Interworking in Heterogeneous Wireless Networks: Comprehensive Framework and Future Trends

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ACCEPTED FROM OPEN CALL

INTERWORKING IN HETEROGENEOUS WIRELESS NETWORKS: COMPREHENSIVE FRAMEWORK AND FUTURE TRENDS

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

IP ABSTRACT text an appropriate interworking of different router wireless access systems is crucial to meet mobile Interworking mechanisms are of prime impor- users’ expectations while making possible the IP tance to achieve ubiquitous access and seamless coexistence of diverse RATs. layer mobility in heterogeneous wireless networks. In The development of interworking solutions this article we develop a comprehensive frame- for heterogeneous wireless networks has spurred (Out work to categorize interworking solutions by a considerable amount of research in this topic, WLAN defining a generic set of interworking levels and especially in the context of IEEE 802.11 wireless access Control network plane its related key interworking mechanisms. The local area networks (WLANs) and cellular net- proposed framework is used to analyze some of work integration. Interworking is linked to many Data plane the most relevant interworking solutions being technical challenges such as the development of considered in different standardization bodies. enhanced network architectures [1, 2], new 802.11 More specifically, I-WLAN and GAN approach- mechanisms and protocols for seamless hand- radio stack es for WLAN and cellular integration, solutions over [3], and advanced management functionali- for WiMAX and 3GPP LTE/SAE interworking, ty for the joint exploitation of heterogeneous Access and the forthcoming IEEE 802.21 standard are wireless networks [4, 5]. Accordingly, interwork- points discussed from the common point of view pro- ing aspects are receiving a lot of attention in vided by the elaborated framework. standardization forums such as the Third Gener- The authors develop ation Partnership Project (3GPP), 3GPP2, Inter- INTRODUCTION net Engineering Task Force (IETF), WiMAX a comprehensive Forum, and IEEE 802 LAN/MAN Committee, An intrinsic characteristic in current and future the new IEEE 802.21 standard [6] for media- framework to wireless communication scenarios is heterogene- independent handover (MIH) being a clear categorize ity, which refers to the coexistence of multiple exponent of such an effort. and diverse wireless networks with their corre- While most published work is focused on par- interworking sponding radio access technologies (RATs). Het- ticular interworking solutions for specific wire- erogeneity is directly associated to the fact that less technologies, this article establishes a solutions by defining no single RAT is able to optimally cover all the comprehensive framework aimed at categorizing different wireless communications scenarios. and analyzing interworking solutions. The pro- a generic set of Hence, a radio technology optimized to provide posed framework is based on the definition of a outdoor coverage to high mobility users may fail generic set of interworking levels along with a interworking levels to meet more demanding data rates in low classification of the related key interworking and its related key mobility indoor scenarios and vice versa. Hetero- mechanisms envisioned so far for heterogeneous geneity is also inherent to technological evolu- wireless networks. The elaborated framework is interworking tion since many new wireless networks are then used to analyze from a common perspective deployed while supporting legacy infrastructures. some of the most relevant interworking solutions mechanisms. Despite RAT heterogeneity, the service proposed for 3GPP, WLAN, and WiMAX net- model pursued under next-generation wireless works. In particular, concerning the integration networks is intended to facilitate the deployment of WLAN and cellular networks, two interwork- of applications and services independent of the ing architectures specified by 3GPP, interwork- underlying RAT. Hence, it is expected that ing WLAN (I-WLAN) [7] and generic access mobile users could eventually enjoy truly seam- network (GAN) [8], are discussed. Next, in the less mobility and ubiquitous service access in an context of coexisting mobile broadband access always best connected mode, employing the networks, interworking solutions for Mobile most efficient combination of available access WiMAX and 3GPP Long Term Evolution/Sys- systems at any time and anywhere. In this con- tem Architecture Evolution (LTE/SAE) net-

