GSM & WCDMA-Common NW Approach

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Introduction

The introduction of GPRS established theveracity of data services in mobile environ-ments and pointed the way toward rich-multimedia services. However, much of thevision that spawned from GPRS could notbe realized until greater bandwidth andhigher bit transfer rates became available,for example, via WCDMA and upgrades ofGSM with EDGE.

To bring multimedia services to marketwithout jeopardizing their business (interms of revenue and capital expenditure,CAPEX) operators need a smooth networkupgrade. GSM has already been widely de-ployed and many operators are currently inthe process of deploying WCDMA. Theirobjective, of course, is to derive increasedstreams of revenue from past and present in-vestments. An important factor in this con-textgiven that the systems will run in par-allel for many yearsis efficient co-exis-tence. Ericsson has had this in mind whiledeveloping its products.

Many of todays GSM operators view

WCDMA as a technology enhancement that enables

them to offer a plethora of new data ser-vices; and

a cost-effective extension in capacity forvoice service.

Initially, GSM operators who deployWCDMA will experience significant gainsin capacity. Eventually, however, as theyreach the limits of that capacity they willwant to use all available radio spectrum as acommon resource. This will require mech-anisms for steering traffic betweenWCDMA and GSM. One objective of doingso will be to balance load between the twosystems. One other objective will be to steertraffic by servicefor example, operatorsmight want to steer voice traffic to GSM tofree up WCDMA resources for high-bit-rateservices and video telephony.

Because enhanced services are expected toincrease operator revenues, it stands to rea-son that operators are anxious to offer asmany services as possible across a broad areaof coverage. The service offerings inWCDMA and GSM must thus be alignedso that new service roll-outs are not entire-

ly dependent on WCDMA coverage. In-stead, new services can be rolled out to takeadvantage of WCDMA and GSM. Continu-ity mechanisms, which facilitate smoothhandover between GSM and WCDMA dur-ing ongoing sessions, will guarantee that

82 Ericsson Review No. 2, 2004

GSM and WCDMACommon network approach

Ros-Marie Furtenback, Torsten Hunte, Dalibor Turina and Ulrik Wahlberg

Many of todays GSM operators view WCDMA as a technology enhance-

ment that enables them to offer new data services and as a cost-effective

extension in capacity for voice service.

Operators who are rolling out WCDMA and EDGE are advised to pay

extra attention to the end-user experience, because end-users expect the

quality of the new bearers to exceed that of GSM. Operators who intro-

duce WCDMA coverage are thus advised to build a WCDMA layer that

provides continuous coverage and offers sufficient capacity. They should

also build out WCDMA indoor coverage.

Initially, those GSM operators who deploy WCDMA will experience sig-

nificant gains in capacity. But eventually they will want to use all available

radio spectrum as a common resource. Doing so, however, will requiretraffic-steering mechanisms. Ericsson has already begun introducing

mechanisms for steering traffic between WCDMA and GSM. Parameters

that trigger redirection are load, coverage, a specific service, or QoS

requirement.

Giving ample consideration for end-user perception and the optimiza-

tion of network resources, Ericsson has identified and addressed an

enhanced approach to idle-mode distribution. This, in turn, has resulted in

four main assertions: (a) the mobile station should camp in the network

that can most efficiently provide the requested subscriber service; (b) lack

of coverage is the prime justification for employing mechanisms that steer

traffic between different radio access technologies; (c) traffic-steering

mechanisms for balancing load should be designed for the dominant

application; and (d) end-users need not be aware of which underlyingradio access technology is currently serving them. To steer the traffic of

ongoing sessions, the performance of the underlying service-continuity

mechanism must match the QoS class, and relevant services must be

available both in WCDMA and GSM.

3GPP Third-generation Partnership Project

CAPEX Capital expenditureCS Circuit-switchedEDGE Enhanced data rates for global evo-

lutionGGSN Gateway GSN

GPRS General packet radio serviceGSM Global system for mobile communi-

cationGSN GPRS service nodeGUI Graphical user interface

HLR Home location registerIPMM IP multimedia

MGW Media gateway

MMS Multimedia messaging serviceMSC Mobile switching centerOPEX Operating expensePS Packet-switchedQoS Quality of service

RAT Radio access technologySGSN Serving GSNSMS Short message serviceSMSC SMS centerWCDMA Wideband code-division multiple

access

BOX A, TERMS AND ABBREVIATIONS


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Ericsson Review No. 2, 2004 83

end-users experience a seamless transition ofservice.

