Author Topic: An Overview on Decision Techniques for Vertical Handoffs across Wireless Heterog  (Read 2470 times)

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Author : Akhila S, Jayanthi K Murthy, Arathi R Shankar, Suthikshn Kumar
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
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Abstract- Wireless communication   of  the future will comprise of  several heterogeneous networks whose  access technologies will  vary to a large extent on the network capacity, data rates,  bandwidth, power consumption, Received Signal Strength and coverage areas. With their complementary characteristics, integration of these networks to offer overlapping coverage to mobile users pose many interesting research challenges to bring about anytime, anywhere connectivity. The  best of these networks with their   varying characteristics can be brought about through a process called vertical handoff. Vertical handoff is the seamless transfer of an ongoing user session between these networks  and   requires accurate and precise decisions about the  availability of the networks and their resources for connection. A good handoff decision should avoid unwanted handoffs which  leads to an increased computational load  or should not miss making a handoff  leading to an ongoing  service being dropped causing  packet loss. Many techniques for vertical handoffs have  been proposed in literature which are based on several parameters,  but there still exists some ambiguity as  to which of these parameters give an optimum performance. This paper aims at providing an account on the various policies  developed in the decision phase of the vertical  handoff. 

Index Terms-  Heterogeneous Networks, Mobility Management, Vertical handoff, handoff decision.
The main attraction of wireless communication lies in the ability to communicate and exchange information on the move. The demand for the available services anytime anywhere is accelerating at a very high rate which calls for an integration of the  various wireless access technologies. With the current technologies varying widely in their bandwidths, latencies, frequencies and access methods, the next generation systems will allow global roaming among a range of mobile access networks.
This calls for a seamless transfer of the Mobile Terminal (MT) to the best access link among all available candidates with no perceivable interruption to an ongoing conversation[1]. It should also provide an end-to-end optimization that takes into account variables such as throughput optimization, routing optimization, delay profiles and economical profitability. The actual trend is to integrate complementary wireless technologies with overlapping coverage, to provide the expected ubiquitous coverage and to achieve the Always Best Connected (ABC) advantage [2]. The Always Best Connected concept should enable a user, to choose among a host of networks that best suits his or her needs and to change when something better becomes available. It requires a framework that supports mobility management, access discovery and selection, authentication, security and profile server. This calls for an efficient Vertical Handoff Decision(VHO) scheme which involves a tradeoff among several handoff parameters such as network conditions, system performance, application types, power  requirements, mobile  node conditions, user preferences, security  cost  and the Quality of Service(QoS). These parameters may have varying levels of importance in the decision process [3]. Also, the handoff solution should be network-layer-transparent and infrastructure-modification-free so that existing Internet server and client applications can painlessly survive the rapid pace of wireless technology evolution [4].

       Fig. 1: Horizontal and vertical handoff
The handoffs are classified into two main streams, Horizontal Handoff (HHO) and Vertical Handoff (VHO). Figure1 illustrates horizontal and vertical handoff. The main distinction between Vertical Handoff and Horizontal handoff (HHO) is symmetry.

   VHO   HHO
Access Technology   Changed   Not Changed
QoS Parameters   May be changed   Not Changed
IP Address   Changed   Changed
Network Interface   May be Changed   Not Changed
Network Connection   More than one connection   Single connection
Table 1: Difference between Vertical  and Horizontal Handoff.
While HHO is symmetric or an intra-technology based process, VHO is an asymmetric or an inter-technology based process in which the MT moves between two different networks with different characteristics [5]. The vertical handoff process involves three main phases [6] [7], namely system discovery phase, decision phase and execution phase.
During the system discovery phase, the MT scans for  available candidate network for connection which may include several parameters like  the supported data rates and QoS parameters. This phase needs to be invoked periodically, since the users are mobile.
In the decision phase, the mobile terminal determines whether the connections should continue using the existing network or be switched to another network depending  on various parameters like the type of the application (e.g., conversational, streaming), minimum bandwidth available, delay constraints, cost, transmit power and the userís preferences.
In the execution phase, the connections of the mobile terminal are handed over to the new network in a seamless manner. Authentication, authorization, and transfer of a userís information is done during this phase.
Handover discovery and decision phase can sometimes overlap, since some situations may require more additional probing of the network condition.  A delay in handoff process can be differentiated into three main mechanisms [10].
Discovery Time (td):  During this period, the mobile terminal perceives its new wireless network range either through the trigger-based router solicitation or waits to receive a router advertisement from an access router in the visited network and gets its router advertisement (RA) from the new access router.
Address Configuration Interval (tc): During this period,  the mobile device receives the  Router advertisement and  updates its routing table. A new care-of- address (CoA) will be based on the prefix of the new router that is obtained from the RA.
Network Registration Period (tr): This is the period during which the binding update(i.e., the association of home address with a care-of address) to the home agent as well as the correspondent node is sent and first packet from the correspondent node is received. Since the binding acknowledgment from correspondent node is elective, optimizing IP-level vertical handoff delay would involve minimizing the discovery time and network registration period. The decision phase is the most important phase in VHO since it determines how meaningful the handoff is to the user. This needs an extensive research to find accurate ways of discovering precise decision techniques which may include one or more parameters. The objective of this paper is to show how decision parameters or policies affect VHO.  A brief survey of the various decision making techniques used has been provided.

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