Optimum Location Area Planning

7/27/2019 Optimum Location Area Planning

1/26

Optimum Location Area PlanningEdition 02 01.03.99

Alcatel MobileCommunication LAC_PLAN.DOC ACS/SR 3DF 00993 7000 PGZZA 1/26

SiteLudwigsburg

Cellular Operations Department

Originator(s)B. Viviand

T. Quick

Optimum Location Area Planning

Domain : MCDDivision : OperationsRubric : Radio Network PlanningType : GuidelineDistribution codes :

Distribution:

To: cc:Mr. K. Eckert OC/NPL Mr. H. Derrey OCDr. R. Collmann OC/NPL Dr. C. Brechtmann OC/NPLMr. J.-B. Leprince OC/NOD Mr. P. Godet OC/NODMr. C. Blachier OC/NOD

Abstract:This document shows approaches how to achieve an optimum location area planningin order to avoid SDCCH congestion and to optimize the load at MSC and BSC.

Approval

Name C. Brechtmann K. Eckert R. Collmann

Signature


2/26



Table of contents

1 History ............................................................................................................ 4

2 Referenced Documents ...................................................................................4

3 Abbreviations.................................................................................................5

4 Scope .............................................................................................................. 6

5 Introduction....................................................................................................7

6 System capabilities.........................................................................................8

6.1 Interfaces............................................................................................................ 86.1.1 Air Interface .................................................................................................... 86.1.2 A Interface .................................................................................................... 10

6.2 BSC.................................................................................................................. 116.3 MSC................................................................................................................. 12

6.3.1 E10 switch..................................................................................................... 126.3.2 S12 switch..................................................................................................... 13

7 Planning of LAs ............................................................................................14

7.1 Methodology..................................................................................................... 147.2 Topological planning.........................................................................................157.3 Tools for LA planning ........................................................................................ 17

7.3.1 A955 ............................................................................................................ 177.3.2 MNDT 1.32 Traffic......................................................................................... 177.3.3 "BSS Telecom Traffic Model" Tool .................................................................... 187.3.4 MSC dimensioning......................................................................................... 18

7.4 Know problem concerning LAC planning............................................................ 187.4.1 Concerned software releases.......................................................................... 187.4.2 Description of the problem ............................................................................. 187.4.3 Work-arround ................................................................................................ 18

8 Optimisation of LAs...................................................................................... 19

8.1 Optimisation Tools ............................................................................................ 198.2 Detection of LU problems .................................................................................. 198.3 Countermeasures .............................................................................................. 21

9 Summary ......................................................................................................23

10 Appendix ......................................................................................................24

10.1 Calculation of PCH and AGCH capacity............................................................. 2410.1.1 PCH capacity................................................................................................. 24


3/26



10.1.2 AGCH capacity ............................................................................................. 2410.2 Calculation of SDCCH capacity..........................................................................2510.3 A Interface Dimensioning...................................................................................25


4/26



1 History

Date Edition Status Author Comments

19 02 1997 01 Draft B.Viviand /T.Quick Document Creation

23 01 1998 01 Released B.Viviand /T.Quick

01 03 1999 02 Released M. Hahn Chapter 7.4 added

2 Referenced Documents

[1] Alcatel 900/1800 BSS System Description Alcatel document 3BK 02974 AAAA TQZZA Ed. 05

[2] Traffic Mix Alcatel document 3DC 21203 0001 TQZZA

[3] Engineering Rules for Radio Networks Alcatel document 3DF 00995 0000 UAZZA

[4] Engineering Rules for Radio Networks: Tables & Figures Alcatel document 3DF 00995 0001 UAZZA

[5] BSS Telecom Traffic Model Release 4 Alcatel document 3BK 11203 0032 DSZZA

[6] A955 User's guide V4.22 Alcatel document 3DF 00985 0422 PCZZA

[7] Quality of Service and Traffic Load Monitoring Alcatel document 3DF 00933 0001 TQZZA

[8] POST User Guide

Alcatel document 3BK 20137 AAAA PCZZA [9] DICO User's Guide Alcatel document 3DF 00936 0001 TQZZA

[10] AGLAE User's Manual8BL 00600 0003 HAZZA


5/26



3 Abbreviations

AGCH Access Grant ChannelB4 BSS Software Release 4

BCCH Broadcasting control channel (GSM)BSC Base Station ControllerBSS Base Station System (BSC and BTS)BTS Base Transceiver StationCCCH Common Control ChannelCPR Common Processing UnitFCCH Frequency Correction ChannelDTC Digital Trunk ControllerGoS Grade of ServiceHLR Home Location RegisterHR Half rateIMSI International Mobile Subscriber Identity LA Location AreaLAC Location Area CodeLU Location updateMS Mobile StationMSC Mobile Switching CenterOML Operation & maintenance linkPCH Paging ChannelQoS Quality of ServiceR3 BSS Software Release 3RACH Random Access ChannelRCP Radio Control PointRNA Radio Network AdministrationRSL Radio signalling linkSACCH Slow Associated Control ChannelSCH Synchronisation ChannelSDCCH Stand-Alone Dedicated Control ChannelSM SubmultiplexerSMS Short message ServiceTC Transcoder

