(PDF) Technology Integration Framework f

Hindawi Publishing Corporation Journal of Computer Systems, Networks, and Communications Volume 2010, Article ID 205786, 21 pages doi:10.1155/2010/205786 Research Article Technology Integration Framework for Fast and Low Cost Handovers—Case Study: WiFi-WiMAX Network Mohamed Kassab,1 Jean-Marie Bonnin,1 and Abdelfettah Belghith2 1 Telecom Institute/Telecom Bretagne/RSM Department, Universit´e Europ´eenne de Bretagne, 35510 Cesson Sevign´e, France 2 ENSI/CRISTAL Lab/HANA Research Group, University of Manouba, 2010 Manouba, Tunisia Correspondence should be addressed to Mohamed Kassab,

[email protected]

Received 1 October 2009; Revised 14 February 2010; Accepted 18 April 2010 Academic Editor: K. Daniel Wong Copyright © 2010 Mohamed Kassab et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Next Generation Wireless Networks (NGWNs) are seemed to be heterogeneous networks based on the integration of several wireless technologies. These networks are required to achieve performances equivalent to classic wireless networks by ensuring the continuity of communications and the homogeneity of network management during horizontal and vertical handovers. This task is even more important when management services, like security and quality of service (QoS), are deployed at access technology level. In this paper, we propose a framework for heterogeneous wireless technology integration based on network architecture skeleton and a handover management mechanism. This framework optimizes the layer-2 handover procedure to achieve performances required by sensitive applications while ensuring the minimization of signaling overhead required for operated networks. As an application example, we make use of this framework to propose a heterogeneous network based on WiFi and WiMAX technologies. We present an application example of the framework using the specification of a WiFi-WiMAX network. We propose several performance evaluations based on simulation tests based on this application. The latter confirm the eﬃciency of handover delay optimization and the minimization of management signaling costs. 1. Introduction and usability domains have converged so that terminals and communication means have evolved to integrate multiple The growth of wireless communication has been, in a few technologies. years, important thanks to the advantages they oﬀer such The result of this evolution is a multitechnology environ- as deployment flexibility and user mobility during com- ment that can be exploited to oﬀer an enhanced connectivity munications. Several wireless technologies have emerged. to users. The Next Generation Wireless Networks (NGWNs) These technologies have been designed independently and appear to be the integration of already existing and newly intended to cover specific service types, user categories, developed wireless technologies that oﬀers a heterogeneous and usability domains. Among these technologies, there access to the same global core network. A multi-technology is not one good and generic enough to replace all the terminal will be able to change its access technology each others; each technology has its own merit, advantages, and time its environment changes. For example, it will be development possibilities. For example, 3G technologies, for connected to a WiFi access point when it is in the mall; it will example, UMTS and CDMA2000, propose network access handover to the WiMAX when it will move to the street and associated to telephony services. WMAN technologies, for it will use UMTS in the train. This could be a great advance example, WiMAX and HyperMAN, are used to deploy depending on the adequate mechanisms which are available outdoor metropolitan networks. WLAN technologies, for to ensure a seamless mobility. example, WiFi, have been developed to be an extension of On the other hand, wireless technologies are no longer already existing wired LANs; they are also used to deploy limited to be a basic communication medium. They eval- local public wireless networks. In addition, user categories uate by integrating several management services such as 2 Journal of Computer Systems, Networks, and Communications user authentication, data exchange confidentiality, and QoS of this framework to an actual wireless network based management. However, the integration of these services at on the WiFi and WiMAX technologies. We make use of the access technology level with specific designs will aﬀect the this application to demonstrate the ability of the proposed handover performances in NGWNs. In fact, the change of the framework to enable the enhancement of HO performances serving Point of Attachment (PoA) requires the renegotiation while ensuring a reduced signaling overhead. of management services between the terminal and the This paper is organized as follows. In Section 2, we network in addition to the redirection of data traﬃc to the propose an overview of solutions adopted for wireless tech- new terminal location. As a result, the HO execution time nology integration. In Section 3, we detail the specification may increase significantly, which should induce significant of the technology-integration framework. We propose, in latency to exchanged data and even the break of the ongoing Section 4, the specification of wireless network based on session. the WiFi and WiMAX technologies. We demonstrate the Public wireless networks have to guarantee a good level advantages oﬀered by this architecture based on perfor- of service while insuring the transparency of management mances evaluations in Section 5. We detail how the proposed to users. The deployment of such networks using het- framework can get along with layer-3 mobility management erogeneous technologies will require a good connectivity mechanisms in Section 6. We propose, in Section 7, a during handovers, by reducing latency, and the homogeneity discussion about heterogeneous technology integration. We of management services such as authentication and QoS. draw up main conclusions and propose future trends of our This is possible by deploying anticipation mechanisms that work in Section 8. reduce negotiation exchanges between the terminals and the network, such as context transfer and proactive negotiation 2. Technology Integration in the Literature [1], and accelerate the redirection data traﬃc during the execution of the HO. Heterogeneous-technology integration has been studied by Researchers have been interested in this problem and sev- several researches. Most studies focused on networks inte- eral papers have proposed models for eﬃcient technology- grating UMTS and data wireless technologies, that is, WiFi integration solutions that deal with network access provider [2–6] and WiMAX [7–9]. Two inter working architectures requirements. However, the mobility management oﬀered have been proposed: loosely and tightly coupled architectures by these solutions does not ensure yet seamless handovers [2, 10]. during heterogeneous mobility. Indeed, most solutions oﬀer With loosely coupled architecture, the interconnected roaming possibilities based on the sharing of user databases. technologies are considered as independent networks con- At best, the integration architectures oﬀer to graft one tech- cerning the handling of data traﬃc and the management nology to another and to manage heterogeneous mobility of network services such as authentication and QoS. Each based on Mobile IP and extensions. These solutions enable technology has a separate user subscription and profile the optimization of the network reattachment (i.e., the layer- management systems. Roaming privileges are assigned to 3 HO) by limiting the heterogeneous handover to the re- subscriptions related to one network. This helps to minimize attachment to the new PoA (i.e., layer-2 HO). This does not session disruption based on the cooperation of account- solve the connectivity disruption due to the re-establishment ing entities. The tightly coupled architecture proposes the of network services defined at the technology level. On the integration of wireless technologies in the same network other hand, the structure of these technology-integration architecture. This integration may be performed in diﬀerent solutions is not suited to heterogeneous mobility. Indeed, levels of the management architectures of the considered the organization of the PoAs in the core network is based technologies. User subscriptions and profiles are manage- on the access technology they oﬀer rather than the closeness ment based on common centralized entities. In all cases, of radio coverage while the executed HOs will be based on user mobility is managed using Mobile IP and its extensions the latter closeness. As a consequence, the HO management [11]. mechanisms based on exchanges between heterogeneous The main advantage of loosely coupled architectures entities will result in a nonnegligible overhead that could is the few modifications to technologies and their core disrupt the network performances. network architectures. However, due to the high level of In this work, we propose a technology-integration frame- integration, the mobility management mechanisms are not work that provides a new approach to deploy next generation able to optimize significantly the performance of layer- wireless networks. This framework oﬀers a heterogeneous 3 handover. Thus, the roaming mechanisms are not able access to a global network with optimized mobility perfor- to reduce suﬃciently the session disruption to deal with mances regarding HO execution time and signaling cost. requirements of sensitive applications. The idea is to optimize the layer-2 HO execution in a The tightly coupled architectures propose integration heterogeneous and homogeneous mobility and to adapt at lower level of network architecture. The complexity of the network architecture so that this optimization yields the implementation increases, and more modifications must to a minimum signaling surplus. The framework defines a be operated to technologies and core network architecture. network architecture skeleton and HO management mecha- Nevertheless, the lower level of integration ensures a very nisms. They tend to optimize the layer-2 HO execution while interesting enhancement of HO performances [4, 5]. This is ensuring the continuity of management services defined at due to the fact that the