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  1. 1. Vertical Handoff and TCP Performance Optimization using Cross Layer Approach Thesis Submitted to Indian Institute of Technology Kharagpur for the partial fulfillment of the requirements for the award of the degree of MASTER OF TECHNOLOGY in Telecommunication Systems Engineering by Anurag Mondal Roll No : 10EC64R04 Under the guidance of Prof. Rajashri Roy Department of Electronics & Electrical Communication Engineering Indian Institute of Technology, Kharagpur Kharagpur-721302, India April 2012
  2. 2. Vertical Handoff and TCP Performance Optimization using Cross Layer Approach Anurag Mondal
  3. 3. Certificate This is to certify that the project report titled ”Vertical Handoff and TCP Performance Optimization using Cross Layer Approach” submitted by Anurag Mondal(10EC64R04) to the Department of Electronics and Electrical Communication Engineering, Indian Institute of Technology, Kharagpur, India, during the academic session 2011-2012 is partial fulfillment for the award of the postgraduate degree of Master of Technology (M.Tech.) in Telecommu- nication Systems Engineering, is a bona-fide record of work carried out by him under my supervision and guidance .The thesis has fulfilled all the requirements as per the regulations of this institute and, in my opinion reached the standard for submission Dr.Rajashri Roy Asst. Professor Department of E& EC Engineering, Indian Institute of Technology, Kharagpur, India-721302. Place: I.I.T. Kharagpur Date: April 2012
  4. 4. Abstract Handover means transferring an on going call or data sessions one cell to another. Handovers occur due to the movement of the mobile user from one area to another area. Handovers are used to prevent an on going call to be disconnected. If we donˆat use handovers then whenever a user leaves the area of a particular cell then its on going call is immediately disconnected. The process of handovers requires a number of parameters e.g. what is the handover scheme we are using, how many channels are free. Vertical handover may be referred to a process of transferring call connected to a network/data session from one channel connected in a cell to the core network of another. A suitably equipped device may be able to use both technologies at a time, the high speed Wireless LAN and cellular technology .Wireless LAN connections generally provide higher bandwidth but smaller coverage area as compared cellular networks which have lower bandwidth and wide coverage. Thus the user can use a Wireless LAN connection whenever it’s available, while when it isn’t available can switch to a cellular connection as an alternative. Vertical handover refers to automatic switching the communication/data session from one technology to the other. So, it’s different from a horizontal handover among various wireless access points using the same technology. In this thesis, a study of Vertical Handover is done in different real-time situation and the pattern of data transmission is analysed. This thesis mainly focuses on the Transport Layer(TCP protocol) and its characteristics during vertical handovers. An effort is made to improve the performance in terms of data rate and to increase the quality of service. A cross- layer approach based on MAC & Transport Layer is being used in this thesis which shows clear improvement in TCP performance.
  5. 5. Acknowledgement I am greatly indebted to my project supervisor Prof.Rajarshi Roy for inspiring and motivating me to develop new ideas and implementing them. I am grateful to him for his invaluable guidance and encouragement throughout the course of this project. He has been the true torch bearer. I would like to take this opportunity to express my sincere and profound gratitude to him for the pains he has taken to accomplish this project. I humbly extend my words of gratitude to other faculty members and our Head of the De- partment Prof C.K. Maiti, faculty advisor Prof S. S. Pathak and Prof. Mrityunjoy Chakroborty, staff and administration of E& ECE department for providing me the valuable support and time whenever it was required. My sincere gratitude is extended to Krishanu Sen Laskar and Gobind Prasad for their help and valuable suggestions. I humbly extend my words of gratitude to my friends Arghyadip Roy, Abhishek Verma,Subhajit Paul & Ketan Das with whom I had very good time throughout the year and also they have providing me the timely help and suggestions.I would also like to thank all others whose direct or indirect help has benefited me during my stay at IIT Kharagpur. Furthermore, I would like to thank all other friends and fellow students of IIT Kharagpur and particularly my classmates from TSE for sparing great time throughout my M.Tech and very good support I received from them during my stay and during the preparation of my thesis. I express my gratitude towards all the people associated with Internet, IEEE and the plethora of technical websites, forums, & groups which have helped me with study material for my project work. Last but not the least, I thank my parents, Himanshu Kr Mondal and Monisha Mondal for their support and motivation they have provided throughout my project. Anurag Mondal
  6. 6. Dedicated to My Parents
  7. 7. Project Report on Vertical Handoff and TCP Performance Optimization using Cross Layer Approach By Anurag Mondal Under the guidance of Prof.Rajashri Roy April.2012
  8. 8. Contents List of Figures iii 1 Introduction 1 1.1 Problem Statement and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Scope and methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Literature Survey 4 3 Introduction to Vertical Handover 7 3.1 A Brief Background of Vertical Mobility . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1.1 A Brief Background of Vertical Mobility . . . . . . . . . . . . . . . . . . . . 9 3.1.2 Interconnecting heterogeneous networks . . . . . . . . . . . . . . . . . . . 10 3.2 System architecture and design issues . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.1 Features of B3G systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.2 Features of Wireless LAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 Introduction to Transmission Control Protocol 13 4.1 General Features of TCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1.1 Connection- Oriented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1.2 Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1.3 Byte Stream Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.2 TCP Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.3 Slow Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.4 TCP Time-Out Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.5 TCP on Wireless Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.5.1 Split-Connection Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.5.2 Link-Layer Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.5.3 Explicit Notification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.5.4 Ends-To-End Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5 Vertical Handover Algorithms and Cross Layer Optimization 20 5.1 Handover Decision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1.1 Handover Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.2 Vertical Handoff based on Hysteresis Technique and Predictive RSS . . . . . . . . 22 5.3 Vertical Handoff based on Polynomial Regression-Based Prediction . . . . . . . . 23 5.4 Cross Layer-Based Predictive Bandwidth for Increasing TCP Throughput in Ver- tical Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 i
  9. 9. 5.5 The Proposed Algorithm Based on Cross Layer Optimization to increase TCP Throughput in Vertical Handoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.5.1 The Proposed Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6 Simulation and Results 27 6.1 Simulation Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.2 Simulation Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 7 Conclusion & Future Work 33 7.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 7.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 ii
