LOCATION AREA DIMENSIONINGGUIDELINETHE ERICSSON GSM SYSTEM R7 Ericsson Radio Systems AB 1998The contents of this product are subject to revision without notice due tocontinued progress in methodology, design and manufacturing.Ericsson assumes no legal responsibility for any error or damage resultingfrom usage of this product.This product is classified as Ericsson Wide Internal. 1(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINE2(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7Contents1 Revision history..................................................................52 Introduction.........................................................................53 Background.........................................................................54 Paging capacity in the BTS................................................64.1 Channel configurations .......................................................................... 64.2 Paging block struc ture ........................................................................... 84.3 Paging groups ........................................................................................ 84.4 Queuing in the BTS..............................................................................104.5 Paging strategies .................................................................................104.6 Calculating the BTS paging capacity ...................................................125 Estimated maximum LA size ...........................................146 Paging capacity in the BSC .............................................177 LA dimensioning strategies.............................................198 Parameters ........................................................................198.1 BSC parameters .................................................................................. 198.2 MSC parameters ..................................................................................209 References ........................................................................24 3(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINE4(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R71 Revision historyRevision Date DescriptionA 1998-12-11 Created. Based on ref. 1 and ref. 2.Traffic estimations revised.Paging capacity figures for BSC revised.Parameter summary included.Editorial improvements.2 IntroductionA Location Area (LA) is a group of cells in which a page will be broadcasted.The upper boundary for the size of an LA is the service area of a Mobileservices Switching Centre (MSC). In general the LAs are smaller than that,mainly due to the excessive paging load that a large LA would have to handle.The lower boundary for the size of a LA is set by the Location Updating load,causing increased demand for Stand alone Dedicated Control CHannel(SDCCH) resources in the LA border cells.By analyzing statistics it is possible to get information on how many pages,first and second, which are issued. However this does not actually indicatewhether the paging load should be considered low, medium or (too) high. Agauge such as the maximum possible paging rate for each unit involved (e.g.BSC, BTS) is needed in the evaluation of the paging load.The purpose of this document is to serve as a guideline on LA-dimensioningfor the Ericsson GSM system R7. The objective of the dimensioning in thisguideline is to avoid having a too high paging load or Location Updating loadrather than to find the optimal size for LAs. Some functions that affect thepaging load and thereby the dimensioning are pointed out.3 BackgroundWhen a Mobile Station (MS) is paged, a Paging message is sent from theMSC to each Base Station Controller (BSC) belonging to that MSC's servicearea (global page), or to those BSCs serving at least one cell belonging to theLA where the MS is registered (local page).For each Paging message received by the BSC, Paging Command messageshave to be sent to all cells belonging to the LA where the target MS isregistered. The number of cells in an LA ranges from a few tens up to perhaps 5(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINEone hundred cells, sometimes even more. This means that one incomingPaging message to the BSC leads to a considerably larger number of outgoingPaging Commands from the BSC.The BTSs have to broadcast all the incoming pages. The Paging Requestmessages are sent on the Paging Channel (PCH) on the Common ControlCHannel (CCCH). Too large LAs may lead to a too high paging load in theBTS resulting in congestion and lost pages.Smaller LAs