Media-Independent Handover for Seamless Service ... - CiteSeerX

PASSAS LAYOUT

12/14/07

5:18 PM

Page 64

MOBILE INTERNET TECHNOLOGIES AND APPLICATIONS

Media-Independent Handover for Seamless Service Provision in Heterogeneous Networks George Lampropoulos, University of Athens Apostolis K. Salkintzis, Motorola Nikos Passas, University of Athens

ABSTRACT The performance of current Internet applications is based mainly on the capabilities of the underlying network technologies. Modern access systems usually can satisfy delay, loss, or bandwidth requirements; however, design inconsistencies can lead to service degradation as the terminals move across different systems. In this article, the focal point is the satisfaction of service requirements during mobility and more specifically, how the emerging IEEE 802.21 standard enables seamless, inter-technology handover. Based on prior work and a well-known example of seamless mobility, the main seamless mobility principles are identified and used as the basis for further evaluating the potential of the IEEE 802.21 standard to meet the requirements of applications for minimum disruption during an inter-technology handover.

INTRODUCTION

1

The term MIH UE or simply UE hereafter refers to the mobile terminal combining 3GPP Release 8 and IEEE 802.21 capabilities.

64

New Internet applications and services are constantly being introduced. To be successful, they must guarantee a specific quality-of-service (QoS) to the user. Modern access systems have the capability to fulfill this requirement, but the problem is more complicated in heterogeneous environments, where transition between diverse types of access technologies (e.g., Bluetooth, WiFi, WiMAX, CDMA, GSM, GPRS, and UMTS) can lead to reduced performance, mainly due to incompatibility problems [1]. This leads to a requirement to make the transition from one access network to another as seamless as possible for the user, that is, to offer a seamless mobility experience. With seamless mobility, users can exploit a variety of access networks to best meet their charging and QoS requirements, while at the same time, operators can offer compelling, value-added services, as well as improve their

0163-6804/08/$25.00 © 2008 IEEE

network capacity and the availability of their services. Seamless mobility can be achieved by enabling mobile terminals to conduct seamless handovers across diverse access networks, that is, seamlessly transfer and continue their ongoing sessions from one access network to another. A seamless handover is typically characterized by two performance metrics: • The handover latency should be no more than a few hundred milliseconds. • The QoS provided by the source and target systems should be nearly identical (or the user should not perceive any change to his communication experience after the handover). These two performance requirements are not trivial to satisfy when different access networks are combined in a single architecture, because minimum service data flow interruption usually can be achieved by networks with tight-coupling mechanisms across them (such as GSM EDGE radio access network (GERAN) and UMTS terrestrial radio access network (UTRAN) [2], specified by 3GPP), and QoS provision at the target system requires mapping of specific QoS attributes from the source network. In general, before seamless inter-radio access technology (inter-RAT) mobility is widely deployed in modern heterogeneous networks, many technical challenges still must be addressed. These challenges are the main focus of this article, in which we study the new trends and technical advances associated with seamless mobility in heterogeneous networks. Another question we address in this article is how efficiently the emerging IEEE 802.21 specification [3], whose purpose is to support inter-technology handovers between IEEE 802 and non-IEEE 802 (e.g., 3GPP, 3GPP2) access technologies, enables seamless mobility in next generation heterogeneous wireless networks. The rest of this article is organized as follows.

IEEE Communications Magazine • January 2008

Authorized licensed use limited to: National Chung Cheng University. Downloaded on January 12, 2009 at 02:17 from IEEE Xplore. Restrictions apply.

PASSAS LAYOUT

12/14/07

2:48 PM

Page 65

We discuss the prior art in the area of seamless mobility across heterogeneous networks and focus on explaining how this is achieved between GERAN and UTRAN radio technologies. Taking this GERAN/UTRAN inter-RAT handover as a good practice case, we identify the principles and mechanisms that can facilitate seamless mobility between diverse radio technologies in general. We provide a brief overview of IEEE 802.21 for the sake of completeness, and we exemplify how inter-RAT handovers can be conducted with the aid of IEEE 802.21. Subsequently, we discuss if and how IEEE 802.21 can address the general principles of seamless mobility. Then, we conclude with some final considerations.