  10. 10. List of Figures 3.1 The main components of a GSM/GPRS system . . . . . . . . . . . . . . . . . . . . 8 3.2 Evolution of communication systems with data rates . . . . . . . . . . . . . . . . 10 3.3 Wireless overlaid heterogeneous network architecture . . . . . . . . . . . . . . . . 11 3.4 Wireless data access in heterogeneous wireless network standards. . . . . . . . . 12 4.1 Three-Way Handshake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2 TCP Congestion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.3 Splitting the TCP Connection into two separate connections . . . . . . . . . . . . 18 4.4 A link-layer approach to improve the TCP performance. . . . . . . . . . . . . . . 18 5.1 Thresholds of three hysteresis.[6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.2 Handoff points in different handoff approaches.[6] . . . . . . . . . . . . . . . . . . 23 5.3 Vertical Handoff based on RSS prediction . . . . . . . . . . . . . . . . . . . . . . . 24 5.4 Message flow in the cross-layer mechanism [6] . . . . . . . . . . . . . . . . . . . . 25 5.5 Heterogeneous network architecture integrating with Wi-Fi, and UMTS/B3G used in the algorithm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 6.1 Signal strength received by the Mobile Terminal . . . . . . . . . . . . . . . . . . . 28 6.2 Random Movement of the Mobile Terminal . . . . . . . . . . . . . . . . . . . . . . 29 6.3 No. of Vertical Handoff against velocity of the Mobile Terminal . . . . . . . . . . 29 6.4 Data received by the Mobile Terminal . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.5 Fig.14. Comparison of the Data rate of TCP [6] and Modified TCP proposed with no. Vertical Handoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.6 Comparison of the Congestion Window of TCP [6] and Modified TCP proposed during Vertical Handoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 6.7 Comparison of the Packet Drop of TCP [6] and Modified TCP proposed . . . . . 31 6.8 Comparison of the Efficiency of TCP [6] and Modified TCP proposed . . . . . . . 32 iii
  11. 11. 1Introduction The topic of this thesis ”Vertical Handoff and TCP Performance Analysis” describes an idea towards new generations of mobile networks where seamless mobility across heterogeneous networks and services is made possible. These new generations are referred to as 3G+, Beyond 3G (B3G) or 4G indicates the fact that seamless mobility among heterogeneous wireless access networks and services have scope of development and research. High demand for multime- dia rich applications in contemporary internet services brings the necessity of achieving good performance by using advanced mechanisms. The development on infrastructure of wireless technology has brought improved performance, reduced cost and comprehensive accessibility to todayˆas mobile networks. The mobile networks need to care about user requirements such as Always Best Connected (ABC). However, additional work for mobile networks is needed to choose the best connection among other possible available connections on air. Therefore, the engineering process on the infrastructure should be done in a way that the end user should note experience any hazardous connection during an interaction with the mobile device.This thesis contributes to the evolu- tion of networking technology by making vertical mobility more understandable as a technical problem, and by helping to make it transparent for the average user. This first chapter discusses problem the statement and motivation. The scope and the method- ology are described, and an overview of the contributions is given. Finally, the outline of the thesis is introduced. 1.1 Problem Statement and Motivation The vertical mobility paradigm deserves research attention from many perspectives. This the- sis focuses on finding a solution to the vertical mobility problem together with data transmis- sion over TCP. The basic question is how to utilize the asymmetric data rates in overlapping heterogeneous wireless networks to the fullest. Elaboration of this problem requires devel- oping an analytical framework for handoff analysis. The objective is to find a method that 1
  12. 12. will give better performance than a traditional handoff algorithm based on a received signal strength threshold. The third question is how to analyze vertical mobility and handoff in a TCP based network. The motivation for addressing the given problem areas is to develop wireless packet switched communications by facilitating a holistic approach for handoff and mobility management in heterogeneous wireless networks. The cross-layer approach involves taking into consideration on handoff technique that are employed in the physical layer solutions in cellular networks and on the other hand mobility management solutions that are employed in the Transport layer. Lower layer (Physical Layer/MAC Layer) parameters and context information such as location- awareness can be used as triggers in preparing for the handoff decision. The whole problem involves designing systems that can meet the requirements of managing the system capacity for various applications and mobility scenarios . The basic requirement is to maintain high wireless bandwidth, low packet loss rates and low latencies . For system design, selections have to be done between different implementation strategies for how to utilize available alter- native protocols, algorithms and other technical solutions 1.2 Scope and methodology Vertical handoff efficiency is one important area for traffic optimization in the next generation wireless networks. It is essential to manage the delay and throughput in the vertical handoff process. The characteristics of delay and throughput in vertical handoff between non homo- geneous systems such as 802.11 WLAN and cellular networks (GPRS, EDGE or UMTS) are analyzed with simulations. This thesis has a specific emphasis on the handoff situation with the limited scope of a transition region where the received signal strength varies depending upon the current location of the WLAN receiver and TCP performance based on that. Transi- tion analysis provides information about the key parameters affecting algorithm performance measured as mean throughput perceived by a single user. No. of Vertical Handoff is compared with the velocity of mobile terminal/user by the user in a particular scenario. An analytical model is developed to include two overlapping heterogeneous wireless technologies The validity of the results must be evaluated in proportion to the limitations of the used sim- ulation model. A simulation model is always an approximation of a real system, expressing only a portion of the whole truth of the studied phenomenon. The simulation model helps in understanding a real life scenario also gives an estimation of such kind of situation which provides us theoretical values. This kind of methodology provides a wide aspect for over- all analysis. The thesis develops wireless packet-based communications in hybrid wireless networks. The objective of the work is to understand the behavior of selected protocols in a wireless environment prone to communication errors and undeterministic delays. For vertical mobility management WLAN is the primary overlay technology. 2
  13. 13. 1.3 Outline of the thesis The outline of this thesis is as follows. In this chapter the importance of the studied topics for next generation networks was discussed. In Chapter 2, the summary of contributions in the original papers is given. In Chapter 3, an overview of the vertical mobility technology evolution and background is provided going through the current state-of-the-art technologies towards future B3G and 4G systems. In chapter 4, TCP basics and also the characteristics of TCP over wireless environment is described. In chapter 5 , different algorithms based on verti- cal handoff is descrided and the proposed algorithm is presented. In Chapter 6, Methodology and the Simulation Environment is described in detail together with the simulation results. In Chapter 7, conclusions and directions for future work is provided. 3