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works are analyzed [9–11]. Finally, the MIH solution elaborated within IEEE 802.21 [6] is addressed. Specific Common services Specific The rest of this article is organized as follows. services services First, we describe a generic interworking scenario for heterogeneous wireless networks and bring up some major considerations about network architectures and multimode terminals. From such a basis, the proposed interworking framework is elaborated and the above mentioned interworking solutions are analyzed. The final section includes our main concluding Wireless Interworking Wireless mechanisms remarks and discusses future trends. network network A B WIRELESS HETEROGENEOUS INTERWORKING SCENARIO Figure 1 illustrates a generic interworking scenario for two coexisting wireless networks with partially overlapped coverage. It is assumed that the RAT used in each wireless network can be different, and terminals have multimode capabilities. Concerning service provisioning, a common set of services can be offered through both wireless networks (e.g., voice calls to/from public switched telephone networks), but there can also Figure 1. Generic interworking scenario for heterogeneous wireless networks. be some specific services only available when connected to a given wireless network (e.g., instant messaging and presence services). Besides, as for network access rights, it is tion, this NG can allocate mechanisms to dynam- considered the most general view where two ically acquire operator policies related to QoS wireless networks could belong to different and accounting, and enforce them on a packet- administrative domains (e.g., networks operated by-packet basis for each mobile user. On the by different network service providers) but users other side, a RAT-specific radio link protocol can potentially be attached to either network stack would be used in the air interface. This (e.g., proper roaming agreements exist). Notice radio protocol stack can be entirely allocated in that a more particular case would be where the base stations (BSs) or distributed in a hierarchi- two networks form part of the same administra- cal manner between BSs and some type of radio tive domain (e.g., a mobile operator with Global controllers. The radio link protocol stack com- System for Mobile Communications [GSM] and prises physical, medium access control, and radio Universal Mobile Telecommunications System link control layers. Through this radio protocol [UMTS] networks). In any case, in this article stack, data transfer in the radio interface can be we denote as home network the network from managed, attending to each mobile user’s specif- which a user has obtained his/her credentials, ic needs while simultaneously pursuing an effi- and as visited network any other network to cient usage of radio resources by means of which the user can be connected. appropriate radio resource management (RRM) In such a context, different interworking mechanisms. Hence, BSs and NGs constitute the mechanisms would be needed to attend to the two key elements within the data plane functions set of requirements imposed in terms of ubiqui- (i.e., those functions that are executed directly tous and seamless service access as well as of on the flow of data packets). Additionally, the overall network resource optimization. data plane between BSs and NGs can also comprise mobility anchoring functions in charge of WIRELESS NETWORKS CHARACTERISTICS receiving data destined for a given mobile and Attending to current architectural trends in next- redirecting the data (usually through tunneling) generation networks [12], wireless networks are to the mobile’s serving BS. mainly devoted to providing network connectivity The management of the overall connectivity services (i.e., bearer services) that may be charac- service is achieved by a network control plane. terized by a given quality of service (QoS) profile. Unlike the data plane, the control plane func- Then, end user service provisioning is supported tions are those that do not directly operate on by means of specialized service platforms (e.g., IP the data flow. This network control plane would multimedia subsystem [IMS]) that become acces- be in charge of handling mechanisms such as sible to the users via those bearer services. network access control (e.g., authentication and Accordingly, Fig. 2 illustrates a generic wire- authorization), accounting and charging func- less network architecture in terms of its main tions, mobility management (e.g., location and network nodes and protocol layer allocation. As paging), security management, and session man- shown in the figure, the wireless network pro- agement. For the sake of brevity, all of the above vides network layer connectivity (e.g., IP connec- mentioned control plane mechanisms are tivity) to external networks and service platforms referred to as wireless network control (WNC) via some type of network gateway (NG). In addi- mechanisms throughout this article. Thus, as

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External networks, service platforms

Network gateway Application layer

Network Transport layer layer Network Common databases Network layer protocols

WNC WNC WNC Control Transport WNC plane protocol protocol protocol network stack stack servers stack Data plane Radio Radio link link

protocol protocol protocols Radio stack stack RAT-specific link Base protocol stations/radio stack RAT A network RAT B network controllers interface interface

Wireless access network Multimode terminal

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

shown in Fig. 2, a set of WNC servers, along ments. The definition of interworking levels can with some network databases (e.g., subscriber be conducted attending to, say, network archi- profile databases), are assumed to allocate all tecture aspects or the level of support for specif- these functions. ic service and operational capabilities. In this Finally, general-purpose packet-switched network a definition exclusively based on the level works would constitute the backbone transport of service integration among networks is consid- network that interconnects the different network ered because of its independence from underly- nodes. ing network technologies and architectures. Hence, in a general case, four interworking lev- MULTIMODE TERMINAL CHARACTERISTICS els are distinguished. Focusing now on the key terminal characteristics, a simplified protocol stack of a multimode LEVEL A: VISITED NETWORK SERVICE ACCESS terminal is depicted in Fig. 2. This protocol stack This level of interworking would allow a user consists of RAT-specific protocols for the lower to get access to a set of services available in a layers (i.e., physical and link layers) and a com- visited network while relying on his/her home mon set of protocols for the higher layers (i.e., network credentials. As well, the user could be network, transport, and application layers). Con- charged for service usage in the visited net- cerning the RAT-specific protocols, they would work through its own home network billing sys- comprise the correspondent radio link protocol tem. An example could be the case of a cellular stack to handle data transfer in the air interface subscriber, equipped with a laptop with both along with the protocols used for WNC-related cellular and WLAN network interfaces, able to functionality in each wireless network. As to the log into a public WLAN hotspot using its cellu- common protocol layers, the network layer (e.g., lar smart card credentials and get high-speed IP) has a fundamental role in the interworking Internet access from the hotspot service model since it provides a uniform substrate over provider. which transport (e.g., Transmission Control Pro- tocol [TCP] and User Datagram Protocol LEVEL B: INTERSYSTEM SERVICE ACCESS [UDP]) and application protocols (e.g., Session In this level, users connected through a visited Initiation Protocol [SIP]) can efficiently run network would also be able to get access to spe- independent of the used access technologies. cific services located in his/her home network. In addition to protocol stack considerations, Hence, coming back to the example given in whether the terminal is able to transmit and receive level A, the cellular subscriber using a simultaneously on both radio links (dual-radio cellular/WLAN laptop would also enjoy his/her operation) or only on one at a time (single-radio cellular IMS services while attached to the pub- operation) has important implications on required lic WLAN hotspot. Neither level A nor level B interworking mechanisms, as discussed later. would support service continuity when the user moves between networks. INTERWORKING LEVELS LEVEL C: INTERSYSTEM SERVICE CONTINUITY Bearing in mind all the above considerations, This level extends the previous ones so that the several interworking levels can be envisioned user is not required to re-establish active ses- with a different range of interworking require- sion(s) when moving between networks. How-

