sallent layout 4/8/10 12:57 pm page 22 accepted from …

IEEE Wireless Communications • April 201022 1536-1284/10/$25.00 © 2010 IEEE

Control plane

(Out

802.11radiostack

Data plane

IPlayer

IP router

Access points

WLANaccess

network

AC C E P T E D F R O M OP E N CALL

RAMON FERRUS, ORIOL SALLENT, AND RAMON AGUSTI,UNIVERSITAT POLITECNICA DE CATALUNYA

INTERWORKING IN HETEROGENEOUS WIRELESSNETWORKS: COMPREHENSIVE FRAMEWORK AND

FUTURE TRENDS

INTRODUCTIONAn intrinsic characteristic in current and futurewireless communication scenarios is heterogene-ity, which refers to the coexistence of multipleand diverse wireless networks with their corre-sponding radio access technologies (RATs). Het-erogeneity is directly associated to the fact thatno single RAT is able to optimally cover all thedifferent wireless communications scenarios.Hence, a radio technology optimized to provideoutdoor coverage to high mobility users may failto meet more demanding data rates in lowmobility indoor scenarios and vice versa. Hetero-geneity is also inherent to technological evolu-tion since many new wireless networks aredeployed while supporting legacy infrastructures.

Despite RAT heterogeneity, the servicemodel pursued under next-generation wirelessnetworks is intended to facilitate the deploymentof applications and services independent of theunderlying RAT. Hence, it is expected thatmobile users could eventually enjoy truly seam-less mobility and ubiquitous service access in analways best connected mode, employing themost efficient combination of available accesssystems at any time and anywhere. In this con-

text an appropriate interworking of differentwireless access systems is crucial to meet mobileusers’ expectations while making possible thecoexistence of diverse RATs.

The development of interworking solutionsfor heterogeneous wireless networks has spurreda considerable amount of research in this topic,especially in the context of IEEE 802.11 wirelesslocal area networks (WLANs) and cellular net-work integration. Interworking is linked to manytechnical challenges such as the development ofenhanced network architectures [1, 2], newmechanisms and protocols for seamless hand-over [3], and advanced management functionali-ty for the joint exploitation of heterogeneouswireless networks [4, 5]. Accordingly, interwork-ing aspects are receiving a lot of attention instandardization forums such as the Third Gener-ation Partnership Project (3GPP), 3GPP2, Inter-net Engineering Task Force (IETF), WiMAXForum, and IEEE 802 LAN/MAN Committee,the new IEEE 802.21 standard [6] for media-independent handover (MIH) being a clearexponent of such an effort.

While most published work is focused on par-ticular interworking solutions for specific wire-less technologies, this article establishes acomprehensive framework aimed at categorizingand analyzing interworking solutions. The pro-posed framework is based on the definition of ageneric set of interworking levels along with aclassification of the related key interworkingmechanisms envisioned so far for heterogeneouswireless networks. The elaborated framework isthen used to analyze from a common perspectivesome of the most relevant interworking solutionsproposed for 3GPP, WLAN, and WiMAX net-works. In particular, concerning the integrationof WLAN and cellular networks, two interwork-ing architectures specified by 3GPP, interwork-ing WLAN (I-WLAN) [7] and generic accessnetwork (GAN) [8], are discussed. Next, in thecontext of coexisting mobile broadband accessnetworks, interworking solutions for MobileWiMAX and 3GPP Long Term Evolution/Sys-tem Architecture Evolution (LTE/SAE) net-

ABSTRACTInterworking mechanisms are of prime impor-

tance to achieve ubiquitous access and seamlessmobility in heterogeneous wireless networks. Inthis article we develop a comprehensive frame-work to categorize interworking solutions bydefining a generic set of interworking levels andits related key interworking mechanisms. Theproposed framework is used to analyze some ofthe most relevant interworking solutions beingconsidered in different standardization bodies.More specifically, I-WLAN and GAN approach-es for WLAN and cellular integration, solutionsfor WiMAX and 3GPP LTE/SAE interworking,and the forthcoming IEEE 802.21 standard arediscussed from the common point of view pro-vided by the elaborated framework.

The authors developa comprehensiveframework to categorize interworking solutions by defininga generic set ofinterworking levelsand its related keyinterworking mechanisms.

SALLENT LAYOUT 4/8/10 12:57 PM Page 22

Authorized licensed use limited to: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 at 14:58:00 UTC from IEEE Xplore. Restrictions apply.

IEEE Wireless Communications • April 2010 23

works are analyzed [9–11]. Finally, the MIHsolution elaborated within IEEE 802.21 [6] isaddressed.

The rest of this article is organized as follows.First, we describe a generic interworking sce-nario for heterogeneous wireless networks andbring up some major considerations about net-work architectures and multimode terminals.From such a basis, the proposed interworkingframework is elaborated and the above men-tioned interworking solutions are analyzed. Thefinal section includes our main concludingremarks and discusses future trends.

WIRELESS HETEROGENEOUSINTERWORKING SCENARIO

Figure 1 illustrates a generic interworking sce-nario for two coexisting wireless networks withpartially overlapped coverage. It is assumed thatthe RAT used in each wireless network can bedifferent, and terminals have multimode capabil-ities. Concerning service provisioning, a commonset of services can be offered through both wire-less networks (e.g., voice calls to/from publicswitched telephone networks), but there can alsobe some specific services only available whenconnected to a given wireless network (e.g.,instant messaging and presence services).

