The International Conference on WIreless Technologies embedded and intelligent Systems WITS-2016 ENSA of Kenitra 21-22 April 2016
Vertical Handover Methods for Heterogeneous Wireless Network A Review
AZEDDINE KHIAT
SSDIA Laboratory, ENSET Mohammedia HASSAN II University of Casablanca Mohammedia, Morocco
[email protected] Abstract Nowadays the development of heterogeneous network becomes a paramount necessity; current computing and communication devices are omnipresent and operate in a heterogeneous environment. The users have the privilege to stay connected to the Internet by using mobile terminals equipped with multiple networking interfaces. Thus, the users have the ability to use the services of their choice at anytime and anywhere. These processes of switching between different wireless technologies (Wireless LAN, WiMax, Cellular, UMTS, and LTE etc.) are referred to as Vertical Handovers/Handoffs.
JAMILA BAKKOURY
[email protected] MOHAMED EL KHAILI
[email protected] connection, while, a communication session (or call) is in progress.
The main objective of this contribution is to provide an overview of the state of the art concerning the basics of the handover, its classifications, its desirable characteristics and the analysis of Vertical Handover decision (VHD) algorithms. Index Terms Handover, Handoff process, Heterogeneous Network, Vertical Handoff (VHO), Hard Handoff, Soft Handoff, VHD algorithms, Multi Attribute Decision Making (MADM).
I. INTRODUCTION
The evolution of wireless working technologies (Figure 1), mobile devices, content and services create heterogeneous environments with increasing complexity. In this context, a compromise between mobility, transparency and performance is required. Mobile users, with different profiles and preferences, look forward to be connected to the best network at any time, without having to take care about the details of switching between heterogeneous networks. Given this complexity, it seems necessary to propose new approaches to make these systems seamless and decisions processes more efficient for vertical handover (VHO). of transferring a mobile communication between two heterogeneous networks. One example, handoff is a continuous transfer of an ongoing voice or video conversation from one channel served by a core network to another channel. In particular, handoff is the process of changing a communication channel (frequency, data rate, modulation scheme, spreading code, or their combination) associated with the current
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Fig. 1. Evolution on Wireless Communications
II. HANDOFF PROCESS BACKGROUND
A. Handoff Classification Several classifications of handoff can be established as presented below: Horizontal and Vertical Handoff: Depending on the type of network technologies involved, handoff can be classified as either horizontal or vertical [2]. Traditional handoff, also called horizontal or intra-system handoff occurs when the Mobile Station (MS) switches between different Base Station (BSs) or Access Point (APs) of the same access network. One example, horizontal handoff typically happens when the user moves between two geographically adjacent cells of a third generation (3G) or Four generation (4G) cellular network. On the other hand, vertical handoff or inter-system handoff involves two different network-interfaces representing different wireless access networks or technologies. Figure 2 depicts two types of handoffs in heterogeneous wireless networks, where horizontal handoff occurs between two WLANs, and vertical handoff occurs between a WLAN and a UMTS network.
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The International Conference on WIreless Technologies embedded and intelligent Systems WITS-2016 ENSA of Kenitra 21-22 April 2016
Fig. 2. Horizontal and Vertical Handoff in Heterogeneous Wireless Networks
Hard and Soft Handoff: This classification of handoff (illustrated in figure 3) depends on the number of BSs and/or APs to which a Mobile Terminal (MT) is associated with at any given moment [3 break its connection from the current access network before it switch communicate and connect with more than one access network during the handoff process. Mobile-controlled, Mobile-assisted, and Networkcontrolled Handoff: These types of handoff classifications are based on the entity, MS or access network, which make the handoff decisions [3]. Mobile-assisted handoff is the hybrid of mobile-controlled and network-controlled handoff where the MS makes the handoff decisions in cooperation with the access network.
Some of these features, depicted in figure 4, are described below: Speed: Handoff should be fast enough to avoid service degradation and/or interruption at the MS. Mobility of an MS at high speed requires the handoff to be done promptly. Reliability: Handoff should be reliable such that the MS will be able to maintain the required Quality of Service (QoS) after handoff. Successful: Free channels and resources must be available at the target access network in order to make the handoff successfully. Number of Handoffs: The number of handoffs must be minimized. Excessive number of handoffs results in a poor QoS and excessive processing overheads increasing the power loss, which is a critical issue in MSs limited battery power. Multiple criteria Handoff: The target access network should be properly chosen based on multiple criteria. Identification of a correct Access Network prevents unnecessary and frequent handoffs.