PRIOR ART AND SEAMLESS MOBILITY PRINCIPLES In recent years, the significance of seamless mobility has evolved rapidly; therefore, the relevant prior work is rather ample. We briefly describe a few indicative proposals related to IEEE 802.21 and one example of seamless mobility in 3GPP networks. In [4], the handover process can be enhanced by incorporating mechanisms for service continuity, mobility policies, power saving, and adaptation support at the application layer. Issues that are addressed include the minimization of communication interruption during handover, the effect of user preferences, cost and security on handover decision, the use of smart techniques to activate interfaces only when required, and so on. To achieve these, a mobility manager is proposed that results in less handover delay for both IEEE 802-to-IEEE 802 and IEEE 802to-non-IEEE 802 handover cases. The claimed improvement reaches as high as 75 percent compared to the handover delay without this mechanism. Seamless behavior also is provided by the mechanism proposed in [5]. However, in this solution, the key element for achieving low handover latency is a mechanism for fast re-authentication, referred to as media-independent pre-authentication. This mechanism works for any IEEE 802 and non IEEE 802 network, and its philosophy is to pre-authenticate, pre-configure the new link, and establish a bidirectional tunnel over the old link with the target network at layer-3. Another case of seamless mobility is a handover between two 3GPP networks [6]. For this purpose, we examine how a data session can be transferred seamlessly from a GERAN to a UTRAN radio environment. The entire handover is divided into a preparation and an execution phase, where the required resources in the target UTRAN are prepared before the user equipment (UE) moves to UTRAN, and the data path is switched from the source to the target path after the UE moves to UTRAN, respectively. To prepare the handover, the UE must take received signal strength (RSS) measurements on neighboring UTRAN channels and report these measurements to the source GERAN network, which will utilize them further for making inter-RAT handover decisions.

To do this, the UE discovers the neighboring UTRAN channels by exploiting neighbor cell information that is broadcast by the source GERAN network. If handover is required, the source GERAN sends the identity of the target UTRAN cell, plus a set of other parameters required by the target UTRAN, to prepare the appropriate radio resources (such as security information and channel configuration) to the core network. After this, transfer of the UE mobility management and QoS contexts may take place between core network entities. Moreover, admission control is performed, and a security context for the UE is established. Then, the resource reservation phase begins. If the target UTRAN manages to reserve the requested trunking and radio resources in the target cell, an acknowledgment is sent back to the core network, indicating that the target radio network is now ready to accept the impending handover. This concludes the handover preparation phase. The handover execution initiates with a confirmation to the source GERAN that the resources in the target network are successfully prepared. Now, the UE can switch to the target UTRAN. This is done with a command from the source GERAN (initially issued by the target UTRAN during the target resource reservation and piggybacked in the final command toward the UE) that contains target channel configuration information for the UE. As soon as it is on the target UTRAN cell, the UE performs the normal access signaling, by means of which its presence is detected. At this point, the transmission of buffered and new downlink traffic begins to the UE over the target UTRAN. When the UE completes the handover process, the data path can be updated, and resources in the source GERAN can be released. At this point, the inter-RAT handover process is finally completed, and the UE resumes its data session over the target UTRAN cell. It is important that the entire handover process takes place as a layer-2 process, so it is completely transparent to higher layers, such as IP and TCP/UDP. From the UE point of view, all the handover signaling is conducted at a radio level (layer-2), and any ongoing IP flows will only experience a short transmission gap. The previous discussion uncovers the following principles that can assure seamless handover. We will discuss these principles again later when we evaluate the potential of IEEE 802.21 to facilitate seamless inter-RAT handover. • The source RAT should make the handover decision by taking into account inter-RAT measurements plus other handover-related information (e.g., QoS information, radio resource availability, link-layer triggers, and user preferences). • The admission control and the reservation of resources at the new RAT should be made in advance to minimize the handover latency. • Critical procedures such as the authentication in the new RAT should be assisted by sending security context and QoS context to the target RAT during the handover preparation.