Besides, as for network access rights, it isconsidered the most general view where twowireless networks could belong to differentadministrative domains (e.g., networks operatedby different network service providers) but userscan potentially be attached to either network(e.g., proper roaming agreements exist). Noticethat a more particular case would be where thetwo networks form part of the same administra-tive domain (e.g., a mobile operator with GlobalSystem for Mobile Communications [GSM] andUniversal Mobile Telecommunications System[UMTS] networks). In any case, in this articlewe denote as home network the network fromwhich a user has obtained his/her credentials,and as visited network any other network towhich the user can be connected.

In such a context, different interworkingmechanisms would be needed to attend to theset of requirements imposed in terms of ubiqui-tous and seamless service access as well as ofoverall network resource optimization.

WIRELESS NETWORKS CHARACTERISTICSAttending to current architectural trends in next-generation networks [12], wireless networks aremainly devoted to providing network connectivityservices (i.e., bearer services) that may be charac-terized by a given quality of service (QoS) profile.Then, end user service provisioning is supportedby means of specialized service platforms (e.g., IPmultimedia subsystem [IMS]) that become acces-sible to the users via those bearer services.

Accordingly, Fig. 2 illustrates a generic wire-less network architecture in terms of its mainnetwork nodes and protocol layer allocation. Asshown in the figure, the wireless network pro-vides network layer connectivity (e.g., IP connec-tivity) to external networks and service platformsvia some type of network gateway (NG). In addi-

tion, this NG can allocate mechanisms to dynam-ically acquire operator policies related to QoSand accounting, and enforce them on a packet-by-packet basis for each mobile user. On theother side, a RAT-specific radio link protocolstack would be used in the air interface. Thisradio protocol stack can be entirely allocated inbase stations (BSs) or distributed in a hierarchi-cal manner between BSs and some type of radiocontrollers. The radio link protocol stack com-prises physical, medium access control, and radiolink control layers. Through this radio protocolstack, data transfer in the radio interface can bemanaged, attending to each mobile user’s specif-ic needs while simultaneously pursuing an effi-cient usage of radio resources by means ofappropriate radio resource management (RRM)mechanisms. Hence, BSs and NGs constitute thetwo key elements within the data plane functions(i.e., those functions that are executed directlyon the flow of data packets). Additionally, thedata plane between BSs and NGs can also com-prise mobility anchoring functions in charge ofreceiving data destined for a given mobile andredirecting the data (usually through tunneling)to the mobile’s serving BS.

The management of the overall connectivityservice is achieved by a network control plane.Unlike the data plane, the control plane func-tions are those that do not directly operate onthe data flow. This network control plane wouldbe in charge of handling mechanisms such asnetwork access control (e.g., authentication andauthorization), accounting and charging func-tions, mobility management (e.g., location andpaging), security management, and session man-agement. For the sake of brevity, all of the abovementioned control plane mechanisms arereferred to as wireless network control (WNC)mechanisms throughout this article. Thus, as

Figure 1. Generic interworking scenario for heterogeneous wireless networks.

Interworking mechanisms

Specific services

Specific services Common services

Wireless network

A

Wireless network

B

SALLENT LAYOUT 4/8/10 12:57 PM 3


IEEE Wireless Communications • April 201024

shown in Fig. 2, a set of WNC servers, alongwith some network databases (e.g., subscriberprofile databases), are assumed to allocate allthese functions.

Finally, general-purpose packet-switched net-works would constitute the backbone transportnetwork that interconnects the different networknodes.

MULTIMODE TERMINAL CHARACTERISTICSFocusing now on the key terminal characteris-tics, a simplified protocol stack of a multimodeterminal is depicted in Fig. 2. This protocol stackconsists of RAT-specific protocols for the lowerlayers (i.e., physical and link layers) and a com-mon set of protocols for the higher layers (i.e.,network, transport, and application layers). Con-cerning the RAT-specific protocols, they wouldcomprise the correspondent radio link protocolstack to handle data transfer in the air interfacealong with the protocols used for WNC-relatedfunctionality in each wireless network. As to thecommon protocol layers, the network layer (e.g.,IP) has a fundamental role in the interworkingmodel since it provides a uniform substrate overwhich transport (e.g., Transmission Control Pro-tocol [TCP] and User Datagram Protocol[UDP]) and application protocols (e.g., SessionInitiation Protocol [SIP]) can efficiently runindependent of the used access technologies.

In addition to protocol stack considerations,whether the terminal is able to transmit and receivesimultaneously on both radio links (dual-radiooperation) or only on one at a time (single-radiooperation) has important implications on requiredinterworking mechanisms, as discussed later.

INTERWORKING LEVELS

Bearing in mind all the above considerations,several interworking levels can be envisionedwith a different range of interworking require-

ments. The definition of interworking levels canbe conducted attending to, say, network archi-tecture aspects or the level of support for specif-ic service and operational capabilities. In thiswork a definition exclusively based on the levelof service integration among networks is consid-ered because of its independence from underly-ing network technologies and architectures.Hence, in a general case, four interworking lev-els are distinguished.

LEVEL A: VISITED NETWORK SERVICE ACCESSThis level of interworking would allow a userto get access to a set of services available in avisited network while relying on his/her homenetwork credentials. As well, the user could becharged for service usage in the visited net-work through its own home network billing sys-tem. An example could be the case of a cellularsubscriber, equipped with a laptop with bothcellular and WLAN network interfaces, able tolog into a public WLAN hotspot using its cellu-lar smart card credentials and get high-speedInternet access from the hotspot serviceprovider.