TCH Traffic channelTCU Terminal Control UnitTDMA Time Division Multiple AccessTMSI Temporary Mobile Subscriber Identity TTCH Terrestrial Traffic ChannelTRX Transceiver

VLR Visitor Location Register


6/26



4 Scope

The planning of location areas has a great influence on all network elements in a GSMsystem. A high number of location update events involves distinct processor load atMSC and BSC and can cause SDCCH congestion at the air interface. SDCCHcongestion reduces drastically the capacity of a network as it disables the call setupprocedure (no SDCCH no TCH). Thus, a suitable planning of location areas isessential for the network quality.

In this document, informations are given on the network elements involved in the location update procedure and the

possible limitations how to determine the maximum number of cells in a location area according to a

given call mix and the configuration of the network elements how to plan the location area borders to avoid the risk of too much location update

events how to optimize LAs according to quality of service measurements in the network

(in a future version of this document)

It is assumed that the reader has a basic knowledge on logical channels, call setupand location update procedures and traffic calculations. These subjects are addressedcomprehensively in [1], [2] and [3].


7/26



5 Introduction

In order to provide an effective paging within mobile terminated calls, roamingprocedures are implemented in the network which involve the grouping of cells tolocation areas (LA). The LA of every mobile station is stored in the HLR and VLR databases. This allows the network to set up a switching path to the appropriate BSS inan effective way. Every base station in this LA forwards paging messages if a certainmobile station is called.

The figure below sketches the network elements which are involved in the locationupdate (LU) process. A detailed description of this process is provided in [1]. For thepurpose of this document, it is sufficient to identify the possible locations that may cause problems due to certain limitations. These are Air interface: limited SDCCH resources A interface: limited #7 signalling resources BSC: limited processor capacity MSC: limited VLR capacity

In the following chapter, the limitations are investigated in more detail, and tables withtypical figures are provided.

BSS

MSC

BSS

VLR A-Itf Air-Itf

VLR capacity

A-Itfcapacity

SDCCHcapacity

BSC pro-cessor load

Fig. 5-1: Location Update in the Network (same MSC area)


8/26



6 System capabilities

6.1 Interfaces

6.1.1 Air Interface Before finding load figures at the air interface caused by location update events, it isnecessary to examine which logical channels are involved and how they are occupied.

Details about the usage of the various logical channels during call setups and locationupdates are found in [1]. For a location update call, a SDCCH is used. Moreover, forthe link establishment procedure, other resources of the air interface are needed.These the common control channels (CCCH) on the BCCH timeslot: RACH, PCH and

AGCH.

6.1.1.1 Channel configuration

It is possible to configure fourSDCCH channels on the BCCHtimeslot together with the CCCH("combined BCCH"). This has animpact on the total CCCH re-sources. Table 6-1 shows thenumber of available frames forthe different logical channels onthe BCCH timeslot dependingon the configuration mode. TheCCCH dl (downlink) frames areshared between PCH and

AGCH.

Not only location updates require free CCCHs, but also other transactions like callsetups and short message services (SMS). Thus, LUs have to be considered assuming a

certain call mix per subscriber [2].The values in Table 6-2 show atypical call mix situation experienced

in a matured European operationalnetwork. They are used as standardvalues in the following calculationsand examples (the LU rate seemsquite high, but this is still amoderate value. Experiences showthat even values in the region 7..12occur!).

Channel BCCHcombined not combined

FCCH 5 5SCH 5 5

BCCH 4 4SDCCH 16 -SACCH 8 -

CCCH dl 12 36DL total 50 50SDCCH 16 -SACCH 8 -RACH 27 51

UL total 51 51

Table 6-1: Nr. of frames according to BCCH configuration

Parameter ValueBusy hour attempts per subscribercall setups 1.3Location updates (LU) 3short message services (SMS) 0.2inter-BSC handover 0.6

Average SDCCH duration 4 sectraffic per subscriber 16 mErlBlockingTCH 2%SDCCH 0.5%Table 6-2: Typical call mix


9/26



6.1.1.2 PCH and AGCH capacity

The downlink CCCHs are shared between PCH and AGCH. The different parameterswhich influence this are :

CCCH_CONF: definition of the CCCH configuration (combined or notcombined)BS_AG_BLK_RES: number of blocks reserved for the AGCH. A block is the

information transmitted on four consecutive TDMA frames onthe CCCH. The default parameters are:BS_AG_BLK_RES = 1 in combined modeBS_AG_BLK_RES = 3 in not combined mode

From Table 6-1, it is seen that there are 12/4 = 3 blocks available for PCH and AGCH in combined mode and 36/4 = 9 blocks in the not combined mode. Thus, with

the above settings for BS_AG_BLK_RES, 2 blocks are available for paging channels incombined mode and 6 blocks in not combined mode, respectively.