IEEE Communications Magazine • January 2008 Authorized licensed use limited to: National Chung Cheng University. Downloaded on January 12, 2009 at 02:17 from IEEE Xplore. Restrictions apply.

When the UE completes the handover process, the data path can be updated, and resources in the source GERAN can be released. At this point, the inter-RAT handover process is finally completed, and the UE resumes its data session over the target UTRAN cell.

65

PASSAS LAYOUT

12/14/07

2:48 PM

Page 66

Upper layers (IP, SIP, transport, application, ...)

Information service

Command service

Event service

Media independent handover function (MIHF)

Information service

Command service

Event service

Lower layers (802 family of networks, 3GPP, 3GPP2, ...)

■ Figure 1. MIH services. • Specific configuration information about the target RAT, such as available radio channels should be provided by the source RAT to the UE. • A unified way to exchange and interpret measurement reports, QoS context, and so on should be provided for inter-technology handover. To achieve seamless mobility especially for real-time applications, tight coupling between the source and target RAT is required (such as the GERAN to UTRAN handover example), but this is complex, expensive, and time consuming. This calls for investigating loose coupling methods for inter-RAT mobility, namely, methods that can be generally applicable (i.e., independent from the involved RATs) and do not require the source and target RAT to exhibit strong interdependencies. Such loose coupling methods can be facilitated by the IEEE 802.21 standard that is described in the next section.

OVERVIEW OF IEEE 802.21 As already mentioned, the scope of the IEEE 802.21 standard is to facilitate the handover between IEEE 802 and non-IEEE 802 access networks (e.g., cellular networks) in a way that is independent from particular access network features (i.e., perform media-independent handover (MIH)). This is realized through its mechanisms that offer, for example, the ability to explicitly indicate an imminent break in communication or link deterioration. This report mechanism is implemented through specific triggers that convey useful information related to mobility to entities where a decision is made and in turn, cause a command to be executed at some specific network elements. In fact, the philosophy of IEEE 802.21 is based on the concept of the previous example

66

and the core entity for this functionality is the MIH function (MIHF). The MIHF is located both in the mobile node (MN) and the network node protocol stack and provides three types of services: • The media-independent event service (MIES) • The media-independent command service (MICS) • The media-independent information service (MIIS) The MIES is responsible for detecting events and reporting them from both local and remote interfaces. This type of service is provided from lower layers to upper layers as depicted in Fig. 1. Link deterioration and link unavailability are examples of such events reported to higher layers. On the other hand, the MICS offers commands to higher layers to control the lower layers regarding handover. Commands follow a top-down direction as opposed to events. Typical commands are the configuration of network devices and the scanning of available networks. The set of service types also include the MIIS, that is less frequently used but is equally important, as it provides the mechanism for retrieving information and assisting the handover decision. Such information can be static link layer parameters, like channel information, or the medium access control (MAC) address of the access point (AP). Inside the IEEE 802.21 standard, the AP is referred to as point-of-attachment (PoA) and describes the network side endpoint connected to the MN with a layer-2 connection. The standard also refers to the PoA associated with the MN as the serving PoA, whereas any other PoA in the vicinity of the MN is a candidate PoA. In addition to the PoA, the MN communicates with a peer MIHF located in a network node. This node is called point-of-service (PoS) and communicates directly with the MN MIHF. If a PoS does not have a direct exchange of MIH messages with a MN, it may act as a PoS for other MNs. It is important to note that PoA and PoS may or may not be collocated. Concerning the scope of IEEE 802.21 in handover, the standard clearly deals more with handover initiation and handover preparation, whereas handover execution is handled by other protocols, such as higher-layer mobility management protocols (e.g., mobile IP v6 [7]). As can be seen in Fig. 2, IEEE 802.21 offers the mechanisms for triggering a handover, as well as preparing the handover to a new link. Handover initiation involves the procedures of old link configuration, radio measurement reporting, and new link discovery. In IEEE 802.21, this means that old devices should be configured to report measurements when specific thresholds are crossed. The type of this measurement report may indicate an urgent handover request or just a periodic informational message. Moreover, new link discovery may be realized with specific triggers from the available link layers. Concerning the handover preparation, additional scanning of RATs in the vicinity of the MN can be performed with the help of different IEEE 802.21 services (i.e., MIES, MICS, and


