i.e. restricted to the LA, or global, i.e. it is sent to all cells in the wholeMSC service area. A global page as second page will of course increase thetotal paging load more than would a local page as second page. Theimprovement in paging success rate that possibly comes with a global secondpage as compared to a local second page should be weighed against theaccompanying increased paging load. It is sometimes questionable if theimproved paging success rate of global second page motivates theaccompanying increased paging load.
3.2 Paging Capacity
3.2.1 BSC
Finding the paging capacity that could be anticipated in a particular BSC isdifficult since the capacity depends on so many things. However, the bottleneckis usually the number of RP signals that can be sent from the CP to the RPDs inthe TRHs. In CME 20 R5 one TRH is normally serving up to four TRXs.Typically up to 1500 RP signals can be sent per second. The figure should be
2 Normally 1-3 BSCs are connected to an MSC.
3 Optionally though the BTS can repeat the page once if the paging load is low enough to allow it. This re-transmission of a page should not be mixedup with a second paging message, which is initiated by the MSC if the mobile does not answer the first page.
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viewed as a hint on the maximum page rate and not as an exact value. Thefollowing calculations can be made:
NO PAGERPSIG
NO TRH RPp_
_=
⋅(1)
( )RPp NO CELLSNO TRX NO LA
TRXpTRH
= − −⋅
1 1_
_ _(2)
where:
RPSIG Maximum number of RP signals per second
TRXpTRH Average number of TRXs per TRH
NO_LA Number of Location Areas
NO_TRH Number of TRHs belonging to the BSC
NO_TRX Number of TRXs belonging to the BSC
NO_CELLS Number of cells
RPp Probability that an RP signal is sent to an TRH
NO_PAGE Number of pages per second
In formula (1) the maximum number of RP signals is divided by the number ofTRH that should have the signal (page). The latter is calculated by multiplyingthe Number of TRHs in the BSC with the probability that a TRH shouldreceive the signal (page). This probability is given by formula (2).
RPSIG was 1000 in early R5 releases but after CNI-180 this figure is 1500,which also is valid for R6.
TRXpTRH, the number of TRXs per TRH, can be set to 4 for R5 and for R6 6is a more reasonable figure. Note that the actual configuration might bedifferent (worse) and will influence the page performance in a negative way.
Example: Assume a scenario with 100 cells in one Location Area with 3 TRXsper cell. Then the minimum number of TRHs needed is 75 (if 4 is assumed to bethe maximum no. of TRXs per TRH). Using the formulas above gives that 25pages/s can be sent from the BSC. Note that this is the maximum number andwith more TRHs, the number decreases. If three LAs is used, 53 pages can besent per second. However, 50 pages per second seems to be the maximum dueto LOAS delay.
In the BSC there also exist a paging queue with 32 positions, i.e. 32 IMSIs orTMSIs can be queued and transmitted when resources are available. Thereforethe time supervision for the page response of the first page,PAGTIMEFRST1LA, should not be too short (shorter than 6 seconds).
In coming CME releases the paging capacity in the BSC will be further
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increased. The things that will be improved are the maximum number of RPsignals/s, lower CP load and maybe also a shorter paging queue.
As mentioned above, there is no measurement function today (CME 20 R6.0),or counter, that tells if the BSC suffers from page congestion, i.e. pages arethrown away. However, it is possible by using the test system to trace avariable, CPAGEDISCARDED in block RMPAG. If the buffer gets full andpages are thrown away, this variable is stepped.
The paging capacity in APZ 212 is slightly higher than APZ 211.
3.2.2 BTS and Paging channel load limit
In case the rate of pages to a BTS becomes higher than what the BTS is able tohandle, the BTS will discard paging commands. At present the BTS does notindicate the overload situation to the BSC in any way. Because of this it isimperative to be able to judge whether or not there is a tangible risk for pagingoverflow in any of the BTSs.
The PCH and AGCH share the same TDMA frame mapping. The channels areshared on a block by block basis, and information within each block whendeinterleaved and decoded, allows an MS to determine whether the blockcontains paging messages or access grants.