  14. 14. 2Literature Survey In this chapter a summary of contributions in the original papers is given. The thesis is re- lated to the analysis of the mobility and handoff in heterogeneous networks at protocol and algorithm levels. Step by step, the work related to the thesis is descried depending upon the analysis and the progress in the thesis. In the first article [1] the requirements of a vertical handoff from the literature surveyed. The evaluation of the existing work is also being done on the basis of required parameters for ver- tical handoff. This paper describes the basis of vertical handoff and together with the basic parameters that can be used analyzes vertical handoff in a 4G wireless network. The author supports the content for the technical information and simulation results. The most important thing in vertical handoff to occur in the particular scenario is when and to which network to do it. So, whenever a mobile terminal or user is moving from one network to other one it have to take some decisions and make some adjustments such that it adapt in the new environment .Some algorithm are employed for this purpose ,these algorithms are known as “Handoff Decision algorithm ” as described in article [2],[3]. The article [3] describes a vertical handoff decision algorithm that enables a wireless access network to not only bal- ance the overall load among all attachment points (e.g., Base Stations (BSs) and Access Points (APs)) but also to maximize the collective battery lifetime of Mobile Nodes (MNs). The fourth article [4] does a comparative study between different decisions algorithms with the aim of understand its performance for different user applications. This kind of comparative analysis helps in developing and understanding the problems and solution associated with different algorithms. In a real life scenario, a mobile device or user always does not follow a particular direction for movement or neither maintain a constant a velocity , so decision making for vertical handoff becomes complex , particularly when the mobile user is moving in and out in a particular net- work very rapidly or it is roaming around the cell edge. So, in such kind of situation predictive 4
  15. 15. algorithm are used to handle the situation. These algorithms help in pre-determining and es- timating the different parameters such as velocity, location and received signal strength (RSS) etc. Depending upon these predicted values different changes are made. These algorithms are called predictive algorithms are described in article [5],[6]. Article [5] describes novel bandwidth utilization & optimization technique is employed in this work to allocate bandwidth more efficiently which is based on a predictive algorithm. The first section of paper [6] describes a “RSS based predictive algorithm” which is used to take handoff decision depending upon the received signal strength. For maintaining proper connectivity with a network, various algorithms are employed. Fuzzy Logic is a method which can be employed in the vertical handoff scenario which run also uses predictive method for vertical handoff as described in Article [7],[8],[9]. Now, in this thesis maintain focus is to analyze the TCP performance or Throughput, so some methods for analysis of TCP performance in a Wireless environment are being described in the Article [10], 11], [12]. Article [10], [12] describes that 3rd third generation (3G) wireless networks provides high data rate. So, optimizing TCP performance over these networks would have a broad and signifi- cant impact on data application performance. TCP performance over the 3G wireless networks is adapting to the significant delay and rate variations over the wireless channel. This paper proposes a network-based solution called the Window Regulator that maximizes TCP perfor- mance. Article [11] describes about the basics of TCP and also proposes explicit feedback algorithm which can be employed in adjustment of size of the TCP packet such as packet loss can be controlled. Article[13] propose and evaluate a vertical handover mechanism for TCP, based on receiver Bandwidth Delay Product (BDP) measurement and congestion control using the receiverˆas ad- vertised window. It has proposed a receiver-based handover mechanism for TCP to deal with two key problems associated with vertical handover between networks with significantly dif- ferent link capacities and latencies. Whenever we consider a wireless environment the entire user subsiding in the current region at a particular time must have fairness in terms the connectivity and the data rate they obtain. So, Article [14] paper is the first to focus on TCP fairness in 802.11 networks in the presence of both mobile senders and receivers. This paper evaluates through analysis, simulation, and experimentation the interaction between the 802.11 MAC protocol and TCP. In a wireless environment, whenever we talk about performance optimization, we have de- ployed different complex technique such that a fairness of among the users is maintained. So, we apply algorithm to a particular network such that a particular parameter is optimized. In 5
  16. 16. such a scenario, we generally focus on a single ISO layer, such as in the case of TCP, we trying to modify a transport layer protocol. Similarly we can come with an approach where we will modify multiple layers protocols such that modifying one protocol in a layer helps in controlling a parameter in a different layer. This technique is known as “Cross Layer Optimization”. Cross layer optimization is a vital technique used in this thesis and it is described in Article [15]. Article [15] describes a cross layer technique between network and transport layer which can be employed in a vertical handover scenario. This paper basically focuses on the performance improvement of Wireless Application Protocol. In Article [6], a cross layer approach for the TCP performance improvement is described in which is a cross layer technique to adjust TCP congestion window is deployed which is based on the MAC layer and Transport layer optimization. This is a fundamental technique which in adopted in this thesis. Article [16], considers downward and upward vertical handovers in integrated wireless LAN and cellular networks and address wireless-profiled Transmission Control Protocol premature timeouts due to step increase of round-trip time (RTT) and false fast retransmit due to packet reordering. This article also uses cross-layer optimization to improve the vertical-handover performance of the WP-TCP in the integrated WLAN and cellular networks. Previously in this chapter we have seen that fairness is an important issue for vertical hand- off. Similarly like fairness, user demands a better Quality of Service (QoS) from a underlying wireless network. So, maintaining a QOS is an important issue to be considered. Article [17] proposes a cross-layer-based Quality-of-Service (QoS) architecture that includes the QoS engine and the cross-layer algorithm. The QoS engine is composed of the QoS dae- mon, QoS agent and control module to guarantee high-quality wireless multimedia services in 4G heterogeneous environment; the cross-layer QoS algorithm monitors and adjusts network resources periodically. In Article [18], an adaptive scheduling with coordination for the performance improvement of delay-sensitive applications over heterogeneous wireless networks is proposed. It uses infor- mation from the physical layer and data link layer to determine the appropriate transmission power level and media encoding rate for a connection, or initialize coordinated scheduling. This paper was helpful in approaching the problem in this thesis. In this paper, a new cross- layer design considering coordinated scheduling for the performance improvement of delay- sensitive applications over heterogeneous wireless networks were described. 6
  17. 17. 3Introduction to Vertical Handover This chapter of the thesis gives an overview of handoff and mobility among heterogeneous networks called vertical handoff. The term vertical refers to overlapping wireless networks and their hierarchical and asymmetric relationship. The problem of vertical mobility can be illustrated by considering a wireless scenario. Let us consider scenario which consists of two vital components - A server and a client. The server can be considered as a sender who has all data required by the client or receiver. Since, we are considering a wireless scenario both the server and client are connected via wireless network. In this thesis, we are considering two types of wireless networks : Wireless local area network (WLAN) and a Beyond 3rd Generation (B3G) cellular network. A WLAN network provides high data rate but its coverage is limited and on the other hand, a B3G network is large in coverage area but provides lower data as compared to WLAN. Now, in the scenario we have considered a user or receiver which is moving and are connected to either of the respective network. The Architecture of a Wireless network is such that user will gets the highest data/Signal Strength near the Base station (BTS) and connectivity gradu- ally declines as the user moves towards the cell boundary. Base station can be considered as a control unit which helps in connecting the Server and Mobile receiver wirelessly. It accepts the data request from the user and forwards the data transmitted by the server. Let the user is residing within the range of cellular network (B3G) and gradually the user come near the cell boundary of a current network. So, the Received Signal Strength (RSS) and other resources obtained by the Mobile user will be low and so, it must be connected to other B3G network in order to fulfil its resource requirements. Depending upon different resource requirement it connects to new network. This phenomenon is known as “Handover”. The figure (Fig.3.1.) below shows the components of a GSM/GPRS system. 7