Roll-out

The end-user experience is very importantwhen operators introduce new mobile accesstechnologies, because end-users expect thequality of the new bearer to exceed (or atleast match) existing GSM bearers. There-fore, when introducing WCDMA coverage,operators should build a WCDMA layerthat provides continuous coverage and suf-ficient capacity to optimize service and sat-isfy end-user perception (minimum numberof handovers, dropped calls, and so on).

Operators should also build out WCDMAindoor coverage to guarantee good user per-ception of new services.1 Likewise, to mini-mize inter-system handover and boost per-ceived service quality, operators should pro-vide coverage along main thoroughfares thatrun between individual islands of WCDMAcoverage.

Some operators have chosen to deployWCDMA; others are upgrading theirGSM/GPRS networks to include EDGE

functionality; and some are doing both.These activities are being carried out in par-allel, initially in densely populated areas.Operators are thus providing new data ser-vices to virtually every customer segment,which will further stimulate data usage.After they have covered densely populatedareas, deployment will continue in otherareas in response to market demand.

Implementing EDGE has only a minorimpact on existing GSM/GPRS networksthat is, operators can upgrade their networksrapidly and easily with only minor hardwareand software upgrades. Herein lies the

GSMRAN

WCDMARAN

CN FM PM SWmgmt

HWmgmt

Inven-tory

Common applications

Platform(Basic services, security, network explorer)

Figure 2Network optimization.

Core network

Same applicationsand devices

Seamless serviceto end-users

Common

GSM/GPRS/EDGE WCDMA

Application servers

Figure 1Complementary technologies enable end-users to use the same applications anddevices seamlessly in GSM and WCDMAnetworks.

Ericsson knows that operators want to runtheir GSM and WCDMA networks as a sin-gle, common network. Therefore, each ofEricssons nodes has been developed tohandle GSM and WCDMA access.2 Having a

common core network enables operators toconnect different kinds of radio access tothe same core network nodes. Ericssonshome location register (HLR), serving GSN(SGSN), gateway GSN (GGSN), mobile

switching center (MSC), IP multimedia(IPMM) and media gateway (MGW) are thuseach fully compatible with GSM andWCDMA. Similarly, to guarantee seamless

service, all of Ericssons service layer andapplication nodes, such as multimedia mes-saging service (MMS) servers and the SMS

centers (SMSC), are fully compliant withGSM and WCDMA.

The operators network management sys-tem must also view and enable GSM andWCDMA networks to serve as a common

network, giving each application a uniformgraphical user interface (GUI, same look andfeel) and greatly facilitating fault manage-ment and performance management in thecore network, GSM radio network and

WCDMA radio network.Ericsson provides solutions for operators

who share sites for GSM and WCDMA radionetworks to greatly reduce their CAPEX and

operating expenses (OPEX).3 The same is truefor operators who use a common transmis-sion solution for GSM and WCDMA accesses.

BOX B, NODE ASPECTS OF A COMMON NETWORK APPROACH


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strength of EDGE: it gives operators theability to win larger market share by launch-ing initial third-generation services earlyand with broader national coverage. Opera-tors can thus offer new nationwide servicesto every user segment.

Likewise, the introduction of HSDPAan additional booster of WCDMA networkcapacity and performance comparable to theintroduction of EDGE in GSM/GPRS net-worksconstitutes only a minor networkupgrade of the installed base. The new func-tionality further enhances the user experi-ence and facilitates the introduction of abroad array of custom-made end-user servicepackages.

Traffic-steeringmechanismsTo help operators to realize a common net-work approach for WCDMA and GSM,Ericsson has begun introducing mecha-nisms for steering traffic between WCDMAand GSM. These mechanisms are needed tosupport the roll-out of WCDMA and to fa-

cilitate subsequent optimizations. A com-mon belief in the industry is that traffic-steering mechanismstriggered either byservice or network loadwill be used fromthe outset to shift every kind of traffic be-tween WCDMA and GSM. This tactic is notviable, however. According to the present3GPP standard, packet-switched servicesare subject to long interruptions duringchangeovers between radio access technolo-gies (RAT). What is more, the 3GPP stan-dard does not sufficiently support RATchangeover for some services. Finally, RATchangeovers degrade end-user perception ofquality of service (QoS), and if frequent, giverise to excessive network load.