reduce the paging load in the BTSs as well as in the BSCs.However, smaller LAs also mean a larger number of LA border cells in thenetwork. Each time an MS crosses the boarder between two LAs, a LocationUpdating is performed. The Location Updatings affect the load on thesignaling subchannels, SDCCH, in the LA border cells. The SDCCH signalingcapacity depends on the SDCCH configuration in the cell and is furtherdescribed in ref. 3 .4 Paging capacity in the BTS4.1 Channel configurationsThe paging is done on timeslot zero on the Broadcast Control CHannel(BCCH) frequency. Timeslot zero consists of several channels; BroadcastCHannel (BCH), CCCH and, if combined BCCH/SDCCH is used, also 1Dedicated Control Channel (DCCH). This is depicted in Figure 1. The BCH consists of Frequency Correction CHannel (FCCH),Synchronization Channel (SCH) and BCCH. The CCCH consists of two subchannels; the PCH and the Access GrantCHannel (AGCH). The PCH is used to page the MSs with Paging Requestmessages. The DCCH consists of SDCCH, Slow Associated Control CHannel (SACCH) and (if cell broadcast is used), Cell Broadcast CHannel(CBCH). 1 A combined BCCH/SDCCH is a combination of BCH, CCCH and DCCH. The CCCHs share its resources on the multiframe with theSDCCH/4s (see Figu re 3).6(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7 BCCH -carrier0 2 4 5 6 7 1 3BCH CCCH DCCHFCCH BCCH AGCH SDCCH/CBCH SACCH SCH PCHFigure 1. Channel configuration of timeslot 0. The dashed squares are used ina combined BCCH/SDCCH configuration.Timeslot 0 is logically divided into multiframes, where each multiframe is235.4 ms. One multiframe equals to 51 frames. The multiframes have differentcontents for the different channel configurations, and therefore the pagingcapacity varies: Non-combined BCCH/SDCCH can fit 9 paging blocks (see 4.2) permultiframe.If the non-combined configuration is used, the CCCH, on timeslot zero onthe BCCH carrier, does not share its timeslot with SDCCHs (see themultiframe mapping in Figure 2). The signaling subchannels are mappedon one or more separate timeslots (8 signaling subchannels, SDCCH/8, isfitted on one timeslot).FC C H F C C H FC C H F C C H FC C H Idle S CH SCH S CH SCH S CHB C C H C C C H C C C H C C C H C C C H C C C H C C C H C C C H C C C H C C C HFigure 2. Non-combined mapping of BCHs and CCCHs on timeslot 0,downlink. One multiframe can fit 9 CCCH blocks. Combined BCCH/SDCCH can fit 3 paging blocks per multiframe.With a combined configuration, the CCCHs are mapped on timeslot zeroon the BCCH carrier together with SDCCH/4 (four signaling subchannels).Combining the CCCHs with the SDCCH/4 means that one extra timeslotcan be used for traffic or signaling. However, a combined configurationalso affects the paging capacity. Each multiframe contains 3 paging blocksinstead of 9 (se Figure 3), which means that the maximum BTS pagingcapacity is reduced to one third. 7(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINEF C C H FC C H F C C H FC C H F C C H Idle SCH S CH SCH S CH SCHB C C H S DCCH C C C H S DCCH C C C H SD CCH S DCCH C C C H SDCCH SD CCHFigure 3. Combined mapping of BCHs, CCCHs and SDCCHs on timeslot 0,downlink. One multiframe can fit 3 CCCH blocks.To choose between combined and non-combined configuration, the parameterBCCHTYPE is used.The AGCH is used to assign an SDCCH (Immediate Assignment) to an MSduring the call set-up phase. The AGCH shares the same resources as thePCH. The CCCH can be set up to either have dedicated blocks for AccessGrant, or Access Grant can work in stealing mode which means replacingpaging blocks with Access Grant blocks when needed. If dedicated blocks areused, each multiframe will contain 2 paging blocks (for combined) or 8 pagingblocks (for non-combined).4.2 Paging block structureEach multiframe contains 3 (combined BCCH/SDCCH) or 9 (non-combinedBCCH/SDCCH) paging blocks. Each paging block can fit up to four PagingRequests.Each paging block can fit either: 2 IMSI Paging Requests (IMSI = International Mobile Subscriber Identity) 4 TMSI Paging Requests (TMSI = Temporary Mobile Subscriber Identity) 1 IMSI + 2 TMSI Paging RequestsIMSI IMSI T = TMSI T T T TIMSI T TFigure 4. A paging block can fit two IMSI pages or four TMSI pages or acombination of one IMSI and two TMSI pages.4.3 Paging groupsAfter an MS tunes to the BCCH carrier and decodes the System Information,it performs an evaluation to which paging group it belongs, and hence, whichparticular paging block of the available blocks on the paging channel that is tobe monitored.8(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7The 