The paging load is further dependent on the paging request type which specifies thenumber of mobiles which can be paged simultaneously with one paging message: Paging request type 1: 2 mobiles (IMSI or TMSI) Paging request type 2: up to 3 mobiles (2 IMSI, 1 TMSI) Paging request type 3: up to 4 mobiles (4 TMSI)

An average value of 3 mobiles per paging message seems reasonable.

The paging subgrouping has no influence on the load, as shown in [2]. It only affectsthe delay between originating the paging messages at the MSC and receiving theanswer, so that the proper timer values must be set accordingly.

With the assumptions from above, it is possible to calculate the capacity of PCH and AGCH channels for the combined and not combined configuration. The maximumnumber of PCH and AGCH messages are presented in Table 6-3. The interestedreader finds the calculation in appendix 10.1.

According to the AGCHmessages, the maximum numberof subscribers was calculated as

well. With the assumed traffic of16 mErl per subscriber, thiscorresponds to the TCH capacity of a 6 TRX BTS for the combinedmode and a 15 TRX BTS (!) forthe not combined mode, respectively. So it seems that there are enough CCCHresources available under normal conditions. However, an AGCH is used in responseto every RACH which the system recognizes. This can also be an invalid RACH due tonoise (spurious RACH).

ModeMessages percell and hour Sub /cellPCH AGCH

combined 58560 15319 2042not combined 176160 45957 6127Table 6-3: PCH and AGCH capacity


10/26



6.1.1.3 SDCCH capacity

SDCCHs are occupied for call setups, location updates, and short message services. A typical average value of the SDCCH holding time for all services is 4 seconds. Thus,the SDCCH traffic can be calculated depending on the call mix and the number ofsubscribers in the cell. The formulas are shown in appendix 10.2.In Table 6-4, maximum LU rates are given for the standard timeslot configurations ofthe Alcatel BTSs [4] as well as configurations with additional SDCCHs for a maximumof 0.5% SDCCH blocking. The values have been calculated using the call mixaccording to Table 6-2.

# of TRX # of TCH # of SDCCH TCH capacity Subscribers max. LU rate[Erl] [LU/call]

1 7 4 2.94 183.5 1.45

1 6 8 2.28 142.2 12.092 14 8 8.2 512.5 2.492 14 12 8.2 512.5 5.933 22 8 14.9 931 0.833 21 16 14.04 877.2 5.194 29 16 21.04 1315 3.065 37 16 28.25 1765.8 1.985 36 24 27.34 1708.9 4.556 44 24 34.68 2167.6 3.347 52 24 42.12 2632.7 2.547 51 32 41.19 2574.3 4.368 59 32 48.7 3043.8 3.5

Table 6-4: Maximum location update rate for 0.5% SDCCH blocking (the standard timeslotconfigurations are shown in bold numbers); other parameters: 2% TCHblocking, 1.3 call setups per subscriber, 0.2 short message services per call

In order to achieve a LU rate of at least 3, additional SDCCH timeslots have to beconfigured for the 1, 2, 3, 5, and 7 TRX configuration, respectively. Restrictionsaccording to the BSC (maximum number of SDCCH per TRX for the TCUs) must betaken into account [4].

6.1.2 A Interface

One single A trunk (2 Mbit/sec PCM line) carries information on 30 TTCHs (Terrestrial Traffic Channels) one #7 signalling link


11/26



The #7 link handles the signalling associated with calls, location updates, shortmessage services and inter BSC handovers. With R3 and the G1 BSC, each #7 linkprovides 60 SCCP (Signalling Connection Control Part) connections which can be usedfor all transactions and additional 4 SCCP connections exclusively for handovers. Each

SCCP connection is established approximately for the duration of one call including itssignalling phase.

Note: with B4 software, 128 SCCP connections are available for the G1 BSC and 256SCCP connections for the G2 BSC.

Besides the already mentioned transactions (calls, LU, SMS, HO), also paging has aninfluence on the #7 link load, but no SCCP connections are used for the handling.Paging commands can be forwarded according to cell identifier lists or with moresimple commands, e.g. paging for the complete BSS. Typically, each #7 link is able tohandle 20.000 paging messages per hour.

The dimensioning of the A interfaces is done as follows: first, the number of TTCHsrequired for the offered traffic is determined (using the Erlang B formula with typically 0.1% blocking), resulting in the required number of A interfaces. Then, a check isperformed whether the available #7 links offer enough resources. If this is not thecase, additional A trunks are required. An dimensioning example is shown in appendix10.3.

General experience within Alcatel shows that two #7 links are required for three A interfaces.

6.2 BSCThe BSC capacity is limitedby the processor load of theinterface boards. Eachtransaction like a call setupor a location update etc.requires a certain processingtime on a related board.There is an Alcatel tool [5]which is capable to computeboth the DTC and the TCUprocessor loads in the BSCs,depending on the traffic mixand the BSC generation.

