Lte and wifi interworking

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Alazemi, Kuwait Habib M. Saeed, 1 Ahmed A. Saeed 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. Recently wireless network interworking has become an important area of research in academia and industry.

This is due to the huge diversity of wireless network types, which range from wireless body area network WBAN covering areas up to a few inches to wireless regional area networks WRANs covering up to several miles. The main challenges in wireless interworking of connect- ing the cellular network with the other wireless networks include issues like security, seamless handover, location and emergency services, cooperation, and QoS.

The developed interworking mechanisms, that is, unlicensed mobile access UMA , IP Multimedia Subsystem IMS , and Media inde- pendent handover MIH , due to the characteristics of wireless channel, need to be analyzed and tested under various circumstances. The aim of this special issue in Journal of Computer Systems, Networks, and Communications JCSNC is to highlight the problems and emphasize and analyze the solutions in this area, which can give a guideline to telecom industry for new techniques and business opportunities.

Kassab, et al. Trimintzios and G. Georgiou, the security intrusion that may occur during handover is discussed. Monteiro, et al. Sun and W. So-In et al. In the same area P. Pham and T. Lin and H. The proposed model is suitable for multihop relay network, where the handover process is frequently performed. Rashid A. Saeed Ahmed A. 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 WiMAXtechnologies. We propose several performance evaluations based on simulation tests based on this application. Several wireless technologies have emerged. Among these technologies, there is not one good and generic enough to replace all the others; each technology has its own merit, advantages, and development possibilities.

WLAN technologies, for example, WiFi, have been developed to be an extension of already existing wired LANs; they are also used to deploy local public wireless networks. In addition, user categories and usability domains have converged so that terminals and communication means have evolved to integrate multiple technologies. A multi-technology terminal will be able to change its access technology each time its environment changes. This could be a great advance depending on the adequate mechanisms which are available to ensure a seamless mobility.

On the other hand, wireless technologies are no longer limited to be a basic communication medium. Public wireless networks have to guarantee a good level of service while insuring the transparency of management to users. The deployment of such networks using het- erogeneous technologies will require a good connectivity during handovers, by reducing latency, and the homogeneity of management services such as authentication and QoS.

These solutions enable the optimization of the network reattachment i. On the other hand, the structure of these technology-integration solutions is not suited to heterogeneous mobility. As a consequence, the HO management mechanisms based on exchanges between heterogeneous entities will result in a nonnegligible overhead that could disrupt the network performances. In this work, we propose a technology-integration frame- work that provides a newapproach to deploy next generation wireless networks.

The idea is to optimize the layer-2 HO execution in a heterogeneous and homogeneous mobility and to adapt the network architecture so that this optimization yields to a minimum signaling surplus. In addition, we propose an application of this framework to an actual wireless network based on the WiFi and WiMAX technologies. We make use of this application to demonstrate the ability of the proposed framework to enable the enhancement of HO performances while ensuring a reduced signaling overhead.

This paper is organized as follows. In Section 2, we propose an overview of solutions adopted for wireless tech- nology integration. We detail how the proposed framework can get along with layer-3 mobility management mechanisms in Section 6. We propose, in Section 7, a discussion about heterogeneous technology integration. We draw up main conclusions and propose future trends of our work in Section 8.

Technology Integration in the Literature Heterogeneous-technology integration has been studied by several researches. Two inter working architectures have been proposed: loosely and tightly coupled architectures [2, 10]. Roaming privileges are assigned to subscriptions related to one network. This helps to minimize session disruption based on the cooperation of account- ing entities. The tightly coupled architecture proposes the integration of wireless technologies in the same network architecture.

In all cases, user mobility is managed using Mobile IP and its extensions [11]. The tightly coupled architectures propose integration at lower level of network architecture.

Nevertheless, the lower level of integration ensures a very interesting enhancement of HO performances [4, 5]. This is due to the fact that the inter-working takes place at a point of the management architecture closer to the mobile terminal. The CXTP proposes a protocol to transfer mobile terminal contexts between Access Routers managing the access control of a wireless network.