With traffic-steering and service-continuity mechanisms, WCDMA andEDGE networks can be used as a commonpool of resources. The ultimate aim, ofcourse, is to optimize the use of these re-sources. A flexible portfolio of traffic-steer-ing mechanisms can give different operator-specific optimizations. Traffic-steeringmechanisms are employed in three mainstages: to distribute mobile stations in idle mode

between GSM and WCDMA (idle-modedistribution);

to redirect calls to accommodate a servicerequest during call or session setup; and

to redirect ongoing calls or sessions.Parameters that trigger redirection are load,


Capacity

Urban Suburban Rural

WCDMA

Upgrade of existing GSM/GPRS to EDGEEDGE (main road coverage)

WCDMA (main road coverage)


GSM/GPRS (existing nationwide coverage)

Capacity


WCDMA

Upgrade of existing GSM/GPRS to EDGE



Capacity


WCDMA including evolution with HSDPA, etc.

EDGE


Figure 3Roll-out evolution.


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coverage, a specific service, or required qual-ity of service.

Giving ample consideration for two es-sential driversend-user perception andoptimization of network resourcesEricsson has identified and addressed an en-hanced approach to idle-mode distribution.This, in turn, has resulted in a number ofassertions as follows: The mobile station should camp in the

network that can most efficiently providethe requested subscriber service.

Lack of coverage is the prime justificationfor traffic-steering mechanisms for inter-RAT changeover.

Traffic-steering mechanisms for balanc-ing load should be designed for the dom-inant application.

End-users need not be aware of which un-derlying radio access technology is cur-rently serving them.

Idle-mode camping

In areas where WCDMA and GSM cover-age overlap, the cell re-selection proceduredirects mobile stations in idle mode to camp

on either a GSM or WCDMA cell. A ran-dom distribution of mobile stations in suchan area might require the system to changeradio access technologies when establishingcalls or sessions. But without adequatetraffic-steering mechanisms in place, thesystem might be unable to establish service.For example, a subscriber who is campingon GSM and requests a 384kbps packet-switched service (which can only be provid-ed via WCDMA) cannot be served unlessthe system supports a redirection mecha-nism to WCDMA.

Consequently, given that mechanisms donot currently exist for steering GSM sub-scriber traffic to every WCDMA service orfor steering WCDMA subscriber traffic toevery GSM service, operators should not usea random idle-mode distribution. Instead,they can configure broadcasted radio para-meters to direct mobile stations to camp ei-ther on WCDMA or GSMthat is, by pe-nalizing surrounding cells, they can controlthe outcome of cell re-selection procedures.

Given the limitations of the current stan-dard, and consequently, implementation,Ericsson recommends that operators set the

radio parameters to direct all dual-modemobile stations to camp on WCDMA.When a mobile station requests service, thesystem can, if necessary, move voice con-nections to GSM. This solution gives directaccess to WCDMA services. At present, no

Campingon GSM

Call/sessioninitiated on GSM

Change RAT?Yes Yes

Yes Yes

No

No

No

No

Call/sessionestablished in GSM

Change RAT?

Call/session in GSM

Campingon WCDMA

Call/sessioninitiated on WCDMA

Change RAT?

Call/sessionestablished in WCDMA

Change RAT?

Call/session in WCDMA

Figure 4

Traffic control between GSM and WCDMA. The illustration describes the complete set ofpossibilities for transferring traffic between WCDMA and GSM. Each arrow represents adifferent technical solution. Note in particular that there are major differences betweeninter-system solutions for circuit-switched calls and packet-switched sessions.

When out of coverage,

the WCDMA voice call ishanded over to GSM

Dual-mode UE

WCDMA coverage

GSM coverage

GSM terminal

When GSM load reaches acustomizable threshold, voice calls

can be moved to WCDMA

Figure 5WCDMA camping and voice handover.


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mechanisms exist for reaching WCDMAservicessuch as video telephony and high-bit-rate packet-switched datafrom GSMduring call or session setup.

Provided the penetration of dual-modemobile stations is relatively low, havingthem camp on WCDMA will not cause in-efficiencies in the network.

One solution to the problems associatedwith idle-mode distribution is to restrict lo-cation registration. The system can use sub-scription information to determine whethera subscriber should be restricted to GSM orWCDMA accessfor example, by definingsubscriptions that nearly always use servicesoffered in GSM and subscriptions that near-ly always use services in WCDMA. This in-formation could then be used to controlwhere mobile stations camp.

In the long term, a more advanced methodof distributing mobile stations in idle modemight predict service from a history ofusage. Operators can thus maximize net-

work capacity by basing the distribution ofmobile stations on actual radio quality.What is more, these two methods may becombined so that the distribution of mobilestations in idle mode minimizes the need for system change

during call setup; and maximizes network capacity.