For paging and location updating, DTC boards are the limiting element. In [2],formulas and dimensioning examples for the G1 BSC with R3 software are given.Table 6-5 shows typical processing times for the different transactions. Note that alocation update event requires the same processing time as ca. 30 paging events!

Transaction #7 DTC (ms) normal DTC(ms)

Originated call 300 35Terminated call 314 33Intra-BSC HO 47 10Inter-BSC HO 90 21Loc. Update 149 not applicableShort Message 149 not applicable

Paging 9.63+(4.5+0.0032

L)

LL: number of cells inlocation area

not applicable

Table 6-5: Processing time of different events at the DTC


12/26



For estimations, a good approach is to take the numbers of transmitted bits of thetransactions according to the call mix, and to relate this result to the message flow andthe busy hour. This number is expressed in %. A processor overhead of ca. 11% has tobe added. The total value must not exceed 60%. The total amount of transactions is

shared between the installed DTC boards. Details on this method are described in [2].In practical work, the BSS dimensioning is best done with the traffic tool described in[5]. This tool considers both G1 and G2 BSC and the possible configurations.

6.3 MSC

The MSC dimensioning is normally not in the scope of radio network planners.However, the MSC performance has an influence on the maximum size of a locationarea, so it is useful to know something about this subject. This will also avoidtroubleshooting at the wrong side.

Alcatel provides two switching products, the E10 switch and the S12 switch. Thearchitecture of these switches are very different, thus, the limitations are dependent onthe used switch as well.

6.3.1 E10 switch

The E10 MSC consists of two separate entities: Switch matrix RCP (Radio Control Point)

where the RCP handles the radio resources and mobility management. Also, the RCPincludes the visitor location register (VLR). A location area can not be bigger than a

VLR. Thus, the VLR capacity sets an upper limit of the size of the LA.

The actual capacity of one RCP is 50000 subscribers. Higher capacities are achievedby further RCPs. This number of 50000 subscribers is valid for a location update rateof 0.5 LU/subscriber/hour. The RCP capacity decreases depending on the call mix.This leads to the notions of static (maximum) and dynamic RCP capacity.

Table 6-6 shows thedynamic capacity Bdepending on different E10configurations. Eachconfiguration ischaracterized by the numberof treatment units (UT), i.e.processors, and a relatedstatic capacity in subscribers.The capacity was estimated

A8330 A8330 A8360

LU rate 6 UT 14 UT 14 UT50000 sub 150000 sub 250000 sub1 32706 85035 2125883 22701 59022 1475557 14084 36619 91548

Table 6-6: E10 capacity depending on LU rate


13/26



based on the call mix of Table 6-2 and a varying LU rate. Naturally, the figures aredependent on far more parameters the description of which is out of the scope of thisdocument.

In an operational network in Paris (with an average LU rate of 3), a value of 23000subscribers was experienced for the RCP capacity. As can be seen, this is in the orderof the dimensioning results.

6.3.2 S12 switch

S12 standard configurations are offered with a static VLR capacity ranging from18000 to 180000 subscribers. However, the number of VLR bases may be configuredindividually. The actual capacity is dependent on the call mix, as in case of the E10MSC. No detailed informations are available yet.

A further restriction is the maximum number of cells in one paging message. Thisnumber is limited to 12 cells. If a location area is bigger (what should be the normalcase), an according number of paging messages is sent to the BSS. This is normally not a restriction, but can cause problems in case of limited software capabilities of theBSC. An interesting problem occured in another European operational network: due tothe R3 software, the G1 BSC only accepts a maximum of 3 consecutive pagingmessages, which lead to a maximum of 36 cells which can be paged simultaneously.In the network, one LA comprised a complete BSC with 40 cells. Due to the limitation,4 cells were not sending the paging message!


14/26



7 Planning of LAs

With the facts described in the previous sections, some planning guidelines can bededuced now. This involves a general methodology and some hints for topologicalplanning.

7.1 Methodology

Basically, to avoid problems with ahigh location update rate, thegeneral goal should be to planlocation areas as big as possible .But obviously, this increases thenumber of cells which have toforward paging messagessimultaneously, and limits are givenby the maximum number of pagingcommands or the maximumnumber of subscribers in the LA.Fig. 7-1 sketches this situation. Oneexpects an optimum location areasize where the LU rate and thepaging load is balanced. But as there are several mechanisms involved in theprocesses, it is not possible to derive a simple formula for this optimum point. A moresuitable procedure is to start with an initial approach for the LAC assignment, then

checking all the system limits, and finally adjusting the assignment until none of thelimits are exceeded. Additionally, there should be some overhead in the figures inorder to be flexible for extensions.

When a network is planned in a first step, a good approach is to define one locationarea code for each BSC . This allows effective paging (i.e. a simple paging messagecan be sent on the A interface which addresses all cells of the BSC). However, caremust be taken about the borders of the LAs . The LU rate can be minimized if theborders are not crossing streets or locations which are characterized by a highsubscriber mobility. The "one BSC / one LA" approach makes sense if there are at least20 cells connected to the BSC. For less cells per BSC, two or even three BSCs should

cover a single LA.