PASSAS LAYOUT

12/14/07

2:48 PM

Page 67

Handover preparation

Handover intitiation

Handover execution

The security check and the implementation of the decision algorithm are out of

• Configuration of old link

• Scanning of alternative RATs

• L2 signaling*

• Radio measurement reports

• Security check*

• Higher layer signaling*

• QoS context transfer

• Transfer of radio paths*

• New link discovery

• Network information retrieval/Radio resource availability check • Handover decision* • Radio resource reservation*

the scope of IEEE 802.21, whereas the resource reservation has not been specified in the current version of IEEE 802.21 draft,

*not included in the scope of IEEE 802.21 yet

but it is expected to be included in the future.

■ Figure 2. Scope of IEEE 802.21 in handover.

MIIS). More specifically, discovering a new link may involve a query to a remote MIIS server that retains information about available networks in the area of a specific MN or a network command to the MN to start scanning. In case of access to new network resources, the MN must authenticate itself to the network before proceeding. If the MN is authenticated at the new network, QoS context must be transferred for a resource availability check. The result of a radio resource availability check and other information from the network (e.g., charging policies) are the input to the handover decision algorithm. Moreover, radio resources must be reserved over the selected RAT. The security check and the implementation of the decision algorithm are out of the scope of IEEE 802.21, whereas the resource reservation has not been specified in the current version of IEEE 802.21 draft, but it is expected to be included in the future. Finally, handover execution is out of the scope of the IEEE 802.21 standard as L2 mobility is treated by network specific procedures, and re-routing of IP traffic is usually performed with other protocols, such as the mobile IPv6. However, IEEE 802.21 may trigger the activation and deactivation of links in a way that may preserve resources over the old link for less handover interruption.

USING IEEE 802.21 FOR INTER-RAT HANDOVERS To demonstrate the way IEEE 802.21 can handle inter-RAT handovers, the architecture in Fig. 3 is assumed. In this figure, WLAN, WiMAX, GPRS/UMTS, and E-UTRAN access networks all connect to the evolved packet core (EPC), which is an evolved 3GPP core network architecture standardized in the context of 3GPP

Release 8 specifications (see [8] for more details). Moreover, 3GPP networks are combined under the same serving gateway (S-GW 1), and the same happens with IP-based networks (WLAN and WiMAX) that are placed under SGW 2. In the case of E-UTRAN, S-GW 1 is directly connected with it, whereas SGSN is the intermediate node when GERAN/UTRAN is used. It is important to mention that a mobility management entity (MME) is incorporated in the architecture for exchanging higher-layer signaling for authentication, security, and mobility. Different paths also are used in the case of WLAN and WiMAX. A WiMAX network is directly connected to S-GW 2, assuming that it offers trusted access. On the other hand, WLAN is accessible through the evolved packet data gateway (ePDG). All data paths from the access networks are combined at the packet data network gateway (P-GW) that incorporates functionality, such as packet filtering, interception, charging, and IP address allocation; and routes traffic to the operator’s network or to an external network. Apart from network entities handling data traffic, the EPC also contains network control entities for keeping user subscription information (home subscriber server — HSS), for determining the identity and privileges of a user and tracking his/her activities (authentication, authorization, and accounting server — AAA server) and for enforcing network policies (policy control rating function — PCRF). Concerning the seamless feature of the architecture, specific network elements are enhanced with extra functionality, based on the IEEE 802.21 protocol. This is accomplished due to MIH functionality placed at the mobile terminal (MIH UE), 1 the radio access networks (MIH PoSs), and the operator’s IP network (MIIS server). In fact, the location of PoS can be either inside the access network (e.g., the ASN GW for WiMAX [9] and the base station controller


67

PASSAS LAYOUT

12/14/07

2:48 PM

Page 68

In any case, information

MIH PoS

exchange between the MIH UE and the

WLAN

Data flow Signaling path Data path

MIH PoSs, as well as between different PoSs, is used to

MIH UE

MIH PoS

choose the best possible target

AAA server

WiMAX

MIIS server

network during

ePDG

handover. This data exchange is realized with IEEE 802.21specific signaling.