The parameter AGBLK sets the number of CCCH blocks reserved for accessgrants. For example, by setting AGBLK to 1, one block is used only for accessgrants and thus the paging capacity will decrease. In the Ericsson system,access grants have priority over pages and therefore it would be preferable tohave AGBLK to 0. However, in the early releases of CME 20 R5 it wasnecessary to set AGBLK to1. AGBLK must also be set to 1 if cell broadcast isto be used on a cell not using combined BCCH and SDCCH/4. As thecalculations later in this chapter shows, if combined BCCH and SDCCH/4 isused AGBLK takes quite a lot of capacity away from paging, but in the non-combined case the paging capacity decrease is limited. It should however now,if the right correction, CNI-229, is loaded, be possible to set AGBLK to 0 alsofor CME 20 R5.
The parameter MFRMS indicates the number of 51-multiframes betweentransmissions of paging messages to the same paging group. The total numberof paging groups is MFRMS * (9 - AGBLK) for the case of non-combinedcontrol channels. If combined control channels are used the number of paginggroups will be MFRMS * (3 - AGBLK).
In the BTS there exist one paging queue for each paging group, which evens outthe paging load at sudden tops in the paging. The setting of the parameterMFRMS is related to the number of paging groups. The parameter does notdirectly influence the paging capacity since it got nothing to do with allocationof resource for paging. However, the setting of MFRMS does affect the pagingqueue length4 and thereby the probability of paging queue overflow. Each
4 The higher the value assigned to MFRMS the shorter the queues become.
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paging queue length, QLength, can be calculated according to:
QLength = 14 - PQmax DIV 10
Where PQmax is equal to the number of paging groups in use in the cell. Thequeues are handled in a FIFO manner when the first IMSI or TMSI intended fora PAGING REQUEST message, is dequeued. When more than one IMSI orTMSI are put into a PAGING REQUEST message, the first suitable IMSI orTMSI (in FIFO order) is dequeued.
Ericsson base stations can be set to re-transmission pages once, provided thatthere are free resources on the PCH. A high load on the paging channels resultsin only a few pages being re-transmitted, if any, and thus entails a deterioratedpaging success rate. A decreased rate of answers to first pages results in more(ordinary) second pages, i.e. an increased load in both MSC and BSC as well asBTS/paging channels. Finally a decreased rate of answers to second pages leadto an overall decreased success rate for paging.
A copy of the transmitted IMSI or TMSI is stored on a stack from theirrespectively queue. There exists one stack per queue and the maximum numberof elements per stack is 4. An IMSI or TMSI stored for retransmission isdiscarded depending on the setting of the parameter MFRMS as specifiedbelow:
If MFRMS <= 3, the IMSI/TMSI is discarded if it has not been retransmittedwithin two schedulings of its paging group.
If MFRMS > 3, the IMSI/TMSI is discarded if it has not been retransmittedwithin one schedulings of its paging group.
It is possible to use extended page mode to meet overload problems on some ofthe paging groups. If the Page Mode element (of the PAGING REQUEST orIMMEDIATE ASSIGNMENT messages) is set to Extended Paging the MSswill not only listen to their normal page group, but they will also listen to nextbut one page group. That is, the MSs listening to group 23 should if extendedpaging is active now also listen to group 25.
Before the pages are transmitted from the BTS a number of IMSIs and/orTMSI are combined in one page. The following configurations are possible inone Paging Request:
Type 1 2 IMSIs
Type 2 1 IMSI and 2 TMSIs
Type 3 4 TMSIs
Thus, by using TMSI the paging capacity can be considerably increased.
As many IMSIs and TMSIs as possible (for a certain paging group) will bepacked into one PAGING REQUEST message. If extended paging is used,IMSIs and TMSIs originating from two different paging groups may be packedinto the same PAGING REQUEST message. IMSIs and TMSIs stored forretransmission (in the corresponding paging stack) are included in the PAGINGREQUEST message if not enough IMSIs or TMSIs are found in the actual
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paging queue.
The following configuration parameter are used to control the retransmissionand extended page mode. The parameters are hard coded in the BSC and canonly be changed using the test system.
CCCH repeatON, OFF Automatic retransmission of pagesand immediate assignments in use(ON), or not in use (OFF).
DRX_DEV_MAX 0-100 0-100% allowed use of extended pagemode per paging group. (For example,50% means that extended page mode isused every other time it is possible to useit).
