Media Independent Handover (IEEE 802.21): Framework for Next Generation Vertical Handover Protocols

Vikas Sharma1, Ankit Agarwal2, Mohammed Abdul Qadeer3

1Dept. of Computer Science & Information Systems, BITS Pilani, Pilani 333031, India 2Adobe Systems India Pvt. Ltd. Adobe Towers, Noida 201301, India

3Dept. of Computer Engg, Aligarh Muslim University, Aligarh 202002, India vikas8536@gmail.com, ankitaga@adobe.com, maqadeer@ieee.org

AbstractAlthough various wireless access network technologies with different characteristics and performance level have been developed, no single network that can satisfy the anytime, anywhere, and any service wireless access needs of mobile users. A truly seamless mobile environment can only be realized by considering vertical and horizontal handoffs together. One of the major design issues in heterogeneous wireless networks is the support of vertical handoff. Vertical handoff occurs when a user with a multi-interface terminal changes association from one type of wireless access technology to another while maintaining an active session. It is believed that vertical handoff decision will be based on multiple criteria. In this paper review of Vertical Handoff Mechanisms (VHM), decision models and IEEE standard 802.21 (also called Media Independent Handover) have been discussed for seamless vertical handover .

Index Terms- VHM, MIH, Vertical Handoff, Handover, IEEE 802.21.

I. INTRODUCTION The next-generation wireless networking is envisioned as a convergence of various wireless access technologies. The currently existing cellular systems, such as GSM and CDMA2000, support low bandwidth over a large coverage area. While WLAN, based on IEEE 802.11 standard, can provide relatively high bandwidth in a small service area. In addition, the IP network will replace several technologies used for various and multiple applications, including standard Internet applications such as file transfer and web browser as well as VoIP, games and different kinds of video applications like video-on-demand and video conference. A single wireless network technology can hardly satisfy the requirements of all current and upcoming services. The complementary characteristics of cellular networks and WLANs make it attractive to integrate these technologies to provide ubiquitous wireless access. The coexistence of heterogeneous wireless networks to provide service anywhere at any time is an inevitable trend to meet application requirements and user expectations. And a multi-mode terminal (MT), containing both cellular and WLAN air interfaces, can select an appropriate network to use. The different technologies should be integrated to form a heterogeneous network (4G) based on an IP core network

infrastructure. This ensures user mobility and service continuity by maintaining connections when switching between various technologies and it introduces new resources and possibilities for applications. An automatic interface selection based on instantaneous and practical constraints and user/application/network references (transmission cost, transmission coverage, QoS parameters, security, power consumption, etc.) is therefore needed to ease the use of these mobile networks. Standardization bodies, such as 3GPP, IETF and IEEE, have already started to tackle this issue. For example, [1] describes the inter-working between the 3GPP systems and IEEE 802.11 WLANs including the access network selection. The Candidate Access Router Discovery (CARD) draft proposed at IETF [2] recognizes the need for an Access Router selection algorithm. In the research community, a number of approaches aiming at providing network support for mobility management have been proposed, and examples are [3] [4] [5]. The IEEE 802.21 working group [6] works on a new standard which aims at facilitating vertical handoffs based on generic triggers and on network assistance for access detection and selection. In [7], the authors presented various studies on how a terminal can be always best connected.

Figure 1: Present Heterogeneous Network

2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 2011 IEEEDOI 10.1109/CICN.2011.106

514

2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 2011 IEEEDOI 10.1109/CICN.2011.106

508

2011 International Conference on Computational Intelligence and Communication Systems

978-0-7695-4587-5/11 $26.00 2011 IEEEDOI 10.1109/CICN.2011.106

507

II. WHAT IS A HANDOFF?

When a mobile user travels from one area of coverage or cell to another cell, within a calls duration, the call should be transferred to the new cells base station. Otherwise, the call will be dropped because the link with the current base station becomes too weak as the mobile recedes. Indeed, this ability for transference is a design matter in mobile cellular system design and is call handoff. Two types of handoff are:

A. Horizontal Handoff Occurs when the user switches between different network access-points of the same kind. Example: Handoff among 802.11 APs.

B. Vertical Handoff Involves two different network interfaces which usually represent different technologies. Example: Handoff from 802.11 to 1xRTT (CDMA 2000).