  18. 18. Figure 3.1: The main components of a GSM/GPRS system Handovers can be considered of two types: Horizontal Handoff and Vertical Handoff. In the above scenario, the type of handover is Horizontal as the type of network within which the handover is done are same i.e. B3G (cellular network). So, Handover between two homoge- nous networks is known Horizontal Handover. Now if we again consider the scenario of the previous situation the user is moving inside the range of the cellular network and it reaches to the cell boundary WLAN network which resides inside the or a the boundary of the cellular network. The user might undergo handover of the connection depending upon the requirement of the resources. So, if handover takes place in between network of different types or Heterogeneous Networks, the handover is known as Vertical Handover. In this thesis, we are mainly considering the situation and problems re- lated to vertical handover. In the following discussion an overview is given for vertical mobility related technologies. The convergence of telecommunication and data networks has been driven by the integration of In- ternet technologies with wireless and cellular networks. This integration involves both radio carrier switching, network level routing information updating, and transport and application level adaptation. The primary goal has been to enable these processes to appear seamless to the users. In addition, services provided by the operators have to adapt to the vertical mobility scenarios, and in some cases provide additional value through content-aware solutions. 3.1 A Brief Background of Vertical Mobility During the past decade both telecommunication and Internet technologies have been in a phase of rapid development. Till the beginning of the new millennium the development was mainly technology driven. The mobile Internet evolution has taken many important steps towards providing better qual- 8
  19. 19. ity wireless data services to a wide audience. In cellular networks, evolution for the first three generations contributed to growing data rates and enhanced communication capabili- ties, achieving its current peak only recently in the third generation (3G) mobile networks and handsets. At the same time wireless local area networks have achieved enormous popularity in providing wireless broadband connection in public, enterprise andesidential environments. The next evolutionary steps after the third generation aim to provide extended mobility with optimized data rates and services. Users have more flexibility when using multiservice net- works that provide services such as seamless connection to the Internet via heterogeneous networks, advanced spatial location and navigation services. One of the key challenges in fu- ture network management is end-to-end optimization that takes into account variables such as throughput optimization, routing optimization, delay profiles for heterogeneous wireless environments and also economical profitability. 3.1.1 A Brief Background of Vertical Mobility The use of “Mobile Internet” was started in the early 1990’s with GSM data connections offer- ing a nominal data rate of 9.6 kbps (in practice much less than that). It was both very slow and expensive when compared to wired use of the Internet. Then, in the latter half of the 1990’s GPRS was introduced offering up to 40 kbps nominal data rates, depending on how many time slots were allocated to use. Yet, GPRS suffers from high peak delays and delay variations. Currently, higher data rate services are being made possible with EDGE and UMTS services. These services still tend to be expensive, especially in comparison to WLAN. Already in the late 1990’s the use of WLANs grew in popularity as an extension to a fast broadband connec- tion in office environments. In the beginning of the new millennium they started to become more common in public set- tings such as airports and hotels, and marching their way also to home environments. This happened partly due to broadband xDSL technologies becoming more common. Optimized connectivity management over heterogeneous networks is driven by all-IP applications that set a continuously growing need for more wireless bandwidth. A mobile user should be able to use them over the TCP/IP protocol suite which has established its role as the de-facto com- munication protocols standard family. All-IP applications include all the standard Internet ap- plications such as WWW, E-mail, file transfer and text chat applications. The figure (Fig.3.2.) below shows the evolution of the communication systems with increasing data rate require- ments. 9
  20. 20. Figure 3.2: Evolution of communication systems with data rates 3.1.2 Interconnecting heterogeneous networks Integration, convergence and interoperability had been an important part in other technology evolutions. Inter-system mobility expands the possibilities of traditional communication net- works by enabling more bandwidth or enriched services for mobile users. Especially, roaming between WLAN and UMTS (and its predecessors GPRS and EDGE) has been seen as an impor- tant scenario for both enabling new services and as a tool for the operator to balance network load. Fig. shows the conceptual architecture of vertically overlapping heterogeneous wireless networks, illustrating also the various domains of operation for the sub networks in the overall architecture. 3.2 System architecture and design issues The evolution and convergence of the cellular and computer industries has opened the ways for many new and exciting applications and services. There is a trend towards the conver- gence, integration and interoperability of various networking related technologies. In the wireless domain, the existing macro-, micro- and picocell networks often have overlap- ping areas of coverage. In next generation wireless systems called B3Gor 4G the mobile users should be able to move among these heterogeneous networks in a seamless manner. 10
  21. 21. Figure 3.3: Wireless overlaid heterogeneous network architecture 3.2.1 Features of B3G systems The first visions of B3G or 4G estimate this next generation to be realized around 2010 and based around five elements: fully converged services, ubiquitous mobile access, diverse user devices, autonomous networks and software dependency [5]. In this vision, the seamless con- nection of heterogeneous networks include cellular data, 3G, WLAN, short range PAN/LAN/MAN and broadcast services integrated through an IP based core network. Implementing this type of integrated system invokes many challenges in mobile handset design, wireless system dis- covery, terminal mobility, topological fault tolerance and survivability. From the service aspect one has to find a balance between public and private service domains by adapting multiple standards and service environments (home, office, outdoors, indoors) across multiple operators and service provider domains with ensured QoS, data privacy and information integrity, and taking into account user profile and terminal characteristics. 3.2.2 Features of Wireless LAN The Wireless Local Area Network (WLAN) is an unlicensed band of 802.11 ISM frequency band. 802.11 is one of the recent communication technologies of IEEE standard. It specifies medium access control (MAC) and physical layer that is why it is called Wireless LAN. It has three widely used types which operates on different frequency bands. These three types are 802.11a, 802.11b and 802.11g. 802.11a operates on 5 GHz frequency band and it gives the max- imum data rate speed of 54 Mbps, which is higher than 802.11b because 802.11b operates on 2.4 GHz frequency band and give the maximum data rate speed of 11Mbps. 802.11b oper- ates. 802.11g is recently developed standards of Wireless LAN. It also operates on 2.4 GHz frequency band and gives the maximum data rate speed of 54 Mbps. 11