Redirection at call setup

There are two main reasons for usingtraffic-steering mechanisms during callsetup: to reach services that are solely available

in the other network; and to balance load.The directed retry from GSM to WCDMAfor video telephony is one example of aservice-reachability mechanism being stan-dardized in 3GPP. One other example istraffic redirection (GSM to WCDMA) forhigh-bit-rate packet-switched services.

The steering of packet-switched services


Low-end subscriptionSubscribers with thisservice set should beon GSM

High-end subscriptionSubscribers with thisservice set should beon WCDMA

WCDMA

GSM

Figure 6Subscriber differentiation.


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is currently an issue because system changeis regulated by the 3GPP cell-re-selectionprocedure, which interrupts data transfer forintervals of up to 10-15 seconds. Obvious-ly, this is unacceptable for some applica-tions. Standardization efforts are underwayto introduce handover for packet-switchedservices that will minimize these interrup-tions. Notwithstanding, the 3GPP muststill address mechanisms for actively steer-ing packet-switched services (by load or con-tent) between WCDMA and GSM.

Ericsson is introducing traffic redirectionfor voice at call set-up, a so-called Directedretry WCDMA to GSM. This mechanism,which helps operators to manage load dur-ing call setup, is triggered by service or aconfigurable load threshold.

Redirection during ongoing calls or

sessions

Ericsson already supports a basic redirectionmechanism for handing over voice calls. Inthe initial releases, handover from WCDMAto GSM is triggered by lack of coverage;handover from GSM to WCDMA is trig-

gered by load (overload).When packet-data traffic begins to dom-

inate, operators will also need to redirecttraffic from ongoing sessions of packet-based services.

Load balancing

Initial parallel deployments of GSM andWCDMA will have WCDMA offloadingGSM. Eventually, however, unless operatorsintroduce load-balancing mechanisms, ashortage of radio resources will give rise toa bottleneck. Two prerequisites for balanc-ing load follow: the service can be offered in WCDMA and

GSM with similar quality; and mechanisms are in place for steering the

service to the other radio accesstechnology.

In particular, given that voice service is ex-pected to continue generating considerabletraffic, load-balancing mechanisms will beneeded to address this service. Ericsson cur-rently supports efficient load balancing forvoice during call setup and during ongoingvoice calls. Directed retry is used during callsetup to offload WCDMA to GSM; likewise,

handover is used during ongoing calls tooffload GSM to WCDMA. However, to ob-tain the most efficient balance of load be-tween WCDMA and GSM radio resources,one should tackle the problem while the mo-bile stations are still in idle mode. This is

because the most efficient way of serving amobile station is to provide it with the re-quested service via the system on which itis camping. Doing so reduces the time ittakes to establish service and makes opti-mum use of network resources. Systemchangeover, by contrast, increases time forcall setup, which has a negative effect on per-ceived quality of service. Also, the increasein network signaling increases the load onnetwork resources.

As packet-data volumes grow and accountfor more traffic, operators might also needto redirect packet-data traffic to balance net-work loads.

Subscriber differentiation

To maximize revenues, some operators mayneed to differentiate between GSM andWCDMA subscriptionsfor example, byadapting the service offering to differentsubscriber groups. This way, operators canoffer distinct charging packages to userswho predominately use a given applicationor who require a specific quality of service.Operators can establish basic differentiation

by offering the subscribed services in eitherGSM or WCDMA. Doing so might also givethem greater control over quality of service.Eventually, operators and end-users mightalso be able to dynamically update sub-scription data according to their currentneeds.

Besides being employed as a traffic-steering mechanism for controlling the dis-tribution of load, restricted subscription-based location registration can also be usedfor differentiating subscribers.

BOX C, EXAMPLE OF POSSIBLE THREE-LEVEL LOAD DISTRIBUTION

Level 1: Load distribution for dual-mode WCDMA/GSM mobile stations in idle mode can restrictregistration by location (location registration restriction)for example, if operators need torestrict certain subscriptions. Operators can base restrictions on subscribed service or historyof service usage. The former criterion can be used where traffic-steering mechanisms do not

exist for subscribed services or in cases where the specific quality of service is available in onlyone of the networks. The latter criterion reduces system changes but requires traffic-steeringmechanisms when the requested service is not available in the current network.

Level 2: Load distribution (also executed in idle mode) can be regulated via the parameters

broadcasted for the cell-selection procedure. By adjusting these parameters, operators candistribute mobile stations between networks, provided the mobile stations can be served

equally well by either network. To avoid local overload while still in idle mode, Ericsson recom-mends that operators set the parameters at cell level.

