Mobile ip and vertical handoff algorithms in wireless networks

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Chapter 1: Introduction

1.1 Introduction

Technology has become synonymous with communication because technology has historically been the method by which communication to or by the general population takes place. From the printing press to the telephone to mobile to radio and TV broadcasting, technology has touched our lives by providing convenient ways for a large population to communicate. Because the intertwining of technology and communication is fundamental to our culture, the technology of communication in a way defines our culture.

As we enter the early years of the 21st century, humanity is awash in instant communication based upon the radio technology that makes it possible. As a society we have near real time access to world events occurring in any corner of the globe, and as individuals we have instant voice communication with each other by virtue of the wired and wireless communication in its many shapes and forms.

Over the last decade cell phone redefined our cultural expectations of communication. The advent of the portable phone along with price competition among hardware and service providers has brought true voice communication service to a large segment of humanity.

The growing volume of the mobile users has made the researchers think about new idea's to use the network resources efficiently and effectively. In other words QoS Vs Security trade off is what researchers are looking at. Beside other QoS concerns mobility management plays an integral role in the overall performance of a network. Usually, continuous service is attained by a user by supporting handoff from one cell to another. Handoff is the process in which channels are changed (time slot, spreading code, frequency or combination of them) related to the present connection while a call is in progress. Sometimes it starts when a mobile terminal either cross a cell boundary or the signal become weak (distortion) in the current channel. Handoff can be categorised as hard and soft handoffs. In hard handoffs, before a new connection is established, current resources are released whereas in soft handoffs there exists both the current and new resources during the handoff process. [ ???]

Handoff process is critical since within cellular communication systems adjacent cells always use disjoint subset of frequency bands, so both current and new base stations must negotiate in order to keep the ongoing call .Handoff significantly affects the QoS of a network [6].

On the basis of the evolving developments of mobile communication, 4G will have high data rates, larger bandwidth, smooth handoff, and deep focus on reduction of fault in services and access to the multiple networks with the help of seamless handoffs. The concept behind this is to combine the existing mobile technologies with the capabilities of 4G. Network management is important among different access systems with respect to horizontal (intra-system) and vertical (inter-system) handoff as well as QoS, seamless mobility and security.

In heterogeneous wireless networks, the mobile devices or mobile terminal will have multiple network interfaces in order to access different wireless networks. Such mobile devices not only support network access and great connection flexibility, but also support mobility between other networks. Users will expect uninterrupted and continuous with minimum or no interruption when they move from one network to another. The process which almost prevent this interruption in wireless networks communication is referred to as vertical handoff or handover.

In a mobile environment, handoff is a challenging problem and the last decade has seen rapid growth in the evolvement of mobile applications. This has increased the demand on the wireless communication services. The ability to achieve wireless access anytime, anywhere and any place has become common expectation as it provides freedom and considerable flexibility in mobility. Seamless vertical handoff is needed to achieve global mobility in heterogeneous networks.

1.2 Organization of Dissertation

In this dissertation report we'll start with some introduction of the mobile network technology evolution from 1G to 4G in Chapter-2. Chapter-3 will explain the basic concept of Vertical Handoffs. The focus will be, especially, to the most popular and researched technology i.e. Mobile IP, so Chapter-4 is dedicated for explaining different perspectives Mobile IP and Vertical Handoffs. Further, Chapter-5 will throw some light on the different vertical handoff algorithms. Then Chapters-6 will present the simulations and results and proceeding to Chapter-7 which finally gives you results and future work.

CHAPTER 2: Mobile Networks and Handoffs

2.1 Evolution of Mobile Networks (From 1G to 4G)

The Evolution of Mobile Networks is divided into four different phases as follow.

2.1.1 First generation (1G)

First generation mobile systems were based on the analogue transmission for speech services. This era of telecommunication introduced in the late 1970s. In 1979 the first operational cellular networks were invented by Nippun Telephone and Telegraph (NTT) in Japan. 1G of mobile phones for the customer was introduced in North America by AT&T Bell in 1980s [13].

2.1.2 Second generation (2G)

Second generation (2G) mobile network systems were launched in the end of 1980s. 2G mobile network systems support data services with the traditional speech service and low bit rate. Second generation (2G) mobile network system was based on the digital transmission instead of analogue transmission. Due to digital transmission, 2G mobile network system offers more advanced roaming, better data services and higher spectrum efficiency by comparison with 1G [13].