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 alonger time than the low number case before the right paging blockarrives. This increases the time for paging. A low number of paging groups shortens the call setup time due to thatthe MS listens to the paging block more frequently. The drawback is thatthe MS power consumption is higher.There are two main parameters that determine the number of paging groups ina cell (besides BCCHTYPE ). The two parameters are AGBLK andMFRMS : AGBLKThe parameter AGBLK determines how many of the paging blocks permultiframe that are reserved for the AGCH (Immediate Assignment).Ericsson's BTSs support AGBLK = 0 (no blocks reserved) andAGBLK = 1 (1 block reserved). The number of paging blocks used forAccess Grant depends on the traffic in the cell. Since ImmediateAssignment has priority over paging in the Ericsson GSM system R7,AGBLK should be set to 0 (no blocks reserved) unless other is required . 2Experiences from existing networks show that the share of CCCHresources used for Access Grant is very low, typically below 5 %. MFRMSMFRMS is the multiframe period and defines the transmission interval ofpaging messages to the same paging group. For example, MFRMS =9means that MSs belonging to a certain paging group is paged every 9thmultiframe. This means that the transmission interval of paging messagesto paging groups is approximately 2.1 second (9 * 235.4 ms). A high valueof MFRMS will lead to more paging groups in the cell.The relation between MFRMS , AGBLK and the number of paging groups is:Combined BCCH/SDCCH cells:Number of paging groups = (3 - AGBLK ) * MFRMSNon combined BCCH/SDCCH cells:Number of paging groups = (9 - AGBLK ) * MFRMSTable 1 shows this relation together with the duration time betweentransmission of each paging group. 2 AGBLK must be set to 1 if Cell Broadcast is used in a cell with a non-combined configuration, or if System Information 7 & 8 are sent on theBCCH. 9(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINETable 1. The relation between MFRMS , AGBLK , number of paging blocksper multiframe, time between transmission of each paging group and numberof paging groups. Number of pagin g groups Number of paging groups(Combined BCCH/SDCCH) (Non combined BCCH/SDCCH) Time between 3 paging blocks 2 paging blocks 9 paging blocks 8 paging blockstransmission per multiframe, per multiframe, per multiframe, per multiframe,of each paging ( AGBLK = 0) ( AGBLK = 1) ( AGBLK = 0) ( AGBLK = 1)MFRMS group2 0.47 sec 6 4 18 163 0.71 sec 9 6 27 244 0.94 sec 12 8 36 325 1.18 sec 15 10 45 406 1.41 sec 18 12 54 487 1.65 sec 21 14 63 568 1.89 sec 24 16 72 649 2.12 sec 27 18 81 724.4 Queuing in the BTSIncoming Paging Commands are buffered in a queue (one for each paginggroup). The queue for each paging group in the BTS can fit between 6 and 14pages, depending on the number of paging groups. The BTS distributes thePaging Commands as Paging Request messages on the radio path when pagingblocks are available. A too high rate of incoming Paging Commands to theBTS increases the queuing time, something that leads to an increase of theaverage time for the paging response. If the queue is full, the incoming pagesare "thrown away". If a page is queued for a too long time in the BTS, thepage may also be lost due to the fact that the MSC does not receive the pagingresponse before the timer ( PAGTIMERREP1LA or PAGTIMEREPGLOB )has expired. The risk of having an excessive delay in the BTS increases if thetime between the transmission of each paging group (set by the parameterMFRMS ) is long.When there are no paging messages to be transmitted to MSs in a certainpaging group, dummy pages are sent instead.4.5 Paging strategies4.5.1 Retransmission of pagesIf the LA of an MS is known in the VLR (normal case), the first page is alocal page. A local page is only broadcasted in the cells belonging to the LAin which the MS is registered. If a page was not successful (the MS did not10(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7respond to the page), the MSC can be set to send a second page. The secondpage can be either local or global. A global page is broadcasted in all cellsbelonging to the same MSC area as the LA in which the MS was registered.When an MS is paged, either TMSI or IMSI can be used to identify the MS.The recommended paging strategy is: If the LA of an MS is known in the VLR, the first page is a local page. Forthe local page, TMSI is used to identify the MS. If the MS did not respond to the first page, a second page is performed.The second page