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Wireless network A Wireless network B

Visited network service access (Level A) WNC WNC Intersystem AAA WNC WNC servers protocol (flexible AAA frameworks; intersystem protocol servers stack A AAA protocols; identity selection support) stack B

Intersystem service access (Level B)

Intersystem user data transfer Network Network nodes nodes (e.g. Intersystem service continuity (Level C) (e.g. network network gateway, Network Network gateway, mobility layer Network layer handover layer mobility anchoring anchoring nodes) Intersystem seamless service continuity (Level D) nodes)

Network layer handover optimisation

Link layer handover optimisation

Base Radio (inter-RAT configuration information; Radio Base stations/ link inter-RAT measurements control and reporting; link stations/ radio protocol inter-RAT resource reservation; protocol radio controllers stack inter-RAT resource availability knowledge) stack controllers RAT A RAT B

Figure 3. Interworking levels and related interworking mechanisms.

ever, a temporary QoS degradation can be tol- NTERWORKING ECHANISMS erated during the transition time. As an exam- I M ple, if the cellular user equipped with the Attending to the four interworking levels identi- cellular/WLAN laptop begins to download a fied, hereafter we provide a classification of large file when attached to the public WLAN those interworking mechanisms that constitute hotspot and the user moves so that WLAN cov- the basic enablers/building blocks in each level. erage is lost, the downloading service will con- Figure 3 illustrates the addressed interworking tinue through the cellular network without user levels and mechanisms, and relates them to the intervention even though a short transfer inter- main network nodes and protocol layers within ruption might be observed during the network wireless networks previously illustrated in Fig. 2. change. LEVEL A: INTERSYSTEM AAA LEVEL D: INTERSYSTEM SEAMLESS Mechanisms included here aim at extending authorization, authentication, and accounting SERVICE CONTINUITY (AAA) functions among wireless networks, This level is aimed to satisfy service require- allowing users to perform authentication and ments also during mobility (i.e., to offer a seam- authorization processes in a visited network less mobility experience). Seamless service attending to security suites and subscription pro- continuity can be achieved by enabling mobile files provided by their home networks. As well, terminals to conduct seamless handovers across the deployment of one bill solutions advocates diverse access networks. A seamless handover is for the existence of mechanisms to transfer commonly related to the achievement of low accounting and charging data between wireless handover latencies (e.g., less than 300 ms could networks. All these functionalities are basically be required for real-time services in intertech- achieved by: nology handovers) so that this interworking level • Adoption of flexible AAA frameworks able to is the one that imposes the hardest requirements support multiple authentication methods (e.g., on the interworking mechanisms. As an example, the Extensible Authentication Protocol [EAP] focusing again on the dual-mode cellular/WLAN defined in IETF RFC 3748 provides support user, a voice over IP (VoIP) call established for the reliable transport of different authenti- when attached to the WLAN hotspot should be cation protocols). seamlessly handed over to the cellular network. • Deployment of additional functionality such as Finally, it is worth noting here that seamless ser- AAA proxy/relay functions and related signal- vice continuity is dependent not only on inter- ing interfaces between networks (e.g., the working mechanisms but also on the consistency Diameter protocol defined in RFC 3588 pro- of QoS characteristics provided by involved net- vides the minimum requirements for an AAA works. protocol between networks).

