gprs performance management & optimization

© 2005 Bechtel Corporation. All rights reserved. 15

INTRODUCTION

Today, although wireless local area network(LAN), worldwide interoperability for

microwave access (WiMAX), and associatedtechnologies can provide a radio access network(RAN) for Internet Protocol (IP)-based services indistinct areas, these hotspots are a long way fromproviding contiguous coverage. Therefore, manywireless operators are focused on providingmobile Internet access. A large variety oftechnologies has contributed to the operators’vision of true mobility and seamless roaming.

The currently prevailing technology providingtrue mobile Internet access on a European scale isgeneral packet radio service (GPRS). Newgenerations of mobile handsets, personal digitalassistants (PDAs), and BlackBerry® devices, aswell as GPRS-enabled PCMCIA1 product cards,are enjoying increasing market penetration. Thedominance of GPRS networks highlights theimportance of taking full advantage ofcapabilities. It remains to be seen whether theongoing deployment of third-generation (3G)access networks will at some point not onlysupplement existing GPRS coverage in Europe,but replace it.

Cellular operator success was built on circuit-switched (CS) services. The popularity of shortmessaging and the growing demand for mobile

data services triggered the implementation of apacket-switched (PS) overlay network, openingthe public land mobile network (PLMN)operators to the market of data networks andInternet-based services. Voice services, however,remain the core business for many EuropeanPLMN operators, who introduced the tendencyto optimize the existing infrastructure from a CSperspective. This tendency and other factors havecontributed to the growing requirement tooptimize GPRS.

This paper focuses on the performancemanagement and optimization of GPRSnetworks. A performance management overviewis presented, including proposed keyperformance indicators (KPIs) for the RAN aswell as the core network (CN). A combination ofnetwork counter and protocol analysis, as well asdrive testing, is introduced as a reliable basis onwhich to isolate problems. This is followed by adiscussion of optimization techniques based onthe configuration of GPRS signaling procedures,the usage of available network features, and theperformance considerations for IP backbones.

GPRS PERFORMANCE MANAGEMENT

Performance management involves capturingand analyzing a set of KPIs that can be

quantified by using different sources ofperformance data. Network operators typicallydefine a set of KPIs and stratified targetthresholds in line with marketing and businesspriorities. Performance management teams—which are responsible, for example, for certain

Dirk Michel [email protected]

GPRS PERFORMANCE MANAGEMENTAND OPTIMIZATIONGPRS PERFORMANCE MANAGEMENTAND OPTIMIZATION

Abstract—This paper discusses performance management and optimization schemes for mature GPRSnetworks. A selection of GPRS KPIs based on network performance counters, drive tests, and active measure-ment systems, as well as possible threshold values and their analysis and interpretation, is presented. Ongoingand problem-triggered optimization and fine-tuning, including considerations regarding network design andconfiguration, are also presented.

Issue Date: August 2005Issue Date: August 2005

____________________________

1 PCMCIA is the name of an international trade associationand standards body (Personal Computer Memory CardInternational Association) and is also used to describe thePC cards themselves (small form factor devices used in avariety of applications and devices).

Bechtel Telecommunications Technical Journal 16

ABBREVIATIONS, ACRONYMS, AND TERMS

3G third generationAPN access point name ARQ automatic repeat request ATM asynchronous transfer modeBCCH broadcast control channelBDP bandwidth delay product BECN backward explicit congestion

notificationBER bit error rateBLER block error rate BSC base station controller BSS base station system BSSGP BSS GPRS protocolBTS base transceiver stationCCCH common control channelC/I carrier-to-interference (ratio)CN core network CPU central processing unitCS circuit switch(ed) CSD circuit switch domain DE discard eligibleDHCP dynamic host configuration

protocolDLCI data link connection identifierDNS domain name service FECN forward explicit congestion

notificationFPDCH fixed packet data channel FR frame relay FRoATM FR over ATMFTP file transfer protocolGGSN gateway GPRS support nodeGPRS general packet radio serviceGSM global system for mobile

communicationGSN GPRS support nodeGTP GPRS tunneling protocol HLR home location registerIMEI international mobile

equipment identity IMSI international mobile

subscriber identityIP Internet ProtocolIPoA IP over ATMISP Internet service providerKPI key performance indicatorLAN local area networkLAU location area update LLC logical link controlMCC mobile country code

MNC mobile network code MRTG Multi Router Traffic GrapherMS mobile stationMSC mobile switching centermulti-RAT multi-radio access technology N-cell neighbor cellNMS network management systemNTE network termination

equipment(P)AGCH (packet) access grant channel PBCCH packet broadcast control channelPCU packet control unit PDA personal digital assistantPDCH packet data channelPDP packet data protocol PDU protocol data unitPLMN public land mobile network(P)RACH (packet) random access channelPS packet switch(ed) PSD packet switch domainQoS quality of service RADIUS remote authentication dial-in user

serviceRAN radio access network RAND random numberRAU routing area update RLC radio link control RTT roundtrip timeRx/TxQual receive/transmit qualityRxQual receive quality SGSN serving GPRS support nodeSI system information SIM subscriber identity module SNMP simple network management protocolSQI speech quality indexSRES signed responseTBF temporary block flowTCH traffic channelTCP transmission control protocolTMSI temporary mobile subscriber identityUMTS universal mobile telecommunications

systemUTRAN UMTS terrestrial radio access

networkVC virtual circuit VLR visitor location registerWAN wide area networkWiMAX worldwide interoperability for

microwave access

17August 2005 • Volume 3, Number 1

districts of major cities, towns of a certain size, orlarge rural areas, as well as routing areas or IPbackbones—periodically monitor various KPIs toisolate areas not achieving target performance.Optimization schemes are then initiated to ensurethat performance targets are met.