is also a local page but IMSI is used to identify the MS.Other strategies than recommended would affect the paging load in thefollowing way: No second page:No second page reduces the paging load in the BTS as well as the BSC.The drawback is the risk of having more unsuccessful MS pagings. Global second page:Compared to a local second page, a global second page increases thepaging load. The advantage is that MSs that for some reason have thewrong LA status in the VLR have a better chance of being successfullypaged. TMSI for second pages:If the second page is global, IMSI has to be used to identify the MS. If thesecond page is local, either IMSI or TMSI can be used to identify the MS.Using TMSI will increase the paging capacity in the BTS. The drawback isthat some pages may be unsuccessful if an MS has the wrong TMSI in theVLR, for example just after having crossed a LA boarder.Note: Ericsson base stations will automatically (if CCCH repeat is turned on)retransmit Paging Requests once, provided that there are free resources on thepaging channel. This has nothing to do with the retransmission of lost pages.Note: In GSM 1900 there is a possibility of restricting the second page to aLocal Access and Transport Area (LATA). A LATA can be defined to consistof one or more LAs, which together correspond to a charging area. It may beserved by one or several MSCs (ref. 4).4.5.2 Reducing unnecessary pagingsThe Location Area dimensioning is mainly based on the paging load in theBTS. Examples of network features that can reduce the required paging rate ina system are: IMSI attach/detach:The ATT parameter informs the MS whether it shall apply IMSI attach anddetach. If ATT is set to ON, the MS will send attach and detachinformation to the MSC/VLR when it is powered on and off. The numberof unnecessary pagings of MSs that are not connected to the network 11(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINEdecreases and therefore, the ATT parameter value should always be set toON. This leads to more signaling on the SDCCH, thereby affecting theSDCCH dimensioning (Ref. 3). Periodic Registration: Periodic Registration is necessary to avoid unnecessary paging attempts foran MS which has lost coverage and has not been able to inform the systemthat it is inactive. The T3212 parameter controls the Periodic Registrationinterval. A shorter interval, together with implicit detach supervisioninitiated in the MSC, will reduce the number of unnecessary pagings but itleads to more signaling on the SDCCH, thereby affecting the SDCCHdimensioning (Ref. 3). The recommended initial setting of T3212 is 40, i.e.registration every 4 hour. t h4.6 Calculating the BTS paging capacityThe paging of an MS is initiated by the MSC sending a Paging message toeach BSC belonging to that service area of the MSC (global page), or to thoseBSCs serving at least one cell belonging to the LA where the MS is registered(local page). Each BSC receiving a Paging message sends a Paging Commandmessage to each of its BTSs. The message contains the MS identity (TMSI orIMSI), the paging group number of the MS and optionally an indication forthe MS about which combination of channels will be needed for thesubsequent transaction related to the paging.If a page is global (only when LA was not known in the VLR with therecommended setting), IMSI has to be used to identify the MS. With acorrectly dimensioned MSC/V LR, less than 5 % of the pages should be globalpages. Therefore, in these calculations, all pages are assumed to be local.The theoretical paging block capacity of a BTS is:Combined BCCH/SDCCH cells:( AGBLK =0): 3 / 0.2354 = 12.75 paging blocks / second ( AGBLK =1): 2 / 0.2354 = 8.50 paging blocks / secondNon combined BCCH/SDCCH cells:( AGBLK =0): 9 / 0.2354 = 38.25 paging blocks / second( AGBLK =1): 8 / 0.2354 = 33.98 paging blocks / secondIn order to calculate the paging capacity in the BTS, the relation between thenumber of pages using IMSI or TMSI has to be estimated. With therecommended setting, all second pages use IMSI to identify the MS.Experiences from live networks show that with the Ericsson's GSM systems,R6 default setting, typically 25 % of the pages of an MS result in a secondpage. In an average, this means that for each MS terminated call attempt(= Paging Attempt (PA)), 1 TMSI and IMSI are transmitted.12(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7Each paging block can fit 4 TMSI pages or 2 IMSI pages. Assuming 25 %second pages (IMSI), and no global pages, the number Paging Attempts perpaging block is:4 / (1 + 2 * 25 %) = 2.7 PA / paging block (1 PA = 1 TMSI + IMSI)The maximum theoretical paging capacity in the BTS is then:Combined BCCH/SDCCH cells:( AGBLK =0): 2.7 * (3 / 0.2354) = 34 PA / second( AGBLK =1): 2.7 * (2 / 0.2354) = 23 PA / secondNon combined BCCH/SDCCH cells:( AGBLK =0): 2.7 * (9 / 0.2354) = 103 PA / second( AGBLK =1): 2.7 * (8 / 0.2354) = 92 PA / secondThe corresponding number of Paging Commands (including 25 % secondpages) the BTS can handle is in each of these cases:Combined BCCH/SDCCH cells:( AGBLK =0): 1.25 * 34 = 42.5 Paging Commands / second( AGBLK =1): 1.25 * 23 = 28.75 Paging Commands / secondNon combined BCCH/SDCCH cells:( AGBLK =0): 1.25 * 103 = 129 Paging Commands / second( AGBLK =1): 1.25 * 92 = 115 Paging Commands / secondIn this document, a maximum allowed paging load of 50 % of the theoreticalmaximum paging capacity in the BTS is assumed. This means that there isroom for all BTS originated retransmissions . When the average paging load 3is 50 % of the maximum theoretical capacity in the BTS, very few BSCoriginated pages will be lost due to that the paging queue (in the BTS) is full.Allowing a maximum paging load of 50 %, the maximum Paging Attemptcapacity in the BTS is:Combined BCCH/SDCCH cells:( AGBLK =0): 0.5 * 2.7 * (3 / 0.2354) = 17 PA / second( AGBLK =1): 0.5 * 2.7 * (2 / 0.2354) = 11.5 PA / second 3 The BTS retransmission of Paging Requests occurs only when there are free recourses on the CCCH, but a margin of 50 % is here applied tobe able to retransmit all Paging Requests. 13(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINENon combined BCCH/SDCCH cells:( AGBLK =0): 0.5 * 2.7 * (9 / 0.2354) = 51 PA / second( AGBLK =1): 0.5 * 2.7 * (8 / 0.2354) = 46 PA / secondThe number of Paging Commands (including 25 % second pages) that theBTS can handle, allowing a maximum paging load of 50 %, is in each of thesecases:Combined BCCH/SDCCH cells:( AGBLK =0): 1.25 * 17 = 21.2 Paging commands / second( AGBLK =1): 1.25 * 11.5 = 14.4 Paging commands / secondNon combined BCCH/SDCCH cells:( AGBLK =0): 1.25 * 51 = 63.8 Paging commands / second( AGBLK =1): 1.25 * 46 = 57.5 Paging commands / second5 Estimated maximum LA sizeWhen doing LA dimensioning it is important not to exceed the maximumpaging capacity. In an existing network, outgoing Paging Commands from theBSCs should be measured. This is done by adding the two countersNLAPAG1LOTOT and NLAPAG2LOTOT that are the counters for thenumber of first and repeated Paging message to an LA. To get PagingCommands per second, divide this sum by the measurement period. Thisshould not exceed the figures calculated in the previous chapter (4.6).If there are no available statistics, e.g. at an initial configuration, a trafficmodel has to be used. The examples below is a calculation of how manyPaging Commands per second that are generated for a traffic model calledBAS1 (Ref. 5) and how many TRXs an LA, in such a network, can serve. TheBAS1 model is based on collected switch data and corresponds in some senseto an average network (Example 1). A fluctuation from the average networkhas to be taken into consideration and in Example 2, a network with higherpaging load is presented. By examine the results of these two examples aconclusion of how to dimension the network can be done. When not beingfamiliar with the subscriber behaviour it is recommended to use thefigures according to Example 2. These recommendations are to usecombined BCCH/SDCCH up to 2100 Erlang ( 500 TRXs) per LA. For morethan 2100 Erlang TRXs per LA, a non-combined configuration should beused. Using this as an initial configuration, the operator can optimize thenetwork by using measurements.Important note: The figures in this section refers to BTS capacity. There arealso capacity limitations in the BSC that must be considered, see section 6.14(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7Example 1: BAS1 modelThese estimations of the LA size require some assumptions regarding thetraffic and subscriber behaviour. In order to estimate the maximum size of aLA, an estimated paging load per Erlang of traffic in the LA is calculated. Thenumber of first pages generated per Erlang traffic is estimated with thefollowing average assumptions (BAS1 default model,Ref. 5): MS originated call attempts: 65 % Fraction of MS originated call attempts with B answer: 77 % Duration of MS originated call with B answer: 75 sec Fraction of MS originated call attempts without B answer: 23 % Duration of MS originated call attempt if no B answer: 20 sec Fraction of MS terminated call attempts leading to a page: 50 % Fraction of MS terminated call attempts with B answer: 90 % Duration of MS terminated call with B answer: 90 sec Fraction of MS terminated call attempts without B answer: 10 % Duration of MS terminated call attempt if no B answer: 20 sec Margin for traffic peaks: 20 %Fraction of all call attempts resulting in a page: (165%) * 50% = 17.5 %The mean holding BTS time for connection (including no B-answer):0.65 / (0.65 + 0.175) * (0.77*75s + 0.23*20s)+ 0.175 / (0.65 + 0.175) * (0.9*90s + 0.1*20s) = 63 seconds1 Erlang of traffic corresponds to an average of 1 / 63 = 0.016 calls per second17.5 % MS terminated calls attempts leading to a page => 0.175 * 0.016 =0.0028 MS terminated call attempts per second and Erlang trafficThe total paging load per Erlang traffic is with the 20 % margin for trafficpeaks:1.2 * 0.0028 = 0.0034 PA per second and Erlang trafficWhich equals to:1.25 * 0.0034 = 0.0043 Paging Commands per second and Erlang trafficHaving estimated the approximate number of Paging Attempts per second forone Erlang of traffic, and the maximum paging load in the BTS, the maximumsize of an LA in terms of served traffic can be calculated: 15(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINECombined BCCH/SDCCH:( AGBLK =0): 0.5 * 2.7 * (3 / 0.2354) / 0.0034 = 4250 Erlang( AGBLK =1): 0.5 * 2.7 * (2 / 0.2354) / 0.0034 = 2833 ErlangNon combined BCCH/SDCCH:( AGBLK =0): 0.5 * 2.7 * (9 / 0.2354) / 0.0034 = 12751 Erlang( AGBLK =1): 0.5 * 2.7 * (8 / 0.2354) / 0.0034 = 11334 ErlangBy doing the estimation that each TRX roughly carry an average of 4 Erlangof traffic during busy hour, the corresponding number of TRXs per LA is:Combined BCCH/SDCCH:( AGBLK =0): 4250 / 4 = 1063 TRXs per LA( AGBLK =1): 2833 / 4 = 708 TRXs per LANon combined BCCH/SDCCH:( AGBLK =0): 12751 / 4 = 3188 TRXs per LA( AGBLK =1): 11334 / 4 = 2834 TRXs per LAExample 2: High paging loadThese estimations of the paging load in a LA use the same assumptions asprevious example except for: MS originated call attempts: 50 % Fraction of MS terminated call attempts leading to a page: 70 %These are harder requirements than the average network in the previousexample.With the same calculations as above, the total paging load per Erlang traffic iswith the 20 % margin for traffic peaks:0.0067 PA per second and Erlang traffic,which equals to:0.0083 Paging Commands per second and Erlang trafficThe size of an LA in terms of served traffic and the corresponding number ofTRXs per LA (by doing the estimation that each TRX roughly carry anaverage of 4 Erlang of traffic during busy hour) are:Combined BCCH/SDCCH:( AGBLK =0): 2168 Erlang = 542 TRXs per LA( AGBLK =1): 1445 Erlang = 361 TRXs per LA16(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7Non combined BCCH/SDCCH:( AGBLK =0): 6503 Erlang = 1626 TRXs per LA( AGBLK =1): 5780 Erlang = 1445 TRXs per LA6 Paging capacity in the BSCThe R7 Extended Paging feature can provide the BSC with a paging capacityof more than 8500 Paging Commands per second (Ref. 6). This was tested bysending 60 Paging messages per second to a BSC (APZ 212 11) with 143 cells(=8580 Paging commands/sec). Due to test limitations, higher load could notbe tested, although it is likely that the BSC can handle higher load. In thisdocument, 8500 Paging commands will be taken to be the maximum limit ofthe BSC. Note that the BSC paging capacity is difficult to estimate. It istherefore recommended to closely monitor the paging performance by STScounters.The BSC paging capacity is no bottleneck when doing LA dimensioning aslong as:#Cells/LA * #Paging Commands/cell and sec < 8500 Paging Commands/secExample 1: BASI model w ith one TRX cellsAn LA with 500 one TRX cells, the Paging Command load is calculated:4 Erlang/TRX * 500 TRXs = 2000 Erlang traffic2000 * 0.0043 Paging Commands/sec*Erlang = 8.6 Paging Commands/sec8.6 * 500 cells = 4300 Paging Commands/secExample 2: High paging load with one TRX cellsAn LA with 500 one TRX cells, the Paging Command load is calculated:4 Erlang/TRX * 500 TRXs = 2000 Erlang traffic2000 * 0.0083 Paging Commands/sec*Erlang = 16.6 Paging Commands/sec16.6 * 500 cells = 8300 Paging Commands/secExample 3: High paging load with 4 TRX cellsAn LA with 250 four TRX cells, the Paging Command load is calculated:4 Erlang/TRX * 1000 TRXs = 4000 Erlang traffic4000 * 0.0083 Paging Commands/sec*Erlang = 33.2 Paging Commands/sec33.2 * 250 cells = 8300 Paging Commands/sec 17(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINEThese examples can be summarized by two graphs using the high paging loadtraffic model. The first one (Figure 5) presents the limitations of the traffic fordifferent number of cells in the BSC. Figure 6 shows how many TRX s per cell(average in BSC) that are recommended without exceeding the PagingCommand capacity of the BSC.250 cells 200 cells 150cells12000 100 cells1000080006000400020000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000Erlang / BSCFigure 5. The graph presents the limitations of the traffic for different numberof cells in the BSC. The thick line represents the highest recommended PagingCommand load for an LA. Note that the graph shows the situation with one LAin the BSC. If the BSC is divided into two LAs, the BSC paging capacity isincreased. 