  22. 22. Figure 3.4: Wireless data access in heterogeneous wireless network standards. 12
  23. 23. 4Introduction to Transmission Control Protocol In this chapter, we will familiarize with TCP to understand the historic, current and future architecture of the Internet protocols. Most applications on the Internet use TCP because its built in reliability and flow control ensure safe delivery of data across an unreliable IP layer below. IP alone is a basic datagram service and does not support any concept of a session or connection. Once a datagram is sent or received, the service retains no memory of the entity with which it was communicating. The abilities to retransmit data or check it for errors are minimal or nonexistent in the datagram services. This chapter provides a brief overview of TCP and its general features. It focuses on TCP and their specific functionalities, with special emphasis on their congestion avoidance and control mechanisms are presented. TCP is both complex and evolving transport protocol. It is a con- nection oriented protocol. For transmission of TCP data a connection must be made between the server and the client. TCP congestion Control, one of the most important TCP related Request for Comment (RFC) in recent years, describes updated congestion control algorithms to avoid congestion. Congestion occurs when the demand is greater than the available resources, such as band- widths of links, buffer space and processing capacity at the intermediate nodes such as routers. Congestion control is concerned with allocating the resources in a network such that network can operate at an acceptable performance level when the demand exceeds the capacity of the network resources. 4.1 General Features of TCP TCP is an end-to-end, point-to-point transport protocol used in the Internet. Being point-to point protocol means that there is always a single sender and a single receiver for a TCP ses- 13
  24. 24. sion. Being an end-to-end protocol, on the other hand, means that TCP session should cover all parameters and transportations involved from the source host to the destination host. TCP provides connection-oriented, reliable byte stream service. 4.1.1 Connection- Oriented Before any data transfer could be started, a connection must be established through a process called three-way handshake. During this process, the TCP sender and receiver come to an agreement in the establishment of a connection and set the relevant parameters such as Max- imum Segment Size (MSS). For example, if a client computer is contacting a server to send it some information, a TCP connection is established by exchanging control messages as follows: 1. The client sends a packet with the SYN bit set and a sequence number N. 2. The server then sends a packet with an ACK number of N+1, the SYN bit set and a sequence number X. 3. The client sends a packet with an ACK number X+1 and the connection is established. Figure 4.1: Three-Way Handshake 4.1.2 Reliability A number of mechanisms, namely checksums, duplicate data detection, sequencing, retrans- missions and timers, help TCP to provide reliable data delivery. All TCP segments carry a checksum, which is used by the receiver to detect corrupted data. TCP keeps track of bytes received in order to detect and drop duplicate transmissions. In packet switched network, 14
  25. 25. packets can arrive out of sequence. TCP delivers the byte stream data to an application in or- der by properly sequencing segments it receives. Corrupted or lost data must be retransmitted in order to guarantee delivery of data. The use of positive acknowledgements by the receiver to the sender confirms successful reception of data. The lack of positive acknowledgements, coupled with a timeout period, calls for a retransmission. TCP maintains a collection of static and dynamic timers on data sent. The TCP sender waits for the receiver to reply with an ac- knowledgement within a bounded length of time. If the timer expires before receiving any acknowledgement, the sender can retransmit the segment. 4.1.3 Byte Stream Delivery TCP interfaces between the application layer above and the network layer below. A stream of 8-bit bytes is exchanged across the TCP connection between the two applications. An applica- tion sends data to TCP in 8-bit byte streams, which is then broken by TCP sender into segments in order to transmit data in manageable pieces to the receiver. The size of the application layer payload is variable but may not be larger than MSS, which is usually announced by the TCP receiver during connection establishment using the MSS option in the TCP header. 4.2 TCP Flow Control TCP flow control is provided through the well-known sliding window mechanism. ACKs sent by the TCP receiver carry the advertised window, which limits the number of bytes the TCP sender may have outstanding at any time. The advertised window corresponds to the size of TCP receiverˆas receive socket buffer. The key feature of the sliding window protocol is that it permits pipelined communication to better utilize the channel capacity. The sender can send a maximum W frames without acknowledgement, where W is the window size of the sliding window. The sliding window maps to the frames in senderˆas buffer that are to be sent, or have been sent and now are waiting for acknowledgement. For maximum throughput, the amount of data in transit at any given time should be the channel bandwidth-delay product, which refers to the product of a data link’s capacity (in bits per second) and its end-to-end delay (in seconds). 4.3 Slow Start When a new TCP connection is established with a host on another network, the CWND (Con- gestion Window) is initialized to 1 (Maximum Segment Size) MSS and slow-start threshold (SSTHRESH), which determines the CWND size at which the slow-start will end and con- gestion avoidance will start, is set to a large value, such as equal to 65 Kbytes. Because the available bandwidth to the connection may be much larger than MSS/RTT, a TCP sender, dur- ing its initial phase, increases its rate exponentially by doubling its CWND value every RTT; 15
  26. 26. the sender generates the exponential growth by increasing the CWND value by 1 MSS every time a transmitted segment is acknowledged. CWND = CWND + MSS The exponential growth continues until there is a loss event, at which time CWND is cut in half, or the SSTHRESH is reached. If an ACK for a given segment is not received in a certain amount of time, known as RTO value, a timeout event occurs and the segment is resent. After a timeout event, a TCP sender enters a slow-start phase; it sets the CWND to 1 MSS and then grows the congestion window exponentially until CWND reaches SSTHRESH. When CWND reaches SSTHRESH, TCP en- ters the CA phase, during which CWND ramps up linearly. Assuming initial value of CWND equals 1MSS, initial value of SSTHRESH is large, i.e. 64 Kbytes, and TCP sender begins in slow stat state. The TCP is based on the notion of the sliding window in order to guarantee reliable and in order packet delivery. All major types of TCP employs congestion control algo- rithms. However, the implementation of the fast recovery and retransmit mechanism is quite different. The figure below (Fig.4.2.) shows the congestion control using slow start mechanism. Figure 4.2: TCP Congestion Control 4.4 TCP Time-Out Mechanism In order to avoid long delays when there is no response from the receiver in a TCP connec- tion, a time-out mechanism is employed. Therefore, after each TCP segment transmission by a sender, a timer is set and it starts counting down. If the TCP sender does not receive a thresh- old number of ACKs before the timer expires, it assumes that either the packet or the ACK is lost, and retransmits the same packet again until an ACK is received. The TCP retransmit 16