2.1.3 Third generation (3G)

During 1990s, tremendous amount of work was done on 3G mobile system in order to eradicate previous incompatibilities and making 3G a truly global system. The 3G system would have higher broadband data capabilities i.e. up to 2Mbps and better quality voice channels [14].

The Figure 2.1 expresses the evaluation of cellular networks as below:

1.1

1.2

1.3

1.4

2.1.4 Fourth generation (4G)

A beyond third generation or 4G is the new technological advancement in the field of wireless communications. It is intended to complement and replace the 3G systems. The development in 4G includes communication with wide range of services and information, and receiving a large size of data, images and videos on internet at any point in time, anywhere and with a seamless connection. The main features of internet in 4G will allow the users to access any application at any environment [14].

2.2 1G, 2G, 3G and 4G features

The following table shows mobile communications history and status to sum up the advancements in the mobile communications with the properties of each generation.

Property

1G

2G

3G

4G

Starting Time

1985

1992

2002

2010-2012

Driven Technique

Analogue signal

Processing

Digital signal

processing

Intelligent signal

processing

Intelligent software

Auto configuration

Representative Standard

AMPS, TACS, NMT

GSM, TDMA

IMT-2000 (UMTS,

WCDMA, CDMA2000)

OFDM, UWB

Radio Frequency (HZ)

400M-800M

800M-900M, 1800M-1900M

2G

3G-5G

Bandwidth (bps)

2.4K-30K

9.6K-14.4K

2M-5M

10M-20M

Multi-address

Technique

FDMA

TDMA, CDMA

CDMA

FDMA, TDMA, CDMA

Cellular Coverage

Large area

Medium area

Small area

Mini area

Core Networks

Telecom networks

Telecom networks

Telecom networks,

Some IP networks

All-IP networks

Service Type

Voice

Mono-service

Person-to-person

Voice, SMS

Mono-media

Person-to-person

Voice, Data

Some Multimedia

Person-to-machine

Multimedia

Machine-to-machine

Table (1) History and status of mobile communications

2.3 Handoffs

Handoff takes place when a cellular phone user move from the range of one cell to another cell's range and the signal is passed from first base station to the next one. During handoff user will not loss the service unless he/she moves out of range of a cell's base station. The process and transition which required making the conversion are both referred to as the handover or handoff [16].

2.4 Classification of handoff

Based on several factors, handoff is classified into major categories as shown in figure 2.2. The network type is not the only handoff classification factor as it can be classified according to many other factors such as: administration domain, number of connections and frequencies engaged [1]. The following are the main handoff classification.

2.4.1 Network types involved

It is the most common handoff classification. According to this classification, handoff is also categorised into two main types; horizontal and vertical depending on the type of networks that involved in the handoff .When the handoff takes place between two networks or more with the same type, it known as horizontal handoff, otherwise it is called vertical handoff.

2.4.1.1 Horizontal handoff (intra-system)

When the mobile terminal moves between two base stations that support the same radio access technology, a horizontal handoff will take place. For example, when mobile terminal moves from an IEEE 802.11b base station to another adjacent IEEE 802.11b base station [1].

2.4.1.2 Vertical handoff (inter-system)

Vertical handoff is occurred when a mobile terminal moves between different access networks which represent different access technologies in other word; vertical handoff is the mechanism of transferred ongoing connection from one base station (BS) to another [5]. For example, when mobile device moves out an 802.11a network into a GPRS network, the handoff that takes place is a vertical handoff.

The term ‘vertical' follows from the overlay network structure [17].Such networks structure with increasing coverage areas at higher levels in the hierarchy as well as homogenous networks structure are shown in figure 2.3

The main concern of vertical handoff is to keep ongoing service not only the change of IP addresses but also the change of Quality of Service (QoS) characteristics, network interfaces, and etc. Due to the benefit of utilizing the higher bandwidth and lower cost of WLAN also better mobility support and larger coverage of UMTS. Vertical handoffs between UMTS (cdma 2000) and WLAN have attracted of attention a great deal to 4G wireless network in all research areas. Furthermore we use vertical handoff in order to maintain communication and also it uses to access supporting infrastructure, generally to support mobility node [5].

The most important features of vertical handoff as compared to horizontal handoff are as follows:

  • Handling several network interfaces.
  • Handling different access technologies.
  • Handling multiple (variable) QoS parameters.
  • Handling multiple network connections.
  • Handling various IP addresses [5].

There are two types of vertical handoff: upward and downward. These types will be discussed in detail in the next chapter.

2.4.2 Frequencies engaged

Handoff can be classified into two types: inter-frequency and intra-frequency handoffs.