Handoff can also be classified as:

C. Soft Handoff A Soft Handoff occurs if the MN (mobile node) can communicate with more than one access points during handoff. If the mobile node (MN) is equipped with multiple network interfaces, with both 802.1lbWLAN card and GPRS card, then it can simultaneously connect to access points in the two different networks during a soft vertical handoff.

D. Hard Handoff In Hard Handoff the link to the prior base station is terminated before or as the user is transferred to the new cells base station. That is to say that the mobile is linked to no more than one base station at a given time.

III. STEPS IN VERTICAL HANDOFF MECHANISM

A. System Discovery The first step is measuring RSS to discover the reachable networks. One way to discover the existence of other wireless networks is to turn on all interfaces of the MT to continually scan new signals and another is to open the interfaces periodically. However, the above methods may result in unnecessary power consumption. In [5], a time-adaptive network discovery method is proposed. Time interval of activating interfaces varies according to the RSS from a new BS to avoid unnecessary interface activation.

And a counter is defined to record the times that the RSS from the new network is larger than a required threshold to avoid ping-pong effect.

B. Score function The score function for candidate network evaluation is k k

Si= wjfj,i , 0

D. Handoff Once the networks are sorted, a handoff is executed to switch to a more (either a more or the most) suitable network as per the specifications of the user.

IV. MEDIA INDEPENDENT HANDOVER (MIH)

The IEEE 802.21 working group started work in March 2004. More than 30 companies have joined the working group. The group produced a first draft of the standard including the protocol definition in May 2005.

A. Problems 1) Incorect Network Selection: With the existing

technologies, the ability of mobile node (MN) to choose its network connection was hopeless, it may connect at layer 2 but not at network layer. If MN were able to connect to any of available access points or network, it may choose it incorrectly based only on one metric (for example: based on signal strength only).

2) Increasing number of interfaces on a mobile node extended the problem as score function is needed to be calculated for each network and that would further make the problem complicated.

3) There was no media independent standard for layer 2 constructs (for example: L2 trigers,events,etc.) and no standard protocol to carry out handover.

B. Overview: 1) 802.21 is a new and emerging IEEE standard

which is developed to implement seamless handover between networks of the same type as well as handover between different network types also called Media Independent Handover (MIH) or vertical handover.

2) It provides a framework for efficiently discovering networks in range and executing intelligent heterogeneous handovers, based on their respective capabilities and current link conditions.

3) The IEEE 802.21 enables optimized handover across heterogeneous IEEE 802 systems as well as between IEEE 802 and cellular (3GPP and 3GPP2) systems. The goal is to provide means to facilitate and improve the intelligence for handover procedure

Figure 2: 802.21 Genesis

C. 802.21 Specifications: 1) Transparent service Continuity:

a) IEEE 802.21 specifies a framework that enables transparent service continuity while a mobile node switches between heterogeneous access technologies. The consequences of a particular handover need to be communicated and considered early in the process and clearly before the handover execution.

b) IEEE 802.21 specifies essential mechanisms to gather all necessary information required for an affiliation with a new access point before breaking (breaking up with or breaking off) the currently used connection.

2) Handover Enabling Functions:

IEEE 802.21 defines a set of handover-enabling functions, which are specified with respect to existing network elements in the protocol stack, and introduces a new logical entity called Media-Independent Handover Function (MIHF). The MIHF logically resides between the link layer and the network layer. It provides, among others, abstracted services to entities residing at the network layer and above, called MIH Users (MIHUs). The primary role of the MIHF is to assist in handovers and handover decision making by providing all necessary information to the network selector or mobility management entities. The MIHF is not meant to make any decisions with respect to network selection.

3) Service Access Point:

SAPs with associated primitives between the MIHF and MIHUs (MIH_SAP) give MIHUs access to the following 3 main mobility services that the MIHF provides:

a) The Media-Independent Event Service (MIES) provides event reporting about, for example, dynamic changes in link conditions, link status, and link quality. Events can be both local and remote. Remote events are obtained from a peer MIHF entity.