  27. 27. timeout (RTO) value must be carefully chosen. If RTO value is too small, the time expires quickly and premature time-outs will be generated during the usual TCP operation and thus unnecessary retransmission will occur. On the other hand, if RTO value is too large, the TCP will slowly respond to the segment loss, which means longer end-to-end delay and can also degrade performance. Therefore, the RTO value must be optimized to the extent possible. 4.5 TCP on Wireless Networks In modern communication system one of the major challenges is to provide wireless access to the Internet. TCP supports the most popular suite of applications on the Internet today and it has been enhanced in recent years to improve robustness and performance over network of varying capacities and quality. In this section, an overview of some optimizations that have been proposed in the literature and describe how they differ in terms of their retransmission and recovery mechanisms. The proposed optimizations can be categorized into four groups; split-connection, link-layer, ex- plicit notification and end-to-end protocols. Split-connection mechanism describes how op- timization at the transport layer can be achieved by splitting the connection at the base sta- tion.Explicit notifications can be used between an intermediate node and the end hosts in order to distinguish the congestion related losses from the wireless error. The End-to-End approaches do not require any intermediate nodeˆas support. 4.5.1 Split-Connection Protocols The idea of split-connection approaches is to divide each TCP connection into two separate connections at an intermediate node; one is between the fix host (FH) and the base station (BS) and the other between the BS and mobile host (MH) as shown in Figure below (Fig.4.3.). Such a subdivision of the traditional end-to-end connection offers several advantages. The trans- mission characteristics of the wireless link such as high bit error probability and disruptions due to radio shadow or handoffs influence only the transport connection over the wireless hop. This way any effects due to wireless link can be hidden from nodes within the wireless network and makes it possible for the wire-line and wireless medium to be employed with different transport protocols. However, since TCP connections are terminated at the base station, data buffers and TCP state information should be maintained at the base station in any split connection scheme. When a TCP connection is created, the socket send buffer and receive buffer are allocated. The buffer size can be specified by the process that creates the socket or a default value can be used. In either case, the TCP send and receive buffer sizes are fixed for the duration of the connection. 17
  28. 28. Figure 4.3: Splitting the TCP Connection into two separate connections 4.5.2 Link-Layer Protocols Link-layer protocols are another alternative for improving the poor performance of TCP over wireless links. The concept behind this is to make the wireless link layer look similar to the wired case for TCP by recovering the wireless error locally. There have been several proposals for reliable link-layer protocols. The main techniques employed by these protocols are for- ward error correction (FER) and automatic repeat request (ARQ). This method is illustrated in figure(Fig 4.4.) . ARQ is frequently used in data communications protocols. When a frame is detected to contain errors after decoding, it is discarded and an ACK is sent back to the sender requesting a retransmission of the frame. This is called a selective retransmission and the most efficient way of retransmission. Cellular networks such as GSM/GPRS and UMTS have recently incorporated the concept of ARQ in order to improve performance of data trans- fers over wireless link. Figure 4.4: A link-layer approach to improve the TCP performance. Snoop Protocol The Snoop protocol is a TCP-aware link layer protocol. It uses link layer retransmission to improve the reliability of the wireless link, and actively tries to avoid unnecessary TCP re- transmissions. In this method, the base station is equipped with a module called snoop agent, the functionality of which is to monitor the TCP packets transmitted from a fixed host to a mobile host and vice versa. The agent caches all these packets locally and uses this informa- tion to detect wireless packet losses and timeouts. In the case of detecting a wireless packet loss, it retransmits the packet promptly and suppresses the duplicate ACK reaching the TCP 18
  29. 29. sender. This way, it prevents the sender from invoking unnecessary fast retransmissions and congestion control algorithms. 4.5.3 Explicit Notification A various explicit notification schemes have been proposed to enable the TCP sender to distin- guish between different types of packet losses. Examples of this approach include Explicit Con- gestion Notification (ECN), Explicit Loss Notification (ELN). The idea behind this approach is to enable the TCP sender to distinguish packet losses due to congestion from wireless error. In the ECN technique, a TCP receiver informs the TCP sender of the network congestion ex- plicitly through one of the reserved bits in the TCP header, called the ECN-Echo (ECE) flag, when it receives an IP packet with the congestion experienced (CE) bit in the IP header set. ECN is an extension proposed to random early detection (RED), which monitors the aver- age queue size and marks packets instead of dropping them based on statistical probabilities. Since ECN marks packets before congestion actually occurs, this is useful for protocols like TCP that are sensitive to even a single packet loss. The packets provided with ECN support is referred as ECN capable packets. In the case of ELN, on the basis of the performance im- provement achieved in TCP Snoop and ECN protocols, a new protocol, namely, explicit loss notification with acknowledgement (ELN-ACK) is proposed that could remedy the limitations of the Snoop protocol . In the ELN-ACK scheme, a new form of acknowledgement packet called ACKELN is used to communicate the cause of packet losses to the TCP sender and no packets are cached at the base station. 4.5.4 Ends-To-End Protocols The standard TCP implementations rely on packet loss as an indicator of network conges- tion and lack the ability to distinguish congestion losses from losses invoked by noisy links. In wireless connections, overlapping radio channels, signal attenuation and additional noises have a huge impact on such losses. As a consequence, the standard TCP reacts with dras- tic reduction of the congestion window, thus degrading the performance of TCP. End-to-end proposals make the TCP sender handle packet losses caused by both congestion and random wireless errors and requires minimal or no processing at the base station. Another advantage of these schemes is that the end-to-end semantics of TCP is maintained. 19
  30. 30. 5Vertical Handover Algorithms and Cross Layer Optimization The vertical handover algorithm has to check different parameters before initiating the process of handover. In heterogeneous technologies, handover decisions should also be based on the type of technology. If all the parameters of handover are satisfied - mean condition of current network, condition of the network in which mobile node wants to switch, user preference, battery power, velocity of mobile station and application requirement etc. - then the decision module makes the decision and initiates the process of handover. In previous research, dif- ferent parameters are used for the handover decision, such as some take the decision on the received signal strength (RSS). As in most previous technologies, the decision algorithm is based on RSS, signal to interference ratio (SIR), bit error rate (BER), blocked error rate (BLER), coverage area of the current base sta- tion and the current status of the neighboring base stations. In heterogeneous networks, some take the velocity of the mobile station as the decision parameter, such as between WLAN and CDMA system. For the best performance, a threshold value of velocity is determined to reduce the unnecessary attempts of handover. 5.1 Handover Decision A seamless handover would only be possible when it has low latency, low handover overhead, low packet loss and low probability of handover discontinuity. All these characteristics of han- dover depend on handover technique and a good and accurate handover decision process. The main characteristics for the handover in heterogeneous networks are discussed below. 20
