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GSM and WCDMA—Common network approach


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GSM and WCDMA—Common network approach

  1. 1. GSM and WCDMA—Common network approach Ros-Marie Furtenback, Torsten Hunte, Dalibor Turina and Ulrik Wahlberg Many of today’s GSM operators view WCDMA as a technology enhance- Introduction ment that enables them to offer new data services and as a cost-effective extension in capacity for voice service. The introduction of GPRS established the Operators who are rolling out WCDMA and EDGE are advised to pay veracity of data services in mobile environ- extra attention to the end-user experience, because end-users expect the ments and pointed the way toward rich- multimedia services. However, much of the quality of the new bearers to exceed that of GSM. Operators who intro- vision that spawned from GPRS could not duce WCDMA coverage are thus advised to build a WCDMA layer that be realized until greater bandwidth and provides continuous coverage and offers sufficient capacity. They should higher bit transfer rates became available, also build out WCDMA indoor coverage. for example, via WCDMA and upgrades of Initially, those GSM operators who deploy WCDMA will experience sig- GSM with EDGE. nificant gains in capacity. But eventually they will want to use all available To bring multimedia services to market radio spectrum as a common resource. Doing so, however, will require without jeopardizing their business (in traffic-steering mechanisms. Ericsson has already begun introducing terms of revenue and capital expenditure, mechanisms for steering traffic between WCDMA and GSM. Parameters CAPEX) operators need a smooth network that trigger redirection are load, coverage, a specific service, or QoS upgrade. GSM has already been widely de- requirement. ployed and many operators are currently in the process of deploying WCDMA. Their Giving ample consideration for end-user perception and the optimiza- objective, of course, is to derive increased tion of network resources, Ericsson has identified and addressed an streams of revenue from past and present in- enhanced approach to idle-mode distribution. This, in turn, has resulted in vestments. An important factor in this con- four main assertions: (a) the mobile station should camp in the network text—given that the systems will run in par- that can most efficiently provide the requested subscriber service; (b) lack allel for many years—is efficient co-exis- of coverage is the prime justification for employing mechanisms that steer tence. Ericsson has had this in mind while traffic between different radio access technologies; (c) traffic-steering developing its products. mechanisms for balancing load should be designed for the dominant Many of today’s GSM operators view application; and (d) end-users need not be aware of which underlying WCDMA as radio access technology is currently serving them. To steer the traffic of • a technology enhancement that enables ongoing sessions, the performance of the underlying service-continuity them to offer a plethora of new data ser- vices; and mechanism must match the QoS class, and relevant services must be • a cost-effective extension in capacity for available both in WCDMA and GSM. voice service. Initially, GSM operators who deploy WCDMA will experience significant gains in capacity. Eventually, however, as they reach the limits of that capacity they will want to use all available radio spectrum as a common resource. This will require mech- anisms for steering traffic between WCDMA and GSM. One objective of doing so will be to balance load between the two systems. One other objective will be to steer traffic by service—for example, operators might want to steer voice traffic to GSM to free up WCDMA resources for high-bit-rate services and video telephony. BOX A, TERMS AND ABBREVIATIONS Because enhanced services are expected to increase operator revenues, it stands to rea- 3GPP Third-generation Partnership Project MGW Media gateway son that operators are anxious to offer as CAPEX Capital expenditure MMS Multimedia messaging service many services as possible across a broad area CS Circuit-switched MSC Mobile switching center of coverage. The service offerings in EDGE Enhanced data rates for global evo- OPEX Operating expense WCDMA and GSM must thus be aligned lution PS Packet-switched GGSN Gateway GSN QoS Quality of service so that new service roll-outs are not entire- GPRS General packet radio service RAT Radio access technology ly dependent on WCDMA coverage. In- GSM Global system for mobile communi- SGSN Serving GSN stead, new services can be rolled out to take cation SMS Short message service advantage of WCDMA and GSM. Continu- GSN GPRS service node SMSC SMS center GUI Graphical user interface WCDMA Wideband code-division multiple ity mechanisms, which facilitate smooth HLR Home location register access handover between GSM and WCDMA dur- IPMM IP multimedia ing ongoing sessions, will guarantee that 82 Ericsson Review No. 2, 2004