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When focusing on • Enhanced network discovery mechanisms for vice continuity can also be provided by the ser- identity selection so that mobile terminals vice itself (e.g., application-based mobility rely- service continuity of could know in advance whether their home ing on SIP as considered in [14]) without the network’s credentials are valid for AAA con- need for network layer handover solutions. common services, trol in a visited network. Notice that nowa- days, wireless networks do not provide such LEVEL D: NETWORK LAYER service continuity can information over the air interface, and the typical approach is to have mobile terminals HANDOVER OPTIMIZATION also be provided by preconfigured with a list of allowed access While simple network or application layer hand- the service itself networks. over solutions may suffice for intersystem service continuity, they may not be able to satisfy the (e.g., application- LEVEL B: INTERSYSTEM USER DATA TRANSFER requirements for seamless mobility. In particular, These mechanisms enable the transfer of user during a handover, latencies related to radio link based mobility data between networks in order to give access to layers (e.g., new radio link establishment) and specific services provided in a network other network layer operation (e.g., movement detec- relying on SIP as than the serving one. A common approach to tion, new IP address configuration, and binding enforce user data transfer between networks updates) can turn into a period during which the considered in [14]) relies on tunneling protocols such as the Layer 2 terminal is unable to send or receive packets. In without the need for Tunneling Protocol (L2TP) defined in RFC 2661 this respect, several optimization mechanisms or the IPsec tunnel mode defined in RFC 2401. have been proposed to reduce the handover network layer han- Tunnels may be established either directly latency due to network layer operations. As an between mobile terminals and remote NGs or example, RFC 4068 defines fast handover exten- dover solutions. may require additional dedicated network nodes. sions for MIPv6 so that terminals can acquire a As an example, 3GPP specifications for WLAN new valid CoA before a handover occurs, and access to 3GPP packet-switched services [7] where tunneling between the old and new CoAs mandates the support of the IPsec Encapsulating reduces the binding update latency. Besides, Security Payload (ESP) protocol described in when handover takes place between networks in RFC 4303 for intersystem user data transfer. different administrative domains, pre-authentication schemes such as the one proposed in [15] LEVEL C: NETWORK LAYER HANDOVER can help reduce non-negligible authentication These mechanisms are required when service and authorization delays. Also, context transfer continuity between wireless networks relies on mechanisms (e.g., Context Transfer Protocol the maintenance of a permanent mobile termi- [CXTP] defined in RFC 4067) can be used to nal IP address. In this regard, Mobile IP (MIP) preconfigure different network (and also link) described in RFC 3344 was the initial proposed layer parameters in the target network and so standard to serve the needs of globally mobile avoid re-initiation of some signaling to and from users who wish to connect to the Internet and the terminal. Finally, network discovery mecha- maintain connectivity as they move from one nisms (see RFC 5113 for a detailed discussion) network to another. MIP is a network layer also have a crucial role in handover optimization mobility solution that covers both handover and since they can convey specific information need- location management aspects. MIP is based on a ed for optimized mobility. redirection approach achieved by a home agent (HA) functionality that maintains a binding LEVEL D: LINK LAYER HANDOVER OPTIMIZATION between the global IP address assigned to the terminal (i.e., home address [HoA]) and the pro- Radio link layer operations could also intro- visional IP address (i.e., care-of address [CoA]) duce delays in the handover process. Hence, set- allocated temporarily within the serving wireless ting up the new radio link could take several network. Within the MIP solution, the mobile steps (e.g., scanning, authentication, and associa- terminal itself is in charge of updating the tion in 802.11) that handover optimization mech- address binding of its HA as the CoA changes anisms should have to either bypass or minimize (i.e., host-based mobility). On such a basis, sev- the latency of while connected to the previous eral IP mobility protocols have been proposed link, especially for the single-radio operation over the past several years to complement or case. At the same time, handover optimization enhance MIP over IPv4 networks (e.g., reverse mechanisms can allow for more efficient radio tunneling in RFC 3024) as well as IPv6 networks resource usage (e.g., network-controlled inter- (e.g., MIPv6 described in RFC 3775 and Hierar- RAT mobility). Hence, among the main aspects chical MIPv6 for localized mobility described in covered by such mechanisms we have: RFC 5380). More recently, network-based IP • Provision of inter-RAT configuration informa- mobility solutions where the terminal is not tion about neighboring BSs to enhance the directly involved in managing IP mobility (e.g., radio scanning process and guide network Proxy MIPv6 defined in RFC 5213) are also selection decisions. being introduced in wireless networks [13]. As • Inter-RAT measurements control and report- an example, the 3GPP LTE/SAE network allo- ing to improve handover initiation. cates HA functionality for MIP-based mobility • Inter-RAT resource reservation. This could anchoring (either host-based or network-based) imply the preconfiguration of some radio link between LTE/SAE and other non-3GPP access contexts (e.g., QoS contexts) in the target networks [10]. It is important to mention here RAT. that, when focusing on service continuity of com- • Inter-RAT resource availability knowledge mon services (i.e., those available in both home (e.g., check or reporting procedures) in order and visited networks as illustrated in Fig. 1), ser- to enhance handover decisions.

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Regarding Level B, External networks, service External networks, service platforms platforms IPsec ESP is used to Level B IP PDG GGSN provide secure router IPsec-based interface tunnels between IP layer IPsec IP IP layer layer terminals connected Diameter- HSS WLAN based interface to the WLAN and a access EAP over EAP- Control UMTS network Control RADIUS/ AKA plane network plane diameter new node named SM/ Data plane Data plane MM WNC servers 3GPP AAA Packet Data (e.g. AAA server WNC servers 802.11 server/proxy) (SGSN control plane) 3GPP Gateway (PDG) radio radio stack stack within the 3GPP Level A Dual-radio operation Access RNC and points NodeBs network. The PDG 802.11 3GPP 3G interface radio radio stack stack Terminal basically behaves equipped EAP- SM/ with 802.11 AKA MM 802.11 interface and 3G as a GGSN for Level B network IPsec IP layer interfaces WLAN users. Transport Application

Figure 4. I-WLAN interworking model.