inter-working takes place at a point of the technology-level. In addition, we propose an application the management architecture closer to the mobile terminal. Journal of Computer Systems, Networks, and Communications 3 The tightly coupled architecture can significantly or based on common management requirements. It also improve the performance of heterogeneous handovers. This remains possible to gather PoAs oﬀering the same wireless can be even more enhanced by using the ConteXt Transfer access technology. We define new management entities: the Protocol (CXTP) [12] in addition to MIP. The CXTP Layer 2 Access Managers (L2-Acc-Mgrs) that manage terminal proposes a protocol to transfer mobile terminal contexts mobility over the network. To each access subnetwork is between Access Routers managing the access control of associated an L2-Acc-Mgr. Figure 1 shows this architecture. a wireless network. CXTP has been designed as a generic The L2-Acc-Mgr integrates several functions to manage protocol that can accommodate a wide range of services. The terminal mobility. It acts as a service proxy regarding context transfer can be reactive, during the HO execution, exchanges between terminals and core network entities or proactive from the serving AR to a possible target AR. during the network entry procedure. For example, terminal CXTP can be useful if some network services such as user authentication is supported by the L2-Acc-Mgr that acts authentication and QoS are integrated to the layer-3 level in as AAA-proxy between the terminal and the AAA server wireless networks [13]. Consequently, several management in the core network. At the end of this procedure, the exchanges between a terminal and the Access Router (AR), L2-Acc-Mgr maintains the terminal authentication profile which controls the access to the network, are required during (authentication keys) to use it for future purposes. the network entry. Thus, the CXTP enables the reduction of The L2-Acc-Mgr supports the Neighborhood manage- exchanged messages between mobile terminal and target AR ment function that maintains the PoAs’ neighborhood. It during the HO execution. provides a list of PoAs to which a terminal may move while However, the latter optimization limits only the eﬀects being associated with a particular PoA. of sub network change during terminal mobility (layer-3 The L2-HO management function integrates the intel- HO optimization). Indeed, all the negotiation exchanges ligence related to the L2-HO management, that is, the and the service establishment procedures defined at access- triggering of HO management exchanges, the execution of technology level must be performed during heterogeneous exchanges and the management of terminal contexts. handover executions. A solution could be the association of the tightly coupled 3.2. L2-HO Management Mechanisms. During the network architectures to an optimization of the terminal to technol- entry, a terminal associates itself with the network and ogy association procedure. This optimization will take into activates a set of services and functionalities. The terminal account the possible resemblances between the definition context includes the parameters negotiated during the net- of services and user profiles of technologies to prevent work entry and states related to network services used by the execution of the negotiations and procedures during the terminal [1]. The acceleration of the establishment of handover executions. This may be based on management this context is required, at the time of handover, to reduce mechanisms like context transfer or proactive execution of the delay that results from the HO execution phase. The exchanges. establishment of the terminal context on the target PoA, based on already available information, is the solution. 3. Technology-Integration Framework The nature of information elements included in the terminal context defines how it can be exploited to perform a This framework aims at defining an optimization of the context re-establishment. This defines values of information handover performances as part of a heterogeneous mobility. elements to be established, when and how they will be We consider an operator network that oﬀers a reliable established, and the network entities that have to manage network access, to mobile terminals, based on several wireless these information elements [1]. Authors in [14] propose a technologies. Network services, such as user authentication, study that define the latter points based on the characteristics QoS management, and billing, have to work properly and of information elements and particularly: seamlessly while terminals are moving over the network. (i) the scope of the information element, We define the network architecture and the position of (ii) the transferability of the information element, management entities that are involved in the handover management procedure. (iii) and the stability of the information element value The proposed framework specifies the skeleton of the over the time. network architecture, the definition of mobility context In the following part, we identify the network entities and the L2-HO management mechanisms. The latter pro- that will manage the context establishment, the values to poses the enhancement of L2-HO performances based on be established, the mechanisms that establish contexts, and mobility-context exchanges. finally when the establishment has to be performed (i.e., before, during, or after the HO execution), while taking 3.1. Network Architecture Skeleton. The global wireless net- into account the network architecture decided upon and work is organized into access subnetworks, each one gathering the nature of information elements that may be included in a set of PoAs. We do away with the classic organization terminal contexts. of wireless networks that separates each technology in an autonomous network. PoAs can be gathered in access sub 3.2.1. Management of Terminal Contexts. Regarding the networks based on the closeness of their wireless coverage scope, a terminal context consists of global session and local 4 Journal of Computer Systems, Networks, and Communications Centralized Layer 2 access servers managers Core network Access Access router router Access Access subnetwork subnetwork PoA PoA PoA Terminal Layer 2 network entry exchanges Figure 1: The L2-Acc-Mgr in the network architecture. association information elements. The global session infor- information elements are held by the L2-Acc-Mgr thanks to mation elements are related to the association established the service proxy function. Second, local information elements between the terminal and the core network entities such as that are conditionally transferable may require centralized AAA servers. The local association information elements are information related to the neighbor PoAs or the terminal to related to the association established between the terminal be translated for re-establishment. The latter information is and the serving PoA. When a terminal executes a HO without held by the L2-Acc-Mgr, so it is the better able to manage performing a new network entry, it maintains its global conditionally transferable local information elements. The session while re-establishing the local association with the new HO management function of the L2-Acc-Mgr is responsible serving PoA. of managing the latter information elements, of the terminal Then, a context information element is transferable context. information when it remains valid while the terminal changes The HO management function defines the values for its serving PoA. Such information element can be reused information elements to be established by the L2-Acc-Mgr. with target PoA to avoid renegotiation during HO execution. The latter values will be derived based on the ones used Other elements are nontransferable context information, their with the current association, cached information elements or current value, associated to a serving PoA, cannot be terminal accounting profile. A Translation function is defined exploited to avoid negotiations between the terminal and as a part of the HO management function. It is responsible target PoA to establish a new association. This type of infor- of defining values to be established for information elements mation has to be re-established through regular exchanges constituting the context terminal. during the HO execution. Finally, an information element This case can be illustrated over a heterogeneous wireless can be conditionally transferable if the value associated to the network oﬀering access to multi-technology terminals. A serving AP is not valid for transfer; however, it can be used mobile terminal can switch between two PoAs oﬀering to define a new value associated to target PoAs. It is possible heterogeneous technologies. In this case, QoS parameters can to define translation rules for this specific set of information be transferred to re-establish the new association since the elements so as to enable their establishment while avoiding two wireless technologies do not necessarily use the same negotiations during HO execution. QoS representation. A QoS translation function can solve the Based on these two classifications we define the content conformity problem as most QoS management mechanisms of terminal contexts and the entities that have to manage have common bases. these contexts, following the recommendation proposed in The definition of new values for a context information [1]. element may result into a synchronization problem between The L2-Acc-Mgr is the most entitled entity to manage the the terminal and the network. Indeed, the terminal must greater part of the terminal context. First, the global session be able to integrate the translation subfunction used by the Journal of Computer Systems, Networks, and Communications 5 L2-Acc-Mgr to define the new information element value. other cases, the information element is established reactively Therefore, the translation rules are defined so that both the during HO execution based on its last update. terminal and the L2-Acc-Mgr can compute a value that corresponds to the new association without performing any 3.2.3. HO Establishment Exchanges. Regarding our speci- exchange. fication, the context transfer is suitable for information The local information elements that have values valid elements managed by the