516510509

b) The Media-Independent Command Service (MICS) enables MIHUs to manage and control the parameters related to link behavior and handovers. MICS provides a set of commands for accomplishing that, as we will see later in this article. Commands can be both local and remote. The information obtained with MICS is dynamic.

c) The Media-Independent Information Service (MIIS) allows MIHUs to receive static information about the characteristics and services of the serving network and other available networks in range. This information can be used to assist in making a decision about which handover target to choose and to make preliminary preparations for a handover.

D. MIH Reference Model: 1) The scope of IEEE 802.21 includes only the

operation of MIHF and the primitives associated with the interfaces between MIHF and other entities. A single media-independent interface between MIHF and MIHU (MIH_SAP) is sufficient.

Figure 3: General MIH reference model

2) There is a need for defining a separate technology-dependent interface, which is specific to the corresponding media type supported, between the MIHF and the lower layers (MIH_LINK_SAP).

3) The primitives associated with the MIH_LINK_SAP enable MIHF to receive timely and consistent link information and control link operation during handovers.

4) IEEE 802.21 specifies a media-independent SAP (MIH_NET_SAP), which provides transport services for Layer 2 (L2) and Layer 3 (L3) MIH message exchange with remote MIHFs.

5) Functions over the LLC_SAP are not specified in IEEE 802.21.

Figure 4: MIHF Message Passing The figures [4] and [5] show messages directions of each MIHF service class, including both local and remote events and commands.

Figure 5: MIHF between L2 and above layers

E. List of MIH Functions:

TABLE 2 MIH FUNCTIONS

Category Function

Event Service Link Event Register Link Event Deregister Link Detected Link UP Link Down Link Going Down Link Event Rollback

Command Service MIH Poll MIH Handover Initiate

MIH Protocol Event Registration Link Events Handover Initiate (request/response) Poll (request/response)

517511510

F. Types of Handover based on control model: 1) Terminal Controlled

Mobile node (MN) or terminal uses some of MIH services and handover is under its control.

2) Terminal Initiated, Network Assisted Terminal uses MIH Information Base to carry out handover.

3) Network Initiated and Network Controlled Network makes use of MIH Event and Command Service, plus Information Service knowledge, to decide if handover is needed/desired, to decide the target, and to command the terminal to handover.

V. CONCLUSION The Heterogeneous Networks are expected to be widely popular in near future as wireless users may like to roam between existing network technologies. The seamless handover is critical for multimedia services. Wireless users expect service continuity when they move between points of attachment. Although there are technologies that exist and can provide seamless vertical handover, yet they do not provide optimized solution as a whole. IEEE 802.21 (Media Independent Handover - MIH) is emerging standard which promises a well optimized solution. IEEE 802.21 standardizes the different functions, services and protocols needed to carry out vertical handover seamlessly.

REFERENCES

[1] 3GPP TS 23.234, 3GPP system to Wireless Local Area Network (WLAN) Interworking, System Description (Release 6) V2.3.0, November 2003.

[2] M. Liebsch, A. Singh (editors), Candidate Access Router Discovery, RFC 4066, System Description (Release 6) V2.3.0, July 2005.

[3] D.Kutscher and J.Ott, Service Maps for Heterogeneous Network Environments,in 7th Mobile Data Managament Conference, 2006.

[4] Y.Khouaja, P.Bertin, K.Guillouard, and J.Bonnin ,Hierarchical mobility controlled by the network in Multiaccess Mobility and Teletrafc for Wireless communications, 2002.

[5] F.Akyildiz, J.Xie ,and S.Mohanty , A survey o fmobility management in next-generation all-IP-based wireless systems, in IEEE Wireless Communications, August 2004.

[6] M. Williams, Directions in Media Independent Handover, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences - Special Section on Multi-dimensional Mobile Information Networks, July 2005.

[7] E. Gustafsson and A.Jonsson, Always Best Connected, IEEE Wireless Communications, February 2003.

[8] Q.Song, and A. Jamalipour, A Time-Adaptive Vertical Handoff Decision Scheme in Wireless Overlay Networks, The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, (PIMRC 2006).

[9] H.J. Wang, R. H. Katz, and J. Giese, Policy-Enabled Handoffs across Heterogeneous Wireless Networks, Proc. of ACM WMCSA, 1999.

[10] M. Angermann and J. Kammann, Cost Metrics For Decision Problem In Wireless Ad Hoc Networking, IEEE CAS Workshop on Wireless Communications and Networking, 2002.

518512511