KPIs are commonly established usingperformance counters provided by networkelements. Statistics based on network elementcounters can be verified and supplemented byadditional measurements obtained by activetesting with test mobile stations (MSs), protocolanalyzers located at different interfaces in thenetwork, and deployed active performancemeasurement systems. Such additional testingsystems can highlight aspects of GPRSperformance not evident from network countersalone, such as duration times for several signalingprocedures that can affect the user-perceivedsystem quality. Figure 1 presents the GPRSnetwork architecture and possible measurementpoints for network counters and protocolanalyzers. Standard network surveillance oralarm monitoring, as well as long-term trendingand analysis of multisource KPIs, can assist inidentifying problems.

GPRS KPIs Selecting meaningful KPIs is a precursor tocontinuous performance monitoring. GPRS KPIsshould encompass the base station system (BSS)and the CN to account for end-to-endperformance, i.e., from the MS to the Gi interfaceon the gateway GPRS support node (GGSN).Typically, KPIs for the BSS and the RAN reflectaccessibility, retainability, and integrity. CN KPIsfocus on routing area update (RAU) behavior,congestion metrics on several interfaces, andsystem accessibility. Table 1 provides anoverview of GPRS KPIs.

GPRS BSS KPIs • BSS Accessibility

The RAN plays a prominent role in overallsystem performance, since it typicallyrepresents the bottleneck in terms ofresources and available physicaltransmission rates on the air interface.Accessibility from a RAN/BSS perspective ismainly a function of radio resources,signaling capacity, and BSS parameterconfiguration. Packet channel requests on the(packet) random access channel ([P]RACH),which constitute the first contact of an MS

Um

Um

ExternalGPRS

Network

GGSN

Internet

Gn GiGn

SGSN

BTS

Gb

BSC/PCU

GGSN

HLRMSC(VLR)

A D

GpNetworkCounters

NetworkCounters

NetworkCounters

GsGc

Gr

Abis

RAN

BSS

Signaling and Data Transfer Interface

Signaling Interface

Test MobileStations for

Active TestingIP Core

Figure 1. Measurement Points

The number ofpacket access

requests and thetransmitted data

volume during CS busy hour can be used to determinewhether the CS resource

requirements are“pushing” the PS traffic intocongestion or

whether the GPRStraffic demand isgenuinely high.


with the network, are transmitted via slottedALOHA2 [1, 2]; unsuccessful access requestsdue to collisions are therefore intrinsic to theaccess method. Increased levels of (P)RACHrequest failures, however, can indicate poorradio conditions. The total number of(P)RACH requests may include instanceswhere the MS sends multiple consecutiveaccess bursts. Immediate assignment—theBSS’ response to the packet channel requestfrom the MS—is pivotal for identifying

congestion; the immediate assignmentrejection rate is based on the number ofimmediate assignment rejections over thetotal number of (P)RACH requests. Suchrejections are due mainly to the lack of packetdata channel (PDCH) resources in the cell,although excessive delays on the Uminterface or lack of resources in the basestation controller (BSC), packet control unit(PCU), or other nodes may cause rejections [3].

Cell congestion levels can be calculated as thetime of zero PDCH availability within theGPRS busy hour, which may be expressed asa percentage of 60 minutes. GPRS congestionlevels should also be interpreted in thecontext of traffic channel (TCH) demand for

NETWORKENTITY

NETWORKELEMENT

ATTRIBUTE KPI

BSS BSC/PCU Accessibility (P)RACH

BSS BSC/PCU Accessibility Immediate Assignment Rejection Rate ([P]AGCH)

BSS BSC/PCU Accessibility Congestion Time

BSS BSC/PCU Accessibility PDCH Allocation Failure Rate

BSS BSC/PCU Accessibility TBF Setup Failure Rate

BSS BSC/PCU Quality 4 out of 4 Success Rate (Multislot Class)

BSS BSC/PCU Quality TBFs per PDCH

BSS BSC/PCU Quality PDCHs per TBF

BSS BSC/PCU Integrity RLC BLER

BSS BSC/PCU Integrity RLC Retransmissions

BSS BSC/PCU Quality Cell Reselection Success Rate

BSS BSC/PCU Quality Cell Reselection Time

BSS BSC/PCU Retainability Percentage of TBF Preemption

CN SGSN Accessibility GPRS Attach Success Rate

CN SGSN Accessibility GPRS Attach Time

CN SGSN Accessibility PDP Context Activation Success Rate

CN SGSN Accessibility PDP Context Activation Time

CN SGSN/GGSN Quality GPRS RAU Success Rate

CN SGSN/GGSN Quality GPRS RAU Success Time

CN SGSN/GGSN Quality Congestion Levels on Gi, Gb, and Gn

System End-to-End Quality Delay

System End-to-End Quality Application Throughput

BLER block error rateBSC base station controllerBSS base station systemCN core networkGGSN gateway GPRS support node

(P)RACH (packet) random access channelRAU routing area updateRLC radio link controlSGSN serving GPRS support nodeTBF temporary block flow

Table 1. KPIs and Thresholds for GPRS System Performance

GPRS general packet radio service(P)AGCH (packet) access grant channelPCU packet control unitPDCH packet data channelPDP packet data protocol

____________________________

2 Slotted ALOHA is a technique devised in 1972 thatdoubled the capacity of ALOHA (a system developed a fewyears earlier at the University of Hawaii to coordinate andarbitrate access to shared communication channels).