L2-Acc-Mgr. Proactive and reactive for diﬀerent local associations (transferable information), exchanges are combined to manage static and dynamic infor- are managed by the PoAs. A serving PoA is responsible for mation elements. The exchange (a) of Figure 2 shows the redistributing them to target PoAs and caching them. proactive establishment procedure involving the L2-Acc-Mgr Finally, there is a set of information elements that current and two neighbor PoAs. The target PoA may execute a reac- values cannot be exploited to avoid management exchanges tive exchange to obtain values related to dynamic informa- between a mobile terminal and the network to establish a tion elements from the L2-Acc-Mgr as shown in Figure 2(b). new association. We name this category: non transferable The establishment of local association information ele- context information. This type of information has to be re- ments managed by serving PoA can be based on context established through regular exchanges during the handover transfer and/or proactive negotiation. These mechanisms may execution. We can mention connection parameters used with be combined to establish one or more information elements a terminal, for example, data rate. These parameters depend in the same procedure or used as alternatives for the same on the position of the terminal in the cell and the serving information element to define diﬀerent procedures since AP capacity, and so they have to be negotiated during the they have diﬀerent properties [1]. Figure 3 shows exchanges association. based on the two mechanisms. The context transfer can be proactive and/or reactive. For 3.2.2. Context Establishment Exchanges. Two options are the proactive one, the establishment exchanges are initiated available for context establishment: the context transfer and by the serving PoA with a list of neighbor PoAs indicated by the proactive negotiation [1]. the L2-Acc-Mgr. During HO execution, a target PoA may The context transfer is an adequate establishment solu- require additional information elements from the serving tion for transferable information elements. It is performed PoA. As such, it can engage reactive context transfers with between the entity managing the information element the previous serving PoA. and one or a set of PoAs. In the same way, condition- Proactive negotiations are engaged between the termi- ally transferable information element re-establishment can nal and neighbor PoAs through the current association be based on a context transfer mechanism. After being (established with the serving PoA). It is mostly used for translated, an information element is transferred to target information elements managed by PoAs that cannot be PoAs. established through context transfer. The context transfer is not the appropriate solution The L2-Acc-Mgr is responsible of managing L2-HO for the re-establishment of non-transferable information management exchanges with entities associated to its access elements. An information element might require to be re- subnetwork (i.e., PoAs and terminals) and L2-Acc-Mgrs established over standard exchanges or the involvement from other access subnetworks. Consequently, the L2-HO of the terminal in the negotiation or generation process. management exchanges are limited to the access subnetwork It remains possible to establish non transferable infor- during intrasubnet mobility. Intersubnetworks exchanges are mation elements using proactive negotiations. The latter relayed by L2-Acc-Mgrs during inter-subnetwork mobility. are based on the standard exchanges usually performed A target L2-Acc-Mgr converses with the serving L2-Acc- during the network entry procedure to generate information Mgr for centralized establishment exchanges as shown in elements. Figure 4. The adequate time to perform a context establishment depends on the stability of the information element value In a nonoptimized architecture, the HO management during the time. There are static information elements exchanges between PoAs are routed through the core net- that values do not change during the local association and work from one access subnetwork to another during inter- dynamic information elements that values change during subnet mobility. The HO management exchanges between a local association based on network conditions, terminal PoAs and centralized entities, during an intra-subnet mobil- behaviors, accounting constraints, and so forth. Proactive ity event, are engaged through the core network while context establishment can be performed with static infor- the terminal mobility is restricted to the access network. mation elements so that it will be available immediately Thus, the use of L2-Acc-Mgrs restricts as much as possible at the HO execution. However, proactive establishment is the HO management operations to intra-access subnetwork not excluded with dynamic context. This depends on the exchanges. This may ensure the eﬃciency of these exchanges frequency of information element update. If an information and reduce the signaling overhead over the core network. element is known not to be frequently updated, it remains possible to perform a conditional proactive establishment. 4. WiFi-WiMAX Network The information element shall be associated to a validity condition. At the time of the handover, the information As an application of the technology-integration framework, element is used only if the validity condition is verified. In we propose the integration of the WiFi and WiMAX 6 Journal of Computer Systems, Networks, and Communications L2-Acc-Mgr L2-Acc-Mgr (2) Access subnetwork Access subnetwork (3) (4) (1) Serving PoA Reactive exchange Serving PoA Reactive exchange Neighbor PoA Proactive exchange Neighbor PoA Proactive exchange Target PoA Target PoA (a) proactive establishment (b) reactive establishment Figure 2: Centralized establishment. Access subnetwork Access subnetwork Serving PoA Proactive negotiation Serving PoA Proactive negotiation Neighbor PoA Context transfer Neighbor PoA Context transfer (a) Proactive negociation (b) Context Transfer Figure 3: Distributed establishment. technologies in a heterogeneous wireless network. This 4.1. WiFi-WiMAX Integration in the Literature. Some network oﬀers to terminals a wireless connectivity adapted researches were interested in the collaboration between to their location. The WiMAX is deployed for an outdoor WiFi and WiMAX technologies. Most of these researches access and the WiFi in building for indoor access. Terminals have proposed to use the WiMAX technologies as backhaul will roam from one technology to another according to support for WiFi hotspot [7, 15, 16]. Therefore, the designed their movements while being attached to the same global networks did not fall within the category of 4G networks, and network. the two technologies do not cooperate to oﬀer the wireless Journal of Computer Systems, Networks, and Communications 7 access to mobile users. More recent research studies were Table 1: User priority to traﬃc class mapping. interested in the inter-working of the WiFi and the WiMAX User Priority Traﬃc Type Description as access technologies in the same heterogeneous network. However, the majority of these studies were limited to the 1 Background Bulk transfers, games, etc. enhancement of the HO decision mechanism between the 2 Spare two technologies and did not discuss the problems related to 0 Best Eﬀort Ordinary LAN priority the integration and the collaboration of these technologies in 3 Excellent Eﬀort Best Eﬀort for important users the same network architecture [17–19]. 4 Controlled Load Some important applications In [20], authors were interested in inter-working of 5 Video Less then 100 millisecond delay the WiFi and the WiMAX technologies. They proposed a 6 Voice less than 10 millisecond delay solution to ensure a continuity of QoS management through 7 Network Control High requirements the heterogeneous wireless access. The solution proposes a mapping between the QoS management parameters of each technology to ensure seamless change of technologies. To (ii) a per-flow QoS management, the parameterized QoS, fix the context of their work, authors tried to define an based on QoS parameters associated to virtual traﬃc interconnection architecture for the network. They proposed stream. The latter are a set of data packets to be the interconnection of separate WiFi and WiMAX access transferred in accordance with the QoS requirements networks through a core network and to manage the layer- of an application flow. 3 HO using Mobile IP. However, no additional management arrangements were proposed (e.g., collaboration between The WiFi equipments and deployed networks are fol- QoS accounting, context transfer between BSs and APs) to lowed by particular evolution. Indeed, the QoS management enable the use of the QoS mapping through the deployed proposed by IEEE 802.11e was not adopted in network access network. deployments. The enhancements of the communication Thus, at the best of our knowledge, there is no serious performances were based on the evolution of the PHY layer work that oﬀers a design of a heterogeneous network performances. integrating the WiFi and the WiMAX technologies. With the WiFi-WiMAX integration, the WiFi technology will coexist with the WiMAX technology, which oﬀers a strong service diﬀerentiation between categories of data 4.2. Technologies’ Overview. We propose an overview of traﬃcs based on user profiling (c.f. the next subsection). So the WiFi [21] and WiMAX technologies [22]. We focus as to oﬀer a homogenous network access service to users over particularly on the network architecture and the layer-2 the network, we propose to adopt a QoS-enabled WiFi access network service defined by each technology and the manners in our specification. We consider the Parameterized QoS as in which they interact with mobility management. it most closely matches the QoS management defined by WiMAX [25]. 