  2. 2. end-users experience a seamless transition of service. Common Same applications Seamless service and devices to end-users Roll-out The end-user experience is very important when operators introduce new mobile access technologies, because end-users expect the quality of the new bearer to exceed (or at GSM/GPRS/EDGE WCDMA least match) existing GSM bearers. There- fore, when introducing WCDMA coverage, operators should build a WCDMA layer that provides continuous coverage and suf- ficient capacity to optimize service and sat- Core network isfy end-user perception (minimum number Figure 1 of handovers, dropped calls, and so on). Complementary technologies enable end- Operators should also build out WCDMA users to use the same applications and indoor coverage to guarantee good user per- Application servers devices seamlessly in GSM and WCDMA ception of new services.1 Likewise, to mini- networks. mize inter-system handover and boost per- ceived service quality, operators should pro- vide coverage along main thoroughfares that run between individual islands of WCDMA coverage. Some operators have chosen to deploy WCDMA; others are upgrading their BOX B, NODE ASPECTS OF A COMMON NETWORK APPROACH GSM/GPRS networks to include EDGE functionality; and some are doing both. Ericsson knows that operators want to run centers (SMSC), are fully compliant with their GSM and WCDMA networks as a sin- GSM and WCDMA. These activities are being carried out in par- gle, common network. Therefore, each of The operator’s network management sys- allel, initially in densely populated areas. Ericsson’s nodes has been developed to tem must also view and enable GSM and Operators are thus providing new data ser- handle GSM and WCDMA access.2 Having a WCDMA networks to serve as a common vices to virtually every customer segment, common core network enables operators to network, giving each application a uniform which will further stimulate data usage. connect different kinds of radio access to graphical user interface (GUI, same look and the same core network nodes. Ericsson’s feel) and greatly facilitating fault manage- After they have covered densely populated home location register (HLR), serving GSN ment and performance management in the areas, deployment will continue in other (SGSN), gateway GSN (GGSN), mobile core network, GSM radio network and areas in response to market demand. switching center (MSC), IP multimedia WCDMA radio network. Implementing EDGE has only a minor (IPMM) and media gateway (MGW) are thus Ericsson provides solutions for operators each fully compatible with GSM and who share sites for GSM and WCDMA radio impact on existing GSM/GPRS networks— WCDMA. Similarly, to guarantee seamless networks to greatly reduce their CAPEX and that is, operators can upgrade their networks service, all of Ericsson’s service layer and operating expenses (OPEX).3 The same is true rapidly and easily with only minor hardware application nodes, such as multimedia mes- for operators who use a common transmis- and software upgrades. Herein lies the saging service (MMS) servers and the SMS sion solution for GSM and WCDMA accesses. Figure 2 Common applications Network optimization. GSM WCDMA CN FM PM SW HW Inven- RAN RAN mgmt mgmt tory Platform (Basic services, security, network explorer) Ericsson Review No. 2, 2004 83
  3. 3. Figure 3 strength of EDGE: it gives operators the Roll-out evolution. ability to win larger market share by launch- ing initial third-generation services early Capacity and with broader national coverage. Opera- tors can thus offer new nationwide services to every user segment. WCDMA Likewise, the introduction of HSDPA— an additional booster of WCDMA network capacity and performance comparable to the WCDMA (main road coverage) introduction of EDGE in GSM/GPRS net- WCDMA (main road coverage) works—constitutes only a minor network Upgrade of existing GSM/GPRS to EDGE upgrade of the installed base. The new func- EDGE (main road coverage) tionality further enhances the user experi- GSM/GPRS (existing nationwide coverage) ence and facilitates the introduction of a broad array of custom-made end-user service packages. Traffic-steering mechanisms Urban Suburban Rural To help operators to realize a common net- work approach for WCDMA and GSM, Capacity Ericsson has begun introducing mecha- nisms for steering traffic between WCDMA and GSM. These mechanisms are needed to support the roll-out of WCDMA and to fa- WCDMA cilitate subsequent optimizations. A com- mon belief in the industry is that traffic- WCDMA (main road coverage) steering mechanisms—triggered either by service or network load—will be used from Upgrade of existing GSM/GPRS to EDGE the outset to shift every kind of traffic be- tween WCDMA and GSM. This tactic is not GSM/GPRS (existing nationwide coverage) viable, however. According to the present 3GPP standard, packet-switched services are subject to long interruptions during changeovers between radio access technolo- gies (RAT). What is more, the 3GPP stan- dard does not sufficiently support RAT changeover for some services. Finally, RAT changeovers degrade end-user perception of Urban Suburban Rural Capacity quality of service (QoS), and if frequent, give rise to excessive network load. With traffic-steering and service- continuity mechanisms, WCDMA and WCDMA including evolution with HSDPA, etc. EDGE networks can be used as a common pool of resources. The ultimate aim, of course, is to optimize the use of these re- sources. A flexible portfolio of traffic-steer- ing mechanisms can give different operator- EDGE specific optimizations. Traffic-steering mechanisms are employed in three main GSM/GPRS (existing nationwide coverage) stages: • to distribute mobile stations in idle mode between GSM and WCDMA (idle-mode distribution); • to redirect calls to accommodate a service request during call or session setup; and • to redirect ongoing calls or sessions. Urban Suburban Rural Parameters that trigger redirection are load, 84 Ericsson Review No. 2, 2004