LEVEL B: INTERSYSTEM SERVICE ACCESSIn this level, users connected through a visitednetwork would also be able to get access to spe-cific services located in his/her home network.Hence, coming back to the example given inlevel A, the cellular subscriber using acellular/WLAN laptop would also enjoy his/hercellular IMS services while attached to the pub-lic WLAN hotspot. Neither level A nor level Bwould support service continuity when the usermoves between networks.

LEVEL C: INTERSYSTEM SERVICE CONTINUITYThis level extends the previous ones so that theuser is not required to re-establish active ses-sion(s) when moving between networks. How-

Figure 2. Generic architecture for a wireless access network and protocol stack of a multimode terminal.

WNCprotocol

stack

RAT B networkinterface

Radiolink

protocolstack

WNCprotocol

stack

RAT A networkinterface

Radiolink

protocolstack

Com

mon

prot

ocol

sRA

T-sp

ecifi

cpr

otoc

ols

Application layer

Network layer

Transport layer

Basestations/radio

controllers

Controlplane

Networklayer

Wireless access network Multimode terminal

Transportnetwork

Dataplane

Networkgateway

Networkdatabases

External networks, serviceplatforms

WNCprotocol

stackWNC

servers

Radiolink

protocolstack




ever, a temporary QoS degradation can be tol-erated during the transition time. As an exam-ple, if the cellular user equipped with thecellular/WLAN laptop begins to download alarge file when attached to the public WLANhotspot and the user moves so that WLAN cov-erage is lost, the downloading service will con-tinue through the cellular network without userintervention even though a short transfer inter-ruption might be observed during the networkchange.

LEVEL D: INTERSYSTEM SEAMLESSSERVICE CONTINUITY

This level is aimed to satisfy service require-ments also during mobility (i.e., to offer a seam-less mobility experience). Seamless servicecontinuity can be achieved by enabling mobileterminals to conduct seamless handovers acrossdiverse access networks. A seamless handover iscommonly related to the achievement of lowhandover latencies (e.g., less than 300 ms couldbe required for real-time services in intertech-nology handovers) so that this interworking levelis the one that imposes the hardest requirementson the interworking mechanisms. As an example,focusing again on the dual-mode cellular/WLANuser, a voice over IP (VoIP) call establishedwhen attached to the WLAN hotspot should beseamlessly handed over to the cellular network.Finally, it is worth noting here that seamless ser-vice continuity is dependent not only on inter-working mechanisms but also on the consistencyof QoS characteristics provided by involved net-works.

INTERWORKING MECHANISMSAttending to the four interworking levels identi-fied, hereafter we provide a classification ofthose interworking mechanisms that constitutethe basic enablers/building blocks in each level.Figure 3 illustrates the addressed interworkinglevels and mechanisms, and relates them to themain network nodes and protocol layers withinwireless networks previously illustrated in Fig. 2.

LEVEL A: INTERSYSTEM AAAMechanisms included here aim at extendingauthorization, authentication, and accounting(AAA) functions among wireless networks,allowing users to perform authentication andauthorization processes in a visited networkattending to security suites and subscription pro-files provided by their home networks. As well,the deployment of one bill solutions advocatesfor the existence of mechanisms to transferaccounting and charging data between wirelessnetworks. All these functionalities are basicallyachieved by:• Adoption of flexible AAA frameworks able to

support multiple authentication methods (e.g.,the Extensible Authentication Protocol [EAP]defined in IETF RFC 3748 provides supportfor the reliable transport of different authenti-cation protocols).

• Deployment of additional functionality such asAAA proxy/relay functions and related signal-ing interfaces between networks (e.g., theDiameter protocol defined in RFC 3588 pro-vides the minimum requirements for an AAAprotocol between networks).

Figure 3. Interworking levels and related interworking mechanisms.

Wireless network A Wireless network B

WNCprotocolstack A

WNCservers

Radiolink

protocolstackRAT A

Basestations/radiocontrollers

Networklayer

Networknodes(e.g.networkgateway,mobilityanchoringnodes)

WNCprotocolstack B

WNCservers

Radiolink

protocolstackRAT B

Basestations/radiocontrollers

Networklayer

Networknodes(e.g.networkgateway,mobilityanchoringnodes)

Intersystem AAA(flexible AAA frameworks; intersystem

AAA protocols; identity selection support)

Link layer handover optimisation

(inter-RAT configuration information;inter-RAT measurements control and reporting;

inter-RAT resource reservation;inter-RAT resource availability knowledge)

Visited network service access (Level A)

Intersystem user data transfer

Intersystem service access (Level B)

Network layer handover

Intersystem service continuity (Level C)

Network layer handover optimisation

Intersystem seamless service continuity (Level D)




• Enhanced network discovery mechanisms foridentity selection so that mobile terminalscould know in advance whether their homenetwork’s credentials are valid for AAA con-trol in a visited network. Notice that nowa-days, wireless networks do not provide suchinformation over the air interface, and thetypical approach is to have mobile terminalspreconfigured with a list of allowed accessnetworks.