The default values are ON for CCCH repeat and 50 for DRX_DEVMAX.
The paging capacity on the air interface is basically determined by the channelconfiguration. Two different configurations are presently supported5; combinedand non-combined BCCH.
It is possible to measure the paging capacity in the BTS to some extent. ForRBS200 the TS monitor in RBS200 Local Maintenance Terminal, LMT, canbe used:
Common channels
- Number of paging request in queues.
- Number of random accesses since previous report.
- Size of largest page request queue.
- Size of all time largest page request queue.
- Current use of extended paging in percent (DRX_DEV)
- All time max DRX_DEV
For details on how to use LMT please refer to [5].
Unfortunately you have to use the Ericsson internal tool DVT (Debug andVerification Tool) to get the same paging related information from RBS2000.OMT2 does not give much information in this area. Some of the counters youcan monitor in DVT:
CCH
- Denied page mode extended
- Discarded channel request
- Discarded immediate assigns
5 Included in the GSM recommendations but not in CME20 R5 is a possibility to extend the paging channel to more than one time slot, thusincreasing the paging capacity.
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- Discarded pagings
- Maximum number of pagins in paging queue
- Maximum number of pagings in paging queue ever
- Pagings in paging queues.
LMT/DVT measurements on one typical BTS with regards to channelconfiguration etc. would give a good picture of the BTS paging performance ofall BTSs in the LA.
Example with combined BCCH:
When using combined BCCH the maximum theoretical paging rate is 51pages/s. To achieve this rate all pages have to use TMSI when addressing themobile and there must not be any messages to send on the AGCH.Assume that:
• maximum three paged MSs per paging message (TMSI is used to addressthe mobile in the first page)
• AGBLK is set to 0 (assume that 10 % of the CCCH resources are occupiedby AGCH)
• reserving about 50% for BTS initiated re-transmissions of pages.
With this assumptions the page rate would be 17 pages/s. Note that if notTMSI is used and especially if AGBLK is not set to 0, the paging capacity willbe significantly lower. If, for example, AGBLK is set to 1, 33 % of the CCCHresources will be occupied by AGCH.
Example with non-combined BCCH:
On a non-combined BCCH the maximum theoretical paging rate is three timesthat of the combined BCCH. A more realistic page rate to use for dimensioningpurposes would be about 52 pages/s, in which the following is assumed;
maximum three paged MSs per paging message (TMSI is used to address themobile in the first page)
AGCH is assumed to take 10% of the CCCH resources. (Approximatelyequivalent to setting AGBLK = 1.)
reserving about 50% for BTS initiated re-transmissions of pages.
As seen from the calculations above, what clearly limits the page capacity is theuse of combined BCCH, especially with a disadvantageous parameter setting.The system should be dimensioned according to its weakest link, i.e. if one cellin the LA uses combined BCCH this cell will dimension the whole LA.
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Note also the affect of the AGBLK setting in the two examples above; with thecombined BCCH setting AGBLK to 1 will severely decrease the pagingcapacity, while in the non-combined case the effect is much smaller.
3.3 MFRMS impactsOverview of MFRMS impacts:
1. MFRMS defines the period of transmission for PAGING REQUESTmessages to the same paging subgroup.
2. Together with AGBLK, MFRMS determines the number of Paging Groups.AGBLK set to 1 is assumed in this case.
3. MFRMS is also used by the MS to determine downlink signalling failure inidle mode. The downlink signalling failure criterion is based on the downlinksignalling failure counter DSC. When the MS camps on a cell, DSC shall beinitialized to a value equal to the nearest integer to 90/N where N is theMFRMS parameter for that cell. For MFRMS = 5 the start value of DSCwill be 18. This value is the maximum allowed value. If the maximum valueis not reached and if the MS successfully decode a paging message, DSCwill be increased by 1. If the MS fail to decode the paging message, DSCwill be decreased by 4. When DSC reaches 0, a downlink signalling failurewill be declared. A downlink signalling failure will result in a cellreselection. A too low value on DSC i.e. a high MFRMS value can result inmore (unnecessary) cell reselections.
4. In the BTS there exist one paging queue for each paging group, which evensout the paging load at sudden tops in the paging. In Table 8 only theQLength for NCOMB is listed.