1 TRX/cell20 00 02 TRXs/cell18 00 016 00 014 00 0 3 TRXs/cell12 00 010 00 04 TRXs/cell80 0060 0040 0020 000 0 10 0 20 0 30 0 4 00 50 0 60 0 70 0 8 00 90 0 10 00 1 10 0TRXs / BSCFigure 6. The graph presents the maximum number of TRXs in the BSC inorder not to exceed the Paging Command capacity. 4 Erlang /TRX isassumed. Note that the graph shows the situation with one LA in the BSC. Ifthe BSC is divided into two LAs, the BSC paging capacity is increased.18(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7As mentioned before it is difficult to estimate the BSC paging capacity.Suspected BSC paging queue overflow can be monitored. The BSC willdiscard paging messages from the MSC when the paging queue is full and theBSC has a counter, TOTCONGPAG, which steps for every discarded page.The number of paging messages received from the MSC is counted byTOTPAG . If the paging queue is congested, consider splitting the LA intotwo or more LAs. This will lower the paging load in the BSC.7 LA dimensioning strategiesThe paging load determines the maximum size of an LA while the LocationUpdating load in the LA border cells sets the minimum size. The mostimportant rule is not to exceed the maximum paging capacity of the BTS orthe BSC.When having defined a combined BCCH/SDCCH configuration in a cell, theLA can be fairly large before the paging capacity becomes a limiting factor inthe BTS. Since all paging messages have to be broadcasted in every cellwithin the LA in question, the combined BCCH/SDCCH cell will be thelimiting factor if at least one combined BCCH/SDCCH cell is used in the LA.(The paging congestion only affects this combined cell, not the entire LA). Byusing combined BCCH/SDCCH configuration an extra SDCCH/4 is gained.Thus if it is applicable from a Location Updating point of view, combinedconfiguration is a recommended solution.In rural areas it is often, because of the lower traffic load, easy to find suitableLA border cells, however there is no reason to have smaller LAs thannecessary. One LA per BSC area is often a good rule of thumb. If an LA isrelatively large, and the paging load is high, one should consider splitting theLA into two or more LAs. This will reduce the paging load in the BTSs aswell as in the BSC.In larger cities, the increased SDCCH load in LA border cells may be morecritical. This can make it more difficult to find suitable LA borders. If theBSC areas are relatively small, and it is difficult to find suitable LA borders,one LA can cover several BSC areas.Regardless of the type of area, rural or urban, it is recommended to have theLA border cells in low subscriber density area. LA borders crossing over highmobility areas, e.g. high ways, should be avoided. The most importantrecommendation is to carefully monitor the performance of the system. 19(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINE8 Parameters8.1 BSC parametersIn this section some of the most important BSC parameters for pagingperformance are explained:MFRMS is the multiframe period and defines the transmission interval ofpaging messages to the same paging group.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 paging capacity.BCCHTYPE can be either: COMB = Combined; The cell has a combined BCCH, CCCH and SDCCH/4. COMBC = Combined with CBCH; The cell has a combined BCCH,CCCH and SDCCH/4 with a CBCH subchannel. NCOMB = Not combined; The cell does not have a SDCCH/4.If COMB is used the maximum capacity of the CCCH will decrease to a third(if AGBLK set to 0), compared to the NCOMB case.T3212 is the time between the periodic registration.ATT determines if attach/detach is allowed.