  4. 4. coverage, a specific service, or required qual- ity of service. Camping on GSM Camping on WCDMA Giving ample consideration for two es- sential drivers—end-user perception and optimization of network resources— Call/session Call/session initiated on GSM initiated on WCDMA Ericsson has identified and addressed an en- hanced approach to idle-mode distribution. This, in turn, has resulted in a number of assertions as follows: • The mobile station should camp in the Change RAT? Change RAT? Yes Yes network that can most efficiently provide the requested subscriber service. No No • Lack of coverage is the prime justification for traffic-steering mechanisms for inter- Call/session Call/session established in GSM established in WCDMA RAT changeover. • Traffic-steering mechanisms for balanc- ing load should be designed for the dom- inant application. Change RAT? Yes Yes Change RAT? • End-users need not be aware of which un- derlying radio access technology is cur- rently serving them. No No Idle-mode camping Call/session in GSM Call/session in WCDMA In areas where WCDMA and GSM cover- age overlap, the cell re-selection procedure directs mobile stations in idle mode to camp Figure 4 on either a GSM or WCDMA cell. A ran- Traffic control between GSM and WCDMA. The illustration describes the complete set of dom distribution of mobile stations in such possibilities for transferring traffic between WCDMA and GSM. Each arrow represents a different technical solution. Note in particular that there are major differences between an area might require the system to change inter-system solutions for circuit-switched calls and packet-switched sessions. radio access technologies when establishing calls or sessions. But without adequate traffic-steering mechanisms in place, the system might be unable to establish service. For example, a subscriber who is camping on GSM and requests a 384kbps packet- Figure 5 switched service (which can only be provid- WCDMA camping and voice handover. ed via WCDMA) cannot be served unless the system supports a redirection mecha- nism to WCDMA. Consequently, given that mechanisms do When out of coverage, When GSM load reaches a the WCDMA voice call is customizable threshold, voice calls not currently exist for steering GSM sub- handed over to GSM can be moved to WCDMA scriber traffic to every WCDMA service or for steering WCDMA subscriber traffic to every GSM service, operators should not use a random idle-mode distribution. Instead, they can configure broadcasted radio para- meters to direct mobile stations to camp ei- WCDMA coverage ther on WCDMA or GSM—that is, by pe- nalizing surrounding cells, they can control the 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-mode mobile stations to camp on WCDMA. GSM coverage When a mobile station requests service, the system can, if necessary, move voice con- Dual-mode UE nections to GSM. This solution gives direct GSM terminal access to WCDMA services. At present, no Ericsson Review No. 2, 2004 85
  5. 5. High-end subscription Subscribers with this service set should be on WCDMA WCDMA GSM Low-end subscription Subscribers with this service set should be Figure 6 on GSM Subscriber differentiation. mechanisms exist for reaching WCDMA work capacity by basing the distribution of services—such as video telephony and high- mobile stations on actual radio quality. bit-rate packet-switched data—from GSM What is more, these two methods may be during call or session setup. combined so that the distribution of mobile Provided the penetration of dual-mode stations in idle mode mobile stations is relatively low, having • minimizes the need for system change them camp on WCDMA will not cause in- during call setup; and efficiencies in the network. • maximizes network capacity. One solution to the problems associated with idle-mode distribution is to restrict lo- Redirection at call setup cation registration. The system can use sub- There are two main reasons for using scription information to determine whether traffic-steering mechanisms during call a subscriber should be restricted to GSM or setup: WCDMA access—for example, by defining • to reach services that are solely available subscriptions that nearly always use services in the other network; and offered in GSM and subscriptions that near- • to balance load. ly always use services in WCDMA. This in- The directed retry from GSM to WCDMA formation could then be used to control for video telephony is one example of a where mobile stations camp. service-reachability mechanism being stan- In the long term, a more advanced method dardized in 3GPP. One other example is of distributing mobile stations in idle mode traffic redirection (GSM to WCDMA) for might predict service from a history of high-bit-rate packet-switched services. usage. Operators can thus maximize net- The steering of packet-switched services 86 Ericsson Review No. 2, 2004