MIH PoS

SGSN HSS

GERAN/UTRAN

PCRF

Operator’s IP network

S-GW2 MME MIH PoS S-GW1

E-UTRAN

P-GW MIP HA

Internet/ Intranet

Evolved packet core (EPC)

■ Figure 3. Next-generation heterogeneous network architecture.

[BSC]/ radio network controller [RNC] for GPRS/UMTS, respectively) or in the core network. However, because one of the MIH PoS functions may be radio resource management, a natural choice is to place it in the access network for less signaling overhead. Moreover, depending on the technology, PoS may not only act as a control node in IEEE 802.21 but also as an intermediate node for data traffic towards the PoA. This can be assumed in the cases of WiMAX, WLAN, and legacy GPRS/UMTS networks, whereas in E-UTRAN, PoA and PoS may be collocated in eNodeB. In any case, information exchange between the MIH UE and the MIH PoSs, as well as between different PoSs, is used to choose the best possible target network during handover. This data exchange is realized with IEEE 802.21specific signaling and may involve link or higherlayer information such as RSS, battery levels, user profiles, and so on. Moreover, information about available networks also can be retrieved at IEEE 802.21-enabled entities from the MIIS server. A typical example of handover that exploits the advantages of the IEEE 802.21 protocol in the aforementioned architecture is the transition from WiMAX to GPRS. The exchanged signaling is based on [3] and described in the following text. Each message is of the form MIH_XXX.YYY (ZZZ), where XXX denotes its functionality, YYY its purpose (i.e., REQUEST = REQ, INDICATION = IND, RESPONSE = RSP, CONFIRM = CNF), and ZZZ its parameters. According to Fig. 2, a handover in IEEE 802.21 consists of three major steps: initiation, preparation, and execution. In our example (Fig.

68

4), the handover initiation phase involves the configuration of wireless devices to generate appropriate triggers towards the network and inform it about important changes in the link quality (1). This means that, at some point in time after the connection has been established, the WiMAX PoS issues a MIH_Configure_Link. REQ message towards the UE to define the required thresholds under which the WiMAX wireless device will report signal strength measurements. In fact, this message indicates the required QoS parameters for the link that are then conveyed to the wireless device and define the thresholds for which a report will be generated. The successful configuration at the wireless device is confirmed at the WiMAX PoS with an MIH_Configure_Link. CNF message. When the previously defined thresholds are crossed, measurement reports (MIH_Link_Parameters_Report. IND) are sent to the WiMAX PoS. These messages can be sent periodically for informational reasons. However, when the deterioration in link quality is unacceptable (2), another type of message can be sent to indicate the necessity of the handover (MIH_Link_Going_Down. IND). This message indicates that the link will go down in a specific time interval and with a specific confidence. It also indicates the reason of this link unavailability. After this indication, the handover preparation phase begins. This phase may include queries to the MIIS server for information retrieval related to the access networks close to the UE, additional scanning commands towards the UE, a preferences exchange between the UE and the WiMAX PoS, a resource availability


PASSAS LAYOUT

UE

12/14/07

2:48 PM

WiMAX PoS

Page 69

GPRS PoS

S-GW 1

S-GW 2

P-GW MIP HA

MIIS server

Resources over the old link either

Traffic flow over WiMAX

can be maintained MIH_Configure_Link.REQ (Link ID, Configuration request list)

1

Handover initiation

or released. This means that if

Configure the network driver with the requested QoS parameters. This triggers another message sequence that sets thresholds for the link quality.

both the old and new links are active

MIH_Configure_Link.CNF (Link ID, Configuration response list)

concurrently, the handover

MIH_Link_Parameters_Report.IND (Link ID = WiMAX, offered QoS)

interruption is 2

minimized.