Since no measured values are available when an early network starts operation, it isreasonable to assume experienced traffic mix values from other networks, such asshown in Table 6-2, and then check for the planned BSS design whether the limits arenot exceeded. This is especially important for the "one BSC / one LA" approach. Thechecklist involves the following points:

LU rate Paging load

Optimum?

Nr. of cells in LA

Fig. 7-1: Influence of the LA size


15/26



are there enough channels available at the air interface? chapter 6.1.1 is the BSC sufficiently equipped (the load is shared between the DTC boards)?

chapter 6.2 is the A interface providing enough traffic and SCCP resources? chapter 6.1.2 are the limits at the MSC not exceeded (max. number of subscribers, pagingcommands etc.)? chapter 6.3. The number of subscribers is calculated out of the

total traffic of all the cells belonging to the location area and the mean traffic persubscriber

If any of these questions are answered with "no", appropriate countermeasures mustbe applied, that is, more resources must be provided by one or more of the followingactions: Air-interface: changing the relation between AGCH and PCH blocks

(BS_AG_BLK_RES), using a not combined BCCH and/or adding SDCCH channels A-interface and BSC: adding more boards at the BSC, i.e. choosing a higher BSC

configuration MSC: choosing a higher MSC configuration

The last two items show that this countermeasures can be a severe cost factor. So, atfirst, the fixed network layout should be checked as well. A re-connection of a singleBTS could be a more feasible and a quite cheaper solution than choosing a BSCconfiguration which requires more ground space due to an additional cabinet, andwhich is ineffective because of many unused resources. A close interworking of theresponsible technical and operational departments will support this methodology.

If limits of the used MSCs are detected, advice should be taken from the fixed networkplanning engineers about re-configuration possibilities and future expansionstrategies. MSC dimensioning is not the task of a radio engineer.

7.2 Topological planning

Besides the capacity problem of the network, a further question for the planner is howto group the cells in the LAs, that is to say how to draw suitable LA borders. If the

number of mobiles crossing the borders are minimized, the LU rate will be minimizedas well. Some guidelines of this border planning are found in this chapter.


16/26



Fig. 7-2 shows the typical shape of a city, with ahigh number of cells in the downtown area and alower cell density in the environment. Obviously, itcan be expected that a high mobility will occur in

the city center. The mobility of the subscribers canbe described by the traffic flow on the streetswhich connects the different cell areas. For theplanner, this means the amount of mobilestations which move from cell A to cell B within acertain time, e.g. the main busy hour. The

planning goal is then not to cross streets of hightraffic flow when drawing the LA borders. It ispossible to convert street traffic data, which a

municipality may use for traffic flow planning, into mobile phone traffic flow figures.However, such a conversion must be done with some simplifying side assumptionswhich are not met in reality at any time. Furthermore, the availability of such data isquestionable. Therefore, it is more effective to perform a LA border planning withqualitative rather than quantitative assumptions. The experience of the staff from thelocal operator and own observations from field surveys can assist in identifying themost populated places and streets.

Fig. 7-3 shows two general ways of dividing the area into location areas: sectors andcircular zones. With the sector solution, many streets are crossed in the densedowntown area which will cause a high rate of location updates there. The solutionusing circular zones optimizes the situation for the inner city, but introduces crossingsat the surrounding streets which have typically a high population at commuting times.The optimum solution is a mixture of sectors and circular zones, where the LA borderscrosses the streets with the lowest traffic flow.

Since the traffic flow is a dynamic value with high variations in time (rush hours!), it isclear that a fixed LA assignment can also only be optimum for a certain moment.

Fig. 7-2: Typical city map

Location areaborder

Location areaborder

Fig. 7-3: Division in LAs: by sectors (left) and circular zones (right)


17/26



Thus, the LA planning should not deal with average figures but rather with peak figures.

If the "one BSC / one LA" solution is used, LA border planning results in a BTS-BSCassignment list. But in reality, there are often not enough degrees-of-freedom,depending on the connection possibilities. For example, microwave engineering has adistinct influence on the site choice for the BSCs and the question which BTSs toconnect to a certain BSC, following the most cost-effective way.

On the other hand, if the LA planning is performed without any consideration of BSCareas, this may result in more paging messages to be scheduled by the MSC, becausethe messages must be forwarded to more than one BSC now. Again, advise may berequired from the MSC planning engineer.

7.3 Tools for LA planning

An effective location area planning can be performed if the planning engineer isprovided with suitable tools. Several programs are available; they are outlined below.

7.3.1 A955

A955, the radio network planning tool of Alcatel, supports the dimensioning of the Airinterface based on a given call mix and provides an automatic assignment of locationarea codes.

The call mix is evaluated when the traffic calculation is carried out. This results in thecell configuration in terms of the number of required carriers and the assignment ofcontrol channels to the different timeslots.