  6. 6. is currently an issue because system change because the most efficient way of serving a is regulated by the 3GPP cell-re-selection mobile station is to provide it with the re- procedure, which interrupts data transfer for quested service via the system on which it intervals of up to 10-15 seconds. Obvious- is camping. Doing so reduces the time it ly, this is unacceptable for some applica- takes to establish service and makes opti- tions. Standardization efforts are underway mum use of network resources. System to introduce handover for packet-switched changeover, by contrast, increases time for services that will minimize these interrup- call setup, which has a negative effect on per- tions. Notwithstanding, the 3GPP must ceived quality of service. Also, the increase still address mechanisms for actively steer- in network signaling increases the load on ing packet-switched services (by load or con- network resources. tent) between WCDMA and GSM. As packet-data volumes grow and account Ericsson is introducing traffic redirection for more traffic, operators might also need for voice at call set-up, a so-called Directed to redirect packet-data traffic to balance net- retry WCDMA to GSM. This mechanism, work loads. which helps operators to manage load dur- ing call setup, is triggered by service or a Subscriber differentiation configurable load threshold. To maximize revenues, some operators may need to differentiate between GSM and Redirection during ongoing calls or WCDMA subscriptions—for example, by sessions adapting the service offering to different Ericsson already supports a basic redirection subscriber groups. This way, operators can mechanism for handing over voice calls. In offer distinct charging packages to users the initial releases, handover from WCDMA who predominately use a given application to GSM is triggered by lack of coverage; or who require a specific quality of service. handover from GSM to WCDMA is trig- Operators can establish basic differentiation gered by load (overload). by offering the subscribed services in either When packet-data traffic begins to dom- GSM or WCDMA. Doing so might also give inate, operators will also need to redirect them greater control over quality of service. traffic from ongoing sessions of packet- Eventually, operators and end-users might based services. also be able to dynamically update sub- scription data according to their current Load balancing needs. Initial parallel deployments of GSM and Besides being employed as a traffic- WCDMA will have WCDMA offloading steering mechanism for controlling the dis- GSM. Eventually, however, unless operators tribution of load, restricted subscription- introduce load-balancing mechanisms, a based location registration can also be used shortage of radio resources will give rise to for differentiating subscribers. a bottleneck. Two prerequisites for balanc- ing load follow: • the service can be offered in WCDMA and GSM with similar quality; and BOX C, EXAMPLE OF POSSIBLE THREE-LEVEL LOAD DISTRIBUTION • mechanisms are in place for steering the service to the other radio access technology. Level 1: Load distribution for dual-mode WCDMA/GSM mobile stations in idle mode can restrict In particular, given that voice service is ex- registration by location (location registration restriction)—for example, if operators need to pected to continue generating considerable restrict certain subscriptions. Operators can base restrictions on subscribed service or history of service usage. The former criterion can be used where traffic-steering mechanisms do not traffic, load-balancing mechanisms will be exist for subscribed services or in cases where the specific quality of service is available in only needed to address this service. Ericsson cur- one of the networks. The latter criterion reduces system changes but requires traffic-steering rently supports efficient load balancing for mechanisms when the requested service is not available in the current network. voice during call setup and during ongoing Level 2: Load distribution (also executed in idle mode) can be regulated via the parameters voice calls. Directed retry is used during call broadcasted for the cell-selection procedure. By adjusting these parameters, operators can setup to offload WCDMA to GSM; likewise, distribute mobile stations between networks, provided the mobile stations can be served handover is used during ongoing calls to equally well by either network. To avoid local overload while still in idle mode, Ericsson recom- offload GSM to WCDMA. However, to ob- mends that operators set the parameters at cell level. tain the most efficient balance of load be- Level 3: Load distribution consists of redirecting traffic during call setup and during ongoing tween WCDMA and GSM radio resources, calls. At this level, Ericsson believes that load will suffice as a trigger for redirecting voice ser- one should tackle the problem while the mo- vices, because once load has been distributed as described in levels 1 and 2, the remaining bile stations are still in idle mode. This is need for distributing load will be minimal thanks to optimized network performance. Ericsson Review No. 2, 2004 87