CXTP has been designed as a generic protocol that can accommodate a wide range of services. The context transfer can be reactive, during the HO execution, or proactive from the serving AR to a possible target AR.

CXTP can be useful if some network services such as user authentication and QoS are integrated to the layer-3 level in wireless networks [13]. Consequently, several management exchanges between a terminal and the Access Router AR , which controls the access to the network, are required during the network entry. A solution could be the association of the tightly coupled architectures to an optimization of the terminal to technol- ogy association procedure. This may be based on management mechanisms like context transfer or proactive execution of exchanges.

Network services, such as user authentication, QoS management, and billing, have to work properly and seamlessly while terminals are moving over the network. The latter pro- poses the enhancement of L2-HO performances based on mobility-context exchanges.

Network Architecture Skeleton. The global wireless net- work is organized into access subnetworks, each one gathering a set of PoAs. We do away with the classic organization of wireless networks that separates each technology in an autonomous network. PoAs can be gathered in access sub networks based on the closeness of their wireless coverage or based on common management requirements.

To each access subnetwork is associated an L2-Acc-Mgr. Figure 1 shows this architecture. The L2-Acc-Mgr integrates several functions to manage terminal mobility. It acts as a service proxy regarding exchanges between terminals and core network entities during the network entry procedure.

It provides a list of PoAs to which a terminal may move while being associated with a particular PoA. The L2-HO management function integrates the intel- ligence related to the L2-HO management, that is, the triggering of HO management exchanges, the execution of exchanges and the management of terminal contexts. L2-HO Management Mechanisms. During the network entry, a terminal associates itself with the network and activates a set of services and functionalities.

The terminal context includes the parameters negotiated during the net- work entry and states related to network services used by the terminal [1]. The acceleration of the establishment of this context is required, at the time of handover, to reduce the delay that results from the HO execution phase. The establishment of the terminal context on the target PoA, based on already available information, is the solution. Management of Terminal Contexts. The global session infor- mation elements are related to the association established between the terminal and the core network entities such as AAA servers.

The local association information elements are related to the association established between the terminal and the serving PoA. When a terminal executes a HOwithout performing a new network entry, it maintains its global session while re-establishing the local association with the new serving PoA. Then, a context information element is transferable information when it remains valid while the terminal changes its serving PoA.

Such information element can be reused with target PoA to avoid renegotiation during HO execution. Other elements are nontransferable context information, their current value, associated to a serving PoA, cannot be exploited to avoid negotiations between the terminal and target PoA to establish a new association.

This type of infor- mation has to be re-established through regular exchanges during the HO execution. The L2-Acc-Mgr is the most entitled entity to manage the greater part of the terminal context. First, the global session information elements are held by the L2-Acc-Mgr thanks to the service proxy function. Second, local information elements that are conditionally transferable may require centralized information related to the neighbor PoAs or the terminal to be translated for re-establishment.

The latter information is held by the L2-Acc-Mgr, so it is the better able to manage conditionally transferable local information elements. The HO management function of the L2-Acc-Mgr is responsible of managing the latter information elements, of the terminal context.

In this case, QoS parameters can be transferred to re-establish the new association since the two wireless technologies do not necessarily use the same QoS representation.

AQoS translation function can solve the conformity problem as most QoS management mechanisms have common bases. A serving PoA is responsible for redistributing them to target PoAs and caching them. Finally, there is a set of information elements that current values cannot be exploited to avoid management exchanges between a mobile terminal and the network to establish a new association. We name this category: non transferable context information.

This type of information has to be re- established through regular exchanges during the handover execution. We can mention connection parameters used with a terminal, for example, data rate.

These parameters depend on the position of the terminal in the cell and the serving AP capacity, and so they have to be negotiated during the association.

Context Establishment Exchanges. Two options are available for context establishment: the context transfer and the proactive negotiation [1]. The context transfer is an adequate establishment solu- tion for transferable information elements. It is performed between the entity managing the information element and one or a set of PoAs.