LEVEL B: INTERSYSTEM USER DATA TRANSFERThese mechanisms enable the transfer of userdata between networks in order to give access tospecific services provided in a network otherthan the serving one. A common approach toenforce user data transfer between networksrelies on tunneling protocols such as the Layer 2Tunneling Protocol (L2TP) defined in RFC 2661or the IPsec tunnel mode defined in RFC 2401.Tunnels may be established either directlybetween mobile terminals and remote NGs ormay require additional dedicated network nodes.As an example, 3GPP specifications for WLANaccess to 3GPP packet-switched services [7]mandates the support of the IPsec EncapsulatingSecurity Payload (ESP) protocol described inRFC 4303 for intersystem user data transfer.

LEVEL C: NETWORK LAYER HANDOVERThese mechanisms are required when servicecontinuity between wireless networks relies onthe maintenance of a permanent mobile termi-nal IP address. In this regard, Mobile IP (MIP)described in RFC 3344 was the initial proposedstandard to serve the needs of globally mobileusers who wish to connect to the Internet andmaintain connectivity as they move from onenetwork to another. MIP is a network layermobility solution that covers both handover andlocation management aspects. MIP is based on aredirection approach achieved by a home agent(HA) functionality that maintains a bindingbetween the global IP address assigned to theterminal (i.e., home address [HoA]) and the pro-visional IP address (i.e., care-of address [CoA])allocated temporarily within the serving wirelessnetwork. Within the MIP solution, the mobileterminal itself is in charge of updating theaddress binding of its HA as the CoA changes(i.e., host-based mobility). On such a basis, sev-eral IP mobility protocols have been proposedover the past several years to complement orenhance MIP over IPv4 networks (e.g., reversetunneling in RFC 3024) as well as IPv6 networks(e.g., MIPv6 described in RFC 3775 and Hierar-chical MIPv6 for localized mobility described inRFC 5380). More recently, network-based IPmobility solutions where the terminal is notdirectly involved in managing IP mobility (e.g.,Proxy MIPv6 defined in RFC 5213) are alsobeing introduced in wireless networks [13]. Asan example, the 3GPP LTE/SAE network allo-cates HA functionality for MIP-based mobilityanchoring (either host-based or network-based)between LTE/SAE and other non-3GPP accessnetworks [10]. It is important to mention herethat, when focusing on service continuity of com-mon services (i.e., those available in both homeand visited networks as illustrated in Fig. 1), ser-

vice continuity can also be provided by the ser-vice itself (e.g., application-based mobility rely-ing on SIP as considered in [14]) without theneed for network layer handover solutions.

LEVEL D: NETWORK LAYERHANDOVER OPTIMIZATION

While simple network or application layer hand-over solutions may suffice for intersystem servicecontinuity, they may not be able to satisfy therequirements for seamless mobility. In particular,during a handover, latencies related to radio linklayers (e.g., new radio link establishment) andnetwork layer operation (e.g., movement detec-tion, new IP address configuration, and bindingupdates) can turn into a period during which theterminal is unable to send or receive packets. Inthis respect, several optimization mechanismshave been proposed to reduce the handoverlatency due to network layer operations. As anexample, RFC 4068 defines fast handover exten-sions for MIPv6 so that terminals can acquire anew valid CoA before a handover occurs, andwhere tunneling between the old and new CoAsreduces the binding update latency. Besides,when handover takes place between networks indifferent administrative domains, pre-authentica-tion schemes such as the one proposed in [15]can help reduce non-negligible authenticationand authorization delays. Also, context transfermechanisms (e.g., Context Transfer Protocol[CXTP] defined in RFC 4067) can be used topreconfigure different network (and also link)layer parameters in the target network and soavoid re-initiation of some signaling to and fromthe terminal. Finally, network discovery mecha-nisms (see RFC 5113 for a detailed discussion)also have a crucial role in handover optimizationsince they can convey specific information need-ed for optimized mobility.

LEVEL D: LINK LAYER HANDOVER OPTIMIZATION

Radio link layer operations could also intro-duce delays in the handover process. Hence, set-ting up the new radio link could take severalsteps (e.g., scanning, authentication, and associa-tion in 802.11) that handover optimization mech-anisms should have to either bypass or minimizethe latency of while connected to the previouslink, especially for the single-radio operationcase. At the same time, handover optimizationmechanisms can allow for more efficient radioresource usage (e.g., network-controlled inter-RAT mobility). Hence, among the main aspectscovered by such mechanisms we have:• Provision of inter-RAT configuration informa-

tion about neighboring BSs to enhance theradio scanning process and guide networkselection decisions.

• Inter-RAT measurements control and report-ing to improve handover initiation.

• Inter-RAT resource reservation. This couldimply the preconfiguration of some radio linkcontexts (e.g., QoS contexts) in the targetRAT.

• Inter-RAT resource availability knowledge(e.g., check or reporting procedures) in orderto enhance handover decisions.

When focusing onservice continuity ofcommon services,service continuity canalso be provided bythe service itself(e.g., application-based mobility relying on SIP asconsidered in [14])without the need fornetwork layer han-dover solutions.




As an example, inter-RAT handover mecha-nisms specified between 3GPP GSM and UMTSnetworks cover most of the aforementionedaspects.

DISCUSSION ONRELEVANT INTERWORKING APPROACHES

According to previous interworking levels andrelated mechanisms, in this section we analyzesome of the most relevant proposed solutions.