4.2.1. WiFi. The WiFi technology is based on the IEEE The Parameterized QoS proposes a QoS management 802.11 standard that defines the PHY and MAC layers for based on virtual connections: the Traﬃc Streams (TSs). The the wireless medium. This standard has been completed by latter are sets of data packets to be transferred in accordance several extensions that define services such as the QoS man- with the QoS requirements of an application flow. A terminal agement and user authentication. The proposed specification specifies TS requirements to the Access Point (AP) using is limited to the management of these services through the the admission control exchange. The requirements can be wireless part of the network and has not defined operations data rate, packet size, service interval, and so forth. An AP that involve centralized entities. may accept or reject new Traﬃc Specification requests based User authentication is proposed by IEEE 802.11i exten- on the network conditions, terminal profile, and so forth. sion [23] that defines a robust securing mechanism oﬀering The traﬃc diﬀerentiation is based on traﬃc specification a privacy equivalent to wired network. It proposes a complete (TSPEC) associated to TSs. The TSPEC element contains a security framework defining the security architecture, the key set of QoS parameters that define the characteristics and the hierarchy, and the cryptographic mechanisms. The 802.11i QoS expectations of a traﬃc flow. In addition User Priorities authentication is based on an authentication key hierarchy (UP) are used to indicate the traﬃc class of the TS. Table 1 and key generation exchanges. They establish mutual authen- presents the mapping between UP values and traﬃc class. tication between peers and generate cryptographic suite to The WiFi technology was developed to be an extension secure data exchanges. of wired networks and not as an operator technology such as The basic IEEE 802.11 standard oﬀered only a best eﬀort WiMAX or UMTS. Thus, the IEEE 802.11 standard and its service to an application flow. The QoS management for extensions have not specified the core network architectures the WiFi technology has been defined by the IEEE 802.11e and mechanisms. The deployment of RSN security and extension [24]. Two operation modes have been defined: parameterized QoS requires an AAA server that manages the identities and the profiles of authorized users. (i) a per-packet QoS management, the prioritized QoS, The negotiations defined by the WiFi authentication based on priorities associated to transmission queues and the parameterized QoS, during the network entry, with diﬀerent channel access priorities, require considerable time, which turns into a connection 8 Journal of Computer Systems, Networks, and Communications Centralized server Serving Core network Target L2-Acc-Mgr L2-Acc-Mgr Access Access subnetwork subnetwork Serving Target Target PoA PoA Target PoA PoA Terminal Inter-subnet exchanges Intra-subnet exchanges Figure 4: HO management exchanges. interruption during a handover. The authentication process mobility management. CSN includes network elements such can last up to 1 s [26]. Several solutions are available to as routers, AAA proxy/servers, and user databases. The QoS ensure reduced authentication delays during horizontal HO management is defined by the NWG specification [30–33] less than 25 milliseconds (ms) [27]. However, these solutions and the IEEE 802.16e-2005 standard [29]. It defines the are not eﬀective for a heterogeneous HO management, which data traﬃc diﬀerentiation mechanism over the wireless link will be the current architecture results to a new network entry and associated management functions included in the core for the target technology. network entities, that is, ANS-GWs and Authorization and Accounting servers. A terminal is associated with a number of service 4.2.2. WiMAX. The WiMAX technology oﬀers a last mile flows characterized by QoS parameters. This information wireless broadband access as an alternative to cable and DSL. is provisioned in a subscriber management system or in a It defines the physical layer design and the wireless medium policy server, typically a AAA server. A service flow is a MAC access mechanism and network services such as the QoS transport service that provides unidirectional transport of management, mobility management, user authentication, packets (uplink or downlink). IEEE 802.16 specifies five Data and accounting for wireless part of the network based on Delivery services in order to meet the QoS requirement of the IEEE 80216 standards [28, 29]. In addition, an end-to- multimedia applications: Unsolicited Grant service (UGS), end network specification is proposed by the WiMAX forum Real-Time Polling Service (rtPS), Non-Real Time Polling [30–33]. It includes the core network architecture reference Service (nrtPS), Extended Real-Time Variable Rate (ERT-VR) models, protocols for end-to-end aspects, procedures for service, and Best Eﬀort (BE). Each Data Delivery Service QoS management, and user authentication. is associated with a predefined set of QoS-related service The reference model defines a logical modeling of the flow parameters. The QoS profile, which is a set resource- network architecture. The Access Service network (ASN) access authorizations and preprovisioned service flows, is is defined as a set of network functions providing radio downloaded from the AAA server to the ASN-GW at access to mobile stations. The Connectivity Service Network the network entry as a part of the authentication and (CSN) is a set of network functions that provides IP con- authorization procedure. Service flows creation is initiated nectivity services to Mobile Stations such as IP parameters based on negotiation exchanges engaged by the terminal, the allocation, Policy and Admission Control, and Inter-ASN BS, and the ASN-GW. Journal of Computer Systems, Networks, and Communications 9 Core network Core network Access router Access Access WiMAX subnetwork WiFi subnetwork subnetwork subnetwork BS AP (a) heterogeneous access subnetworks (b) homogeneous access subnetworks Figure 5: WiFi-WiMAX network. Security in WiMAX network is based on Key manage- technologies. In all types of deployment, a mobile terminal ment protocol (PKM). The latter defines mutual authen- may execute vertical HOs (BS to AP and AP to BS) and tication exchanges between the terminal and the network horizontal HOs (AP to AP and BS to BS). Figure 5 shows the entities, that is, the BSs and the ANS-GWs. These exchanges two deployments. result in the generation of a hierarchical sequence of authentication keys. Each key is related to the authentication of the terminal with a level of the access network: BS, ASN- 4.3.2. The L2-Acc-Mgr. L2-Acc-Mgrs, associated to access GW, and AAA server. After the authentication, the terminal subnetworks, manage the L2-HO for both vertical and hori- negotiates with the serving BS a cryptographic suite for each zontal HOs. They support WiFi and WiMAX specific functions provisioned service flows. that manage authentication and accounting exchanges with terminals during network entries. An L2-Acc-Mgr acts as an The WiMAX network entry procedure requires, as with ASN-GW for the WiMAX terminals and as an AAA proxy WiFi, several exchanges for the authentication and the for the WiFi terminal during the network entries. These establishment of provisioned service flows. The technology functions allow the L2-Acc-mgr to support layer-2 service defines an HO management mechanism based on proactive proxy function. and reactive terminal context transfers from the ASN-GW and the serving BS to target BSs while attempting to ensure This specification defines management exchanges bet- minimal delay and data loss during the HO procedure. The ween L2-Acc-Mgr and PoAs (APs and BSs), the intelligence terminal context includes authentication parameters, service related the triggering of exchanges, and the management of flow parameters (QoS information, cryptographic informa- context information elements. We limit the description of tion, classification rules, etc.), and PHY capabilities of the the neighborhood management function to the definition of terminal. Having these information elements, a target BS will Recommended PoA lists. The actual content is to be defined be able to associate the terminal during the HO procedure by the network operator that can define the neighborhood with the minimum of negotiation exchanges. However, such management function based on wireless cell load, network as the HO management mechanism defined for the WiFi, this topology, PoA geographic neighborhood, link status, and optimization is restricted to horizontal HOs. mobility behaviors. The translation functions define the information element values to be established during HO procedures for both 4.3. WiFi-WiMAX Integration vertical and horizontal HOs. This specification considers the user authentication, the QoS management and WiMAX PHY 4.3.1. Network Architecture. We propose a flexible deploy- layer enhancement as the services to be managed during the ment schema for the network architecture. The access L2-HO preparation procedure. In the next subsection, we subnetworks may oﬀer a homogeneous deployment that detail the specification of this function. gathers PoAs oﬀering the same technology: WiMAX subnet- works including Base Stations (BSs) and WiFi subnetworks including Access Points (APs). It is also possible to oﬀer 4.3.3. Terminal Context Translation. For horizontal HOs, the a heterogeneous deployment that gathers PoAs according translation function provides context information elements to the wireless coverage neighborhood apart from their based on the ones used during actual association. The 10 Journal of Computer Systems, Networks, and Communications Table 2: QoS mapping between IEEE 802.11e and IEEE 802.16e- mapping between QoS parameters, we propose to consider 2005 classes. the matching defined by the IEEE 802.16e-2005 between 802.16e-2005 Scheduling services and QoS parameters as a reference in Data Delivery 802.11e UPs Application the translation procedure. The parameters associated to a service traﬃc flow depend on the traﬃc class associated to it in both UGS 6,7 Voice IEEE 802.11 and IEEE 802.16e-2005. We propose a static translation procedure between QoS parameters to be used by Voice with silence ERT-VR 5 the Translation Function. The translation process depends suppression on the QoS information related to the current terminal RT-VR 4 Video association, that is, the serving technology. NRT-VR 3 FTP BE 1,2,0 Email,Web (i) Terminal associated to a IEEE 802.11 PoA: in this case, the Parameter Translation Function translates the TSPEC list into an SF info list. computation is based on what is defined by each technology for internal HO optimization. Firstly, the