2.4.2.1 Inter-frequency handoff

It is a process in which a mobile terminal roams across access points (AP) operating on different frequencies. This type of handoff is the only one supported in GSM cellular systems and it presents in code division multiple access (CDMA) networks with time-division duplex (TDD).

2.4.2.2 Intra-frequency handoff

It is the roaming of a mobile terminal across access points (AP) operating on the same frequency. This type of handoff is currently used in code division multiple access (CDMA) networks with frequency-division duplex (FDD) [1].

2.4.3 Number of connections involved

Based on the number of connections involved, handoffs can be classified into three categories as described below:

2.4.3.1 Hard handoff

This handoff is called break-before-make. In other meaning the connection to the source is terminated before or once the connection to the target is made. It is used largely in FDMA (frequency division multiple access) and TDMA (time division multiple access), while using different range of frequencies in the neighbouring channel to minimize channel interference. It could not be possible for MS to communicate with both base stations when it moves from single BS to another as each one uses different frequencies.

Advantages of hard handoff

  • One call uses only one channel at any moment in time.
  • The phone's hardware does not need to be capable of receiving two or more channels in parallel, which makes it simpler and cheaper.
  • Efficient use of bandwidth.
  • Increase of reverse link capacity
  • No data overhead.

Disadvantages of hard handoff

  • If handoff fails the call may be interrupted or terminated unusually.
  • Re-establishing the connection may not always be possible and even when possible the process may cause a temporary interruption to the call [2].
  • Sensitive to link and transfer-time (may result in dropped call).

2.4.3.2 Soft handoff

Soft handoff can be also called make-before-break. Mainly the connection to the target is established before the connection to the source is broken. it refers to the overlapping of base stations coverage regions in cellular telephone communication, in order that every cell phone set is always well within range of at least one base station. In some cases, set of mobiles transmit signals to, and receive signals from more than one base station at a moment in time [3].

2.4.3.3 Softer handoff

A softer handoff is very similar to a soft handoff, with the exception of the mobile terminal changes the connections over radio links that belong to the same point of attachment [1].

Advantages of soft handoff

  • Connect to multiple base stations (BSs).
  • Improved power usage.

Disadvantages of soft handoff

  • Based on the additional network resources which are used during a soft handoff. It is more complex.

2.4.4 Administration domain handoff

Administration domains handoff consists of set of systems and networks operated by a particular organization of administrative authorization. This administration domain is concern problem and can be classified as Inter-administrative and Intra-administrative handoff [1].

2.4.4.1 Inter-administrative handoff

In inter-administrative handoff a mobile terminal moves between different networks which supporting the same or different cases of network interfaces that are controlled by different administration domains [1].

2.4.4.2 Intra-administrative handoff

Intra-administrative domain handoff is the opposite side to inter-administrative domain process where the cellular phone workstation moves between different networks that supporting the same or different network interface types and controlled by the same administrative domains [1].

2.4.5 Essential of handoff

In accordance to the need of handoff it can be categorized into two classes: -

2.4.5.1 Mandatory handoff

To avoid disconnection in some cases it is necessary for the mobile terminal to transmit the connection to another access point (AP).

2.3.5.2 Voluntary handoff

In voluntary handoff transmit the connection is optional in some cases and sometimes does not improve the quality of service (QoS).

2.4.6 User control allowance

Handoffs consist of two types which they are proactive or passive.

2.4.6.1 Proactive handoff

Proactive handoff is expected to be one of the fundamental features of 4G wireless schemes. In this type of handoff the mobile terminal's user is allowed to decide when to handoff and the handoff decision can be based on a serial of preferences which specified by the user [1].

2.4.6.2 Passive handoff

The most common type of handoff from first to third-generation wireless schemes is passive handoff and also the user does not have control over the handoff process.

Chapter 3: Vertical Handoff

3.1 Handoff in Mobile Communication Network Systems

One of the most important features of wireless cellular network technologies is mobility. In wireless communication systems continuous service are accomplished by supporting handoff or handover service from one cell to another. Handoff is that the process to avoid call termination of ongoing cell to transfer call or data session from the area covered by one cell to the second cell outside the range of the first cell [9]. Any cellular network that enables user mobility must support handoff and this phenomenon will be clear as shown in Figure 3.1 a mobile user that is in connection with base station (BS1), but move to the coverage area of base station (BS2). The procedure in which the mobile station (MS) is shifted from BS1 to BS2 is called handoff or handover event [8]. Moreover when handoff initiates the system has to allocate new channel to the mobile host (MH) and transfer the connection from the old base station (BS1) to the new serving base station (BS2) [9].