CS connections. On the one hand, CS busyhour traffic can affect available resources forPS connections and obviously depends onthe BSS configuration in terms of preemptionand the allocation of fixed PDCHs. Thenumber of packet access requests and thetransmitted data volume during CS busyhour can be used to determine whether theCS resource requirements are “pushing” thePS traffic into congestion or whether theGPRS traffic demand is genuinely high. On

the other hand, it should be noted that GPRSbusy hour patterns are not necessarilycongruent, but are offset to those of thecircuit switch domain (CSD). It may,therefore, be useful to consider CS traffic(expressed in Erlangs) and PS traffic(expressed in bytes) busy hour statisticswhen assessing GPRS congestion. Forillustrative purposes, sample traffic profileshave been provided in Figure 2. Figure 2apresents CS and PS traffic distributions based

Figure 2a. Sample Traffic Profile – Routing Area

Time of Day

140,000

160,000

O:00 2:00 4:00 6:00 8:00 1O:00 12:00 14:00 16:00 18:00 20:00 22:00

20,000

40,000

60,000

80,000

100,000

120,000

1000

0

2000

3000

4000

7000

9000

Dat

a Vo

lum

e (M

B)

Erla

ng [E

]

Network-Wide (1 Month)

5000

6000

8000PS (Data Volume) [MB]

CS (Erlang) [E]

0

Figure 2b. Sample Traffic Profile – Network-Wide

Time of Day

1400

1600

O:00 2:00 4:00 6:00 8:00 1O:00 12:00 14:00 16:00 18:00 20:00 22:00

200

0

400

600

800

1000

1200

20

0

40

60

80

100

120

Dat

a Vo

lum

e (M

B)

Erla

ng [E

]

Routing Area (1 Month)

PS (Data Volume) [MB]

CS (Erlang) [E]


on 4-week averages for a routing areacontaining 120 cells. Each point on the graphrepresents the average value calculatedacross 20 weekdays during a particular hourof the day. The average total data volumefrom 24:00 to 01:00 is 39.61 MB, based on 20 values at the same hour. The figure showsthat both CS and PS distributions arecorrelated and that their peaks fall into thesame period (17:00 to 18:00). The voice curveexhibits the expected pattern, with asecondary peak at lunchtime and the primarypeak close to 18:00, followed by a rapiddecline in traffic. The PS curve shows asimilar trend up to the primary peak, fromwhich the traffic levels are sustained forapproximately another 3 hours beforedeclining. This GPRS traffic curve resemblesthose of typical Internet service providers(ISPs). Figure 2b provides a network-widereference graph for the same period of time toverify the traffic patterns shown for arandomly chosen routing area, although theprimary peak for CS and PS falls between16:00 and 17:00.

The PDCH allocation failure rate (sometimesreferred to as GPRS blocking) and thetemporary block flow (TBF) setup failure rate are additional KPIs that can flagcongestion situations. A certain amount of GPRS blocking may be acceptableconsidering that CS-centric cell dimensioningstrategies may allow for approximately 2 percent blocking or quality of service (QoS).QoS levels may vary, however, depending onthe operator’s preference.

• BSS Retainability

RAN CS congestion, genuinely high volumesof data traffic, or a combination of both cangive rise to a series of other symptoms thatmay affect not only the system accessibilitybut also the retainability and quality forongoing TBFs, i.e., GPRS connections. TBFs

are subscriber-specific, unidirectional, logicalconnections associated with a varyingnumber of PDCHs, established to facilitatedata transfer between the MS and theBSC/PCU [3]. In other words, the dataservice may be accessed successfully, but theconnection quality can range fromunacceptably bad to very good.

The PCU reserves the requested number ofPDCHs for new connections during thepacket assignment procedure. PDCHsavailable in the packet switch domain (PSD)at any given moment are used for theassignment, although more than one TBFmay be assigned to a PDCH. The GPRSmultiplexing capability can support multipleTBFs per PDCH and allocate new TBFs onalready active PDCHs. The absolute oraverage number of TBFs on a single PDCH istypically governed by system parametersettings, which also regulate when newrequests for on-demand PDCHs can beissued. On-demand PDCHs are requestedwhen the limit of allowable TBFs per PDCHhas been reached, which is captured by theTBF-per-PDCH KPI. Note that the number ofPDCHs requested by an MS is determinedmainly by its multislot class. On-demandPDCHs make temporary use of idle CS TCHs,which are transferred back to the CSD whenthey have not been used for a defined periodof time.

Additionally, fewer than the supportednumber of PDCHs for a given multislot classmay be assigned to the MS if the total numberof requested PDCHs cannot be provided dueto failed on-demand and fixed packet datachannel (FPDCH) requests. The successfulassignment of all requested PDCHs permultislot class can be monitored by the 4 (or 3) out of 4 (or 3) success rate (multislotclass)KPI. When active resources are releasedby other connections, TBF upgrades may beinitiated to allow the allocation of the PDCHsto TBFs that have fewer resources than thosesupported by the MSmultislot class.

New GPRS users can be blocked if the PSDcannot be expanded to accommodate newTBFs when the TBF-per-PDCH limit has beenreached, resulting in PDCH allocationfailures (packet access reject) and TBF set-upfailures. Note that PDCH allocation failuresmay occur after successful immediateassignment. These failure rates can be biasedby TBF upgrade failures. System vendorsmay provide a feature that allows the TBF to

Interfaces

Gb interface between the SGSN and the BSC/PCU

Gi interface between the GGSN and thepublic data network

Gn interface between the GSNs

Gr interface between the SGSN and the HLR

Gs interface between the SGSN and the MSC

Um air interface between the MS and the BSSthat provides circuit and packet dataservices over the radio interface to the MS

21

request additional PDCHs if it has been setup initially with fewer PDCHs than themultislot class of the MS allows.

Although TBF multiplexing and theallocation of fewer resources than the MS cansupport work together to provide flexiblesystem accessibility, both mechanisms canhave an undesirable effect on throughputperformance and bandwidth delay product(BDP) per user. Additionally, the absolute oraverage number of allocated PDCHs per TBFin a given cell provides another perspectiveon GPRS QoS. Multiplexing is indicatedwhen this KPI increases beyond 1. Becausethe logical link control (LLC) throughputperformance per TBF is affected by themultislot class of an MS, the distribution ofdifferent multislot MSs in the network shouldbe identified to set achievable throughputtargets. Analyzing Gb interface log files canprovide the dominant multislot class MS.