In the same way, condition- ally transferable information element re-establishment can be based on a context transfer mechanism. After being translated, an information element is transferred to target PoAs.

The context transfer is not the appropriate solution for the re-establishment of non-transferable information elements. An information element might require to be re- established over standard exchanges or the involvement of the terminal in the negotiation or generation process.

It remains possible to establish non transferable infor- mation elements using proactive negotiations. The latter are based on the standard exchanges usually performed during the network entry procedure to generate information elements. The adequate time to perform a context establishment depends on the stability of the information element value during the time. There are static information elements that values do not change during the local association and dynamic information elements that values change during a local association based on network conditions, terminal behaviors, accounting constraints, and so forth.

Proactive context establishment can be performed with static infor- mation elements so that it will be available immediately at the HO execution. However, proactive establishment is not excluded with dynamic context. This depends on the frequency of information element update. If an information element is known not to be frequently updated, it remains possible to perform a conditional proactive establishment.

The information element shall be associated to a validity condition. In other cases, the information element is established reactively during HO execution based on its last update. HO Establishment Exchanges. Proactive and reactive exchanges are combined to manage static and dynamic infor- mation elements. The exchange a of Figure 2 shows the proactive establishment procedure involving the L2-Acc-Mgr and two neighbor PoAs. The target PoA may execute a reac- tive exchange to obtain values related to dynamic informa- tion elements from the L2-Acc-Mgr as shown in Figure 2 b.

Figure 3 shows exchanges based on the two mechanisms. For the proactive one, the establishment exchanges are initiated by the serving PoA with a list of neighbor PoAs indicated by the L2-Acc-Mgr. As such, it can engage reactive context transfers with the previous serving PoA.

Proactive negotiations are engaged between the termi- nal and neighbor PoAs through the current association established with the serving PoA. It is mostly used for information elements managed by PoAs that cannot be established through context transfer. The L2-Acc-Mgr is responsible of managing L2-HO management exchanges with entities associated to its access subnetwork i. Consequently, the L2-HO management exchanges are limited to the access subnetwork during intrasubnet mobility.

Intersubnetworks exchanges are relayed by L2-Acc-Mgrs during inter-subnetwork mobility. In a nonoptimized architecture, the HO management exchanges between PoAs are routed through the core net- work from one access subnetwork to another during inter- subnet mobility. The HO management exchanges between PoAs and centralized entities, during an intra-subnet mobil- ity event, are engaged through the core network while the terminal mobility is restricted to the access network.

Thus, the use of L2-Acc-Mgrs restricts as much as possible the HO management operations to intra-access subnetwork exchanges. Terminals will roam from one technology to another according to their movements while being attached to the same global network. Most of these researches have proposed to use the WiMAX technologies as backhaul support for WiFi hotspot [7, 15, 16].

More recent research studies were interested in the inter-working of the WiFi and the WiMAX as access technologies in the same heterogeneous network. However, the majority of these studies were limited to the enhancement of the HO decision mechanism between the two technologies and did not discuss the problems related to the integration and the collaboration of these technologies in the same network architecture [17—19].

They proposed a solution to ensure a continuity of QoS management through the heterogeneous wireless access. The solution proposes a mapping between the QoS management parameters of each technology to ensure seamless change of technologies. However, no additional management arrangements were proposed e. User authentication is proposed by IEEE The They establish mutual authen- tication between peers and generate cryptographic suite to secure data exchanges.

The basic IEEE The WiFi equipments and deployed networks are fol- lowed by particular evolution. The enhancements of the communication performances were based on the evolution of the PHY layer performances. The requirements can be data rate, packet size, service interval, and so forth. Thus, the IEEE The authentication process can last up to 1 s [26]. Several solutions are available to ensure reduced authentication delays during horizontal HO less than 25 milliseconds ms [27].

It includes the core network architecture reference models, protocols for end-to-end aspects, procedures for QoS management, and user authentication. This information is provisioned in a subscriber management system or in a policy server, typically a AAA server. IEEE These exchanges result in the generation of a hierarchical sequence of authentication keys.