3.2 Vertical handoff types

There are two types of vertical handoff: upward and downward. An upward vertical handoff occurs from a network with small cell size and high data rate to a network with a larger cell size and lower date rate, while downward vertical handoff occurs from a network with lower data rate and wider coverage to a network with high date rate and small coverage. According to this classification cellular networks and WLANs can be taken as example. The WLAN system is the one with the small coverage network and high data rate whereas the cellular system can be considered as wider coverage with lower data rate. So the handoff process from WLAN to a cellular network is called upward vertical handoff, whilst the changing from cellular network to WLAN is called download vertical [18].

3.3 Horizontal and Vertical handoff terminologies

There are different subclasses below to represent the terminology of horizontal and vertical handoff. It can be reflect on the wireless access network technology as an alternative of the administrative domain by comparison to macro- and micro mobility.

  • Horizontal macro mobility:It is refer to mobility between different administrative domains using the same wireless technologies.
  • Vertical macro mobility: This refers to mobility among different administrative domains with different wireless technologies.
  • Horizontal micro mobility: It can be denote to mobility within the same administrative domain using the same wireless technology.
  • Vertical micro mobility: It refers to mobility within the same administrative domain controlled by different wireless technologies [19].

3.4 Handoff Metrics

We briefly introduce some ideas about traditional handoffs, such as a handoff between cellular networks; the handoff decision is basically based on Relative Signal Strength (RSS) in the limit area of two cells, and may also be based on call drop rate, etc. In vertical handoff, the state is more complex. For instance, there are two different types of wireless networks WLAN compared to UMTS normally have incomparable signal strength metric. Both WLAN and UMTS networks cover an area at the same time. The Handoff Metrics in this state should include network conditions, user preference, application types, cost and relative signal strength (RSS) etc [7].

3.5 Dual Mode Card

A mobile terminal requests to have a dual mode card in order to support vertical handoff. It can be work under both UMTS and WLAN scheme [7].

3.6 Vertical handoff process

In this subsection we discuss vertical handoff process which consists of three phases, namely system discovery, handoff decision and handoff execution.

3.6.1 System Discovery

It can be called also handoff information gathering or handoff initiation phase [20]. Basically at this stage the mobile terminals equipped with multiple interfaces that have to determine the available services in each network and which networks can be used.

3.6.2 Handoff Decision

It can be also called system selection. At this phase the mobile device determines which network it should connect to. This decision may depend on different parameters including, delay, jitter, the available bandwidth for each wireless access network, access cost, the existing battery status of the mobile device, the user's preferences and transmit power. The decision-making process of handoff might be as centralized or decentralized (i.e., the handoff decision may be made at the mobile station (MS) or network).

From the decision process point of view, there are several ways to perform the handoff; one can find at least three basic types of handoff decisions described as follows:

3.6.2.1 Network-Controlled Handoff

Network-controlled handoff is used in analog systems of first generation for examples advanced mobile phone system (AMPS), total access communication system (TACS), and Nordic mobile telephone (NMT).

In a network-controlled handoff protocol, the network is responsible of making decision for handoff depending upon the measurements of the mobile stations (MSs) at different base stations (BSs). The handoff process takes about 100-200 ms to perform the channel switching, data transmission and network switching. The signal quality information for all users is available at a single point in the network that facilitates appropriate resource allocation.

3.6.2.2 Mobile-Assisted Handoff

In a mobile-assisted handoff process, the mobile station (MS) is responsible of making the required measurements and finally the decision is made by the network. In the circuit-switched global system mobile (GSM), the base station controller (BSC) takes care of radio interface management. For such a circuit-switched GSM the time taken between the handoff decision and execution is approximately 1 second.

3.6.2.3 Mobile-Controlled Handoff

In mobile-controlled handoff, the handoff process is completely controlled by the mobile station (MS). There is a very short reaction time for such a handoff (approx 0.1 second). The responsibility of Mobile station (MS) is to measures the interference level on all channels and signal strengths from surrounding base stations (BSs). If by a certain threshold, the signal strength of the serving base station (BS) is lower than that of another base station (BS), a handoff can be initiated.

3.7 Handoff decision and execution

During handoff execution point connections need to be re-routed from the current network to the new network in a seamless manner. It also contains the authorization and authentication, and the transfer of user's context information.