  7. 7. Continuity of service suspended while the procedures for inter- between WCDMA and RAT changeover are executed. GSM Service fallback Two prerequisites for steering the traffic of For the majority of applications, it might be ongoing sessions follow: acceptable to lower data rates for packet- • the performance of the underlying ser- data applications when changing RAT, for vice-continuity mechanism must match example, when changing from WCDMA to the QoS class; and GSM. However, for video telephony, a bet- • relevant services must be available in ter solution might be to fall back to voice WCDMA and GSM. when changing from WCDMA to GSM. One major mechanism for guaranteeing ser- End-users might accept this solution, be- vice continuity is seamless voice handover cause it is similar to voice/video toggling, in each direction (GSM-to-WCDMA, which is currently being specified in 3GPP. WCDMA-to-GSM). Ericsson’s implemen- tations already provide this mechanism. At present, the services that distinguish Conclusion WCDMA from GSM are video telephony GSM operators with a WCDMA license are and high-bit-rate (384kbps) packet-data not forced to choose between EDGE and service. The introduction of EDGE reduces WCDMA. Instead, the question is how can the gap between the packet data services of they best take advantage of each technolo- GSM and WCDMA by increasing available gy? GSM networks will continue to run for data rates in GSM (yielding 160kbps on many years. Therefore, Ericsson recom- downlink packet-data channels over four mends that operators upgrade their GSM time slots). networks to EDGE, to increase data rates Long interruptions (10-15s) are a serious throughout the entire network. WCDMA drawback to steering packet-switched ser- and EDGE can be rolled out in parallel, ini- vices between WCDMA and GSM. Due to tially with coverage in densely populated limitations in the service-continuity mech- areas. anism, packet-data traffic is temporarily Traffic-steering and service-continuity 88 Ericsson Review No. 2, 2004
  8. 8. mechanisms are needed to guarantee a mize system changeover while subscribers smooth introduction of WCDMA. At the are still in idle mode. same time, to maximize performance, inter- In the long run, as the role of packet data system changes should be kept to a mini- increases, operators may address traffic- mum. Notwithstanding, system change- steering and service-continuity mecha- over is inevitable where coverage is lack- nisms for packet-data services. A significant ing—for example, when users leave a given enabler will be a mechanism for shortening area of coverage. WCDMA coverage must the interruptions to packet-data services thus be as contiguous as possible in the during network changeover. The introduc- planned coverage area. Furthermore, indoor tion of packet-handover techniques, which coverage solutions play an important role, are in the process of being standardized in even if deployed on a small scale. 3GPP, is expected to resolve this issue. In the short run, the preferred strategy for Given that adequate traffic-steering mech- giving users complete access to third- anisms are in place, operators can use broad- generation services is to have them camp on casted radio parameters to randomly dis- WCDMA. By providing efficient traffic- tribute load from subscribers in idle mode. steering mechanisms (including handover) As an alternative, the system might analyze for voice service in the initial WCDMA and end-user histories to predict service re- corresponding GSM releases, Ericsson gives quests and to steer mobile stations in idle operators a tool for balancing load and guar- mode to the network that is most apt to anteeing service continuity. Because voice is serve them. expected to be a dominant application for Ericsson understands the complexity and the foreseeable future, it will suffice to have implications of adopting a common net- a load-balancing solution based on voice. work approach, which is why it actively dri- Looking further down the line, as the pen- ves the standardization of inter-system etration of WCDMA increases, Ericsson be- steering mechanisms in coming 3GPP re- lieves that administrative roaming restric- leases. Ericsson’s aim is to optimize the end- tions can be used to distribute load from sub- user experience and to help operators max- scribers in idle mode. The most efficient imize their revenue through the timely in- method of distributing load seeks to mini- troduction of needed mechanisms. REFERENCES 1. Beijner, H.: “The importance of in-building solutions in 3G networks,” Ericsson Review, Vol. 81(2004):2, pp. 90-97 2. Heickerö, R., Jelvin, S. and Josefsson, B.: “Ericsson seamless network,” Ericsson Review, Vol. 79(2002):2, pp. 76-83 3. Höglund, K. and Ternby, B.: “Co-siting solutions,” Ericsson Review, Vol. 80(2003):2, pp. 72-79 Ericsson Review No. 2, 2004 89