Level 3: Load distribution consists of redirecting traffic during call setup and during ongoingcalls. At this level, Ericsson believes that load will suffice as a trigger for redirecting voice ser-vices, because once load has been distributed as described in levels 1 and 2, the remainingneed for distributing load will be minimal thanks to optimized network performance.


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Continuity of servicebetween WCDMA andGSMTwo prerequisites for steering the traffic ofongoing sessions follow: the performance of the underlying ser-

vice-continuity mechanism must matchthe QoS class; and

relevant services must be available inWCDMA and GSM.

One major mechanism for guaranteeing ser-vice continuity is seamless voice handoverin each direction (GSM-to-WCDMA,WCDMA-to-GSM). Ericssons implemen-tations already provide this mechanism.

At present, the services that distinguishWCDMA from GSM are video telephonyand high-bit-rate (384kbps) packet-dataservice. The introduction of EDGE reducesthe gap between the packet data services ofGSM and WCDMA by increasing availabledata rates in GSM (yielding 160kbps ondownlink packet-data channels over fourtime slots).

Long interruptions (10-15s) are a serious

drawback to steering packet-switched ser-vices between WCDMA and GSM. Due tolimitations in the service-continuity mech-anism, packet-data traffic is temporarily

suspended while the procedures for inter-RAT changeover are executed.

Service fallback

For the majority of applications, it might beacceptable to lower data rates for packet-data applications when changing RAT, forexample, when changing from WCDMA toGSM. However, for video telephony, a bet-ter solution might be to fall back to voicewhen changing from WCDMA to GSM.End-users might accept this solution, be-cause it is similar to voice/video toggling,which is currently being specified in 3GPP.

Conclusion

GSM operators with a WCDMA license arenot forced to choose between EDGE andWCDMA. Instead, the question is how canthey best take advantage of each technolo-gy? GSM networks will continue to run formany years. Therefore, Ericsson recom-mends that operators upgrade their GSMnetworks to EDGE, to increase data ratesthroughout the entire network. WCDMA

and EDGE can be rolled out in parallel, ini-tially with coverage in densely populatedareas.

Traffic-steering and service-continuity



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mechanisms are needed to guarantee asmooth introduction of WCDMA. At thesame time, to maximize performance, inter-system changes should be kept to a mini-mum. Notwithstanding, system change-over is inevitable where coverage is lack-ingfor example, when users leave a givenarea of coverage. WCDMA coverage mustthus be as contiguous as possible in theplanned coverage area. Furthermore, indoorcoverage solutions play an important role,even if deployed on a small scale.

In the short run, the preferred strategy forgiving users complete access to third-generation services is to have them camp onWCDMA. By providing efficient traffic-steering mechanisms (including handover)for voice service in the initial WCDMA andcorresponding GSM releases, Ericsson givesoperators a tool for balancing load and guar-anteeing service continuity. Because voice isexpected to be a dominant application forthe foreseeable future, it will suffice to havea load-balancing solution based on voice.

Looking further down the line, as the pen-etration of WCDMA increases, Ericsson be-

lieves that administrative roaming restric-tions can be used to distribute load from sub-scribers in idle mode. The most efficientmethod of distributing load seeks to mini-

mize system changeover while subscribersare still in idle mode.

In the long run, as the role of packet dataincreases, operators may address traffic-steering and service-continuity mecha-nisms for packet-data services. A significantenabler will be a mechanism for shorteningthe interruptions to packet-data servicesduring network changeover. The introduc-tion of packet-handover techniques, whichare in the process of being standardized in3GPP, is expected to resolve this issue.Given that adequate traffic-steering mech-anisms are in place, operators can use broad-casted radio parameters to randomly dis-tribute load from subscribers in idle mode.As an alternative, the system might analyzeend-user histories to predict service re-quests and to steer mobile stations in idlemode to the network that is most apt toserve them.

Ericsson understands the complexity andimplications of adopting a common net-work approach, which is why it actively dri-ves the standardization of inter-systemsteering mechanisms in coming 3GPP re-

leases. Ericssons aim is to optimize the end-user experience and to help operators max-imize their revenue through the timely in-troduction of needed mechanisms.

REFERENCES

1. Beijner, H.: The importance of in-buildingsolutions in 3G networks, EricssonReview, Vol. 81(2004):2, pp. 90-97

2. Heicker, R., Jelvin, S. and Josefsson, B.:Ericsson seamless network, EricssonReview, Vol. 79(2002):2, pp. 76-83

3. Hglund, K. and Ternby, B.: Co-siting

solutions, Ericsson Review, Vol.80(2003):2, pp. 72-79

GSM & WCDMA-Common NW Approach

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