The automatic assignment of location area codes is carried out if the CAE parametersof the planning projects are exported. It is based on traffic flow values which arederived out of the offered traffic of the cells [6]. This assignment may serve as a firstsolution, however, a visual check of the LAs and a manual adjustment isrecommended.

In the current version, A955 does not provide BSC dimensioning.

7.3.2 MNDT 1.32 Traffic

This tool is part of the tool chain used in the MND department. It provides help for the Air interface dimensioning, again based on a call mix. The parameters are entered inflow chart boxes, which provide a good understanding of the influence of the inputs oncertain parameters.


18/26



7.3.3 "BSS Telecom Traffic Model" Tool

This tool is capable to calculate the BSC processor load according to a call mix and anumber of cells and considers different hardware and software configurations as well.The used model is described in detail in [5].

7.3.4 MSC dimensioning

For MSC dimensioning, proper tools are available at the switching departments. Theirusage require a much deeper knowledge of the architecture of the proper MSC. Moreeasy to operate are "configurators", i.e. tools used for offer work. They can providecoarse values of the capacity of the used MSC configuration.

Naturally, the LU rate is only one parameter which has an influence on the MSCperformance. Therefore, it is recommended to discuss capacity problems with theresponsible engineers from the related switching departments.

7.4 Know problem concerning LAC planning

7.4.1 Concerned software releases

The afterwards described problem occurs under certain conditions for the softwarereleases B4 and B5. In B4 the problem is solved with BSCSAD11F/61F included inBSSSAE02H-2. In B5 the problem is planned to be corrected with the delivery BSSSAH06A.

7.4.2 Description of the problem

When the BSC receives a paging command from the MSC with 260 or 261 byteslength, all #7 links get instable. The critical length of 260/261 bytes can be reachedusing three different paging scenarios:

1. Paging mode LAC+CI without TMSI and with IMSI of 8 bytes, 55 cells within the LA

2. Paging mode LAC+CI with TMSI but without IMSI, 55 cells within the LA

3. Paging mode LAC+CI with TMSI and with IMSI of 8 bytes, 54 cells within the LA

7.4.3 Work-arround

The best way to avoid the above described problem is not to use 54 or 55 cells withinone LAC.

If you have 54 or 55 cells planned to be included within one LAC, dont use theLAC+CI paging mode. The paging mode LAC only should be used instead.


19/26



8 Optimisation of LAs

The possibilities of LA optimisation are strongly dependent on the capabilities of theactual BSS software release. Therefore, it is sometimes quite tricky to prove whethercongestion is coming from LU problems or whether it is based on other effects. In thefollowing, possible clues for LU problems are discussed and what actions are requiredto investigate the problem in more detail, respectively, how to overcome it.

8.1 Optimisation Tools

The problem identification is based on OMC-R performance measurements andsupported by a optimization tool chain. The tools applicable for LA optimisation are the peri-OMC tools OBSTRT and OBSYNT [8]

OBSTRT: traffic analysis and alarm generation

OBSYNT: collection of measured data and formatting in spreadsheet files DICO: Database of Indicators and Counters from the OMC-R, for problemidentification [9]

AGLAE: A-Interface GLobal Analysis and Elaboration, based on protocol analyzermeasurements (K1103 traces), for further investigation [10]

MAFIA: Macros d'Analyse de Fichiers Interface A: for the display of the AGLAEresults

8.2 Detection of LU problems

In normal operational networks, the typical problems are starting when SDCCHcongestion occurs. This leads to the question of a suitable monitoring procedure.

An appropriate methodology is the QoS monitoring [7]. The application of thismethodology requires the following steps: Generation of OMC-R performance measurement files Usage of OBSTRT and OBSYNT Post processing of OBSYNT files and display of the suitable QoS figures with DICO

(for R3 and B4) or - in future - Alcatel's A985 NPA tool (based on METRICA; for B4only)

The QoS monitoring procedure supports the quality management of the network. TheSDCCH traffic is provided daily per cell. Hence, problems with SDCCH congestion willbe recognized and localized fastly. The question for the reason, however, is not foundthat easy. Especially for the R3 software, the available counters are not allowing tolook for details; they can be used for the evaluation of SDCCH traffic and congestion.Further counters allowing calculations for PCH, AGCH, and RACH are available in B4.