WiMAX PoS is ready to begin a handover as soon as the provided QoS is below thresholds

MIH_Link_Going_Down.IND (Link ID = WiMAX, Time interval, Confidence level, Reason code)

MIH_Get_Information.REQ (PoA location)

3

Handover preparation

WiMAX PoS, based on PoA location, requests from the MIIS Server to be informed about neighboring PoA and their characteristics

MIH_Get_Information.RSP (candidate PoAs, PoAs characteristics) MIH_Scan.REQ (Scan link identifier = WiMAX + GPRS) MIH_Scan.CNF (Scan link identifier = WiMAX + GPRS, Scan response sets = PoAs identifiers + signal strength) MIH_Net_HO_Candidate_Query.REQ (Suggested new link list = WiMAX/PoA or GPRS/PoA) MIH_Net_HO_Candidate_Query.RSP (Handover ack = TRUE, Preferred link list = GPRS/PoA or WiMAX/PoA) MIH_N2N_HO_Query_Resources.REQ (Query resource list)

4

Serving PoS examines the available resources in the target PoS/PoA MIH_N2N_HO_Query_Resources.RSP (Resource status = 1(available), Available resource set)

5

Make the decision about the target PoA

6

Reserve resources

Handover execution MIH_Net_HO_Commit.REQ (Link ID, Link action) PDP context establishment

Mobile IP registration/binding update MIH_Net_HO_Commit.RSP (Status = 0 (success) MIH_N2N_HO_Complete.REQ (Handover result = 0 (success) MIH_N2N_HO_Complete.RSP (Resource status = FALSE (release resources) Traffic flow over GPRS

■ Figure 4. WiMAX to GPRS handover.


69

PASSAS LAYOUT

12/14/07

2:48 PM

Page 70

Handover principles

IEEE 802.21 supported

IEEE 802.21 not supported

Multicriteria handover decision (1)

Application layer information, QoS, radio resource availability, link layer information

—

Admission control and resource reservation (2)

Admission control

Resource reservation

Context transfer (3)

QoS context exchange

Security context exchange

Extra information about the new connection (4)

—

Channel configuration information to the UE

Unified information representation (5)

RDF/XML schema for information retrieval

Translation mechanism

■ Table 1. Support of seamless mobility principles in IEEE 802.21.

check at the candidate networks, handover decision, and resource reservation. More specifically, immediately after receiving an indication for handover, the WiMAX PoS asks the MIIS server for information about available networks near the UE with an MIH_Get_Information. REQ message (3). This query is based on the current PoA location, and its result returns the candidate networks (PoAs) near the UE, along with their characteristics (MIH_Get_Information. RSP). The last information is quite useful because it can help the UE search for specific radio networks rather than every available network. At this point, the UE still is unaware of the networks that are reachable in terms of RSS. Therefore, the WiMAX PoS orders it to begin scanning for PoAs mentioned in the response from the MIIS server, and the signal measurements are sent back to the WiMAX PoS (MIH_Scan. REQ and MIH_Scan. CNF messages, respectively). In addition, the WiMAX PoS issues a MIH_Net_HO_Candidate_Query REQ message to verify the intention of the UE to hand over. With this message, the WiMAX PoS also proposes target networks to the UE and is informed about the UE preferences (MIH_Net_HO_Candidate_Query RSP). This means that the preferred link list of the UE may differ from the suggested new link list of the network. In parallel, the network checks for available resources in the candidate networks (4). The WiMAX PoS communicates with its peer entity in the GPRS (GPRS PoS) for this reason (MIH_N2N_HO_Query_Resources request and reply). It is important to note here that the translation of resource information across different technology networks still is an open issue in IEEE 802.21. Having all the information in the network and the UE side related to the handover, the WiMax PoS can decide about the target access network (5) and reserve resources (6). If resource reservation is guaranteed both in the new access network and the core network, the UE may enjoy a seamless or make-before-break handover. After the reservation stage, the handover execution phase starts, and the WiMAX PoS orders the UE to begin handover by indicating the actions over the old and the new link