Fig. 4. Desirable Handoff Features
C. Vertical Handoff Process In the development of solutions supporting mobility management, the key aspect is handover management. Handover management is the process by which the mobile terminal uses to maintain its connection active while moving from access network to another. Heterogeneous Networks are the networks that support different technologies. Fig. 3. Handoff Classification Tree
B. Desirable Features of Handoff The efficiency of a handover's algorithm is estimated by a set of criteria or desirable characteristics [4]. These
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The handover process can be carried out (as shown in Figure 5), in three phases [5]: Handoff Information Gathering: Also known as System Discovery, information required to identify the need handover are collected. In this phase, the information of all neighbor
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The International Conference on WIreless Technologies embedded and intelligent Systems WITS-2016 ENSA of Kenitra 21-22 April 2016 networks are collected it can also be called system discovery phase. Handover Decision: This process finds the appropriate candidate network to which the MT can be handed over based on certain criteria decision algorithm. Handover Execution: Finally signaling exchange for establishment of the new communication path has been performed along with data re-routing through this path
Fig. 6. Metrics used in Vertical Handoff Decisions Fig. 5. Process Handover Vertical
D. Vertical Handoff Criteria and Metrics In the literature several criteria and metrics have been proposed for VHD Algorithms. They are depicted in figure 6 and briefly explained below: Received signal strength (RSS): RSS is the one of the key criteria for VHD. The RSS is easy to measure and it is directly related to the quality of service. The majority of existing horizontal handover algorithms use RSS as the main decision criterion. Network connection time: to choose the accurate moment to trigger a handover, it is very important to determine exactly the network connection time which refers to the duration that a mobile terminal remains connected to a point of attachment. Available bandwidth: It is a measure of available data communication resources expressed in bit/s. Power consumption: is low the power consumption becomes a critical issue. In such situations, it would be preferable to handover to a point of attachment and avoid extensive handovers to optimise the battery life. Roaming cost: In some situations the cost of a network service should be taken into consideration in making handover decisions because for different networks, there would be different charging policies. Security: A network with higher security level may be chosen over another providing lower level of data security when confidentiality or integrity of the transmitted data can be critical. Use access network could lead to the selection of one type of network over the other available networks.
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III. CLASSIFICATION AND ANALYSIS OF VHD ALGORITHMS
There are various ways to classify VHD algorithms [6]. Here, we have chosen to outline some algorithms, Method used for decision, Input parameters, Complexity of algorithms, Reliability, Advantages and Drawbacks. a) RSS based algorithms: RSS is used as the main handover decision criterion in this group. Various strategies have been developed to compare the RSS of the current point of attachment with another candidate point of attachment [7]. (Figure 7)
Fig. 7. RSS based algorithms
b) Bandwidth based algorithms: Available bandwidth for a mobile terminal is the main criterion in this group [8][9][10]. In some algorithms, both bandwidth and RSS information are used in the decision process [7]. Depending on
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The International Conference on WIreless Technologies embedded and intelligent Systems WITS-2016 ENSA of Kenitra 21-22 April 2016 whether RSS or bandwidth is the main criterion considered, an algorithm is classified either as RSS based or bandwidth based. (Figure 8)
d) Combination algorithms: These VHD algorithms attempt to use a richer set of inputs than the others for making handover decisions. When a large number of inputs are used, it is usually very difficult or impossible to develop analytical formulations of handover decision processes. Due to this reason, researchers apply machine learning techniques to formulate the processes. (Figure 10)
Fig. 8. Bandwidth based algorithms
c) Cost function based algorithms: This class of algorithms combine metrics such as roaming cost, security, bandwidth and power consumption in a cost function, and the handover decision is made by comparing the result of this function for the candidate networks [11][12]. Different weights are assigned to different input metrics depending on the network conditions and user preferences. (Figure 9)
Fig. 10. Combination algorithms
e) Multiple Attributes Decision Making Based Algorithms: The multiple attributes decision making based algorithm (MADMA) [13] calculates the quantitative value of each normalized attribute and evaluates the target systems through the weighted function of the quantitative values, the final decision can then be made. (Figure 11)
Fig. 9. Cost function based algorithms Fig. 11. Multiple Attributes Decision Making Based Algorithms
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The International Conference on WIreless Technologies embedded and intelligent Systems WITS-2016 ENSA of Kenitra 21-22 April 2016 f) Authentication Based Algorithms: Authentication during handover is one of the main challenges. The user has to execute multi-pass authentication procedures in order to get access to the other network. This causes overhead on the controlling servers and increases the delay of authenticating the user due to unnecessary and repeated procedures and protocols. These algorithms provide proactive handover and authentication process that maintains QoS and reduces the handover delay. (Figure 12)
[5] L. Xia, L.handover algorithm with aid of differential prediction and pre-
[6]
[7] [8]
[9]
[10]
[11]
Fig. 12. Authentication Based Algorithms
IV. CONCLUSION
Through this review novice wireless network researchers shall be able to get a comprehensive idea about user mobility and Handovers, which is a key underlying for mobile wireless communications. In addition, the analysis outlines the handover vertical decision algorithms, to give a global idea about VHD as well as its advantages and disadvantages.
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