20/26



Counter Mnemonic Description C01 NBR_DCCH_TERM_TRANS successful SDCCH seizures for terminating

transactions as seen by the mobile (pagingresponse)

C02 NBR_DCCH_ORIG_TRANS successful SDCCH seizures for originatingtransactions as seen by the mobile (other cases andemergency calls)

C04 NBR_NO_DCCH_AVAIL The subset of SDCCH seizures attempts which areunsuccessful because all SDCCH are busy

C05 NBR_SDCCH_FAIL_BSS_PBL Number of SDCCH transactions failures due to BSSproblems

Table 8-1: OMC-R counters for used for SDCCH statistics

Before any further step is taken, the problem has to be examined in more detail. This

can be done by taking a trace on the A interface at the BSC where the congestionproblem occurs, using e.g. the K1103 protocol analyzer. This trace can be analyzedfurther by following the procedure described below: Identify the BSS with SDCCH congestion Take an A interface trace (K1103) Perform an analysis with AGLAE Display the results with MAFIA

BSC 8 Date: 14.9.97 Begin time 13:34:50Total Trafic 15196 Stop time 15:35:01

Time interval 2:00:11Procedures % Type Nbr % Messages Nbr %

Assign Request 3836 95.68%MO 4009 65.19% Alerting+Progress 3219 80.29%

Connect Ack 2308 57.57%Calls Mean Call setup Time (s)

Mean call Holding time upon no answer (s)31.37% Mean call Holding time (s)

Assign Request 2021 94.40%MT 2141 34.81% Alerting+Progress 1986 92.76%

Connect Ack 1565 73.10%Mean Call setup Time (s)Mean call Holding time upon no answer (s)Mean call Holding time (s)

Normal 6333 70.67% Mean Holding Time

Location Update 45.71%Periodic 2628 29.33% Mean Holding Time

BSS Internal 2158 48.95% HO Performed 2158Handover 22.49%

BSS External 2251 51.05% HO Request 1428HO Command 823

Supplementary Service 0.42% 83Emergency Call 0.01% 2

Table 8-2: Call mix output of MAFIA


21/26



Using AGLAE together with MAFIA allows the evaluation of a call mix out of the A trace. This helps in investigating the reason for SDCCH congestion. A high LU rate canbe directly identified. An example of a measured call mix is shown in Table 8-2.

The figure "Total traffic" is the number of events which require a SDCCH (withouthandover events). The percentages in the second column are calculated by relating thenumber of counted events to the total number of events. If the transaction times areknown, these values could also been converted to figures per subscriber. The exampleshows, anyway, that there are ca. 1.5 times more location updates than calls. The LUpercentage can be used as a direct indicator of LU problems. A "problem threshold" isin the range of 70..80%.

8.3 Countermeasures

Before one starts looking for a higher-level solution against high LU rates, one can try to adjust another cell parameter which influence the stability of location updateprocesses: the CELL_RESELECT_HYSTERESIS.

A practical example is presented in Table 8-3. For five cells with a high LU rate, theparameter CELL_RESELECT_HYSTERESIS was adjusted from 6dB to 10dB. This has animpact on both the LU rate and the average SDCCH congestion.

Cell Hysteresis = 6dB Hysteresis = 10dBLU/call SDCCH cong. LU/call SDCCH cong.

1 3.14 1.06% 2.57 0.6%2 12.9 6.09% 10.32 4.01%3 9.55 1.75% 7.83 2.82%4 6.59 6.13% 6.11 4.57%5 5.66 4.02% 3.91 3.42%

Table 8-3: Influence of CELL_RESELECT_HYSTERESIS parameter

It is seen that in most of the cases, there is a distinct decrease of the LU rate. Even forcell 3, which shows an increasing SDCCH congestion rate due to more traffic, animprovement was achieved. However, the results also show that, though the situationhas become better, this is not the right method to remove the basic problem. It is asolution for cells with rather low SDCCH congestion.

It is suitable to draw the results in cartographic form, e.g. by plotting a dot in thecenter of the cell. A large dot may reflect a high LU rate. This is sketched in Fig. 8-1.One can see that the LU rate of two cells which are serving a motorway and which aresituated at a location area border is very high. This information is, on one hand,useful for re-assigning the LACs of the cells, i.e. moving the LA border. On the otherhand, care must be taken in drawing simple conclusions out of such plots. Let us


22/26



assume that an engineer decides to include one of the critical cells into the LA of theother cell - this seems to be a reasonable solution. If the parameters are changed atthe OMC-R and a new measurement is made, the result may look like in Fig. 8-2.

The problem has not been solved, it has only been shifted. This is because themotorway will cause a high mobility not only between the two considered cells but inthe whole area of all the cells which will serve it. This example shows that it is very important not to loose the global view when such LU problems are investigated.

Motorway

LA border

Dots identifyingthe LU rate

Fig. 8-1: Visualization of the LU rate

Motorway

new LA border

Fig. 8-2: Moving the LA border


23/26



9 Summary

This paper showed approaches for a suitable location area planning. It has outlinedthat high location update rates will influence the network performance at severallocations, from the air interface resources to the MSC capacity. The contributions ofeach of this possible problem locations have been evaluated.

With the achieved results, location area planning guidelines have been defined. Basedon an assumed initial LAC assignment and call mix values experienced from realnetworks, it is possible to check the limits of the system. The LAC assignment can bemodified then until the optimum solution is found.

For high location update rates in running networks, the paper showed ways toexamine the problem based on Alcatel's optimization tools and methodologies andpresented proper countermeasures.