Having these information elements, a target BS will be able to associate the terminal during the HO procedure with the minimum of negotiation exchanges. Network Architecture. Figure 5 shows the two deployments. The L2-Acc-Mgr. These functions allow the L2-Acc-mgr to support layer-2 service proxy function. Terminal Context Translation. For horizontal HOs, the translation function provides context information elements based on the ones used during actual association.

We suggest the static association between class of services of both technologies shown in Table 2. Therefore, when the IEEE The IEEE However, IEEE We propose a static translation procedure between QoS parameters to be used by the Translation Function. The translation process depends on the QoS information related to the current terminal association, that is, the serving technology.

Table 3 presents the mapping used to compute IEEE These parameters do not have an equivalent in When the current serving technology is the In the reverse case, the value of the User Priority parameter is obtained based on the Data Delivery Service as previously indicated.

However, we propose to compute a corresponding value based on available parameters. Thus, max D corresponds to the Delay Bound Additionally, min D can be computed based on the data rate perceived by the A 3-way-handshake exchange is performed between the terminal and the BS based on the AK.

This key is used to perform the 4-way-handshake between the terminal and the serving AP. Similarly, the When the terminal is associated with a BS, it shares an Figure 6 details related exchanges. When the terminal is associated with an Figure 7 details related exchanges. Thus, we maintain a caching mechanism for PHY-layer capabilities managed by the translation function.

PHY-layer capabilities of terminals are maintained during the ongoing session. Additionally, it indicates to the terminal, in the recommended Candidate PoA List, to execute proactive exchanges to negotiate these parameters with target BSs. Context Establishment Procedure. The L2-HO opti- mization is based on the establishment of terminal contexts on target PoAs to avoid their re-negotiation and conse- quently reduce the HO delay.

The context establishment procedure is mainly proactive. The neighborhood man- agement function provides the Recommended PoA List to which the establishment is initiated. The cryptographic suites are established based on a context transfer between the serving PoA and target PoAs preparation of a horizontal HO or proactive negotiation between the terminal and target PoAs preparation of a vertical HO.

The translation function computes values for the information elements to be established based on the available terminal context. Figure 8 shows an example of the proactive phase of the context establishment procedure.

The terminal is associated with a serving AP. The context establishment is performed with an AP and a BS. Based on target PoA responses, which indicates the support of terminal requirements, the HO management function builds the PoA List that is forwarded to the serving AP.

The serving AP transfers the list to the terminal. The cryptographic suites are established, with available PoAs, using a context transfer with target APs and a proactive negotiation with the target BSs. The previous example describes a preparation procedure performed with target PoAs in the same access network as the serving PoA. The serving L2-Acc-Mgr is the manager of the preparation procedure while the target L2- Acc-Mgr relays the messages between the latter entity and the target PoA.

Figure 9 shows the exchange. Regarding context transfers between PoAs and proactive negotiations between the terminal and the target PoAs, we make the choice not to execute these exchanges during the inter-subnet preparation procedure. Therefore, the prepa- ration will be limited to centralized exchanges performed between the L2-Acc-Mgr and the PoAs. The evaluation has shown that exchanges performed between PoAs and particularly proactive negotiations are not adapted to inter- subnet mobility.

In fact, they increase the signaling cost of the preparation procedure and reduce the HO performance in high mobility environments. Translation procedure Context req crypt. HO Execution Optimization. The HO preparation procedure, presented in previous sections, establishes a set of context information elements and parameters in target PoAs.

The exchanges engaged during the HO execution depend on the information elements that were established proactively during the HOpreparation procedure or requested reactively during the HO execution. We consider optimal scenarios where target PoAs were able to acquire all context information elements. The establishment of the terminal context results in an important optimization of the L2-HO execution procedure for both vertical and horizontal HOs. The target AP acquires also the PTK based on a context transfer or computes this key with a proactive negotiation performed with the AP.

The Ranging Response RNG- Rsp indicates the re-entry steps that are omitted thanks to the availability of terminal context information elements obtained during HO execution.