3.7.1 Vertical handoff decision

Vertical handoff decision is made according to both quality of service parameters (QoS) such as (delay, bandwidth, etc) and handoff metrics are required. The information of different handoff metrics is collected during the system discovery stage and the decision is formulated as an optimization problem. Each candidate network is linked with a utility function. The decision is to select the network which has provides highest utility value. The utilization function is a weighted sum of different normalized QoS parameters. Each candidate network is linked with a cost function. The chosen network is the one which has the lowest cost value. The cost function relies on number of criteria, including, power requirement, delay and bandwidth.

3.8 Limitations of Vertical Handoff

There are two limitations of vertical handoff described as follows:

  • When all TCP/IP connection automatically transfers from one interface to another, in this condition only single wireless interface is used at that moment. The term which is normally used for end user specific application is known as “the best one”. In most cases it use different techniques for multiple connections and it may also appropriate for finer grained approach this approach refers to sequential instruction accomplished in parallel.
  • Vertical handoff needs the similar network interface. All the wireless interfaces must be used as element of the same Mobile IP and DNS communications. The reason is that mobile nodes and peers must be able to reach the Mobile IP and DNS server.

Regarding these two limitations vertical handoff cannot be possible to bring together the ad-hoc technologies, for instance ad-hoc IrDA, Bluetooth and 802.11b.

The ad-hoc networks consist of some nodes that may not be the part of direct peer to peer infrastructure. Because sometimes peer to peer connections offer the shortcuts for slow and expensive infrastructure they are most efficient [21].

CHAPTER 4: Mobile IP and Vertical Handoffs

Mobile IP is an IETF standard protocol for communication and is designed to allow a user device to maintain a permanent IP address while the user moves from one network to another. Mobile IP in wireless networks is intended to be a direct extension of the existing fixed/wire-line networks and guarantees uniform end-to-end QoS.

4.1 Overview of Mobile IP

With Mobile IP, the IP packets are transparently routed to mobile nodes in the internet. Regardless of the current attachment of the MN (Mobile Node) in the internet, it is identified by its home address. A host that communicates with the MN is called a CN. The CN does not have to keep log of the mobility of the MN. It considers MN to be a normal stationary host in the Internet.

HA and FA are also two types of mobility agents identified by Mobile IP protocol as shown in Figure 4.1. The HA is present at the home network of the MN which is the same network as the one in which the home address of the MN is allocated. Packets sent from the CN to the MN are routed using the home address. It is the responsibility of HA to tunnel the packets to the current location of the MN by intercepting them. To be able to tunnel packets to the MN, the HA must be aware of the current location of the MN. For this purpose, the MN acquires a CoA from the network it is visiting is represented in Table 4.1. The home address of the MN is192.168.16.5 and the CoA of the MN is 195.152.12.8 while it's away from HA and visits this FA.

Home Address

Care-of-Address

Lifetime in (s)

192.168.16.5

195.152.12.8

500

192.168.16.45

120.12.15.65

100

Table 4.1 Mobility Binding

Every time the MN moves, the current CoA is registered to the HA. A FA resides in the visited network. It offers routing services for the registered MN. The FA provides the MN with a CoA and detunnels and delivers datagrams to the MN that were tunneled by the HA. The FA can also serve as a default router for the registered MNs. Each entry in the visitor list is identified by the permanent home address, a HA address, media address of the MN, and associated lifetime, which is represented in Table 4.2.

Home Agent

Home Agent Address

Media Address

Lifetime in (s)

192.168.16.5

192.168.16.8

00-50-FC-8E-1D-26

120

120.126.10.45

120.126.10.3

00-50-08-68-A2-56

300

Table 4.2 Visiting List

Datagrams are tunnelled by HA to the CoA of the MN when CN sent them to the home address of MN sent by the CN to the home address of the MN. In Mobile IP, IP encapsulation [43] is the default tunneling method. The routing is done according to the outer IP header in which the source address is the IP address of HA and the destination address is the CoA. The original packet remains unchanged inside the tunnel. The inner IP packet is then extracted and delivered to MN at the CoA (Care-of Address).

From the application point of view, the home address can be used as a static IP address of the host. The MN uses the home address as the source address while sending packets to the CN. Packets can be tunnelled through the HA or routed directly to the CN. Figure 4.2 shows the tunnelling of the packets and the Mobile IP triangular routing. The tunnel endpoint of datagrams from the HA to the MN is determined by CoA.

There CoAs are divided into two types. The FA's CoA, which serves as the tunnel end-point on behalf of the MN, is the IP address of the FA. The arriving packets are decapsulated by FA and then the packets are delivered to the MN. The MN listens to the advertisements of the agent sent by FAs in the network and obtains the FA's CoA. Multiple MNs are allowed by FA to use its IP address as a CoA. A visitor list is also maintained by FA containing the home addresses and the link layer addresses of the registered MNs for easily delivering packets to their right recipients. In this way the internet address space is saved as every MN does not need its own CoA.