• BSS Integrity

The varying nature of radio propagation andRAN interference conditions are the mainfactors affecting data integrity in the BSS. Thecarrier-to-interference (C/I) ratio has acritical impact on the maximum physicaltransmission rate [4, 5] of a PDCH. The GPRScoding schemes, whose ranges of optimumperformance are related to the C/I ratio, helpto reduce the impact of radio interference onthe radio channel’s block error rate (BLER).Remaining uncorrectable bits and corruptedradio blocks trigger the radio link control(RLC) automatic repeat request (ARQ)mechanism if the TBF is operated in RLC-acknowledged mode [3]. The proba-bility of RLC retransmissions thus reducesthe physical transmission rate of the channel,adds additional delays to the air interface,and affects the end-to-end roundtrip time(RTT) of higher layer protocols [4, 5].Increased delays due to RLC retransmissions,however, appear to affect the quality lessthan operating RLC in unacknowledgedmode and leaving possible retransmissions tothe upper protocol layers, bearing in mindthat ARQ on the RLC and the transmissioncontrol protocol (TCP) do not pose anincreased risk of protocol inefficiencies [6].The bit error rate (BER) of a radio channel(also categorized into receive/transmitquality [Rx/TxQual] levels ranging from 0 to 7) and the percentage of RLC retrans-missions should be closely monitored.

Desirable RLC retransmission rates aregenerally below 1 percent.

Additionally, TCP window sizes, TCPretransmission time-outs, typical IP packetsize distributions, and the applicationprotocol affect the effective throughput onthe application layer [5, 7, 8]. For a detailedanalysis of higher layer performance overGPRS, refer to the aforementioned references.

• BSS Quality and Mobility

Cell reselection, as well as possible locationand routing area updates (LAUs and RAUs),has a deteriorating effect on throughputperformance. During cell reselection, the MSreleases all channels to read the systeminformation of the target cell. The radiooutage time begins with the channel releaseand ends with the completion of the cellreselection process and the establishment ofdownlink and uplink TBFs on PDCHs of thetarget cell. Cell reselection may also requireLAUs and RAUs if the target cell is part ofanother location and routing area. Thisintroduces extra signaling sequences with thenetwork as discussed below and increases thetime until new TBFs are established on thetarget cell. It should be noted that the radiooutage also affects the higher layerthroughput. The corresponding IP outagetime begins with the last received IPdatagram on the source cell and ends withthe first received IP datagram on the targetcell. During this outage time, the networkbuffers arriving datagrams, which introducesincreased delay times and decreased BDP.The number of cell reselections andassociated procedures are directly correlatedto the mobility of the MS [9].

GPRS CN KPIsThe GPRS CN plays a vital role in assessingoverall system performance. Among the keyelements of CN performance assessments aresuccess rates for GPRS attach, packet dataprotocol (PDP) context activation, RAU, andpaging. The average amount of time required toperform the above procedures may also berelevant when determining system performance,although it should be noted that this dependsheavily on the feature set used by the operator.This makes it very difficult to directly compareapproximate GPRS attach times and other delaysamong operators. Variations among systemvendors may also exist. Benchmark valuesprovided by system vendors (dependent on thesoftware and hardware revision of the network

August 2005 • Volume 3, Number 1

The PDCHallocation failurerate (sometimes

referred to asGPRS blocking)

and the TBF setup failure rate

are additional KPIs that can

flag congestionsituations.


nodes) can be used. However, long-term trendingof such procedure completion times can helpidentify critical network variations.

• CN Accessibility

The GPRS attach request message is the firstcontact of the MS with the serving GPRSsupport node (SGSN) after a TBF has beenestablished successfully. It contains thetemporary mobile subscriber identity (TMSI),mobile network code (MNC), and mobilecountry code (MCC), as well as location androuting area identity. Subsequent identitychecks of the international mobile equipmentidentity (IMEI) or the international mobilesubscriber identity (IMSI), as well asauthentication and ciphering requestmessages, may be issued by the SGSN. Toretrieve the IMSI if no entry is found for it,the SGSN uses the old location areainformation to identify the old SGSN wherethis terminal was being served last. TheSGSN may require home location register(HLR) signaling over the Gr interface toidentify profiles for unknown IMSIs.Unsuccessful authentication and cipheringmay also contribute to the attach rejects if theoperator has chosen to activate those featuresfor attach procedures. The SGSNauthenticates the GPRS mobile by sending arandom number (RAND). The subscriberidentity module (SIM) applies GSMalgorithms and the authentication key (Ki) tothis RAND to obtain the ciphering key (Kc)and the signed response (SRES), which arethen sent back to the SGSN in theauthentication response [10]. If successful,the Kc is used to cipher the payload.

SGSN counter statistics for the Gb interfacecan be used to establish the probability ofattach failures and the distribution of failurecauses. Some of the common failure causesare attach requests from users who are not subscribed to the GPRS network,international roamers from networks withoutroaming agreements, software and hardwarelevels on specific MSs, attach time-outs(excessive delays, failed responses from theMS), and network failures. Typical networkfailures include lower layer link errors, as well as HLR signaling failures on the Gr interface.

Gb interface congestion and dimensioningshould be considered when assessing theGPRS attach success rate. The Gb interfaceinterconnecting the BSS with the SGSN is aframe relay (FR) connection over an E1/T1link [11, 12]. Depending on the CN designstrategy, this FR connection may betransported over intermediate networks,such as asynchronous transfer mode (ATM)backbones supporting FRF5 (FR over ATM[FRoATM]). FR congestion control anddiscard mechanisms, i.e., forward andbackward explicit congestion notifications(FECNs and BECNs), as well as the discardeligible (DE) flags, can be monitored toidentify congestion levels on the FR data linkconnection identifier (DLCI) carrying the BSS GPRS protocol (BSSGP) protocol dataunits (PDUs).