With the information of this paper and the referenced documents, the radio networkplanner is able to handle location area planning in an effective way. If newexperiences from Alcatel's projects or new hardware or software features will beavailable in future, this paper will be updated in order to provide the most actual andmost simplified planning and optimization methods.


24/26



10 Appendix

10.1 Calculation of PCH and AGCH capacity

10.1.1 PCH capacity The capacity is computed by

Paging capacity [pagings

hour]

#of paging blocks of mobiles / paging block 3600duration of 51-multiframe [sec]

= #

For paging request type 3, one gets for

Combined mode : 2 4 3600 / 0.235 = 122000 pagings / hour

Not combined mode : 6 4 3600 / 0.235 = 367000 pagings / hour

Restricting to a maximum paging load of 60% and taking into account that, besidesTMSI numbers, also IMSI numbers are used for paging (this reduces the availablecapacity by further 20%), the following values are yielded: Capacity of the PCH in combined mode: 58560 pagings/hour Capacity of the PCH in non combined mode: 176160 pagings/hour

10.1.2 AGCH capacity

One access grant message is carried by one AGCH block. The capacity is therefore :

AGCH capacity [ AG msg

hour]

#of AGCH blocks 3600duration of 51-multiframe [sec]

=

For the different modes:

Combined mode : 1 3600 / 0.235 = 15319 AG msg / hour

Non combined mode : 3 3600 / 0.235 = 45957 AG msg / hour

According to the capacity available on the AGCH channel, the maximum number ofsubscribers in the cell can be computed. With the typical value of 4.5 access grantmessages per hour and per subscriber and considering a maximum load of 60%, oneyield: Capacity of the AGCH in combined mode: 2042 subscriber Capacity of the AGCH in not combined mode: 6127 subscriber


25/26



10.2 Calculation of SDCCH capacity

The maximum SDCCH capacity is calculated out of the maximum traffic capacity ofthe cell, via the maximum number of subscribers:

# of subscriberstraffic capacity of the cell

traffic per subscriber=

SDCCH traffic [Erl] = # of call setupssubscriber

# of subscribers call mix mean SDCCH duration3600

where call mix = 1 + +# #LUcall

SMScall

(the "1" represents the call setup of the call itself)

As an example for the 1 TRX standard configuration [4] (7 TCH, 4 SDCCH on thecombined BCCH timeslot) and given the call mix according to Table 6-2, the numberof subscribers is

2 94183

. Erl16 mErl / sub

subscribers=

and the SDCCH traffic

1.3call setups

sub

183 sub 4.2 4 sec= 1.11 Erl

3600 sec

With 4 SDCCH channels, this leads to a blocking rate of 2.1%. One can see that,considering a typical SDCCH maximum blocking rate of 0.5%, the given call mixwould cause problems at the air interface.

10.3 A Interface Dimensioning

For the TTCH dimensioning, Erlang B statistics with typically 0.1% blocking is used,with the total number of channels in all connected A trunks (1 A trunk comprises 30channels, according to one 2 Mbit/sec PCM line). The following examples clarify the

dimensioning.Case 1: 30 cells, 2 TRX per cell

For 2% TCH blocking, the maximum load of each cell is 8.2 Erlang. So the maximumcapacity for the BSC will be 30 8.2 Erlang = 246 Erlang. With 0.1% blocking, this

requires 287 channels 287 channels30 channels / trunk

= 9 57. 10 A trunks needed.


26/26


Alcatel Mobile LAC PLAN DOC ACS/SR 3DF 00993 7000 PGZZA 26/26

Case 2: 15 cells, 4 TRX per cell

The maximum load of each cell is 21 Erlang, hence, maximum capacity of the BSC:15 21 Erlang = 315 Erlang 360 channels required at 0.1% blocking 12

A trunks needed.

It has to be proven now whether the available #7 links (= number of calculated A trunks) are sufficient for signalling. The signalling load on one SCCP connection isequal to the traffic load and the SDCCH load according to the typical call mix of Table6-2. With an assumed mean time of 3 seconds for these transactions and 1.3 callsetups per subscriber and hour with typical 4.6 seconds setup time, the additional

signalling load is ( . . ) .3 0 2 0 6 13+ +

+

=3sec

3600 sec4.6 sec

3600 sec4.83

mErlsub

.

The number of subscribers for case 1 is 246 Erl / 16 mErl/sub = 15375 subscriberand, for case 2, 315 / 16 = 19687 subscriber, respectively. Thus,Case 1: 15375 subscriber (16 + 4.83) mErl/sub = 320 ErlangCase 2: 19687 subscriber (16 + 4.83) mErl/sub = 410 Erlang

Following Erlang B, for 0.1% blocking, 366 SCCP connections are required for case 1and 460 SCCP connections for case 2, i.e. six to seven #7 links are needed for case 1and eight #7 links for case 2 (60 SCCP connections per #7 link). Comparing theseresults with the TTCH requirements, it is seen that enough #7 capacity is available.

END OF DOCUMENT