The MN of CoA itself also acquires co-located Care-of Address (CCoA). One of the network interfaces of the MN is associated with the CCoA. In this scenario the MN itself performs the encapsulation and decapsulation of the packets and acts as a tunnel endpoint. A CCoA can be acquired dynamically using Dynamic Host Configuration Protocol (DHCP) [16]. In other ways, the address may be owned by the MN for its use only as a long term address, while visiting a specific foreign network. The mode of using a CCoA has the advantage that it allows the MN to function without FA.

4.2 The Need for Mobile IP

The most widely researched and improved protocol is Mobile IP. It has been expected that mobile computing devices will become more useful, pervasive, and powerful in the near future. The efficiency and power will come from being able to integrate and extend the functionality for overall types of communication such as email, phone calls, Web browsing, information recovery, and perhaps even video transmission.

There are certain advantages of using Mobile IP to sum up as follow [5]:

  • Mobile IP provides a large range of applications from Internet access and e-mail to E-commerce.
  • Users can be maintained permanently on connect with their Internet provider and charged only for the data packets which are sent and received.
  • A user can take a laptop computer anywhere and keep the connectivity to the home network without any loses.
  • Mobile IP provides low cost, fast and continuous access to corporate networks in the distant areas where there is no cellular coverage or public telephone system.
  • Mobile IP can move from one type of medium to another and keeping ongoing connection services. It is unique in its capability to adapt heterogeneous mobility further to homogenous mobility.
  • Authentication or verification is achieved to guarantee that rights are being protected.
  • On the other hand Mobile IP network is characterized by some disadvantages described below [6]:
  • The most important problem facing Mobile IP is Security risks. Moreover the traditional security risks with IP, one has to worry about faked Care of Addresses (CoA). By obtaining a MN's CoA and rerouting the data to itself, an attacker can obtain unauthorized information.
  • Nevertheless another problem associated to the security is that how to make Mobile IP coexist with the security features available working within the Internet.

4.3 Mobility Management

It uses to allow the mobile station to communicate with the corresponding node and to find it correctly and also to avoid the session from termination which occur when a mobile station transfer a user's session from one network to another. In order that will cause to change the IP address. The problem in mobility management can be solved in different layers, such as IP layer, application layer, etc.

Mobility management can be categorized into two components either location management or handoff management [21-23].

4.3.1 Location Management

As shown in the above diagram, location management comprise of two stage process that enables the network to discover the current attachment point of the mobile user for call delivery.

In the first stage there is location registration and update and the Mobile Node (MN) informs the network of its new Access Point (AP) after equal interval of time. In this way user's location profile is verified by the network [11]. The second stage is call delivery. In this stage, queries to the network is made for finding the user location profile and therefore the received communication for the MN can be routed to the corresponding location.

When current location information is available, location registration requires the updating of location databases. On the other hand, call delivery involves the querying of location databases to determine the current location of a called MN.

1.5

4.3.2 Handoff Management

The most significant issue in mobility management is handoff management. It can be refer to the ability of the network to allow a call in progress to maintain a user's connection as the mobile terminal continues to move and change its access point. The handoff processes can be classified to three activities, initiation, new connection generation and data flow control as shown in Figure 4.2. Firstly, initiation phase where network agent, the user or changing network conditions identify the need for handoff. Secondly, a new connection generation, where is the network must find new resources for handoff, and perform any additional routing operations. Finally; data flow control, where the delivery of the data from the old connection path to the new connection path is maintained according to agreed-upon service guarantees [12].

4.4 Handoffs in mobile IP

A handoff in Mobile IP occurs when a Mobile Node (MN) moves between two Foreign Agents (FAs). Actually, an FA periodically broadcasts agent advertisement messages that carry important information for MNs to establish a successful connection with the FA. The MN ensures that it has released from the old FA, and then it sends a registration request to the new FA. This registration request message is constructed using the agent advertisement message received from the new FA. When the new FA receives this request, it assigns a Care-off Address (CoA) to the MN. On top of receiving this CoA, the MN sends a registration request message to its Home Agent (HA) to associate its home address with the recently allocated CoA. After that the HA acknowledges this request by sending a secured registration reply message to the MN [11].