  31. 31. 5.1.1 Handover Characteristics Some phases of handover are common in both horizontal and vertical handover. Such as: • Available network domain detection • Selecting the best network among the available • Deciding the handover policy • Handover execution based on decision policy In horizontal handover, the decision of a handover mainly depends on the value of the signal strength of the received signal. If the received signal strength lies below a threshold value and the neighbour base station provides more signal strength than the previous base station, the MN will switch to the neighbour base station. In vertical handover, only the received signal strength is not adequate as the only decision pa- rameter. All these parameters mentioned below, collectively decide whether or not handover should take place, while all these parameters are not required in horizontal handover. Fol- lowing are the proposed parameters which are taken under consideration in the decision of vertical handover: 1. Network condition: The purpose of handover is to provide better conditions of signal strength and high performance. Before the handover, some measurements of the target and present networks are taken to see if target network is able to provide the require- ments of application for a continuous connectivity. These parameters are transmission rate, error rates and other characteristics can be measured. 2. Application requirements: An application running on the mobile node has its own re- quirements such as quality of service requirement which can affect the vertical handover. So handover decision also based on QoS as well. 3. Bandwidth: Different applications require some specific amount of bandwidth, such as FTP or video streaming. These applications perform well when they get desired amount of bandwidth if we provide them more bandwidth then they can give us too good per- formance. 4. Packet error rate: Some applications can bear packet loss to some degree but not more than that for example voice conversation and video streaming can bear packet loss whereas FTP and email application can not. 5. Latency: Non-real time applications are not sensitive to latency while real-time applica- tions need only low latency (such as in video streaming and voice conversation). 6. Power Requirements: Every wireless device requires some amount of battery power. If the level decreases, it is necessary to remain in low power consuming technology or 21
  32. 32. handover to a technology which is using less power. WLAN uses less power compared to UMTS. If the mobile device is connected to the UMTS and battery power is going to finish and WLAN is available, the mobile device should move to WLAN for the long life. 7. Interference: Interface, such as co-channel or adjacent channel interference, is also a big problem. Many wireless technologies suffer from interference. Before vertical handover takes place, the network is examined regarding the co-channel or inter-channel interfer- ence which exponentially degrades the network. 8. Velocity: In horizontal handover, the velocity does not have the same effect just like in vertical handover. As WLAN covers a considerably smaller area than UMTS, if the mobile device is moving very fast, it is better to remain in UMTS to reduce the frequency of handover. 5.2 Vertical Handoff based on Hysteresis Technique and Predictive RSS The RSS-based scheme with hysteresis is adopted to avoid unnecessary handoff due to the ping-pong effect. For instance, H1 (Fig.5.1.) indicates the hysteresis between B3G and WMAN, where the margins of H1 are bounded by the lower threshold SWMAN,1 and the higher thresh- old SWMAN,2, and the margin interval is denoted as ΓH1 = |SWMAN,2 − SWMAN,1|. Although mobile nodes transmit/receive well between these two thresholds and thus avoid unnecessary handoff, they suffer from low data rate and weak RSS when the received RSS of the serving network is close to the low threshold. Such a low data rate and weak RSS cause serious low utilization and high dropping probability. The main reason is explained below and shown in Fig. 9. The figure shows a mobile node MN moving from network 1 to network 2. In the RSS-based handoff approach, the mobile node performs handoff when it is in the overlap area; therefore, the handoff point may occur at point 2, 3, or 4. This causes a serious ping-pong effect if the mobile node moves around the overlap area. In the RSS with hysteresis approach, the mobile node performs handoff at point 4. Therefore, the mobile node receives RSS that is too weak from network 1. This results in a low data rate and high dropping probability. Later, In this chapter, a predictive RSS-based handoff approach to perform handoff at point 2 when the mobile node predicts that it is moving toward network 2is discussed. The predictive RSS mechanism has two advantages. First, the handoff process can be performed before RSS becomes weak and thus obtains better quality of service (QoS) and higher data rate. Second, it not only avoids unnecessary handoff but also obviously minimizes the dropping probability. 22
  33. 33. Figure 5.1: Thresholds of three hysteresis.[6] Figure 5.2: Handoff points in different handoff approaches.[6] 5.3 Vertical Handoff based on Polynomial Regression-Based Predic- tion Based on the hysteresis mechanism, this paper proposes a polynomial regression predictive RSS handoff approach that consists of two steps, i.e., the preprocess step and the RSS predic- tion step. • The Reprocess Step: In prediction, some previous RSSs are important for determin- ing the next predictive RSS. To intensify the polynomial regression-based curve fitting, the preprocess of accumulated generating operation, which is based on, is adopted to 23
  34. 34. achieve the accuracy of the prediction results. • RSS prediction Step: After the preprocess step, the new data sequence is used as the in- put data for curve fitting in the RSS prediction step. This process uses polynomial func- tion to determine the predicted RSS from previous samples of RSS values. The flowchart below shows the steps of the algorithm. Figure 5.3: Vertical Handoff based on RSS prediction 5.4 Cross Layer-Based Predictive Bandwidth for Increasing TCP Through- put in Vertical Handover This section first describes the TCP transmission of vertical handoff in a heterogeneous wire- less network. The proposed cross-layer-based mechanism between layers 2 and 4 to increase the TCP Throughput when the available bandwidth significantly and dynamically changes . TCP is a connection-oriented transmission protocol within the transport layer, which provides reliable transmission for Internet applications, such as those for File Transfer Protocol(FTP) and the Web. Some well-known TCP congestion control mechanisms, including Reno, NewReno, and Westwood+, provide TCP connections that yield high goodput and good fairness.when a mobile node performs vertical handoffs in heterogeneous wireless networks that provide dif- ferent maximum and available bandwidth. Although the TCP protocol provides dynamic congestion control based on the received roundtrip time (RTT) at the sender, it suffers from the large change in bandwidth that occurs. Conse- quently, TCP transmissions easily yield timeout while a mobile node performs vertical handoff from a network with higher available bandwidth to one with low bandwidth and slack change of congestion window when a mobile node performs vertical handoff from a low-bandwidth network to a high-bandwidth network. Additionally, TCP is designed to transmit packets in wired networks rather than for wireless networks, because wireless networks may frequently 24