UP related to the TS is translated into When the context establishment is executed to prepare a Data Delivery Service in accordance to mapping a vertical HO (serving PoA and target PoA with diﬀer- proposed in Table 2. The retained Data Delivery ent technologies), the computation of values of context Service indicates the IEEE 802.11e QoS parameters information elements is less obvious than with horizontal to be determined using the translation. Secondly, HOs. However, we have found a similitude between the the Parameter Translation Function defines values QoS and authentication management of WiMAX and WiFi. related to the Data Delivery Service parameters based Therefore, we define a mapping between the terminal context on the mapping in Table 3. of the WiFi and WiMAX that enables the translation function (ii) Terminal associated to IEEE 802.16 PoA: in this case, to define values for WiFi context information-elements the Parameter Translation Function translates the SF (resp., for WiMAX context information-elements) based on info list into a TSPEC list. values related to a WiMAX association (resp., for WiFi association). SF info includes the Data Delivery Service and related QoS parameters. The Parameter Translation Function translates the Data Delivery Service into (a) QoS Information Elements. Regarding QoS management, a UP based on mapping defined in Table 2. Then, the traﬃc diﬀerentiation defined by IEEE 802.11e parame- it defines which parameters to be included in the terized QoS mechanism and the WiMAX QoS management TSPEC and their values. are very similar, particularly Traﬃc Stream and Service Flow concepts. Table 3 presents the mapping used to compute IEEE We specify an association between User Priorities used in 802.16e-2005 QoS parameters based on the IEEE 802.11e IEEE 802.11e and IEEE 802.16e-2005 Data Delivery services. parameters. These two types of information are used to characterize in We now discuss some translation choices and diﬀerence each technology the class of the traﬃc flow. We suggest with mapping used in the reverse translation (i.e., from the static association between class of services of both 802.16e-2005 parameters to 802.11e ones). technologies shown in Table 2. Classes are mapped according to the key QoS requirement for each Data Delivery Service. (a) Unsolicited Grant Interval parameter indicates the As shown in the mapping table, more than one User Priority nominal interval between successive grant oppor- correspond to UGS and BE data delivery service. Therefore, tunities for UGS and ERT-VR flows. Unsolicited when the IEEE 802.16e-2005 is the serving technology, we Polling Interval parameter indicates the same QoS propose to map Service Flows with data delivery service characteristic for RT-VR flows. These parameters do corresponding to UGS into TSs with UP equal to 6 and those not have an equivalent in 802.11e QoS parameters. with data delivery service corresponding to BE into TSs with However, the TSPEC include Maximum Service UP equal to 1. Interval and Minimum Service Interval that defines, In addition, we propose a mapping between QoS param- respectively, maximum and minimum of the interval eters associated to each IEEE 802.16e-2005 Data Delivery ser- between the start of two successive transmission vice and IEEE 802.11e QoS parameters defined in the TSPEC opportunities. Thus, we use these two parameters information element. The IEEE 802.16e-2005 defines specific to define a mean value corresponding to the IEEE QoS parameters for each Data Delivery Service. However, 802.16e-2005 parameter: (MinimumServiceInterval IEEE 802.11e defines a list of parameters used for QoS + MaximumServiceInterval)/2. When the current characterization that may be more extensive than needed serving technology is the 802.16e-2005, we may or available for any particular instance of parameterized allocate the same value to Maximum and Minimum traﬃc. The specification does not define a correspondence Service Interval 802.11 parameters. This value tallies between traﬃc categories (defined using UPs) and possible to Unsolicited Grant Interval or Unsolicited Polling lists of associated parameters. To be able to ensure a Interval value depending on Data Delivery Service. Journal of Computer Systems, Networks, and Communications 11 MSK L2-Acc-Mgr PMK PMK AK AK HO-Req HO-Req(AK) HO-Req(PMK) Serving BS AK Target BS Target AP MSK MS PMK AK Figure 6: Proactive key distribution, Scenario 1. (b) The correspondence between Traﬃc Priority and to the authenticator. The authenticator computes a Pairwise User Priority is defined only for mapping from 802.11 Master Key (PMK) and an Authorization Key (AK); it specification to the 802.16 one. In the reverse case, the transfers the AK to the Base Station. A 3-way-handshake value of the User Priority parameter is obtained based exchange is performed between the terminal and the BS on the Data Delivery Service as previously indicated. based on the AK. The exchange results in the generation of (c) The Tolerated Jitter parameter do not have an Traﬃc Encryption Keys (TEK). equivalent in 802.11e QoS specification. However, we The IEEE 802.11i authentication results to an MSK propose to compute a corresponding value based on negotiated between the terminal and the AAA server. The available parameters. The jitter value is defined as J = latter generates a PMK key, based on the identity of the max(D) − min(D) where D is the delay imposed to serving AP, that it transfers to the AP. This key is used to exchanged data packets. We have D = Dl + Dn , where perform the 4-way-handshake between the terminal and the Dl is local delay due to buﬀering and scheduling and serving AP. This exchange computes the Pairwise Transient Dn is the network delay due to the transmission of the Key (PTK) used to secure data transfer. packet. We suppose that Dl is negligible compared to Conforming to the WiMAX specification, the AK is Dn , and thus the latter equation will be D = Dn . Thus, generated by the L2-Acc-Mgr, which acts as an ASN-GW, max(D) corresponds to the Delay Bound 802.11 and delivered to the BS. Similarly, the 802.11 PMK is parameter. Additionally, min(D) can be computed generated by the L2-Acc-Mgr (the 802.11 AAA proxy) and based on the data rate perceived by the 802.11 station. delivered to the AP. The 802.16 AK and the 802.11 PMK The Parameter Translation Function can obtain a have the same functionality in authentication procedures. We Mean Data Rate value based on information gathered consider these two keys as the starting point to define the by the L2-Acc-Mgr about mobile connectivity and inter technology translation for security parameters. cell states. When the terminal is associated with a BS, it shares an 802.16 PMK with the L2-Acc-Mgr. This key is used to (b) Authentication Information Elements. The authentication compute the AK that the L2-Acc-Mgr transfers to the BS. procedures defined by the WiFi and the WiMAX are both During the HO preparation procedure, the L2-Acc-Mgr uses based on negotiation exchanges that result to the generation the 802.16 PMK to generate keys for target PoAs. 802.16 AKs of hierarchical sequences of authentication keys. The two are generated for BSs, and 802.11 PMK are generated for APs. key sequences are similar and have a common root key, the Figure 6 details related exchanges. Master Session Key (MSK), negotiated between the AAA When the terminal is associated with an 802.11 AP, it server, and the terminal for WiFi and WiMAX. Thus, it shares an 802.11 PMK with the L2-Acc-Mg.During the HO is possible to define a mapping between levels of two key preparation procedure, the L2-Acc-Mgr uses the 802.11 PMK sequences. to generate keys for target PoAs. 802.16 AKs are generated for The WiMAX authentication procedure results to the BSs, and 802.11 PMK are generated for APs. Figure 7 details establishment of the MSK transferred from the AAA server related exchanges. 12 Journal of Computer Systems, Networks, and Communications Table 3: QoS mapping between IEEE 802.11e and IEEE 802.16e-2005 classes. IEEE 802.16e-2005 parameter IEEE 802.11e parameter Description Maximum Sustained Traﬃc Rate Peak Data Rate The peak information rate in bit per second The latency period starting at the arrival of a packet at the MAC till Maximum Latency Delay Bound its successful transmission to the destination Minimum reserved Traﬃc rate Minimum Data Rate The minimum data rate required by the traﬃc flow The maximum continuous burst the system should accommodate for Maximum Traﬃc Burst Burst Size the traﬃc flow SDU size Nominal MSDU size Number of bytes in a fixed size packet The maximum nominal interval between successive polling grant Unsolicited Polling Interval (a) opportunities for the traﬃc flow The nominal interval between successive grant opportunities for the Unsolicited Grant Interval (a) traﬃc flow The priority among two IEEE 802.16e-2005 service flows identical in Traﬃc Priority User Priority (b) all QoS parameters. Tolerated Jitter (c) The maximum delay variation (jitter) (in milliseconds) (c) WiMAX PHY Information Elements. The WiMAX tech- HO-Request, which includes QoS information elements sent nology defines parameters related to PHY-layer capabilities by the serving AP to the L2-Acc-Mgr. The translation func- of terminal. These parameters have no equivalent in the WiFi tion builds the contexts related to PoAs in the Recommended specification. Thus, we maintain a caching mechanism for PoA List. The HO management function initiates context PHY-layer capabilities managed by the translation function. transfer to PoAs using HO Request messages that includes PHY-layer capabilities of terminals are maintained during terminal contexts. Based on target PoA responses, which the ongoing session. When preparing an HO with target BSs, indicates the support of terminal requirements, the HO if a terminal has never been attached to a BS in previous management function builds the PoA List that is forwarded associations, the L2-Acc-Mgr sends an HO-Req to target to the serving AP. The serving AP transfers the list to BSs without these parameters. Additionally, it indicates to the terminal. The cryptographic suites are established, with the terminal, in the recommended Candidate PoA List, to available PoAs, using a context transfer with target APs and a execute proactive exchanges to negotiate these parameters proactive negotiation