I-WLANI-WLAN architecture [7], commonly referred toas loose coupling interworking, is targeted tocover interworking levels A and B for packetservices (i.e., General Packet Radio Service[GPRS]). Following the basic architecture repre-sentation for a generic wireless network intro-duced in Fig. 2, Fig. 4 illustrates both the mainnetwork elements within 3GPP UMTS andWLAN networks and the new main networkfunctions (marked within circles) and interfaces(highlighted in italics) added for I-WLAN inter-working support.

As to the 3GPP UMTS network, networkgateway functions are provided by the gatewayGPRS support node (GGSN); WNC functionssuch as mobility management (MM) and sessionmanagement (SM) are mainly handled withinthe control plane of the serving GSN (SGSN)and with the support of a home subscriber server(HSS) database; and UMTS radio link protocolstack and related functions (e.g., RRM) are dis-tributed among NodeBs (i.e., naming conventionfor UMTS BSs) and radio network controllers(RNC). As for the WLAN access network, theIEEE 802.11 protocol stack (e.g., MAC and

physical layer) is entirely handled by accesspoints (APs); WNC functions such as access con-trol can be supported by means of AAA serversusing IETF protocols (e.g., RADIUS protocolbetween APs and AAA servers for EAP/IEEE802.1X access control); and generic routers canprovide NG functions to external networks orservice platforms.

Thus, interworking level A is achieved by theallocation of a 3GPP AAA server in the 3GPPnetwork and AAA proxy functions within theWLAN network. The interface between them isbased on the Diameter protocol, which can beused for the transfer of, say, EAP messages forauthentication and authorization. In this respect,specific EAP extensions for 802.1X access controlhave been defined to allow authentication basedon UMTS credentials (i.e., EAP-AKA defined inRFC 4187). As well, the list of available inter-working UMTS networks can be provided throughthe WLAN connection by means of the networkdiscovery mechanism defined in RFC 4284.

Regarding level B, IPsec ESP is used to pro-vide secure tunnels between terminals connectedto the WLAN and a new node named packetdata gateway (PDG) within the 3GPP network.The PDG basically behaves as a GGSN forWLAN users.

From the terminal side, the I-WLAN modelbasically requires a conventional 3G networkinterface, a 802.11 network interface supportingEAP/802.1X authentication (e.g., Wireless Pro-tected Access [WPA] certification) and supportfor IPsec within the IP protocol stack.

GENERIC ACCESS NETWORKThe GAN [8], also referred to as tight couplinginterworking, provides a solution to extend cellu-lar circuit and packet services over IP broadband

Figure 4. I-WLAN interworking model.

Level B

IPsecIP layer

GGSNPDG

3GPP AAAserver

HSS

WNC servers(SGSN control plane)

IPlayer

802.11radiostack

EAP overRADIUS/diameter

Diameter-based

interface

IPsec-basedinterface

WNC servers(e.g. AAA

server/proxy)

Transport

Level B

Level A Dual-radio operation

IPsec

802.

11 in

terf

ace

3G interfaceEAP-

AKA

EAP-AKA

SM/MM

SM/MM

3GPPradiostack

RNC andNodeBs

UMTSnetwork

802.11radiostack Terminal

equippedwith 802.11and 3Gnetworkinterfaces

3GPPradiostack

IP layer

Application

Data plane

WLANaccess

network

IProuter

Accesspoints

Controlplane

Controlplane

Dataplane



IPlayer

Regarding Level B,IPsec ESP is used to

provide secure tunnels between

terminals connectedto the WLAN and a

new node namedPacket Data

Gateway (PDG) within the 3GPP

network. The PDGbasically behaves

as a GGSN for WLAN users.




access networks. Hence, the GAN model is notrestricted to 802.11 access networks, yet deploy-ing GAN over 802.11 is the most commonapproach. Moreover, dual-mode cellular/802.11terminals compliant with GAN specifications aremore widely known as unlicensed mobile access(UMA)-enabled terminals.

The GAN model provides interworking levelsB, C, and D. Figure 5 illustrates the main aspectsof the GAN interworking approach within thesame UMTS/WLAN scenario previouslydescribed for I-WLAN. The three key elementsof the GAN model, as shown in Fig. 5, are a newnetwork element named GAN controller(GANC) located within the 3GPP network, aUMA-enabled terminal, and a new interfacebetween both elements specified by 3GPP.

The GANC serves much like an RNC, reusinglegacy 3GPP interfaces towards the core net-work. Regarding the UMA-enabled terminal, itis a 3GPP terminal with embedded 802.11 com-munications. Hence, the interface definedbetween GANC and UMA-enabled terminalscomprises new protocols with functions similarto the UMTS radio resource control (RRC) pro-tocol (e.g., Generic Access RRC [GA-RRC])along with legacy 3GPP control plane protocols(e.g., SM and MM). The transfer of all the infor-mation between the terminal and the GANCthrough the WLAN network uses IPsec tunnel-ing mechanisms.

On this basis, interworking level B is provid-ed by the fact that GANC actually constitutes agateway toward 3GPP services. Also, level C isbuilt up on legacy 3GPP handover procedures sothat established data sessions or calls could be

transferred between the GANC, acting as anRNC from the network side, and the corre-sponding RNCs within the UMTS network with-out service disconnection. Concerning level D,handover latencies similar to those betweenUMTS cells can be achieved by the fact thatUMA-enabled terminals support dual-radiooperation. Hence, a UMA-enabled terminal canconnect to the GANC via a WLAN while main-taining the UMTS network connection so thatthe aforementioned interworking mechanismsfor network or link layer handover optimizationare not necessary. On the contrary, level A is outof the scope of the GAN specifications since theGAN model only requires IP connectivity fromthe WLAN access network and does not dealwith 802.11-specific issues such as access control.