As an example, inter-RAT handover mecha- physical layer) is entirely handled by access nisms specified between 3GPP GSM and UMTS points (APs); WNC functions such as access con- networks cover most of the aforementioned trol can be supported by means of AAA servers aspects. using IETF protocols (e.g., RADIUS protocol between APs and AAA servers for EAP/IEEE 802.1X access control); and generic routers can DISCUSSION ON provide NG functions to external networks or RELEVANT INTERWORKING APPROACHES service platforms. Thus, interworking level A is achieved by the According to previous interworking levels and allocation of a 3GPP AAA server in the 3GPP related mechanisms, in this section we analyze network and AAA proxy functions within the some of the most relevant proposed solutions. WLAN network. The interface between them is based on the Diameter protocol, which can be I-WLAN used for the transfer of, say, EAP messages for I-WLAN architecture [7], commonly referred to authentication and authorization. In this respect, as loose coupling interworking, is targeted to specific EAP extensions for 802.1X access control cover interworking levels A and B for packet have been defined to allow authentication based services (i.e., General Packet Radio Service on UMTS credentials (i.e., EAP-AKA defined in [GPRS]). Following the basic architecture repre- RFC 4187). As well, the list of available inter- sentation for a generic wireless network intro- working UMTS networks can be provided through duced in Fig. 2, Fig. 4 illustrates both the main the WLAN connection by means of the network network elements within 3GPP UMTS and discovery mechanism defined in RFC 4284. WLAN networks and the new main network Regarding level B, IPsec ESP is used to pro- functions (marked within circles) and interfaces vide secure tunnels between terminals connected (highlighted in italics) added for I-WLAN inter- to the WLAN and a new node named packet working support. data gateway (PDG) within the 3GPP network. As to the 3GPP UMTS network, network The PDG basically behaves as a GGSN for gateway functions are provided by the gateway WLAN users. GPRS support node (GGSN); WNC functions From the terminal side, the I-WLAN model such as mobility management (MM) and session basically requires a conventional 3G network management (SM) are mainly handled within interface, a 802.11 network interface supporting the control plane of the serving GSN (SGSN) EAP/802.1X authentication (e.g., Wireless Pro- and with the support of a home subscriber server tected Access [WPA] certification) and support (HSS) database; and UMTS radio link protocol for IPsec within the IP protocol stack. stack and related functions (e.g., RRM) are distributed among NodeBs (i.e., naming convention GENERIC ACCESS NETWORK for UMTS BSs) and radio network controllers The GAN [8], also referred to as tight coupling (RNC). As for the WLAN access network, the interworking, provides a solution to extend cellu- IEEE 802.11 protocol stack (e.g., MAC and lar circuit and packet services over IP broadband

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The initial interwork- External networks, service ing solution consid- platforms IP 3GPP ered within WiMAX router standardised GGSN interface Forum and 3GPP is IP layer IP layer based on the I-WLAN Data plane HSS architecture by 3GPP. (Out of scope of GAN) WLAN Control UMTS access AAA plane network Control protocols Hence, such a network plane Levels B,C GA-RPC Data SM plane solution is already Data plane IPstack MM

WNC servers WNC servers available for the first (e.g., AAA server) GANC 802.11 (SGSN control plane) 3GPP radio radio release of the stack stack Dual-radio operation WiMAX architecture Access RNC and points NodeBs (WiMAX NWG Level D 802.11 3GPP stack radio stack Release 1) and Levels B,C IP stack covers interworking GA-RRC UMA-enabled

3GPP GAN interface terminal Levels A and B. SM/MM IP layer Transport Application

Figure 5. GAN interworking model.