The evaluation criteria will highlight both the contributions of new mech- anisms and the limits of their application. Handover Delay. The most obvious criterion that must be evaluated is the HO delay. Therefore, the HO delay includes the time required to detect the need to perform a handover, to choose a target PoA, and to perform re-association exchanges. We adopt the network simulator SimulX [35] that supports features that enable the design and the evalu- ation of future communication protocols like cross-layer interactions, multi-interface inter-working in terminals, and heterogeneous network environments.

Both have been validated through simulation tests that result in well-known performances of Table 4: Handover delay. Target technology Opt. HO ms Non-opt. The WiFi-WiMAX architecture and the L2-HO optimization mechanism proposed in this researches have been implemented in the simulator based on the latter architectures [25]. A terminal 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.

The delay due to non-optimized HOs is eval- uated to ms when the WiMAX is the target technology and ms when the WiFi is the target technology. Figures 10 and However, these phases are well optimized in handover procedure of WiMAX.

For example, there is no search phase at the time of HO as the serving BS sends a recommended neighbor list to terminal. The L2-HO management mechanisms ensure a uni- form execution time for both intratechnology and inter- technology HOs limited to a mean value of 24,63 ms. This is obtained thanks to the context establishment mechanism that ensures the same optimization of the HO execution regardless of the target PoA type. In WiFi networks, the performance of terminal exchanges depends on the cell load because of the contention-based medium access [27].

In a previous research, we were interested in the evaluation of HO performances in WiFi networks. We evaluated a management mechanism that ensures the same optimization of HO execution for WiFi terminals. Results demonstrated that such optimization ensures a limited execution time lower than 50 ms even with high loads. The performance of WiMAX wireless access is not sensitive to the cell load as the medium access is managed by the BS that allows transmission opportunities to the medium modeled by transmission frame [28].

The duration of the IEEE In a previous research, we have evaluated the variation of the regular WiMAX network entry as a function of the frame duration. Results have shown that the network entry duration vary from ms to 1 s with frame duration that varies from 3 ms to 12 ms. Figure 12 plots the delay due to optimized WiMAX handover as a function of the This curve shows that the handover delay increases when the lEEE However, even with frame duration of 12 ms the handover delay remains reasonable and does not exceed the value of 50 ms tolerable threshold of real-time applications.

The second parameter considered for WiMAX cells is the contention-based transmission period. It is used by a terminal that starts an HO procedure or an association 0 2 4 6 8 10 12 14 This period has a limited duration during a single frame. The exchanges over it will be impeded by the number of terminals trying to communicate.

Figure 13 plots the evolution of the HO delay as a function of the number of terminals. This parameter exceeds 50 ms as soon as the number of terminals that try to associate exceeds 5. Journal of Computer Systems, Networks, and Communications 17 5. Signaling Cost. This evaluation aims to compare the new architecture with alternative network deployments under the same conditions.

We compare the performances of the integration archi- tecture optimized architecture to an architecture that does not integrate an L2-Acc-Mgr non-optimized archi- tecture. In the latter architecture, we suppose that the HO management functions, for example, neighborhood management and context establishment, are supported by centralized network servers.

Four network architectures are considered: non-optimized architecture with homogeneous deployment, non-optimized architecture with heterogeneous deployment, optimized architecture 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. To each link we associate a weight that models the cost of transmitting of one byte over this link. The sub-formula S HOpreparation of 1 resp. We make use of the VanetMobiSim software to emulate the terminal mobility over the considered wireless deploy- ment [37].

The combination of the signaling cost formulas and the mobility statistics allow us to evaluate the signaling cost average of the HO management over the considered deployment [25]. We assume a mix of three types of mobility model: walking users, slow cars, and fast cars.

These values indicate that the transmission cost of a management message over the core links is twice the transmission cost over the local links. The transmission cost over the wireless links is fourfold the transmission cost over local links. Both the optimized architecture and the heterogeneous deployment reduce the signaling cost of an HO. As a result, there is no more exchanges with centralized servers for HO management.