WIMAX AND 3GPP NETWORKS INTERWORKINGThe initial interworking solution considered withinthe WiMAX Forum and 3GPP is based on the I-WLAN architecture by 3GPP. Hence, such a solu-tion is already available for the first release of theWiMAX architecture (WiMAX NWG Release 1[9]), and covers interworking levels A and B.Then, in the context of 3GPP LTE/SAE networks,architecture enhancements are being specified forproviding IP connectivity using non-3GPP accesses[10]. In particular, support for MIP and PMIP-based mobility is being introduced to achieve levelC. As well, functionality intended to provide net-work discovery and selection assistance data isbeing specified (i.e., access network discovery andselection function [ANDSF]).

Besides, optimized interworking solutionsbetween WiMAX networks and 3GPP LTE/SAE

Figure 5. GAN interworking model.

3GPPradiostack

RNC andNodeBs

HSS

GGSN

GANC

IPstack

GA-RPCSMMM

IPlayerData plane

WNC servers (SGSN control plane)

UMTS network Control

plane

Data plane

External networks, service platforms

UMA-enabled terminal

IP stack GA-RRC

3GPPSM/MM

Levels B,C

Levels B,C

Dual-radio operation

Control plane

3GPP standardised

interface

AAAprotocols

(Out of scope of GAN)

WNC servers (e.g., AAA server) 802.11

radiostack

Data plane

IPlayer

IP router

Access points

GA

N in

terf

ace

802.11 stack

3GPPradiostack

Transport IP layer

Application

Level D

WLANaccess

network

The initial interwork-ing solution consid-ered within WiMAXForum and 3GPP isbased on the I-WLANarchitecture by 3GPP.Hence, such a solution is alreadyavailable for the firstrelease of theWiMAX architecture(WiMAX NWGRelease 1) and covers interworkingLevels A and B.




networks are also under consideration to achieveinterworking level D [11]. In this regard, anapproach targeted to cover interworking levels A,B, C, and D is illustrated in Fig. 6 and discussednext for a 3GPP LTE/SAE network coexistingwith a WiMAX access service network (ASN). Asshown in Fig. 6, regarding the 3GPP LTE/SAEnetwork, radio link layer protocols and functionsare now entirely allocated in an enhanced NodeB(eNB), and an evolved packet core (EPC) han-dles WNC functions via so-called MME serversand provides NG functionalities through PDNgateways (PDN-GWs). As for the WiMAX ASN,the IEEE 802.16e radio protocol stack is alsoentirely allocated to BSs, and a separate networknode named ASN gateway (ASN-GW) could holdboth data plane anchoring functionalities (e.g., formobility anchoring within WiMAX ASN) andcontrol plane WNC functions (e.g., authenticationbased on an EAP framework).

According to this interworking solution, inter-working level A is achieved as in I-WLAN solu-tion by means of AAA proxy functions in theASN-GW, a 3GPP AAA server located in the3GPP LTE/SAE network, and the use of specificEAP authentication mechanisms. Then levels Band C are built up on MIP-based mobility solu-tions already supported within the WiMAX ASN(e.g., the ASN-GW data plane can allocate for-eign agent [FA] functions for MIPv4 or accessrouter [AR] functions for MIPv6) and on addingdata plane anchoring functions within the 3GPPLTE/SAE network (e.g., HA functions forMIPv4 or MIPv6 are allocated in the PDN-GW).The MIP client can be located in the terminals,as shown in Fig. 6, or in the ASN-GW if PMIPsolutions are used instead between the ASN-GWdata plane and the 3GPP PDN-GW.

Concerning interworking level D, intersystemhandover optimization mechanisms are dis-cussed in [11] to provide seamless mobility forsingle-radio operation terminals, a conditionthat imposes the hardest requirements on theinterworking solution. In this respect, as illus-trated in Fig. 6, both WiMAX and 3GPPLTE/SAE radio link protocol stacks would needto be modified to add support for inter-RATmeasurement control and reporting along withthe delivery of inter-RAT BS information.Besides, support for resource reservation, aswell as pre-registration (i.e., covering aspectssuch as access control, context transfers anddefault bearer service establishment), is envis-aged by means of tunneling legacy signalingmessages between terminals and BSs or WNCservers in the target network while beingattached to the serving network. Hence, asexample, a terminal connected to an eNB caninitiate a handover to WiMAX by tunneling a802.16e Handover Request message throughthe 3GPP LTE/SAE network towards the targetWiMAX BS. Transparent transfer of intersystemhandover signaling messages requires each radioprotocol stack to provide transport functionalityfor signaling messages of a different technology.Moreover, transparent signaling transferbetween networks requires the deployment of aninterface among MME and ASN-GW ControlPlane as shown in Fig. 6 (e.g., S101 interface in[11]) for intersystem handover signaling. Finally,it is worth noting that for dual-radio operation,previous mechanisms would not be necessary.Nevertheless, dual-radio operation may not befeasible due to e.g., radio frequency coexistenceissues when radio frequencies used in the twoRATs are close to each other.

Figure 6. WiMAX and 3GPP LTE/SAE interworking model.