with the target BSs. with target BSs. The previous example describes a preparation procedure performed with target PoAs in the same access network as the 4.3.4. Context Establishment Procedure. The L2-HO opti- serving PoA. The HO messages are exchanged between PoAs, mization is based on the establishment of terminal contexts and the L2-Acc-Mgr managing the subnetwork and context on target PoAs to avoid their re-negotiation and conse- messages are exchanged between involved PoAs. When a quently reduce the HO delay. The context establishment target PoA is located in an access network diﬀerent from the procedure is mainly proactive. The neighborhood man- serving PoA one, the HO management exchanges are relayed agement function provides the Recommended PoA List to between the serving L2-Acc-Mgr and the target L2-Acc-Mgr which the establishment is initiated. The QoS parameters, to reach the involved entities. The serving L2-Acc-Mgr is the the authentication keys, and the WiMAX PHY profiles are manager of the preparation procedure while the target L2- established based on a context transfer managed by the L2- Acc-Mgr relays the messages between the latter entity and the Acc-Mgr. The cryptographic suites are established based on target PoA. Figure 9 shows the exchange. a context transfer between the serving PoA and target PoAs Regarding context transfers between PoAs and proactive (preparation of a horizontal HO) or proactive negotiation negotiations between the terminal and the target PoAs, we between the terminal and target PoAs (preparation of a make the choice not to execute these exchanges during the vertical HO). The translation function computes values for inter-subnet preparation procedure. Therefore, the prepa- the information elements to be established based on the ration will be limited to centralized exchanges performed available terminal context. between the L2-Acc-Mgr and the PoAs. This is justified by In addition to proactive establishment, the specifica- results we have obtained in work related to HO preparation tion defines reactive establishment exchanges that may be mechanisms proposed for the IEEE 802.11 networks regard- engaged by the target PoA during the HO execution. ing velocity support and signaling cost [34]. The evaluation Figure 8 shows an example of the proactive phase of the has shown that exchanges performed between PoAs and context establishment procedure. The terminal is associated particularly proactive negotiations are not adapted to inter- with a serving AP. The context establishment is performed subnet mobility. In fact, they increase the signaling cost of with an AP and a BS. When a mobile terminal associates itself the preparation procedure and reduce the HO performance through an AP, the context establishment is started using an in high mobility environments. Journal of Computer Systems, Networks, and Communications 13 MSK L2-Acc-Mgr PMK PMK AK HO-Req HO-Req(AK) HO-Req(PMK) Serving AP PMK Target BS Target AP MSK MS PMK Figure 7: Proactive key distribution, Scenario 2. Serving AP L2-Acc-Mgr Target AP Target BS Terminal HO-Req (QoS param.) Translation procedure HO-Req (QoS param., auth. key) HO-Req (QoS param., auth. key, PHY param.) HO-Resp HO-Resp HO-Ack HO-Resp (PoA List) (PoA List) HO-Ack HO-Ack Context req (crypt. suite) Context Rprt 3-way-handshake Key derivation Figure 8: Example of context establishment. 4.3.5. HO Execution Optimization. The HO preparation The establishment of the terminal context results in an procedure, presented in previous sections, establishes a set of important optimization of the L2-HO execution procedure context information elements and parameters in target PoAs. for both vertical and horizontal HOs. The terminal no The exchanges engaged during the HO execution depend on longer needs to reauthenticate itself and to renegotiate QoS the information elements that were established proactively parameters and PHY profile (when the WiMAX is the target during the HO preparation procedure or requested reactively technology) during the L2-HO execution. during the HO execution. We present in the following para- Figure 10 presents a regular WiFi network entry that graphs possible HO execution scenarios for both WiMAX may be executed during a first network association and and WiFi technologies. We consider optimal scenarios where an optimized reassociation procedure that may be executed target PoAs were able to acquire all context information during HO with an AP. In the first case, the terminal elements. performs a regular 802.11i authentication (2, 3, 4, and 5), 14 Journal of Computer Systems, Networks, and Communications Serving Serving Target Target PoA L2-Acc-Mgr L2-Acc-Mgr PoA Ho-Req Ho-Req HO-Req HO-Rsp HO-Rsp HO-Ack HO-Ack Selection of target PoAs for the candidate PoA List HO-Rsp HO-Ack Figure 9: Inter-subnet HO preparation exchanges. Terminal AP L2-Acc-Mgr AAA (1) 802.11 exchange (2) EAP Terminal AP (3) EAP success (1) 802.11 exchange (5) EAPoL-key-1 (4) PMK transfer (5) EAPoL-key (5) EAPoL-key-2 (5) EAPoL-key (5) EAPoL-key-3 (TS List) (5) EAPoL-key-4 Optimized HO execution (6) Connection establishment Regular WiFi network entry Figure 10: Association versus Re-association with a WiFi Access Point. including exchanges with the AAA server, and the 802.11e Figure 11 presents a regular WiMAX network entry traﬃc streams’ establishment (6). that is executed during a first network association and an During a HO preparation, a target AP may acquire the optimized re-association procedure that have to be executed Traﬃc Stream (TS) list and the PMK during the first phase during an HO with a BS. In the first case, the terminal of the procedure based on exchanges performed with the performs all steps of regular WiMAX association: synchro- serving L2-Acc-Mgr. The target AP acquires also the PTK nization (1), ranging (2), basic capabilities negotiation (3), based on a context transfer or computes this key with a authentication (4,5, and 6), cryptographic key negotiation proactive negotiation performed with the AP. Therefore, in (7,8), and connection establishment (10,11) [29]. the second case of Figure 10, the terminal starts the HO During handover preparation, a target BS may acquire execution with the legal IEEE 802.11 re-association and proactively the authentication key AK, the encryption key authentication. Over Authentication Req/Resp, the terminal list TEK list, the SF list, and the WiMAX PHY capabilities and the target AP inform each other about the preestablished of the terminal. So in the second case of Figure 11, The keys. Then, they engage a key-handshake to exchange the HO execution starts with a Ranging exchange between the Group Temporal Key (GTK). If this part of the authenti- terminal and the target BS. The Ranging Response (RNG- cation exchange succeeds, the new serving AP sends to the Rsp) indicates the re-entry steps that are omitted thanks terminal the TS List (including TSPECs), and the latter can to the availability of terminal context information elements start data exchange. obtained during HO execution. Then, the target BS sends an Journal of Computer Systems, Networks, and Communications 15 Terminal BS L2-Acc-Mgr AAA (1) MAC synchronization (2) (RNG-Req/Rsp) MS BS (3) SBC-Req (3) SBC-Rsp (2) (RNG-Req/Rsp) (4) EAP (9) Reg-Rsp (5) EAP success Bandwidth request (6) AK (7) SA-TEK challenge transfer Optimized WiMAX HO (7) SA-TEK-request (7) SA-TEK-responce (8) Key-request (8) Key-reply (9) Reg-Req (9) Reg-Rsp (11) Connection (10) Data path establishment establishment Regular WiMAX network entry Figure 11: Association versus Re-association with a WiMAX Base station. unsolicited Registration Response (REG-Rsp) that includes Table 4: Handover delay. information about connections. Finally, the terminal sends a Target technology Opt. HO (ms) Non-opt. HO (ms) Bandwidth Request header with zero BR field to the target BS that regards this message as a confirmation of successful WiFi 24, 67 1000 re-entry registration. WiMAX 23, 16 700 As shown in Figures 10 and 11, the handover execution is significantly reduced for both WiFi and WiMAX. both technologies. The WiFi-WiMAX architecture and the L2-HO optimization mechanism proposed in this researches 5. Performance Evaluation have been implemented in the simulator based on the latter In this section, we evaluate the performances of the L2-HO architectures [25]. management for WiFi-WiMAX network. This evaluation In the first scenario, we evaluate the HO delay performed requires the definition of parameters and metrics that will when we use the L2-HO optimization mechanism. We constitute the reference of the evaluation. The evaluation consider a wireless network with a single access subnetwork criteria will highlight both the contributions of new mech- that includes all the PoAs (two BSs and two APs). A terminal anisms and the limits of their application. moves with a straight path to cross the wireless coverage of all PoAs of the network. We measure the delay involved by the executed L2-HOs. To show the contribution of L2- 5.1. Handover Delay. The most obvious criterion that must HO optimization mechanism, we can compare the inter- be evaluated is the HO delay. The latter is defined as the technology HO delay to the network entry delay of the time during which the station is not connected to any PoA. WiFi and WiMAX technologies, which correspond to non- Therefore, the HO delay includes the time required to detect optimized HOs. the need to perform a handover, to choose a target PoA, and Table 4 lists HO delay values obtained with diﬀerent to perform re-association exchanges. types of HOs. The delay due to non-optimized HOs is eval- We adopt the network simulator SimulX [35] that uated to 700 ms when the WiMAX is the target technology supports features that enable the design and the evalu- and 1000 ms when the WiFi is the target technology. Let’s ation of future communication protocols like cross-layer note that the WiFi handover delay is larger than the WiMAX interactions, multi-interface inter-working in terminals, and handover delay although that nonoptimization handover heterogeneous network environments. We have integrated execution of WiMAX seems to engage even more exchanges to SimulX the IEEE 802.11 architecture [14] and the than the WiFi handover execution (c.f. Figures 10 and 11). WiMAX architecture [36]. Both have been validated through Actually, the detection and the search phases contribute simulation tests that result in well-known performances of largely to the delay induced to traﬃc during the handover 16 Journal of Computer Systems, Networks, and Communications procedure of WiFi. However, these phases are well optimized 35 in handover procedure of WiMAX. For example, there is no 30 HO delay (miliseconds) search phase at the time of HO as the serving BS sends a recommended neighbor list to terminal. As a consequence, 25 the overall HO delay of WiFi network entry during HO is 20 larger that of the WiMAX. 