5. BTS re-transmission. Ericsson base stations can be set to re-transmit pagesonce, provided that there are free resources on the PCH.
6. Mobile Station power consumption. The impact of MFRMS on the batteryconsumption may differ for different mobiles but the more frequent the MShas to leave sleep mode to listen to it's paging group the more power will beconsumed.
7. The average call setup time for mobile terminated calls will slightly increaseif MFRMS is high.
8. MSC and BSC load. MFRMS should not impact the load very much but theload will increase slightly if the number of BTS re-transmissions is low. Thisis because more second page attempts are needed.
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MFRMS1 Timebetween
pages [s]
Paginggroups2
NCOMB
Paginggroups2
COMB
DSC3 QL4 BTSre-
tran5
MSpower
consum.6
MSC/BSCload8
Callsetuptime7
2 0,47 16 4 45 13 few higher higher shorter
3 0,71 24 6 30 12
4 0,94 32 8 23 11
5 1,18 40 10 18 10
6 1,41 48 12 15 10
7 1,65 56 14 13 9
8 1,89 64 16 11 8
9 2,12 72 18 10 7 many lower lower longer
Table 8. Summary of the most important MFRMS impacts.
The risk of BTS overflow is not in proportion with MFRMS. In the extremecase with MFRMS=2 and BCCHTYPE=COMB the BTS queue for eachpaging group will be 14. With the longest possible queue and the shortestpossible transmission period the BTS is able to handle a normal paging load.
Theoretical paging capacity for MFRMS=2, AGBLK=1 and assuming 25%pages with IMSI and no global pages:
NCOMB: (16*3.2)/0.47 = 109 pages/second
COMB: (4*3.2)/0.47 = 27 pages/second.
This theoretical paging capacity is almost the same for different values onMFRMS.
Allowing the paging load to be 40% of the capacity:
NCOMB: 109*0.40 = 43.6 pages/second
COMB: 27*0.40 = 10.8 pages/second.
3.4 Analysing the PerformanceWhen analysing the paging performance several things should be taken intoaccount. The present paging performance should be evaluated; by monitoringthe STS statistics for the LAs in the MSC the page load can be calculated andcompared to the estimated, maximum, page load that the system can handle.,see previous chapters. Note that the system should be dimensioned after itsweakest link, e.g. if combined control channels are used, this sets the maximumpaging load for the LA.
If the page load is found to be higher than acceptable, several actions can betaken: firstly, if not already in service, TMSI should be used, secondly, avoidusing combined control channels and thirdly, introduce more LAs. The lastproposition is the last action that should be taken, due to the negative effect onthe SDCCH load. Splitting a LA should thus be the last action to meet pagingcongestion.
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4 Location Updating
4.1 GeneralA too high paging load can be alleviated through a reduced LA size. SmallerLAs though tends to generate a larger location updating load since the rate ofmobiles crossing LA borders increases as the size of the LAs decreases. Thisincreased location updating load has a restraining effect on the desire to reducethe size of the LAs. The increased location updating load is mainly manifestedin an increased SDCCH traffic in the LA border cells. The limiting factor willthus be the SDCCH capacity requirement.
It is of course desirable to avoid paging a mobile which for some reason is notpossible to reach. A page to a turned off mobile is a wasted page or rather twowasted pages since a second attempt will be made. Avoiding to page turned offmobiles will thus improve the paging success rate and also reduce the pagingload. The function IMSI attach/detach enables the system to tell in advancewhether a particular mobile is attached or detached. Thus using this function alot of unnecessary pages could be avoided to the prize of an increased load dueto frequent attach/detach messages sent to the MSC/VLR.
Radio coverage, or rather lack of coverage, and radio disturbance are alsosources to unnecessary pages for obvious reasons. Forcing the mobiles toregister at uniform intervals, i.e. using the function periodic registrations, willreduce the number of pages to mobiles temporarily in blind spots. Shorterregistration intervals will lead to fewer unnecessary paging messages sent andlonger intervals to more. The function periodic registration does however alsointroduce an increased load in the system, which increases with a shorterregistration interval.
To continue the analysis from Figure 2 the SDCCH (and TCH) congestion timefor the cells with highest Location Updatings should be measured. These cellsare mainly LA border cells.