SMMM

3GPPradiostack

3GPP cells measurements

Level D

IP layer

PDN-GW

EAP- AKA

HA fornon-3GPP

IP layer Transport

Application

Level D

Level A

MIP Dua

l-mod

ene

twor

k in

terf

ace

Dual-mode WiMAX/3GPP LTE terminal

EAPLevel B,C

Levels B,C

Level A IP

layer

EAP overdiameter

802.16radiostack

802.16 radio stack

ASN-GW data plane

Intersystem handover signaling interface

Single-radio operation

Diameter- based

interface

3GPP AAA server

HSS

eNB

MIP-based interface

WNC servers (MME)

WNC servers (ASN-GW

control plane)

WiMAX ASN Control

plane

802.16 BS

Radio stack modifications to account for inter-RAT issues Transfer capabilities for intersystem handover signalling

3GPP neighbouring BS information

Controlplane

DataplaneData plane

3GPP LTE/SAE network



3GPPradiostack

SM/MM

Level D

It is worth notingthat for dual-radio

operation, previousmechanisms would

not be necessary.Nevertheless,

dual-radio operationmay not be feasible

due to, say, radio frequency

coexistence issueswhen radio

frequencies used inthe two RATs are

close to each other.




IEEE 802.21 INTERWORKING

The standard IEEE 802.21 “Media-IndependentHandover (MIH) Services” [6] is mainly con-ceived as an enabler of interworking level Dbetween heterogeneous link-layer technologies(e.g., IEEE 802 and cellular networks) andacross IP subnet boundaries. The standarddefines a new link layer functionality (i.e., MIHFunction [MIHF]) to be added within terminalsand networks together with the protocol requiredfor message exchanging between them. ThisMIHF entity is allowed to have some control onlink layer behavior (e.g., link actions such aspower-up and link configuration) and can collectlink information (e.g., link status polling orevent-triggered information). To that end, radiolink layers have to add support for a media spe-cific interface with the MIHF function. On theother hand, the MIHF entity provides a mediaindependent interface to upper layers of the pro-tocol stack (denoted as MIH users in 802.21standard’s notation) and offers a set of genericservices classified as event, command and infor-mation services [3]. Through these services,upper layers can control lower layer’s behavior(e.g., remote/local link actions and handovercommands) and acquire relevant information formore efficient handover decisions (e.g.,local/remote link status and information aboutdifferent available networks and their services).The 802.21 service model offers a flexible frame-work to facilitate different handover approaches.Hence, while served by a given wireless network,the MIHF entity of the mobile terminal couldinteract with a MIHF entity in the serving net-work in order to retrieve inter-RAT networkinformation and initiate an inter-technology han-dover by indicating a preferred list of candidateaccess networks. As well, handover could also beinitiated from the network side. In both cases,802.21 signaling enables intersystem radioresource availability check and resource prepara-tion (e.g., intersystem resource reservation)between involved networks and provides the ter-minal with the required configuration of thereserved resources at the target network. It’sworth noting that the scope of 802.21 is limitedto handover initiation and preparation phases,while the execution phase is not covered (e.g.,mobility handling in upper layers is still requiredfor service continuity between networks).

The adoption of the 802.21 solution in aWiMAX-3GPP LTE/SAE interworking scenariosuch as the one described in Fig. 6, wouldrequire the allocation of functional MIHF enti-ties in the terminal and within both wireless net-works along with the correspondent transportcapabilities to exchange MIH protocol messages.In this sense, MIH signaling to/from terminalscould be transferred by using specific mecha-nisms introduced in the radio link layers (IEEE802.16g extension adds such a support but 3GPPdoes not consider it yet). Another possibilitywould be the transfer of MIH signaling op top ofthe IP bearer service provided by the network(e.g., IETF MIPSHOP working group is specify-ing support for sending 802.21 messages over IPnetworks).

CONCLUDING REMARKS ANDFUTURE TRENDS

This article has elaborated a comprehensiveframework to analyze and categorize interwork-ing solutions in heterogeneous wireless net-works. Over such a basis, some of the mostrelevant interworking solutions being consideredin different standardization bodies have beendiscussed.

As to WLAN and cellular interworking solu-tions, I-WLAN has been noted to constitute asimple but versatile solution to extend cellularpacket services access. I-WLAN requires littlechanges in terminals and networks, though it isnot able to provide seamless mobility and net-work-controlled mobility. Currently, I-WLANinterworking architecture is being extended tohandle mobility between I-WLAN and 2G/3G bymeans of adding support for MIP-based mobili-ty, thus converging to the type of solutions beingconsidered within LTE/SAE for interworkingwith non-3GPP access networks. On the otherhand, 3GPP GAN solution requiring dual-radiooperation UMA-enabled terminals was firstlydeveloped for 2G, then extended to 3G but noextension is envisioned so far within LTE/SAE,where PDN-GW intersystem mobility anchoringwith MIP and PMIP-based solutions are favored.Nevertheless, UMA-terminals are a reality todayas some operators are embracing such a technol-ogy both in business and in the home 802.11/cel-lular access.