4.5 Mobile IP requirements

It may be summarized as follows [7]:

  • In order to protect all messages must be authenticated against redirection attacks which it used to update another node with the location of MN.
  • A MN must be able to communicate with other nodes after changing its link layer point of attachment to the Internet, yet without changing its IP address.
  • Application programs must be able to operate continuously over a single session while the network attachment point of the MN changes.
  • A MN must be able to communicate with other nodes that do not implement these mobility functions.

4.6 Problems of IP Mobility

The infrastructure of internet is built upon a collection of protocols, called the TCP/IP protocol suite. It is the requirement of IP to identify uniquely the location of any connected host based on the assigned IP address. Therefore the most important issue to be considered is mobility because its IP address has to be changed while the host moves from one physical location to the other [8].

To understand this one has to ponder on how IP addresses are used today in the Internet. Basically the IP addresses are used to identify both the position and the node itself. In this sense the work of IP addresses is quite similar to that of Domain Name Server's (DNS) or Fully Qualified Domain Names (FQDNs). In other words, IP address or FQDN can be used to identify a particular node out of the mesh of millions of nodes that makes up the Internet. The FQDN is a DNS domain name that has been stated unambiguously for the purpose of indicating with absolute certainty its location in the domain namespace tree. Together, the DNS namespace and the host name make up the FQDN [9].

There is a search being carried out by the IP community for a best IP-Mobility solution. As the function of IP layer is to deliver packets, also the IP layer is considered to be the correct place for mobility handling. This practically means that the other internet protocols should work perfectly. Therefore this concludes that for terminal mobility Network Layer (L3) and Data Link Layer (L2) solutions are appropriate where as for personal mobility application layer solutions are sometimes applicable.

In the IP header the source and destination IP addresses are present and help the packet to be transmitted from source to the destination over the internet. There are indexed connections in TCP that are indexed by a quadruplet containing the IP addresses and port numbers of both connection endpoints. If any four of these numbers are changed the connection will be disrupted and lost.

4.7 Mobile IP Shortcomings and Improvement

Although Mobile IP has solved the problem of mobility still there are various unresolved problems and issues. Reverse tunnelling [17, 18] offers a solution to the asymmetry problem. Route optimization [19] has been proposed as a solution to inefficiencies caused by tunnelling. Also, when an MN moves from one FA to another, there is no way to inform the old FA (oFA) about the movement of MN. Hence packets already tunnelled to the old CoA (oCoA), and in flight can not be delivered to the MN and are lost.

Chapter 5: Vertical handoff algorithms

In this chapter we will discuss different types of vertical handoffs based on which handoffs between different networks take place.

5.1 Multiple attributes decision strategies (MAD)

[20] There are several vertical handoffs decision algorithm which is currently introduced. The handoff decision problem deal with making selection between limited numbers of candidate networks which provide varies technologies and services with respect to different criteria. This is a type of Multiple Attribute Decision Making (MADM) problem. For decision making multiple attribute, multiple criteria and multiple objectives are the terms are often used. Although the difference between them can be made based on different concepts Multiple Criteria Decision Making (MCDM) is applied to decisions that involve multiple attributes or multiple objectives, but in general when they both apply. Multiple Attribute Decision Making (MADM) deals with the problem of choosing an alternative from a set of alternatives which are considered in terms of their attributes while Multiple Objective Decision Making (MODM) contains a set of conflicting objects or goals that cannot be achieved simultaneously. The most popular classical MADM methods are:

5.1.1 (SAW) Simple Additive Weighting

The SAW approach is a renowned method of MADM. The value of a candidate network in SAW is calculated by weighted sum of all the attribute values. The score of each alternative or interface is obtained by adding the normalized contribution of each value Xij multiplied by the assigned importance weight Wj [12]. Equation (5.1) shows the selected interface of SAW:

Equation 5.1 SAW approach

Whereon N denotes the number of parameters, and M refers to the number of candidate networks.

5.1.2 (WP) Weighting Product

The WP approach is similar to SAW but the scaled property values of each alternative or interface are powered by Wj and the overall score is a product of the values made across the attributes [12]. The selected interface as the following equation:

Equation 5.2 WP approach

5.1.3 (TOPSIS) Technique for Order Preference by Similarity to Ideal Solution

[10, 12] TOPSIS is an algorithm widely used for mobile terminal interface selection using multiple attributes. The selected candidate network is the one which is the closest to the ideal solution (and the farthest from the worst case solution). There are several steps to determine the chosen alternative described below:

Step 1: Construction of a normalized decision matrix. Each element in the Euclidean normalized decision matrix R can be calculated as follows:

R=

Equation 5.3 Euclidean normalized decision matrix

Step 2: a weighted normalized decision matrix may be calculated by multiplying each column of the Euclidean normalized decision matrix R with its associated weight.