On the other hand, the heterogeneous deployment allows to gather neighbor PoAs in the same access network. Figure 16 plots the evolution of the handover signaling cost as a function of the core-link weight.

In fact, the signaling exchanges related to a mobile terminal will be enclosed in the wireless cells and access subnetworks in its mobility areas. Thus, the proposed designs ensure the enhancement of HO performances while reducing the core network resources. Indeed, a multiple- hop neighborhood should ensure a good support of fast moving terminals. The neighbors of an AP are the APs that surround within two hops and the BS that covers the area if it is reachable by a terminal on two hops.

The neighbors of a BS are the APs on its coverage zone reachable at most with two hops and the BSs in its immediate wireless neighborhood. The results are shown in Figure This combination allows the operator to design wireless network with better mobility support without increasing the HO management signaling overhead. Interaction with Layer-3 Handover Management Mechanisms In this study, we are interested in optimization of HO performances in heterogeneous networks. Our proposals have been limited to the management of layer-2 handovers L2-HO.

Thus, it seemed interesting to study the interaction of this framework with additional HO management mech- anisms, proposed in the literature, that may be deployed in heterogeneous networks. Collaboration with FMIP. The terminal may request information, about all wireless links, to the current router. The reply can be received on the old link or on the new link reactive HO. The latter HO execution is optimized thanks to the preparation procedure of the L2-HO management mechanism.

The link availability indication may also be used to trigger the preparation of following handovers. Collaboration with the MIH. Particularly, it manages exchange of information elements between the terminal and the network to enhance the decision and search phases of the handover procedure.

It also helps the preparation of the HO execution between heterogeneous technologies. For example, the MIH provides to upper layers, link-layer triggers based on reactive and predictive local link state changes and network information load balancing information, operator preferences that enhance the HO detection.

It also supports the transfer of global network information list of available networks, neighbor maps and higher layer network services from network servers to the terminal to help it on the HO preparation procedure.

However, the handover execution optimization is not part of the MIH functions. The mechanisms, proposed by the MIH, are complemen- tary to the solution we have proposed. Indeed, it is possible to make use of the MIH with our solution. Its role will be to manage exchanges between the terminal and the network entities during the HOpreparation procedure and to interact with heterogeneous interfaces for the optimization of HO execution based on context information elements established proactively.

Discussions about Heterogeneous Technology Integration It is obvious that the mobility management in the het- erogeneous wireless networks is more complex than classic wireless networks. In this research, we have been able, as well, to propose a layer-2 handover optimization solution based on general and technology-agnostic framework.

Another interesting point related to this framework is the ability of the proposed architecture to facilitate the extension of heterogeneous networks based on additional technologies. Based on this framework, it is possible to propose a new organization of heterogeneous networks where hetero- geneous PoAs are gathered in the same access subnetwork based on the neighbor of their wireless coverage. At least, network providers have to retain that with the growth of heteroge- neous mobility there is a need to consider wireless coverage neighborhood between heterogeneous PoAs to ensure a reasonable signaling overhead above the core network.

Conclusion In this work, we have been interested in the integration of heterogeneous wireless technologies in the same network. This application demonstrates the utility of this framework based on a practical network deployment and enables the performance of evaluation tests. We have shown the interest for network access providers to upside the conventional network architecture by merging the backbones of heterogeneous wireless access networks.

Such network deployments are more adapted to Next Generation Wireless Networks where vertical HOs will be more frequent and trivialized. In future work, we are interested in proposing an application of this framework for the deployment of com- munication systems for transport context and especially rail transport.

The latter are required to operate in extremely varied environments, such as urban and suburban environ- ments, countryside, sparsely or very low populated, tunnels, and railway stations. In addition, transport systems have very high constraints regarding transmission delays, robustness, and reliability.

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This special issue would not have come true without the tight guidelines and support from the Editor-in-Chief Professor Hsiao-Hwa Chen and Mariam Albert the editorial staff in Hindawi Publishing Corporation. Saeed Ahmed A. Daniel Wong. Saeed et al.