access networks. Hence, the GAN model is not transferred between the GANC, acting as an restricted to 802.11 access networks, yet deploy- RNC from the network side, and the corre- ing GAN over 802.11 is the most common sponding RNCs within the UMTS network with- approach. Moreover, dual-mode cellular/802.11 out service disconnection. Concerning level D, terminals compliant with GAN specifications are handover latencies similar to those between more widely known as unlicensed mobile access UMTS cells can be achieved by the fact that (UMA)-enabled terminals. UMA-enabled terminals support dual-radio The GAN model provides interworking levels operation. Hence, a UMA-enabled terminal can B, C, and D. Figure 5 illustrates the main aspects connect to the GANC via a WLAN while main- of the GAN interworking approach within the taining the UMTS network connection so that same UMTS/WLAN scenario previously the aforementioned interworking mechanisms described for I-WLAN. The three key elements for network or link layer handover optimization of the GAN model, as shown in Fig. 5, are a new are not necessary. On the contrary, level A is out network element named GAN controller of the scope of the GAN specifications since the (GANC) located within the 3GPP network, a GAN model only requires IP connectivity from UMA-enabled terminal, and a new interface the WLAN access network and does not deal between both elements specified by 3GPP. with 802.11-specific issues such as access control. The GANC serves much like an RNC, reusing legacy 3GPP interfaces towards the core net- WIMAX AND 3GPP NETWORKS INTERWORKING work. Regarding the UMA-enabled terminal, it The initial interworking solution considered within is a 3GPP terminal with embedded 802.11 com- the WiMAX Forum and 3GPP is based on the I- munications. Hence, the interface defined WLAN architecture by 3GPP. Hence, such a solu- between GANC and UMA-enabled terminals tion is already available for the first release of the comprises new protocols with functions similar WiMAX architecture (WiMAX NWG Release 1 to the UMTS radio resource control (RRC) pro- [9]), and covers interworking levels A and B. tocol (e.g., Generic Access RRC [GA-RRC]) Then, in the context of 3GPP LTE/SAE networks, along with legacy 3GPP control plane protocols architecture enhancements are being specified for (e.g., SM and MM). The transfer of all the infor- providing IP connectivity using non-3GPP accesses mation between the terminal and the GANC [10]. In particular, support for MIP and PMIP- through the WLAN network uses IPsec tunnel- based mobility is being introduced to achieve level ing mechanisms. C. As well, functionality intended to provide net- On this basis, interworking level B is provid- work discovery and selection assistance data is ed by the fact that GANC actually constitutes a being specified (i.e., access network discovery and gateway toward 3GPP services. Also, level C is selection function [ANDSF]). built up on legacy 3GPP handover procedures so Besides, optimized interworking solutions that established data sessions or calls could be between WiMAX networks and 3GPP LTE/SAE

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It is worth noting External networks, service External networks, service platforms platforms that for dual-radio

Levels B,C MIP-based PDN-GW operation, previous ASN-GW interface data plane HA for mechanisms would IP 3GPP AAA non-3GPP layer WNC servers Level A server (ASN-GW IP layer not be necessary. control plane) HSS Diameter- 3GPP based Nevertheless, WiMAX Control LTE/SAE Control interface EAP- plane ASN EAP over AKA network plane diameter SM dual-radio operation MM Data Data plane plane Intersystem may not be feasible handover WNC servers (MME) 802.16 signaling 3GPP radio 3GPP cells interface radio due to, say, stack measurements stack Single-radio operation radio frequency 802.16 3GPP eNB BS neighbouring coexistence issues BS information 802.16 3GPP Level D radio radio Level D stack stack when radio Level D Level A Dual-mode EAP WiMAX/3GPP LTE frequencies used in Level B,C SM/MM terminal MIP Dual-mode network interface Radio stack modifications to account for inter-RAT issues the two RATs are IP layer Transfer capabilities for intersystem handover signalling Transport close to each other. Application

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

networks are also under consideration to achieve Concerning interworking level D, intersystem interworking level D [11]. In this regard, an handover optimization mechanisms are dis- approach targeted to cover interworking levels A, cussed in [11] to provide seamless mobility for B, C, and D is illustrated in Fig. 6 and discussed single-radio operation terminals, a condition next for a 3GPP LTE/SAE network coexisting that imposes the hardest requirements on the with a WiMAX access service network (ASN). As interworking solution. In this respect, as illus- shown in Fig. 6, regarding the 3GPP LTE/SAE trated in Fig. 6, both WiMAX and 3GPP network, radio link layer protocols and functions LTE/SAE radio link protocol stacks would need are now entirely allocated in an enhanced NodeB to be modified to add support for inter-RAT (eNB), and an evolved packet core (EPC) han- measurement control and reporting along with dles WNC functions via so-called MME servers the delivery of inter-RAT BS information. and provides NG functionalities through PDN Besides, support for resource reservation, as gateways (PDN-GWs). As for the WiMAX ASN, well as pre-registration (i.e., covering aspects the IEEE 802.16e radio protocol stack is also such as access control, context transfers and entirely allocated to BSs, and a separate network default bearer service establishment), is envis- node named ASN gateway (ASN-GW) could hold aged by means of tunneling legacy signaling both data plane anchoring functionalities (e.g., for messages between terminals and BSs or WNC mobility anchoring within WiMAX ASN) and servers in the target network while being control plane WNC functions (e.g., authentication attached to the serving network. Hence, as based on an EAP framework). example, a terminal connected to an eNB can According to this interworking solution, inter- initiate a handover to WiMAX by tunneling a working level A is achieved as in I-WLAN solu- 802.16e Handover Request message through tion by means of AAA proxy functions in the the 3GPP LTE/SAE network towards the target ASN-GW, a 3GPP AAA server located in the WiMAX BS. Transparent transfer of intersystem 3GPP LTE/SAE network, and the use of specific handover signaling messages requires each radio EAP authentication mechanisms. Then levels B protocol stack to provide transport functionality and C are built up on MIP-based mobility solu- for signaling messages of a different technology. tions already supported within the WiMAX ASN Moreover, transparent signaling transfer (e.g., the ASN-GW data plane can allocate for- between networks requires the deployment of an eign agent [FA] functions for MIPv4 or access interface among MME and ASN-GW Control router [AR] functions for MIPv6) and on adding Plane as shown in Fig. 6 (e.g., S101 interface in data plane anchoring functions within the 3GPP [11]) for intersystem handover signaling. Finally, LTE/SAE network (e.g., HA functions for it is worth noting that for dual-radio operation, MIPv4 or MIPv6 are allocated in the PDN-GW). previous mechanisms would not be necessary. The MIP client can be located in the terminals, Nevertheless, dual-radio operation may not be as shown in Fig. 6, or in the ASN-GW if PMIP feasible due to e.g., radio frequency coexistence solutions are used instead between the ASN-GW issues when radio frequencies used in the two data plane and the 3GPP PDN-GW. RATs are close to each other.