Step 3: the equations 5.4, 5.5 respectively determine ideal and negative ideal solution:

Equation 5.4 ideal solution

Equation 5.5 negative ideal solution

Step 4: The distance between alternatives are measured by m-dimensional Euclidean distance. The distance between each alternative and the positive ideal solution is:

Equation 5.6 the distance between each alternative and the positive ideal solution

The distance between each alternative and the positive ideal solution is:

Equation 5.7 the distance between each alternative and the negative ideal solution

Step 5: The relative closeness to the ideal solution is calculated as follows:

Equation 5.8 the relative closeness to the ideal solution

Step 6: Rank the preference order. According to the decreasing order of Cj a set of alternatives can now be ranked.

Chapter 6: Simulations and Results

Chapter 7: Conclusion and Future Work

References

[1] Nidal Nasser, Ahmed Hasswa, Hossam Hassanein, “Handoffs in Fourth Generation Heterogeneous Networks”, Communications Magazine, IEEE, Volume 44, Issue 10, pp 96-103, Oct 2006.

[2] http://en.wikipedia.org/wiki/Handoff.

[3] http://searchmobilecomputing.techtarget.com/dictionary/definition/what-is-soft-handoff.html.

[4] Ling-Jyh Chen*, Tony Sun*, Benny Chen*, Venkatesh Rajendran†, Mario Gerla*, “A Smart Decision Model for Vertical Handoff”,???.

[5] INTERNATIONAL TELECOMMUNICATION UNION, Considerations of horizontal handover and vertical handover, 2007

http://www.itu.int/net/itu_search/index.aspx?cx=001276825495132238663%3Anqzm45z846q&cof=FORID%3A9&ie=UTF-8&q=Consideratons+of+horizontal+handover+and+vertical+handover#955.

[6] Qing-An Zeng and P.Agrawal, “Handoff in Wireless Mobile Networks”, University of Cincinnati.

[7] http://en.wikipedia.org/wiki/Vertical_handoff

[8] J. D. Bakker, R. Prasad, Handover in a Virtual Cellular Network, 1999.

[9] Carolin I. Bauer, Dr. S. John Rees, “CLASSIFICATION OF HANDOVER SCHEMES WITHIN A CELLULAR ENVIRONMENT”, 2002.

[10] Enrique Stevens-Navarro and Vincent W.S. Wong, Comparison between Vertical Handoff Decision Algorithms for Heterogeneous Wireless Networks, 2006.

[11] J.-Z. Sun, "A review of vertical handoff algorithms for cross-domain mobility," in Wireless Communications, Networking and Mobile Computing, 2007. WiCom 2007. International Conference on, 2007, pp. 3156-3159, 2007

[12] Phuoc Nguyen Tran, Nadia Boukhatem, “Comparison of MADM Decision Algorithms for Interface Selection in Heterogeneous Wireless Networks”, MOBIWAC, pp 61-68, 2008

[13] Yue Chen; “Soft Handover Issues in Radio Resource Management for 3G WCDMA Networks”. Queen Mary University of London, 2003.

[14] B.G Evans and K Baughan, “Vision of 4G”, Electronic and Communication Engineering Journals, 2002.

[15] Jun-Zhao Sun, Jaakko Sauvola, and Douglas Howie, “Features in Future: 4G Visions from a Technical Perspective”,2001.

[16] http://www.webopedia.com/TERM/H/handoff.html.

[17] Mark Stemm, “Vertical Handoffs in Wireless Overlay Networks”,1998.

[18] Enrique Stevens-Navarro, “Vertical Handoff in Heterogeneous Wireless Networks”, Department of Electrical and Computer Engineering, University of British Columbia, October 6, 2006. Available at <http://www.acmcog.ca/femex/congreso_2006/24_Navarro_Eng.pdf>

[19] Marry Nariestti, “Implementation of Vertical Handoff Algorithm Between IEEE 802.11 WLAN and CDMA Cellular Network”, ??.

[20] Meriem Kassar, Brigitte Kervella, Guy Pujolle, “An overview of vertical handover decision strategies in heterogeneous wireless networks”, Received 5 June 2007; received in revised form 14 January 2008; accepted 15 January 2008

Available online 31 January 2008.

[21] Gregory P. Pollioni, “Trends in Handover Design”, IEEE Communications Magazine, vol. 34, March 1996, pp. 82-90.

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