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 with manyLocation Updatings.Table 2. BSC parameter table.Parameter Default Recommended Value rangename value valueMFRMS 6 4 2-9 CCCH multiframesAGBLK 1 0 0-7 (0-1 for RBS 200,2000)BCCHTYPE NCOMB see section 3.6 COMB, COMBC,NCOMBT3212 40 40 0-255 YES YES YES, NO ATTCRH 4 4 0-14 (in steps by 2) dB20(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R78.2 MSC parametersThe following MSC parameters and exchange properties are relevant forpaging and Location Updating:BTDM implicit detach supervision should be equal (or longer) than T3212 inthe BSC. If T3212 is increased, BTDM must also be increased. Note thatBTDM is set in minutes and T3212 is set in deci-hours.GTDM is an extra guard time in minutes before the subscriber is set todetached.TDD sets the time (in days) that an inactive IMSI is stored in the VLR beforeit is removed.PAGTIMEFRST1LA is the time supervision for the page response of thefirst page. The MS is paged in the LA with the first page if the Location AreaIdentity (LAI) information exists in the VLR. The parameter is set accordingto the default values.PAGETIMEFRSTGLOB is the time supervision for the first global page. Itis used instead of PAGTIMEFRST1LA if the LAI information does not existin 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 IMSI3 Paging is repeated as global paging with IMSIPAGEREPGLOB defines how global paging is repeated according to:0 Global paging is not repeated1 Global paging is repeated with IMSIPAGTIMEREP1LA is the time supervision for the second page to LA. Thisis the timer used for the second page when PAGEREP1LA is 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 allocated. Note that this setting this means thatTMSI is not used. The paging capacity will be decreased ifTMSI is not used.1 TMSI is allocated only on encrypted connection2 TMSI is allocatedTMSILAIMSC states if a new TMSI shall be allocated at a change of LAIwithin the MSC/VLR. Only applicable if TMSIPAR is not equal to 0.0 Allocation only once1 Allocation on every change of LAI 21(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINEIf 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 that 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 from the VLR,due to being inactive for too long (see parameter TDD above). At an incomingcall, the HLR has information about the most recent location, i.e. VLR, wherethe MS was registered. Then, when the call is connected to the VLR a globalpage will be sent out due to that no information exists in the VLR about thisparticular MS. If the MS would have been registered in the VLR but notactive, no page would have been sent out.The following LATA exchange properties are valid only if the function EqualAccess and Transit Network Selection in MSC/VLR and Gateway MSC(GMSC) is implemented. This is an optional GSM 1900 function.LATAUSED defines the usage of LATA administration:0 LATA administration is not used1 LATA administration is usedPAGLATA defines if LATA paging is used for mobile terminating calls ornot: 0 LATA paging is not used1 LATA paging is usedPAGREPCT1LA defines how the paging in one location area is repeated, ifthe first Paging Attempt was local. This parameter is only valid whenPAGLATA is set to 1.0 Paging in one LA is not repeated1 Paging in one LA is repeated with either TMSI or IMSI2 Paging in one LA is repeated with IMSI3 Paging is repeated as call delivery LATA paging with IMSIPAGTIMEREPLATA defines the time supervision for page response ofrepeated LATA paging. After expiration of this timer no new pagingrepetition for this call is done.22(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7Table 3. MSC parameter table.Parameter name Default Recommended Value rangevalue value OFF BTDM T3212 * 6 6-1530 (steps of 6) minutes orOFFGTDM - - 0-255 minutesTDD OFF - 1-255 days or OFF 4 - 2-10 sec PAGTIMEFRST1LAPAGETIMEFRSTGLOB 4 - 2-10 secPAGEREP1LA 2 - 0-3PAGEREPGLOB 0 - 0-1PAGTIMEREP1LA 7 - 2-10 secPAGTIMEREPGLOB 7 - 2-10 secTMSIPAR 0 1 or 2 0-2TMSILAIMSC 0 - 0-1LATAUSED 0 - 0-1PAGLATA 0 - 0-1 2 - 0-3 PAGREPCT1LAPAGTIMEREPLATA 7 - 2-10 sec 23(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 LOCATION AREA DIMENSIONING GUIDELINE9 Referencesref. 1 Location Area dimensioning guideline CME 20 R6,5/100 56-FCU 101 206 Uen Rev Aref. 2 LA- and paging analysis for radio network investigations,LVR/P-970001 Rev Aref. 3 SDCCH Dimensioning Guideline, 10/100 56-HSC 103 12 UenRev Aref. 4 User Description for Ericsson's GSM systems, R7, Idle ModeBehaviour, 2/1553-HSC 103 12 Uen Aref. 5 BSC capacity report Basic model, Hans Andersson, Z/EP-96:072ref. 6 Capacity test report in BSC/SV for BSS R7/BSC, Jan Gustavsson,3/152 83 4/FCP 103 970/324(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11 THE ERICSSON GSM SYSTEM R7 25(25) 11/100 56-HSC 103 12 Uen Rev A 1998-12-11