15 The L2-HO management mechanisms ensure a uni- form execution time for both intratechnology and inter- 10 technology HOs limited to a mean value of 24,63 ms. This 5 is obtained thanks to the context establishment mechanism that ensures the same optimization of the HO execution 0 0 2 4 6 8 10 12 14 regardless of the target PoA type. 802.16 frame duration (milliseconds) In a second phase of this evaluation, we study the eﬀect of wireless cell conditions on the performances of the L2- WiMAX-WiMAX HO optimization performances. We consider a network WiFi-WiMAX topology integrating six BSs with six APs in each WiMAX cell. The PoAs are attached to two access subnetworks: a WiFi Figure 12: Eﬀect of the 802.16 frame duration on optimized HO subnetwork and a WiMAX subnetwork relayed through a performances. core network, which hosts also the AAA server. A terminal moves with a straight path and a velocity of 10 m/s. We measure the HO delay for WiFi to WiMAX and WiMAX to 120 WiMAX handovers. In WiFi networks, the performance of terminal exchanges HO delay (miliseconds) 100 depends on the cell load because of the contention-based 80 medium access [27]. In a previous research, we were interested in the evaluation of HO performances in WiFi 60 networks. We showed that the wireless cell load has non- negligible eﬀects on the HO execution performances. We 40 evaluated a management mechanism that ensures the same 20 optimization of HO execution for WiFi terminals. Results demonstrated that such optimization ensures a limited 0 0 5 10 15 20 execution time (lower than 50 ms) even with high loads. Number of terminals The performance of WiMAX wireless access is not sensitive to the cell load as the medium access is managed by WiFi-WiMAX WiMAX-WiMAX the BS that allows transmission opportunities to the medium WiMAX-WiFi WiFi-WiFi modeled by transmission frame [28]. However, two param- eters can have an influence on the performances of HO exe- Figure 13: Eﬀect of number of terminals on optimized HO cution: the IEEE 802.16 frame duration and the contention- performances. based transmission period defined for network entry. The duration of the IEEE 802.16 frame, which is config- urable, has an eﬀect on the delay between two transmission opportunities for one terminal, which impacts on the delays procedure with a BS. This period has a limited duration for exchange between the terminals and the BS. In a previous during a single frame. The exchanges over it will be impeded research, we have evaluated the variation of the regular by the number of terminals trying to communicate. WiMAX network entry as a function of the frame duration. To evaluate the eﬀect of the number of terminals Results have shown that the network entry duration vary executing a network entry on the HO delay, we define a from 700 ms to 1 s with frame duration that varies from 3 ms simulation scenario that varies the number of terminals to 12 ms. executing HOs in the same contention-based transmission We evaluate the eﬀect of the frame duration of the period of a cell, and we measure the average of HO delays. optimized WiMAX handover. Figure 12 plots the delay due The simulation scenario defines a set of terminal moving to optimized WiMAX handover as a function of the 802.16 at the same velocity, over similar trajectories, and neighbor frame duration. This curve shows that the handover delay starting points. The network topology includes six BSs with increases when the lEEE 802.16 frame duration increases. six APs in each WiMAX cell. However, even with frame duration of 12 ms the handover Figure 13 plots the evolution of the HO delay as a delay remains reasonable and does not exceed the value of function of the number of terminals. The curves show an 50 ms (tolerable threshold of real-time applications). increase of the HO execution time (WiMAX to WiMAX HOs The second parameter considered for WiMAX cells is and WiFi to WiMAX HOs) with the increase of the number the contention-based transmission period. It is used by a of terminals. This parameter exceeds 50 ms as soon as the terminal that starts an HO procedure or an association number of terminals that try to associate exceeds 5. Journal of Computer Systems, Networks, and Communications 17 5.2. Signaling Cost. We propose to evaluate the signaling overhead of the HO management mechanism associated to the WiFi-WiMAX integration network. This evaluation aims to compare the new architecture with alternative network deployments under the same conditions. We consider a realistic deployment of the WiMAX and WiFi technologies over a city. The WiMAX is used to oﬀer an outdoor access while the WiFi is used to oﬀer indoor accesses. As shown in Figure 14, the WiMAX access is oﬀered to user over a continuous coverage. The WiFi access is oﬀered via WiFi AP coverage scattered areas over the WiMAX coverage. WiMAX BS coverage We compare the performances of the integration archi- tecture (optimized architecture) to an architecture that Figure 14: WiFi-WiMAX wireless coverage. does not integrate an L2-Acc-Mgr (non-optimized archi- tecture). In the latter architecture, we suppose that the ×104 HO management functions, for example, neighborhood 10 management and context establishment, are supported by 9 HO signaling cost centralized network servers. In addition, we evaluate the 8 (bytes∗ weight) influence of the design of access subnetworks (homogeneous 7 deployment versus heterogeneous deployment) on the HO 6 management signaling cost performances. Four network 5 architectures are considered: non-optimized architecture 4 with homogeneous deployment, non-optimized architecture 3 2 with heterogeneous deployment, optimized architecture Heterogeneous Homogeneous with homogeneous deployment, and optimized architecture with heterogeneous deployment. The signaling cost of a management mechanism is the transmission cost of management messages over the network links. We define a signaling cost formula that models the Architecture signaling overhead generated by one HO. This formula takes Optimized into account the proactive exchanges with neighbor PoAs Non-optimized during the HO preparation and the execution exchanges with a target PoA at the time of HO as shown in (1): Figure 15: Basic configuration signaling cost. SHO = SHOpreparation + SHOexecution . (1) integrates PoAs whose coverage areas are tangent to the We consider three types of network links: the local serving PoA one. links (between entities in the same access subnetwork), the In a first evaluation, we consider an arbitrary configu- core network links, and the wireless links. To each link we ration with fixed value for link weight. These values indicate associate a weight that models the cost of transmitting of that the transmission cost of a management message over the one byte over this link. These weights allow to quantify core links is twice the transmission cost over the local links. link transmission costs relatively rather than define absolute The transmission cost over the wireless links is fourfold the values. A signaling cost formula is the sum of subformulas transmission cost over local links. With this configuration, that are products of the messages’ size into the crossed links’ Figure 15 plots the measured HO signaling costs related to weight. network architectures. The sub-formula SHOpreparation of (1) (resp., SHOexecution ) Both the optimized architecture and the heterogeneous is diﬀerent as the HO preparation is engaged from a serving deployment reduce the signaling cost of an HO. Particularly, AP or a serving BS (resp., the HO execution is engaged with a combination of these strategies in the same network oﬀers a a target AP or a target BS). significant reduction of the HO signaling cost. The optimized We make use of the VanetMobiSim software to emulate architecture allows the confining of establishment exchanges the terminal mobility over the considered wireless deploy- at best to an access network and at worst to a connection ment [37]. This software oﬀers the list of executed HOs between two L2-Acc-Mgrs. As a result, there is no more considering a wireless deployment and a mobility model. The exchanges with centralized servers for HO management. On combination of the signaling cost formulas and the mobility the other hand, the heterogeneous deployment allows to statistics allow us to evaluate the signaling cost average gather neighbor PoAs in the same access network. The use of the HO management over the considered deployment of the latter deployment with a non-optimized architecture [25]. We assume a mix of three types of mobility model: enables to reduce inter-PoAs exchanges to the intra-access walking users, slow cars, and fast cars. We consider one networks exchanges, which reduces significantly the HO hop neighborhood definition. The Recommended PoA list management signaling cost. With an optimized architecture, 18 Journal of Computer Systems, Networks, and Communications the heterogeneous deployment enables, as well, to confine ×104 centralized exchanges to into one access network. 24 HO signaling cost (bytes∗ weight) In a second step, we study the eﬀect of architecture 22 parameters on the HO management signaling cost. We con- 20 18 sider the core-link weight and the neighborhood definition. 