Measuring the PDP context activation successrate is equally important. The MS mayinitiate a PDP context activation uponsuccessful completion of the attachprocedure. The MS initiating the PDP contextactivation request message provides certaininformation to the network for an activationattempt to be successful [13]; PDP type, PDPaddress type, access point name (APN),password, QoS profile, and various protocoloptions can all be defined in the requestmessage. The SGSN receiving the activationrequest issues a domain name service (DNS)query to identify which GGSN IP addressholds the appropriate routing table entry tothe requested APN. The SGSN routes thecontext activation request to the identifiedGGSN, which authenticates the GPRSsubscription against a remote authenticationdial-in user service (RADIUS) server [13].Similar to a regular dynamic hostconfiguration protocol (DHCP) client, theGGSN typically follows with a dynamic PDPaddress allocation request to a dedicatedDHCP server. Static and dynamic allocationof IP addresses can be supported, althoughdynamic allocation is generally preferred bymany GPRS operators. The use of externalIPv4 and IPv6 address space can also besupported. The GGSN provides the allocatedIP address to the SGSN via the create PDPcontext response, which, in turn, sends theactivate PDP context accept message to theMS. Thus, a successful PDP context activationestablishes IP connectivity between the MSand the GGSN, which then forwards orroutes the traffic to external networks.

Common causesof PDP context

activation failureinclude network

link failures,incorrect DNSrecords, and

inappropriateconfiguration

of MSs.

KEYS

Kc ciphering

Ki authentication

23

Common causes of PDP context activationfailure include network link failures,incorrect DNS records, and inappropriateconfiguration of MSs. The latter can lead toauthentication failures on the RADIUS serverand failed access requests to unknown APNs.Counters on the SGSN and GGSN providethe basis for establishing the PDP contextactivation failure rates and the distribution ofthe causes. Like the above Gb interfaceperformance assessment, congestion levelsand dimensioning shortfalls can be identifiedon the Gn interface. In contrast to the FRinterfaces used on the Gb interface, the Gninterface typically uses IP interfaces over 100-Base-Tx or ATM/AAL5 STM-1/OC3 toconnect to the GPRS backbone network. TheIP backbone can be based on differentnetwork architectures, although meshedATM platforms with IP over ATM (IPoA)interworking are typically used forconverged, QoS-aware, high-capacitybackbones. When a PDP context is in activestate, GPRS tunneling protocol (GTP)encapsulation is used to create virtual circuit(VC) connections between GPRS supportnodes (GSNs) supporting the data flowacross the backbone. The GTP VC connectionis released as soon as the context becomesinactive. Various ATM performance countersof the Gn interface are provided by the SGSNand GGSN nodes, which typically includeATM adaptation-layer-related counters asdiscarded AAL5 PDUs, and ATM layer-related counters as cell loss ratio, cell losspriority cells, and others. ATM provides amultitude of congestion control schemes thatshould be part of any ATM network design toavoid congestion and accomplish recovery.

Additionally, operators can introducecommercial network management systems(NMSs) that provide real-time APN-specificmetrics for attach time, attach success rate,and context activation time. Such systems areoften provided with Web interfaces and canhelp to quickly assess system performance.

• CN Quality and Mobility

The RAU success rate forms another keymetric for GPRS system performance.Intelligence about this KPI contributes to theunderstanding of end user throughputperformance. A GPRS-attached MS can issueperiodic, intra-SGSN and inter-SGSN RAUs,depending on its situation. Periodic RAUsare used to ensure that the MS is stillreachable and are thus treated as intra-SGSN

updates. The nonperiodic update requestscan be triggered when the MS transcends arouting area boundary during cellreselection. The intra-SGSN RAU procedureis used when the new routing area of thetarget cell is administered by the same SGSN;the inter-SGSN RAU is used when it is not.

During RAU, the SGSN can commence theLAU procedure with the mobile switchingcenter (MSC)/visitor location register (VLR)(known as a combined RAU) if the optionalGs interface is used by the operator. If the Gsinterface is not deployed, the MS itself isrequired to issue the LAU request messageon the new cell before requesting RAU.Similar to the attach procedure, the LAU mayrequire the MS to respond to IMSI or IMEIchecks as well as authentication andciphering requests. From the time the MSreceives the channel release message afterLAU completion until the MS issues thechannel request for RAU, the MS remains inidle mode to read the system information (SI) 13 on the broadcast channel of the targetcell. Consequently, the MS needs to reacquirea channel to initiate the RAU, which may, inturn, require authentication and ciphering.RAU rejects can result from authenticationand ciphering failures, RAU time-outs due toexcessive delays, lower layer link failures,and/or protocol failures.

Figure 3 summarizes the GPRS KPIs and theelements that affect them.

KPIs for End-to-End System PerformanceAside from the ability to access the GPRSnetwork, one of the outstanding KPIs is theachieved application throughput between the MSand the host. Application-specific throughputperformance is also one of the hardest to quantify,bearing in mind that network counters do notprovide direct quantifications for this KPI. Onepossible way to monitor application throughputis to deploy active performance measurementsystems. Such systems often require thatstationary MSs be installed and client hardwarebe distributed among the routing areas as well asthe server platforms at a central location. Theclient typically has protocol analyzer facilitiesand configurable functions that enableconsecutive attach cycles, context activations,different application protocols, and predefinedtraffic profiles. Performance metrics can be used for GPRS attach, context activation,authentication time, DNS response time, TCPconnection establishment time, and additional

August 2005 • Volume 3, Number 1


Using GPRS-specific

broadcastchannels andalgorithms is

a fundamentaldecision that

many Europeanoperators havenot yet made.

application-specific indicators. Figure 4 providesa possible installation scenario for an activeperformance measurement system.