70

(MIH_Net_HO_Commit.REQ). Resources over the old link either can be maintained or released. This means that if both the old and new links are active concurrently, the handover interruption is minimized. In any case, the actions over the new link indicate to set up a new PDP context and perform mobile IP registration or a binding update between the UE and the home agent (HA) in the P-GW. The successful establishment of an IP connection over the GPRS is reported to the WiMAX PoS (MIH_Net_HO_Commit. RSP) and afterwards, to the GPRS PoS through an MIH_N2N_HO_Complete. REQ message. Moreover, resources over WiMAX are released if not released earlier (MIH_N2N_HO_Complete.RSP). After this, data can start flowing over the GPRS.

DISCUSSION In this section, the potential of the IEEE 802.21 standard to efficiently support inter-technology handovers is analyzed, and future directions are provided to the ongoing work within the IEEE 802.21 Working Group (WG) toward the support of seamless mobility. Based on the principles derived in a previous section, IEEE 802.21 capabilities are compared against every single principle. Principle 1 is fully implemented and supported by IEEE 802.21; it offers the capability to gather information from different parts of the network and the mobile terminal. This is performed for all protocol stacks, from physical to application layer, and additional information can be retrieved from the MIIS server. In contrast, principle 2 is partly satisfied. Although admission control is achieved through queries to candidate networks, no primitives define the reservation of resources over the candidate network. Here, it is important to guarantee resource reservation not only in the access part, but also in the core network as it is done with the GERAN to UTRAN handover example. Concerning principle 3, the exchange of security context for fast re-authentication at the candidate network is not considered in IEEE 802.21. However, this is essential for the seamless performance of the handover and special attention should be paid to solutions, such as the one pre-


PASSAS LAYOUT

12/14/07

2:48 PM

Page 71

sented in [5]. Another issue that is important is how the mobile terminal initially connects to the new network (principle 4). In case of GERAN to UTRAN handover, channel configuration information is generated during the handover preparation phase and sent in the handover execution phase. In the case of IEEE 802.21, similar information could be generated during resource reservation and provided at the initialization of the handover execution, probably in the MIH_Net_HO_Commit. REQ message. Finally, principle (5) is handled well by the unified representation resource description framework (RDF)/extensible markup language (XML) schema of the MIIS database in IEEE 802.21, but the translation of information between different technologies is still undefined. The above evaluation is summarized in Table 1.

CONCLUSIONS Seamless mobility is a critical factor if service requirements are to be supported efficiently in next-generation heterogeneous access networks. In this article, the main principles that govern seamless handover have been derived from prior work and presented. These principles have been the basis for evaluating the advantages and disadvantages of the new, emerging IEEE 802.21 standard concerning the provision of seamless mobility. Moreover, the ability of the standard to support seamless mobility has been demonstrated with a WiMAX to GPRS handover case study. This example, along with a brief discussion, has demonstrated that IEEE 802.21 covers many seamless mobility principles, and therefore seamless mobility largely is supported. However, the standard should be extended to further facilitate seamless handover provision.

ACKNOWLEDGMENTS This work was performed in the context of the project “PENED 03ED 909” co-funded by the General Secretariat for Research and Technology of Greece and the European Social Fund. The authors would like to thank the editors and the reviewers for their constructive comments that helped to improve the quality of this article.

REFERENCES [1] S. Hui and K. Yeung, “Challenges in the Migration to 4G Mobile Systems,” IEEE Commun. Mag., Dec. 2003. [2] GSM, GPRS and EDGE Performance: Evolution towards 3G/UMTS, T. Halonen, J. Romero, and J. Melero, Eds., Wiley, 2003. [3] IEEE P802.21/D05.00, “Draft IEEE Standard for Local and Metropolitan Area Networks: Media Independent Handover Services,” Apr. 2007. [4] F. Cacace and L. Vollero, “Managing Mobility and Adaptation in Upcoming 802.21 Enabled Devices,” Proc. 4th Int’l. Wksp. Wireless Mobile Apps. and Services on WLAN Hotspots, 2006. [5] A. Dutta et al., “Secured Seamless Convergence across Heterogeneous Access Networks,” World Telecommun. Cong., Budapest, Hungary, 2006. [6] 3GPP TS 23.009 V7.0.0, “Handover Procedures (Release 7),” Mar. 2007.