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IEEE 802.21 IEEE 802.21 INTERWORKING CONCLUDING REMARKS AND solution is expected The standard IEEE 802.21 “Media-Independent FUTURE TRENDS Handover (MIH) Services” [6] is mainly con- to constitute a cor- ceived as an enabler of interworking level D This article has elaborated a comprehensive between heterogeneous link-layer technologies framework to analyze and categorize interwork- nerstone within IEEE (e.g., IEEE 802 and cellular networks) and ing solutions in heterogeneous wireless net- across IP subnet boundaries. The standard works. Over such a basis, some of the most 802’s proposal for defines a new link layer functionality (i.e., MIH relevant interworking solutions being considered IMT-Advanced, where Function [MIHF]) to be added within terminals in different standardization bodies have been and networks together with the protocol required discussed. enhanced IEEE for message exchanging between them. This As to WLAN and cellular interworking solu- MIHF entity is allowed to have some control on tions, I-WLAN has been noted to constitute a 802.16 and IEEE link layer behavior (e.g., link actions such as simple but versatile solution to extend cellular power-up and link configuration) and can collect packet services access. I-WLAN requires little 802.11 radio link information (e.g., link status polling or changes in terminals and networks, though it is event-triggered information). To that end, radio not able to provide seamless mobility and net- technologies will be link layers have to add support for a media spe- work-controlled mobility. Currently, I-WLAN integrated under the cific interface with the MIHF function. On the interworking architecture is being extended to other hand, the MIHF entity provides a media handle mobility between I-WLAN and 2G/3G by interworking model independent interface to upper layers of the pro- means of adding support for MIP-based mobili- tocol stack (denoted as MIH users in 802.21 ty, thus converging to the type of solutions being offered by 802.21. standard’s notation) and offers a set of generic considered within LTE/SAE for interworking services classified as event, command and infor- with non-3GPP access networks. On the other mation services [3]. Through these services, hand, 3GPP GAN solution requiring dual-radio upper layers can control lower layer’s behavior operation UMA-enabled terminals was firstly (e.g., remote/local link actions and handover developed for 2G, then extended to 3G but no commands) and acquire relevant information for extension is envisioned so far within LTE/SAE, more efficient handover decisions (e.g., where PDN-GW intersystem mobility anchoring local/remote link status and information about with MIP and PMIP-based solutions are favored. different available networks and their services). Nevertheless, UMA-terminals are a reality today The 802.21 service model offers a flexible frame- as some operators are embracing such a technol- work to facilitate different handover approaches. ogy both in business and in the home 802.11/cel- Hence, while served by a given wireless network, lular access. the MIHF entity of the mobile terminal could Concerning possible solutions to achieve opti- interact with a MIHF entity in the serving net- mized seamless handover with single-radio ter- work in order to retrieve inter-RAT network minals between LTE/SAE and WiMAX, two information and initiate an inter-technology han- main approaches have been discussed. While dover by indicating a preferred list of candidate 3GPP envisions a tailored solution, IEEE 802.21 access networks. As well, handover could also be efforts are trying to push for a generic solution initiated from the network side. In both cases, for inter-technology handover. Nevertheless, the 802.21 signaling enables intersystem radio adoption of a generic solution such as IEEE resource availability check and resource prepara- 802.21 in 3GPP networks is not as straightfor- tion (e.g., intersystem resource reservation) ward as within IEEE-based technologies, which between involved networks and provides the ter- already share a common architectural frame- minal with the required configuration of the work. In this regard, the 802.21 standard is reserved resources at the target network. It’s becoming a central piece of an IEEE 802 wide worth noting that the scope of 802.21 is limited initiative so that, in addition to defining the new to handover initiation and preparation phases, 802.21 standard itself, IEEE is making changes while the execution phase is not covered (e.g., to existing access technologies specifications mobility handling in upper layers is still required (e.g., WLAN, WiMAX) to support 802.21 relat- for service continuity between networks). ed handover functionality. Moreover, IEEE The adoption of the 802.21 solution in a 802.21 solution is expected to constitute a cor- WiMAX-3GPP LTE/SAE interworking scenario nerstone within IEEE 802’s proposal for IMT- such as the one described in Fig. 6, would Advanced [3], where enhanced IEEE 802.16 and require the allocation of functional MIHF enti- IEEE 802.11 radio technologies will be integrat- ties in the terminal and within both wireless net- ed under the interworking model offered by works along with the correspondent transport 802.21. capabilities to exchange MIH protocol messages. In this sense, MIH signaling to/from terminals ACKNOWLEDGMENTS could be transferred by using specific mecha- This work has been partly funded by the Span- nisms introduced in the radio link layers (IEEE ish Ministry of Science and Innovation under 802.16g extension adds such a support but 3GPP the TelMAX project, belonging to the Ingenio does not consider it yet). Another possibility 2010 program. This work has also been partly would be the transfer of MIH signaling op top of supporrted by the Spanish Research Council the IP bearer service provided by the network and FEDER funds under a COGNOS grant (e.g., IETF MIPSHOP working group is specify- (ref. TEC2007-60985. The authors are very ing support for sending 802.21 messages over IP thankful to the reviewers for their constructive networks). comments.