Concerning possible solutions to achieve opti-mized seamless handover with single-radio ter-minals between LTE/SAE and WiMAX, twomain approaches have been discussed. While3GPP envisions a tailored solution, IEEE 802.21efforts are trying to push for a generic solutionfor inter-technology handover. Nevertheless, theadoption of a generic solution such as IEEE802.21 in 3GPP networks is not as straightfor-ward as within IEEE-based technologies, whichalready share a common architectural frame-work. In this regard, the 802.21 standard isbecoming a central piece of an IEEE 802 wideinitiative so that, in addition to defining the new802.21 standard itself, IEEE is making changesto existing access technologies specifications(e.g., WLAN, WiMAX) to support 802.21 relat-ed handover functionality. Moreover, IEEE802.21 solution is expected to constitute a cor-nerstone within IEEE 802’s proposal for IMT-Advanced [3], where enhanced IEEE 802.16 andIEEE 802.11 radio technologies will be integrat-ed under the interworking model offered by802.21.

ACKNOWLEDGMENTSThis work has been partly funded by the Span-ish Ministry of Science and Innovation underthe TelMAX project, belonging to the Ingenio2010 program. This work has also been partlysupporrted by the Spanish Research Counciland FEDER funds under a COGNOS grant(ref. TEC2007-60985. The authors are verythankful to the reviewers for their constructivecomments.

IEEE 802.21 solution is expectedto constitute a cor-nerstone within IEEE802’s proposal forIMT-Advanced, whereenhanced IEEE802.16 and IEEE802.11 radio technologies will beintegrated under theinterworking modeloffered by 802.21.




REFERENCES[1] A.K. Salkintzis, C. Fors, and R. Pazhyannur, “WLAN-GPRS

Integration for Next-Generation Mobile Data Networks,”IEEE Wireless Commun., vol. 9, no. 5, Oct. 2002.

[2] A.K. Salkintzis, “Interworking Techniques and Architecturesfor WLAN/3G Integration toward 4G Mobile Data Net-works,” IEEE Wireless Commun., vol. 11, no. 3, June 2004.

[3] L Eastwood et al., “Mobility using IEEE 802.21 in a Het-erogeneous IEEE 802.16/802.11-based, IMT-Advanced(4G) Network,” IEEE Wireless Commun., vol. 15, no. 2,Apr. 2008.

[4] C. Skianis, G. Kormentzas, and K. Kontovasillis, “Interac-tions between Intelligent Multimodal Terminals and aNetwork Management System in a B3G Context,” Wire-less Commun. Mobile Comp., vol. 5 , no. 6, Sept. 2005.

[5] C. Skianis et al., “Efficiency Study of the InformationFlow Mechanism Enabling Interworking of Heteroge-neous Wireless Systems,” J. Sys. Software, vol. 80, no.10, Oct. 2007.

[6] IEEE Std 802.21, Draft D14.0, “IEEE Standard for Localand Metropolitan Area Networks: Media IndependentHandover Services,” Sept. 2008.

[7] 3GPP TS 23.234 v. 7.7.0, “3GPP System to WirelessLocal Area Network (WLAN) Interworking; SystemDescription (Release 7),” June 2008.

[8] 3GPP TS 43.318 v. 8.3.0, “Radio Access Network; GenericAccess Network; Stage 2 (Release 7),” Aug. 2008.

[9] WiMAX Network Forum Architecture, “Stage 2, 3GPP-WiMAX Interworking,” Release 1, v. 1.2, Jan. 2008.

[10] 3GPP TS 23.402 v. 8.2.0, “Architecture Enhancementsfor Non-3GPP Accesses (Release 8),” June 2008.

[11]. 3GPP TR 36.938 v. 8.0.0, “Improved Network Con-trolled Mobility between E-UTRAN and 3GPP2/MobileWiMAX Radio Technologies,” Mar. 2008.

[12] R. Agrawal and A. Bedekar, “Network Architectures for4G: Cost Considerations,” IEEE Commun. Mag., vol. 45,no. 12, Dec. 2007, pp. 76–81.

[13] K.-S. Kong et al., “Mobility Management for All-IP MobileNetworks: Mobile IPv6 vs. Proxy Mobile IPv6,” IEEE WirelessCommun., vol. 15, no. 2, Apr. 2008, pp. 36–45.

[14] S. Salsano et al., “SIP-based Mobility Management inNext Generation Networks,” IEEE Wireless Commun.,vol. 15, no. 2, Apr. 2008, pp. 92–99.

[15] A. Dutta et al., “Media-independent Pre-authenticationSupporting Secure Interdomain Handover Optimiza-tion,” IEEE Wireless Commun., vol. 15, no. 2, Apr.2008, pp. 55–64.

BIOGRAPHIESRAMON FERRUS ([email protected]) is an associate professor atUniversitat Politècnica de Catalunya (UPC), Spain, since 2002.His actual research interest is focused on architectures, QoS,mobility, and resource management in the context of hetero-geneous IP-based mobile communications systems. He hasparticipated in several research and technology transfer pro-jects. He is co-author of more than 50 papers published ininternational journals/magazines and conference proceedings.

ORIOL SALLENT ([email protected]) is an associate profes-sor at UPC. His research interests are in the field of radioresource and spectrum management for heterogeneouscognitive wireless networks, where he has published 100+papers in IEEE journals and conferences. He has participat-ed in many research projects and consultancies funded byeither public organizations or private companies.

RAMON AGUSTI ([email protected]) has been a full profes-sor at UPC since 1987. For the last 20 years he has mainlybeen interested in mobile communications topics and haspublished about 200 papers in these areas. He has alsoparticipated in the COST and many European research pro-grams, and managed many private and public funded pro-jects. His actual research interests include radio networks,cognitive radio, radio resource management, and QoS.



sallent layout 4/8/10 12:57 pm page 22 accepted from …

Documents