16 Figure 16 plots the evolution of the handover signaling 14 cost as a function of the core-link weight. Both the optimized 12 architecture and the heterogeneous deployment reduce the 10 eﬀect of core link cost on the HO signaling cost. The com- 8 bination of an optimized architecture and a heterogeneous 6 deployment oﬀers the better optimization. These results 4 confirm that the design of a network architecture based 2 on this combination reduces the consumption of the core 1 2 3 4 5 6 7 8 9 10 network resources by HO management signaling overhead. Core links’ weight In fact, the signaling exchanges related to a mobile terminal Non-optimized homogeneous architecture will be enclosed in the wireless cells and access subnetworks Non-optimized heterogeneous architecture in its mobility areas. Thus, the proposed designs ensure the Optimized homogeneous architecture enhancement of HO performances while reducing the core Optimized heterogeneous architecture network resources. The enlargement of neighborhood definition is impor- Figure 16: Core Link weight eﬀect on HO signaling cost. tant to ensure a better mobility support. Indeed, a multiple- hop neighborhood should ensure a good support of fast ×104 moving terminals. However, this neighborhood definition 12 may result to an increase of the signaling cost of HOs. HO signaling cost (bytes∗ weight) To study the eﬀect of the neighbor list size, we assume 10 a second neighborhood definition including PoAs that are 8 reachable within two hops. The neighbors of an AP are the APs that surround within two hops and the BS that covers 6 the area if it is reachable by a terminal on two hops. The 4 neighbors of a BS are the APs on its coverage zone reachable 2 at most with two hops and the BSs in its immediate wireless heterogeneous Non-optimized Non-optimized heterogeneous homogeneous homogeneous Optimized neighborhood. Optimized We compare the HO signaling costs of this neighborhood definition to those obtained with the one-hop neighborhood definition proposed in the basic network configuration. The results are shown in Figure 17. Both the optimized archi- Architecture tecture and the heterogeneous deployment reduce the eﬀect One hop of the growth of the neighbor-list size on the HO signaling Two hop cost. As in the previous evaluation, the combination of Figure 17: Neighborhood definition eﬀect on HO signaling cost. these network designs oﬀers the better results regarding HO management signaling cost. This combination allows the operator to design wireless network with better mobility support without increasing the HO management signaling 6.1. Collaboration with FMIP. The Fast handover for Mobile overhead. IPv6 (FMIPv6) [38] proposes an improvement to the MIPv6 that reduces the layer-3 handover latency. FMIPv6 defines a collaboration between access routers (ARs) to accelerate 6. Interaction with Layer-3 Handover the acquisition of link configuration parameters and the for- Management Mechanisms warding of data traﬃc when a terminal executes a handover from a previous AR (PAR) to a new AR (NAR). It enables In this study, we are interested in optimization of HO the mobile terminal to learn the IPv6 link configuration performances in heterogeneous networks. Our proposals parameters (IP subnet) related to links, that it detects, before have been limited to the management of layer-2 handovers it starts eﬀectively the HO execution. The terminal may (L2-HO). Thus, it seemed interesting to study the interaction request information, about all wireless links, to the current of this framework with additional HO management mech- router. The reply can be received on the old link or on anisms, proposed in the literature, that may be deployed the new link (reactive HO). During the HO execution, the in heterogeneous networks. We consider in particular the terminal sends a message to the NAR to inform it about the mobility management based on FMIPv6 and the Media movement. Independent Handover (MIH) mechanism proposed by the The framework, proposed in this research, enables two IEEE 802.21 standard to optimize vertical HOs. possible configurations regarding L3-HOs. In the first case, Journal of Computer Systems, Networks, and Communications 19 access subnetworks oﬀers heterogeneous access technologies, MIH to the specification we have proposed. MIH functions which allow having several technologies on the same IP can be used, for example to, transfer the Recommended PoA subnetwork (with the same prefix). This approach avoids list to the terminal during HO preparation. the need to define a relation between the L2-HO mecha- nisms and a possible L3-HO, since the latter is no longer necessary. With the other possible configuration, each access 7. Discussions about Heterogeneous subnetwork oﬀers a single access technology, that is, WiFi Technology Integration access subnetworks and WiMAX access subnetworks. With this architecture, a vertical HO leads to a L2-HO associated to It is obvious that the mobility management in the het- an L3-HO. Therefore, in addition to the L2-HO management erogeneous wireless networks is more complex than classic mechanism we have defined, there is a need to ensure a wireless networks. Indeed, the more we try to optimize management of the L3-HO. This can be possible by defining the HO at a low level (to ensure better performances), the an interaction between the latter mechanism and FMIPv6. more proposed solutions are dependent on the specificities The L2-HO management mechanism defines the reception of technologies. This makes diﬃcult the optimization of of neighboring PoAs list with which the HO preparation the L2-HO between heterogeneous technologies, particularly has been performed. This list may be used, by the FMIPv6 when their designs are based on diﬀerent principles, for module, to engage the management procedure defined pre- example, the network accesses (connected mode or shared viously with ARs attached to PoAs in the list. Upon receiving access mode), core network organization, and so forth. In an indication of the imminent HO execution, the terminal this research, we have been able, as well, to propose a knows its next AR; so it can prepare the configuration of its layer-2 handover optimization solution based on general interface with new IP parameters and wait for the indication and technology-agnostic framework. This framework oﬀers of the L2-HO handover execution success. The latter HO mechanisms that optimize the L2-HO delay independently of execution is optimized thanks to the preparation procedure the engaged mobility type (homogeneous or heterogeneous), of the L2-HO management mechanism. The link availability which is a novel idea. indication may also be used to trigger the preparation of Another interesting point related to this framework is following handovers. the ability of the proposed architecture to facilitate the extension of heterogeneous networks based on additional technologies. In fact, the location of HO management 6.2. Collaboration with the MIH. The Media Independent functions at L2-Acc-Mgr allows avoiding the modification Handover (MIH), proposed by the IEEE 802.21 [39], defines of technology specific network entities, for example, PoAs, tools to manage multiple interfaces in the same terminal. and functions, for example, authentication and accounting Particularly, it manages exchange of information elements during these possible extensions. Modifications are restricted between the terminal and the network to enhance the to the adaptation of the L2-Acc-Mgr and their functions. decision and search phases of the handover procedure. It Let us consider the extension of the WiFi-WiMAX network, also helps the preparation of the HO execution between we have proposed in Section 4, based on a UMTS access. heterogeneous technologies. For example, the MIH provides This will require, first, to define the possible associations to upper layers, link-layer triggers based on reactive and between the QoS and security parameters in UMTS, WiFi, predictive local link state changes and network information and WiMAX to include adequate translation rules at the (load balancing information, operator preferences) that Translation function. Second, we have to define at UMTS enhance the HO detection. It also supports the transfer core network entities that manage terminal active contexts, of global network information (list of available networks, for example, Radio Network Controllers (RNCs) or Serving neighbor maps and higher layer network services) from GPRS Support Node (SGNC), a context exchange with network servers to the terminal to help it on the HO L2-Acc-Mgrs. Therefore, the latter will be able to execute preparation procedure. However, the handover execution translation rules and to engage context establishment over optimization is not part of the MIH functions. WiMAX BSs and/or WiFi AP. The mechanisms, proposed by the MIH, are complemen- Based on this framework, it is possible to propose a tary to the solution we have proposed. Indeed, it is possible new organization of heterogeneous networks where hetero- to make use of the MIH with our solution. Its role will be geneous PoAs are gathered in the same access subnetwork to manage exchanges between the terminal and the network based on the neighbor of their wireless coverage. Although, entities during the HO preparation procedure and to interact this organization remains far from current deployments’ with heterogeneous interfaces for the optimization of HO organization, it is very interesting to consider these aspects execution based on context information elements established for future network deployments as we have demonstrated proactively. that such a configuration enables optimized heterogeneous In the integration example we have proposed in IV, we HOs with very low singling overhead, which is not the use mechanisms oﬀered by WiFi and WiMAX to perform case with classic network configuration. At least, network actions related to the heterogeneous HO management. providers have to retain that with the growth of heteroge- The IEEE 802.21 proposes media-dependent interfaces and neous mobility there is a need to consider wireless coverage primitives to be used with the WiFi and the WiMAX neighborhood between heterogeneous PoAs to ensure a technologies. This will make easier the integration of the reasonable signaling overhead above the core network. 20 Journal of Computer Systems, Networks, and Communications Finally, we return to the fact that the use of this International Conference on Wireless and Mobile Communica- framework remains interesting with classic architectures and tions (ICWMC ’07), p. 16, March 2007. that this configuration does not have as many constraints as [4] R. Samarasinghe, V. Friderikos, and A. Aghvami, “Analysis is believed. 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(PDF) Technology Integration Framework for Fast and Low Cost Handovers—Case Study: WiFi-WiMAX Network