GPRS NETWORK OPTIMIZATION

GPRS network optimization schemes can varyfrom operator to operator and may be tied to

specific circumstances, although certain practicesand approaches have a more universal character.On the one hand, optimization should beunderstood as an ongoing activity in the contextof network rollout and expansion, and one that isconcerned mainly with an optimal integration ofnew base stations, BSCs, and CN elements. TheRAN, however, more than any other part of theGPRS system, changes constantly as new cells are introduced and existing ones aredecommissioned, relocated, or otherwisemodified. Optimization teams also supportnetwork operations on an ongoing basis. On theother hand, optimization activities may betriggered when certain areas of the network fail tomeet performance targets. Performance reviewsand subsequent optimization can be used totarget specific shortfalls. On the whole, GPRSoptimizers are generally grouped into RAN/BSSand CN teams.

GPRS RAN/BSSThe RAN plays a prominent role in GPRSoptimization. Coverage planning, frequencyplanning, and neighbor cell definitions have anobvious impact on radio conditions andinterference levels, which should be carefullycontrolled. The RAN can aid in understanding CSretainability and handovers of a given cell whenassessing radio conditions. Interference levels candeteriorate receive quality (RxQual), speech qualityindex (SQI), and other CS-related KPIs. Frequencyretunes and neighbor cell corrections oftenimprove high percentages of intra-cell andimperative inter-cell handovers or TCH drops andhandovers due to downlink quality. A “clean”radio environment is a prerequisite for GPRSperformance due to its impact on possibletransmission rates of the air interface (Um) [4, 5, 6, 7]. Adequate channel dimensioningstrategies of base transceiver stations (BTSs) for CS and PS services should ensure minimumcongestion levels.

Supported BSS features and parameteroptimization can further improve systemperformance. Using GPRS-specific broadcastchannels and algorithms is a fundamentaldecision that many European operators have not

BSS

CN

Figure 3. KPI Impact Matrix


yet made. A principal concern is the fact that notall MSs in the operator’s network support packetbroadcast control channels (PBCCHs). Roaming canoften pose a compatibility issue, and a dedicatedUm time slot is required for GPRS signaling.

An important feature that can be provided by PSbroadcasts, however, is network-assisted cellchange, which allows a considerably reducedradio outage time during cell reselection. When inpacket transfer mode, the MS sends packet cellchange notifications to the BSS before enteringthe cell reselection procedure. The network’sresponse contains minimum required systeminformation about the target cell, which allowsthe MS to initiate TBF connections in the targetcell before cell reselection is actually completed.

The PBCCH also enables the operator to useGPRS-specific cell plans. Typically, GPRS cellreselection decisions are controlled by CS idlemode parameters because the MS remains in idlemode during packet idle and packet transfermodes. The MS behaves, therefore, as it does inCS idle mode, even though it is GPRS attached.GPRS-specific cell plans allow the independentoptimization of GPRS cell reselection in packettransfer mode, i.e., reselection is not governed byCS parameter settings. Prominent idle modeparameters include cell reselection offset andhysteresis, penalty timers and penalty offsets, andminimum required receive levels for accessingthe system.

The possibility of prioritizing cells for GPRStraffic is another feature available with GPRS-specific signaling. A behavior similar to CShierarchical cell structure can then be used to“move” traffic from certain cell types to others. Itcan be desirable, for example, to prioritize cells ofindoor schemes, thus minimizing traffic causedby overlapping macro cells.

A general decision is also made in the context ofCS and PS prioritization or preemption.Depending on GPRS priority and preemptionparameters, on-demand PDCHs can be treated as available or blocked for CS connections,regardless of a PDCH’s current usage state.Generally, priority and preemption algorithmsare affected by using dynamic allocation oradapting half-rate channels for CS connections,but the implementation can vary among system vendors. Operators typically allow thepreemption of on-demand PDCHs to allocatenew CS connections when needed. Thisapproach, however, begins to conflict with abaseline performance guarantee for GPRS usersin congested situations.

Where TCH requirements are high and PDCHpreemption causes GPRS blocking (packetimmediate assignment rejects) to be 2 percent orhigher during the CS busy hour, a strategy forallocating FPDCHs can improve accessibilityperformance. FPDCHs may not be preempted, sothe defined number of time slots is guaranteed forGPRS users. Additionally, if GPRS blocking is 2 percent or higher during the GPRS busy hour,the allocation of FPDCHs can be considered.FPDCHs can also increase throughputperformance during blocked and congestedsituations. Software-based simulations and testtrials on the production network can be used toidentify the optimal number of FPDCHs for atarget GPRS congestion time, target throughput,packet size, TCP window size, and MS multislotclass. Target values for immediate assignmentreject rate and TBF setup failure rates aretypically between 1 and 2 percent. It should benoted, however, that the allocation of FPDCHsreduces the number of Um time slots available tothe CSD, introducing the potential for degradingthe CS service. Ensuring an acceptable level of

Cell reselectionsuccess rates andcompletion times,

including theirimpact in terms ofradio outage andIP outage, can be

established byactive drive

testing in the field.

Um

Um

Gn

BTS

Gb

SGSNBTS

IPBackbone

SGSN

GGSN

WAN

NTE

NTE

StoreServer

AMSServer

Server HardwareLogically Connectedto Client Hardware

AMS FixedClient Hardware,

Including MS

AMS FixedClient Hardware,

Including MS

BSS

BSS

Gn

GnGb

Data Center

Figure 4. Sample Active Performance Measurement System


GPRS performance without significantly affectingthe CS service becomes part of the operator’sFPDCH allocation methodology.