  35. 35. cause link error. Moreover, few works have discussed the cross layer- based mechanism to improve this problem. Specifically, although the TCP protocols adopt automatic repeat request (ARQ) to determine cwnd, it causes inaccurate window control and low utilization in a wireless network with sudden unpredictable wireless link error. Therefore, this paper proposes a cross-layer-based mechanism to yield high TCP goodput in heterogeneous wireless networks. The key idea is to notify the layer 4 TCP protocol by the layer 2 MAC protocol at the TCP receiver when the network types of these two wireless networks are different. The TCP receiver then replies to the TCP sender with the MAC type of the target network by using the optional field of the TCP ACK segment. Consequently, the sender determines the accurate cwnd from the TCP ACK rather than waiting for multiple delayed ACKs or timeout. This brings some advantages, including quick discovery of network congestion or wireless link error, accurate determination of cwnd, and increased network utilization. Fig. 7 depicts the message flow of the cross-layer mechanism. A mobile node is aware of the network type of the handoff-in target network when it performs vertical handoff. Figure 5.4: Message flow in the cross-layer mechanism [6] 5.5 The Proposed Algorithm Based on Cross Layer Optimization to increase TCP Throughput in Vertical Handoff In this section, a vertical handoff algorithm is proposed which uses the basic idea of cross layer optimization described in the above. Similar to the above technique, a cross- layer approach 25
  36. 36. using MAC layer and TCP layer is can be used for the modification of cnwd for TCP. In the above algorithm, server increases or decreases the Congestion Window and Slow Start Thresh- old depending on type of the network receiver is in. For example if the receiver moves from the lower data rate network to higher one, the congestion window is gradually increasing. But, the above algorithm is not considering the situation of packet loss at cell boundary ,since the RSS is gradually decreasing in current network and also due to ping-pong effect at the cell boundary. So, the solution to the problem proposed is to suspend TCP transmission and set the conges- tion window at a low constant value at cell boundary until connection is established to a new network and RSS reaches a threshold .By this method, TCP packets loss can be reduced as a result TCP throughput increases. 5.5.1 The Proposed Algorithm For describing the algorithm, let consider the scenario of vertical handoff earlier discussed in chapter three. Let a MT (Mobile Terminal) is moving and after a certain instance of time makes a hand off. The handoff is RSS based with Hysteresis technique, means it makes sure it remains connected to the best network available based on handoff. The Mobile Terminal is receiving data in the form of TCP segment. The Mobile Terminal is both WLAN and B3G enabled. When the Mobile Terminal come near the cell boundary and it changes the MAC type in the TCP ACK. When the server the receives the ACK, the server understands that MT is try- ing to make a vertical handoff. Server suspends the packet transmission for a predetermined period of time until MT the mobile terminal connects to a new network. Figure 5.5: Heterogeneous network architecture integrating with Wi-Fi, and UMTS/B3G used in the algorithm. 26
  37. 37. 6Simulation and Results For obtaining vertical handoff using simulation, a wireless environment is created using MAT- LAB. 6.1 Simulation Parameters 1. Number of mobile Node: 1 2. Number of WLANs hotspots: 10 3. Number of B3G / UMTS BTS :1 4. Transmission range of Cellular Network (B3G/UMTS) = 3000m 5. Transmission range of WLAN = 200m 6. Transmit Power for Cellular Network (B3G/UMTS) = -40dBm 7. Transmit Power for WLAN=-50dBm 8. Path loss constant ,S= 5 9. Path Loss exponent ,n=3.5 10. RSS factor=2.8 (Signal Delay Factor for WLAN) 11. Data rate for Cellular Network (B3G/UMTS) = 384kbps 12. Data rate for WLAN = 2Mbps 13. Mobilty Model: Random walk 14. Velocity of the Mobile Terminal: 1-25m/s 15. TCP segment size = 1kbytes 16. Maximum Congestion Window Size= 64kbytes 27
  38. 38. 6.2 Simulation Characteristics In a simulation, we need to take many considerations such that we can generate an environ- ment which resembles real life environment keeping in mind the limitations of the simulation platform. The RSS received by a mobile node is different when it uses different wireless networks. In WLAN, the RSS is computed based on RSS = 10 log10 100 (39.37 × distance)RSS factor (6.1) In the case of B3G/UMTS Network ,RSS is computed as RSS(d) = Ptrans − PL(d) (6.2) where, Ptrans is the transmit power, and PL(d) is the path loss at distance d. The path loss at distance d is defined as PL(d) = S + 10.n. log(10) (6.3) Figure 6.1: Signal strength received by the Mobile Terminal For simulating the motion of the mobile terminal, Random walk mobility model is used. In this model, the mobile terminal is considered to be moving randomly with a certain probability in forward, backward direction. Also, the mobile node is considered to stay a certain position (no 28
  39. 39. movement). Figure 6.2: Random Movement of the Mobile Terminal Now, using the above simulation characteristic, the number of vertical handover is calculated over different velocity (1-25m/s). Figure 6.3: No. of Vertical Handoff against velocity of the Mobile Terminal Now, the mobile terminal is considered to moving in the wireless environment, and the situ- ation of data transmission is considered. It is considered that mobile terminal receiving data throughout the simulation. Data received is considered as TCP packets. 29
  40. 40. Figure 6.4: Data received by the Mobile Terminal Next, a comparison between the throughput of the TCP (proposed by [6]) and throughput of TCP proposed by thesis is plotted against the no. of Vertical Handoff. Figure 6.5: Fig.14. Comparison of the Data rate of TCP [6] and Modified TCP proposed with no. Vertical Handoff The above comparison clearly shows that there is an increase of the throughput of “TCP Mod- ified” as proposed by the thesis. The plots below show a comparison between the congestion windows during vertical handoff. 30
  41. 41. Figure 6.6: Comparison of the Congestion Window of TCP [6] and Modified TCP proposed during Vertical Handoff Now a comparison between the number of packet drops for both the algorithm is calculated. We can see that the no. of packet drop is comparatively lower in proposed algorithm. Figure 6.7: Comparison of the Packet Drop of TCP [6] and Modified TCP proposed Similarly, a plot for comparison of Efficiency of TCP for both the algorithm shows a better re- sult in the proposed algorithm. 31
  42. 42. Figure 6.8: Comparison of the Efficiency of TCP [6] and Modified TCP proposed 32
  43. 43. 7Conclusion & Future Work 7.1 Conclusion In this thesis, vertical handover and and different algorithms are being studied. Knowledge from the prior analysis is used to understand the architecture of Wireless Hetrogeneous Net- work and is also helpful in developing a simulation environment for vertical Handoff. Basics of TCP and Characteristics of TCP over a wireless environment are being analysed. The problem regarding the TCP data transmission over a vertical handover situation is being acknowledged and a suitable solution to this problem based on cross-layer optimization is being proposed. The proposed algorithm shows an improvement on TCP performance on a wireless network while performing a vertical handover. 7.2 Future Work In this thesis, cross layer optimization is being to acknowledge the problem of vertical han- dover and TCP performance. Here ,cross-layer optimization is done between MAC-Transport Layer. Such optimizations can be done using other layers of protocol stack. Network Layer can be a valid candidate for such optimizations.Mobile IP can be also be included for optimization as a part of future work. 33
  44. 44. List of Acronyms 3G 3rd Generation 4G 4th Generation AP Access Point ARQ Automatic Repeat Request B3G Beyond 3rd Generation BDP Bandwidth-Delay Product BER Bit Error Rate BLER Blocked Error Rate BS/BTS Base Station CA Congestion Avoidance CNWD Congestion Window ECN Explicit Congestion Notification EDGE Enhanced Data Rates for GSM Evolution ELN Explicit Loss Notification FER Forward Error Correction GPRS General Packet Radio Service GSM Global System for Mobile Communications LAN Local Area Network MAC Media Access Control MAN Metropolitan Area Network MH Mobile Host MN Mobile Node MSS Maximum Segment Size PAN Personal Area Network QOS Quality of Service QOS Quality of Service RED Random Early Detection RSS Received Signal Strength RTO Retransmit Timeout RTT Round Trip Time SIR Signal to Interference Ratio SSTHRESH Slow Start Threshold TCP Transmission Control Protocol UMTS Universal Mobile Telecommunications System UWB Ultra Wideband WiMAX Worldwide Interoperability for Microwave Access WLAN Wireless Local Area Network WMAN Wireless Metropolitan Area Network 34
  45. 45. WPAN Wireless Personal Area Network WWAN Wireless Wide Area Network 35
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