[7] D. Johnson, C. Perkins, and J. Arkko, “Mobility Support in IPv6,” RFC 3775, June 2004. [8] 3GPP TS 23.402 V1.0.0, “3GPP System Architecture Evolution: Architecture Enhancements for Non-3GPP Accesses (Rel. 8),” May 2007. [9] WiMAX Forum, http://www.wimaxforum.org/

BIOGRAPHIES GEORGE LAMPROPOULOS [S] ([email protected]) received his B.Sc. and M.Sc. degrees from the Department of Informatics and Telecommunications, University of Athens, Greece, in 1999 and 2003, respectively. Currently he is working toward his Ph.D. in the area of interworking between heterogeneous networks. Since 2000 he has been with the Communication Networks Laboratory of the University of Athens, participating in both national and European research projects. His research interests are in the integration of WLAN and cellular networks, mobility management schemes, and handover decision algorithms in heterogeneous environments. He has published more than 10 papers in referred journals and conferences. APOSTOLIS K. SALKINTZIS [SM] ([email protected]) received his Diploma (Honors) and his Ph.D. degree from the Department of Electrical and Computer Engineering, Democritus University of Thrace, Xanthi, Greece. Since 1999 he has been with Motorola Inc., working on the design and standardization of wireless communication networks, focusing in particular on IMS, GPRS, UMTS, WLANs, and TETRA. His primary research activities lie in the areas of wireless communications and mobile networking, and particularly on seamless mobility, IP multimedia over mobile networks, and mobile network architectures and protocols. In 1999 he was a sessional lecturer at the Department of Electrical and Computer Engineering, University of British Columbia, Canada, and from October 1998 to December 1999 he was a post-doctoral fellow in the same department. During 1999 he was also a visiting fellow at the Advanced Systems Institute of British Columbia, Canada. During 2000 he was with the Institute of Space Applications and Remote Sensing (ISARS) of the National Observatory of Athens, Greece. He has many pending and granted patents, has published more than 60 papers in referred journals and conferences, and is a co-author and editor of two books in the areas of mobile Internet and mobile multimedia technologies. He is an editor of IEEE Wireless Communications and Journal of Advances in Multimedia, and has served as lead guest editor in a number of special issues of IEEE Wireless Communications, IEEE Communications Magazine, and so on. He is an active participant and contributor in 3GPP and vice chair of the Quality of Service Interest Group (QoSIG) of the IEEE Multimedia Communications Technical Committee. He is a member of the Technical Chamber of Greece.

This example, along with a brief discussion, has demonstrated that IEEE 802.21 covers many seamless mobility principles, and therefore seamless mobility largely is supported. However, the standard should be extended to further facilitate seamless handover provision.

NIKOS PASSAS [M] ([email protected]) received his Diploma (Honors) from the Department of Computer Engineering, University of Patras, Greece, in 1992 and his Ph.D. degree from the Department of Informatics and Telecommunications, University of Athens in 1997. Since 1995 he has been with the Communication Networks Laboratory of the University of Athens, working as a sessional lecturer and research associate on a number of national and European research projects in the ACTS and IST frameworks. Since 2004 he is also a visiting lecturer at the Department of Informatics of the Athens University of Economics and Business. His research interests are in protocol design and performance analysis for mobile multimedia communications. He is particularly interested in QoS for wireless networks, mobility management, mobile network architectures, and protocols. From 1992 to 1995 he was a research engineer at the Greek National Research Center Democritus. He also has served as a guest editor and technical program committee member in prestigious magazines and conferences, such as IEEE Wireless Communications, IEEE Vehicular Technology Conference, and IEEE GLOBECOM. He has published more than 60 papers in peerreviewed journals and international conferences, one book, and six book chapters. He is a member of the Technical Chamber of Greece.


71