Cell reselection success rates and completiontimes, including their impact in terms of radiooutage and IP outage, can be established by activedrive testing in the field. Network counters do notnecessarily provide statistics that can be used toestablish the duration of cell reselection andoutage when in GPRS packet transfer mode.Typically, the number of PDCH requests, TBFrequests, and other counters are provided on aper-cell basis. Identifying cell reselectionperformance on a statistically representative basisis, therefore, challenging; carefully selecting acertain number of sample cells in adequatelocations in different routing areas can help to identify anomalies in the RAN. Themeasurements can be retrieved by drive trialswith test MSs performing bulk downloads usingfile transfer protocol (FTP). Tracing cell reselectionsignaling messages as well as data transferactivities in the source cell and target cell providesthe desired information. Because of the impact ofreselection, usage of cells with small coveragefootprints should be minimized in areas wherehigh levels of mobility are expected, especially formotorway and railway coverage. It should benoted that MS type and software level affect cellreselection times, since these autonomouslytrigger cell reselection. Another element thataffects cell reselection times is the availability ofaccess grant blocks on the common controlchannel (CCCH), which is used by the BSS to issueimmediate assignment messages. SI13 broadcastcycles can vary depending on the BCCHconfiguration. Shorter SI13 broadcast intervalscan, therefore, reduce the amount of idle timeduring cell reselection that the MS uses to read the SI13 of the target cell when a PBCCH is not present.

Delayed TBF release should also be considered.When TBFs are operated in acknowledged mode,uplink and downlink TBFs are released when allRLC blocks have been received and acknowledgedby the MS or the BSS. It may, however, be desirableto retain TBF connections during temporaryinterruptions of data transfer, thus avoidingongoing TBF releases and establishments.Increasing this delay, however, may lead to theinefficient use of radio resources, because theprobability of a TBF remaining established withouttraffic being sent also increases. Typical delayvalues are between 2 and 3 seconds. PDCHutilization can be useful to determine theeffectiveness of the configured release time, i.e.,

the number of allocated PDCHs that carry a TBF provider should be analyzed. Excessively long release times lead to reduced capacityutilization rates.

GPRS CN GPRS CN design and supported features make acritical contribution to overall system performance,especially in the context of processes with an end-to-end character. A clear prerequisite for CNperformance is the analysis of link utilization andcentral processing unit (CPU) load of the involvednetwork interfaces and nodes, including those ofthe IP backbone. CN capacity should be monitoredregularly to accommodate the growth of GPRS anduniversal mobile telecommunications systems(UMTS) subscribers. In multi-radio accesstechnology (multi-RAT) networks, GPRS- andUMTS terrestrial radio access network (UTRAN)-originated traffic can be supported by thesame CN platform, in which case it is imperativethat regular dimensioning audits be performed.Ideally, simple network management protocol(SNMP)-enabled devices should be used to monitorthe IP backbone for packet loss and delay, as well aslink utilization, which can be achieved with SNMP-enabled devices. Multi Router Traffic Grapher(MRTG) [14] and SmokePing [15] are twocommercial tools3 that can help to achieve real-time trending. Typical RTT values in GPRSnetworks vary between 700 and 1200 ms. Packetlosses of 1 to 5 percent are common.

GPRS attach, as well as many other access andchange request operations, can require the MS toperform various security checks. Attachprocedures can be configured with a varyingnumber of security features, including identitycheck, ciphering, and authentication. Thesefeatures can be implemented with differentrequirements (e.g., constant check, selectivecheck, systematic check, and random check) thatregulate when the network issues requests to theMS. Because it is generally desirable to reduce thenumber of signaling handshakes and thus thetime needed to complete GPRS attach, operatorsmust identify the level of security required.

PDP context activation after successful GPRSattach can also introduce substantial delays.Identifying context activation times with protocoltesters on the Gn interface can reveal if a

The Gs interfacecan be useful

to reduce the amount of

signaling betweenthe network andthe MS, becauseit allows location

and paginginformation to be

passed directlybetween theSGSN and the MSC.

____________________________

3 Multi Router Traffic Grapher and SmokePing arefreeware, freely available under the terms of the GNUGeneral Public License.


particular message exchange is causing excessivedelays or failures. RADIUS and DHCP servers,including the available IP address space per APN,should be dimensioned appropriately. TypicalRADIUS setup success rates should be greaterthan or equal to 95 percent, and setup timesshould range between 20 and 150 ms. The CPUload of the servers and the most common failurecauses should also be noted. Operators canchoose to initiate GPRS attach on MS after power-on, thus reducing the user’s perception ofthe time required to establish an IP connection.

One of the basic CN design capabilities is theimplementation of the optional Gs interface. The Gsinterface can be useful to reduce the amount ofsignaling between the network and the MS,because it allows location and paging informationto be passed directly between the SGSN and theMSC. The LAU signaling procedure betweennetwork and MS is not required, consequentlydecreasing RAU times as well as the IP outage time.

The impact of the current SGSN configuration onLAU and RAU completion times can bemeasured by appropriate protocol analyzers onthe Gb interface, as indicated in Figure 1. Theindividual signaling messages can be traced toderive statistically meaningful standard messagetransfer times and anomalies or outliers,accordingly. Conducting such measurements onall available Gb interfaces in the network helps toestablish target times for LAU and RAUcompletion. Due to the different signalingprocedures involved in intra- and inter-RAU,differing RAU success rates are to be expected.Differentiating between intra- and inter-RAUsuccess rates can help isolate possibleperformance issues between neighboring routingareas, without a possible bias due to intra-RAUs.

To assess the IP outage time, active field tests arerequired, since network counters typically do notprovide session-specific performance data. TestMSs engaged in bulk downloads from an FTPserver located on the intranet, including TCP/IPprotocol analyzers and tracers installed on theterminal equipment, can increase understanding ofIP outage times. End-to-end packet loss, TCPretransmission ratios, and delay measurementsprovide additional metrics to characterizethroughput performance. The time required to setup TCP connections is another indicator; thenumber of issued synchronize (SYN), systemacknowledge (SYSACK), and acknowledge (ACK)messages per connection should be close to one.