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[13] K.-S. Kong et al., “Mobility Management for All-IP Mobile REFERENCES Networks: Mobile IPv6 vs. Proxy Mobile IPv6,” IEEE Wireless [1] A.K. Salkintzis, C. Fors, and R. Pazhyannur, “WLAN-GPRS Commun., vol. 15, no. 2, Apr. 2008, pp. 36–45. Integration for Next-Generation Mobile Data Networks,” [14] S. Salsano et al., “SIP-based Mobility Management in IEEE Wireless Commun., vol. 9, no. 5, Oct. 2002. Next Generation Networks,” IEEE Wireless Commun., [2] A.K. Salkintzis, “Interworking Techniques and Architectures vol. 15, no. 2, Apr. 2008, pp. 92–99. for WLAN/3G Integration toward 4G Mobile Data Net- [15] A. Dutta et al., “Media-independent Pre-authentication works,” IEEE Wireless Commun., vol. 11, no. 3, June 2004. Supporting Secure Interdomain Handover Optimiza- [3] L Eastwood et al., “Mobility using IEEE 802.21 in a Het- tion,” IEEE Wireless Commun., vol. 15, no. 2, Apr. erogeneous IEEE 802.16/802.11-based, IMT-Advanced 2008, pp. 55–64. (4G) Network,” IEEE Wireless Commun., vol. 15, no. 2, Apr. 2008. [4] C. Skianis, G. Kormentzas, and K. Kontovasillis, “Interac- BIOGRAPHIES tions between Intelligent Multimodal Terminals and a RAMON FERRUS ([email protected]) is an associate professor at Network Management System in a B3G Context,” Wire- Universitat Politècnica de Catalunya (UPC), Spain, since 2002. less Commun. Mobile Comp., vol. 5 , no. 6, Sept. 2005. His actual research interest is focused on architectures, QoS, [5] C. Skianis et al., “Efficiency Study of the Information mobility, and resource management in the context of hetero- Flow Mechanism Enabling Interworking of Heteroge- geneous IP-based mobile communications systems. He has neous Wireless Systems,” J. Sys. Software, vol. 80, no. participated in several research and technology transfer pro- 10, Oct. 2007. jects. He is co-author of more than 50 papers published in [6] IEEE Std 802.21, Draft D14.0, “IEEE Standard for Local international journals/magazines and conference proceedings. and Metropolitan Area Networks: Media Independent Handover Services,” Sept. 2008. ORIOL SALLENT ([email protected]) is an associate profes- [7] 3GPP TS 23.234 v. 7.7.0, “3GPP System to Wireless sor at UPC. His research interests are in the field of radio Local Area Network (WLAN) Interworking; System resource and spectrum management for heterogeneous Description (Release 7),” June 2008. cognitive wireless networks, where he has published 100+ [8] 3GPP TS 43.318 v. 8.3.0, “Radio Access Network; Generic papers in IEEE journals and conferences. He has participat- Access Network; Stage 2 (Release 7),” Aug. 2008. ed in many research projects and consultancies funded by [9] WiMAX Network Forum Architecture, “Stage 2, 3GPP- either public organizations or private companies. WiMAX Interworking,” Release 1, v. 1.2, Jan. 2008. [10] 3GPP TS 23.402 v. 8.2.0, “Architecture Enhancements RAMON AGUSTI ([email protected]) has been a full profes- for Non-3GPP Accesses (Release 8),” June 2008. sor at UPC since 1987. For the last 20 years he has mainly [11]. 3GPP TR 36.938 v. 8.0.0, “Improved Network Con- been interested in mobile communications topics and has trolled Mobility between E-UTRAN and 3GPP2/Mobile published about 200 papers in these areas. He has also WiMAX Radio Technologies,” Mar. 2008. participated in the COST and many European research pro- [12] R. Agrawal and A. Bedekar, “Network Architectures for grams, and managed many private and public funded pro- 4G: Cost Considerations,” IEEE Commun. Mag., vol. 45, jects. His actual research interests include radio networks, no. 12, Dec. 2007, pp. 76–81. cognitive radio, radio resource management, and QoS.

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