Figure 3 shows that some cells are in immediate need for actions by theoperator. A cheaper alternative than to add more capacity would be to considerto change the LA borders e.g. to avoid major roads crossing between two LAs.The figure also shows that the operator has been very good in dimensioningcells with a high Location Updating load.
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0
2
4
6
8
10
12
14
16
18
A B C D E F G H I J
Cell
[%]
SDCCH CONGESTION TIME
TCH CONGESTION TIME
Figure 3. SDCCH and TCH congestion time for the 10 cells withthe highest rate of Location Updating.
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5 ProcedureThe following procedure should be used as a basis for LA- and paging analysis:
1. Check the parameter setting of the system.
2. Use a map of the network indicating the LAs.
3. Use STS to measure the 24 hours traffic level of the network.
4. Select a daily period with high traffic level.
5. Use STS together with user formulas to measure the location updating andpaging performance for a number of days including the paging success rate.
6. Check if the overall location update load is high in the network.
7. Check if one or more BSCs are suffering from high paging load.
8. Check the average CP load.
9. Check problem cells i.e. cells which suffers from a high location updatinglevel and/or SDCCH congestion. LA border cells are often represented inthis list.
Examples on actions:
1. Consider changing the MSC parameters if the paging success rate is bad e.g.the time supervision parameters.
2. Use TMSI if possible.
3. If the paging load is high, the LA might be too big and an LA split is needed.Also consider moving cells to another LA.
4. If needed add SDCCH capacity in problem cells e.g. by changing thechannel configuration. Normally LA border cells.
5. If the paging load (and traffic) is high consider adding another BSC butestimate the paging load in the BTSs.
6. Increase CRH in areas with many repeated location updatings.
7. Move cells to another LA in areas with many repeated location updatings.
8. Increase T3212 (and BTDM) if the location update load is high but noproblem with paging capacity and paging success rate.
9. Consider moving the LA border if the number of mobiles crossing the LAborder is high. To measure the number of handovers between LA bordercells give a good indication.
10. Use field measurements to verify changes to the network e.g. after an CRHchange.
11. Use STS to verify changes to the network. Measure the location update- andpaging performance, SDCCH load and CP load before and after thechanges. Calculate and/or measure the paging load.
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6 ConclusionsLAs consisting of 10-20 cells could be viewed as a reasonable lower limit onthe size of the LAs in rural areas. Larger LAs are of course also possible up tothe limit set by the paging capacity. As a rule of thumb it is recommended toconsider splitting an LA when the number of cells exceeds 100-150. This doeshowever not represent an absolute upper limit to the size of the LAs. Assumingfor instance that combined BCCH is not used in any of the cells, it could bepossible to have up to 200 cells in an LA.
It is here assumed that TMSI is used in the first page and that no blocks arereserved for access grants. If IMSI is used and CCCH blocks are reserved foraccess grants together with combined BCCH, the LA should be split before itcontains 100 cells. If only non-combined BCCH is used in the area the LAshould be able to cater for more than 100 cells.
Avoid using combined BCCH and SDCCH/4. One extra time slot can be usedfor traffic but the paging capacity is reduced. For AGBLK=0 it is reduced byapproximately 66%.
Use TMSI to improve the paging capacity.
Avoid splitting the LAs in central parts of large cities.
It might be very difficult to find suitable LA borders and by splittingthese LAs the Location Updating load may increase considerably.Monitor the paging load for all MSC/BSCs and if the load isapproaching the limit for the current set-up, try to “move” cells toother MSCs
If the paging demands get too high, split the LAs in the MSCs covering theoutskirts of the city.
The LAs in these MSCs should also be monitored and if the load isapproaching the limit (maybe because cells are “moved” from thecentral MSCs), split the LAs. It should be possible to find suitable LAborder for these MSCs by measuring the handover traffic betweencells in the MSCs.
Change the T3212, periodic location updating time out timer when the pagingcapacity is increased.
If no paging capacity problems exists the timer could be increased from 2hours to 4 hours to decrease the amount of periodic location updatings.
Use STS to measure the location update- and paging performance before andafter the change.
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LA- AND PAGING ANALYSIS FOR RADIO NETWORK INVESTIGATIONS