CONCLUSIONS

GPRS system performance relies onperformance management schemes, which

identify performance shortfalls, and on networkoptimization, which is conducted continuouslyand also when triggered by problems. GPRSperformance from the terminal equipment to thehost is a function of the BSS as well as of the CN;thus, optimization activities should focus on bothareas. C/I and BLER, radio capacity, PCU andGSN dimensioning, RADIUS, DNS and DHCP,mobility, usage of BSS and CN features, andconfiguration all contribute to the end-to-endperformance experienced by the user. Activemeasurement systems can be used to accuratelycapture application throughput and RTTs.Network operators should ensure that globalsystem for mobile communication (GSM) andGPRS networks are balanced in terms ofresourcing performance management andoptimization, acknowledging the increasingimportance of GPRS in the context ofsupplementing IP services over UMTS. �

TRADEMARKS

BlackBerry is a registered trademark of ResearchIn Motion Limited.

REFERENCES

[1] 3GPP, TS 03.64 V8.11.0 (2003–04), TechnicalSpecification Group GSM/EDGE Radio AccessNetwork; General Packet Radio Service (GPRS);Overall Description of the GPRS Radio Interface;Stage 2 (Release 1999).

[2] 3GPP, TS 44.060 V6.10.0 (2004–11), TechnicalSpecification 3rd Generation Partnership Project;Technical Specification Group GSM/EDGE Radio Access Network; General Packet RadioService (GPRS); Mobile Station (MS) – BaseStation System (BSS) Interface; Radio LinkControl/Medium Access Control (RLC/MAC)Protocol (Release 6).

[3] 3GPP, TS 04.60 V8.23.0 (2004–05), TechnicalSpecification Group GSM/EDGE Radio AccessNetwork; General Packet Radio Service (GPRS);Mobile Station (MS) – Base Station System (BSS)Interface; Radio Link Control/Medium AccessControl (RLC/MAC) Protocol (Release 1999).

MESSAGES

ACK acknowledge

SYN synchronize

SYSACK system acknowledge


[4] T. Halonen, J. Romero, and J. Melero, GSM, GPRS and EDGE Performance – Evolution Towards3G/UMTS, John Wiley & Sons, 2nd Edition, 2003.

[5] R. Chakravorty and I. Pratt, “Performance Issueswith GPRS,” Journal of Communications andNetworks (JCN), Vol. 4, No. 2, December 2002, pp. 226–281.

[6] S. Hoff, M. Meyer, and A. Schieder, “APerformance Evaluation of Internet Access via the General Packet Radio Service of GSM,”Vehicular Technology Conference (VTC) ’98, May 1998, Ottawa, ON, Canada.

[7] X. Chen and D. Goodman, “Theoretical Analysisof GPRS Throughput and Delay,” IEEEInternational Conference on Communications(ICC) 2004, June 2004, Paris, France.

[8] D.E. Comer, “Internetworking with TCP/IP –Principles, Protocols, and Architectures,” 4th Edition, Prentice Hall, 2000.

[9] D. Michel and N. Ramasarma, “GPRSMeasurement Methodologies and PerformanceCharacterization for the Railway Environment,”Wireless Communications and NetworkingConference (WCNC) 2005, March 2005, NewOrleans, LA.

[10] G. Heine, “GSM Networks: Protocols,Terminology, and Implementation,” Artech House Publishers, 1999.

[11] 3GPP, TS 04.64 V8.7.0 (2001–12), TechnicalSpecification Group Core Network; DigitalCellular Telecommunications System (Phase 2+);General Packet Radio Service (GPRS); MobileStation – Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) LayerSpecification (Release 1999).

[12] 3GPP, TS 48.016 V6.1.0 (2004–11), TechnicalSpecification Group GSM EDGE Radio AccessNetwork; General Packet Radio Service (GPRS);Base Station System (BSS) – Serving GPRS Support Node (SGSN) Interface; Network Service (Release 6).

[13] 3GPP, TS 07.60 V7.2.0 (2001–03), TechnicalSpecification Group Core Network; General Packet Radio Service (GPRS); Mobile Station (MS) Supporting GPRS (Release 1998).

[14] MRTG (http://mrtg.hdl.com/mrtg.html). [15] SmokePing (http://www.fastmirrors.org/

smokeping/index.en.html).

ADDITIONAL READING

• 3GPP, TS 23.060 V6.3.0 (2003–12), TechnicalSpecification Group Services and System Aspects;General Packet Radio Service (GPRS); ServiceDescription; Stage 2 (Release 6).

BIOGRAPHY

Dirk Michel joined BechtelTelecommunications in 2003.He is currently based in the UKas a senior RF engineer,overseeing the configuration,integration, and optimization of GSM/GPRS cell sites. He isalso responsible for networkperformance analysis andcharacterization of GPRS.

Before joining Bechtel, he worked for AOL Europe(Luxembourg) and Star21 Networks (Germany). WithAOL, he worked for the European Operating Centre,focusing on dial-in and DSL networks. At Star21, heworked as a fault management team leader andnetwork planning and development engineer. Hisresponsibilities included ATM and IP core networkdesign, traffic engineering, and capacity planning. He was also part of the R&D team at Star21Laboratories, responsible for product acceptancetesting, hardware compatibility testing, and the stagingarea for network rollout.

Dirk received a Diplom Geograph degree inGeographical Information Science from the JustusLiebig University, Giessen, Germany. He is a member ofthe Institute of Electrical and Electronics EngineersCommunications Society and the Institution ofElectrical Engineers.

gprs performance management & optimization

Documents