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1.Introduction

“The wireless telegraph is not difficult to understand. The ordinary telegraph is like a long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is the same, only without the cat.”-Albert Einstein.

Today’s wireless users expect great things from tomorrow’s wireless networks.

These expectations have been fueled by hype about what the next generations of wireless networks will offer. The rapid increase of wireless subscribers increases the quality of services anytime, anywhere, and by any-media becoming indispensable. Integration of various networks such as CDMA2000 and wireless LAN into IP-based networks is required in these kinds of services, which further requires a seamless vertical handoff to 4th generation wireless networks.

As mobile computing increases in prevalence and popularity, it is becoming increasingly important to have a vertical handoff solution which can perform a vertical handoff seamlessly and smartly.

Background:
In the past decade, the telecommunications industry has witnessed an ever accelerated growth of the usage of the mobile communications. As a result, the mobile communications technology has evolved from the so-called second-generation (2G) technologies, GSM in Europe, IS-95(CDMA) and IS-136 (TDMA) in USA, to the third generation (3G) technologies. Along with the standards development for providing voice service to mobile users, a group of standards to deliver data to the mobile users have evolved from both SDO’s (Standards development organizations) and industry. Systems and applications, such as Short Message Service (SMS) for sending and receiving short text messages for mobile phone users, have been built and continue to be developed.

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. In a typical analog cell-phone system in the United States, the cell-phone carrier receives about 800 frequencies to use across the city. The carrier divides the entire city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a larger hexagonal grid, as shown in Figure1.1:

Figure 1.1 The Cell Topology

Each cell has a base station that consists of a tower and a small building containing the radio equipment that is used to communicate with Mobile Terminals over pre-assigned radio frequencies. Cell phones have low-power transmitters in them. Many cell phones have two signal strengths: 0.6 watts and 3 watts [1]. The base station also transmits at low power.

Low-power transmitters have two advantages:
• The transmissions of a base station and the phones within its cell do not make it very far outside that cell. Therefore, in Figure1.1, both of the cells in alternate rings and non-adjacent cells can reuse the same frequency. The same frequencies can be reused extensively across the city. • The power consumption of a cell phone, which is normally battery-operated, is relatively low. Low power corresponds to small batteries, and this is what has made handheld cellular phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using cell phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles the entire phone connections to the normal land-based phone system, and controls all of the base stations in the region. Groups of several cells are connected to a Mobile Switching Center (MSC) through which the calls are then routed to the telephone networks. The area serviced by a MSC is called a Registration Area (RA) or Location Area (LA). A group of RA’s composes a Service Area (SA). Each SA is serviced by a Home Location Register (HLR). A wireless network may include several SA’s and thus several HLR’s.

All cell phones have special codes associated with them. These codes are used to identify the phone, the phone’s owner and the service provider. Electronic Serial Number (ESN) (a unique 32-bit number programmed into the phone when it is manufactured), Mobile Identification Number (MIN) (a 10-digit number derived from the owners phone’s number), and a System Identification Code (SID) (a unique 5-digit number that is assigned to each carrier by the FCC-Federal Communications Commission (A U.S. government agency charged with the task of regulating all forms of interstate and international communication)) are a few of the standard cell phone codes employed. While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone when one purchases a service plan and has the phone activated.

2G systems such as GSM, IS-95, and CDMA One were designed to carry speech and low bit rate data. 3G systems were designed to provide higher data rate services. During the evolution from 2G to 3G, a range of wireless systems, including GPRS, Bluetooth, WLAN and Hyper LAN have been developed. All these systems were designed independently, targeting different service types, data rates, and users. As these systems all have their own merits and shortcomings, there is no single system that is good enough to replace all the other technologies.

In cellular networks such as GSM, a call is seamlessly handed over from one cell to another using hard handover without the loss of voice data. This is managed by networks based handover control mechanisms that detect when a user is in a handover zone between cells and redirect the voice data at the appropriate moment to the mobile node via the cell that the MN has just entered. In 4G networks a handover between different networks is required. A handover between different networks is referred to as a vertical handover. Although commercial mobile telephone networks existed as early as the 1940’s, many consider the analog networks of the late 1970’s to be the first generation (1G) wireless networks.

The details of 1G, 2G, 3G and 4 G and their stages of evolution and the concepts involved are discussed in the Literature review of the Chapter 2. Features of 4G networks, possible architectures for 4G and various mobility management issues are discussed. 4G Networks are all IP based heterogeneous networks that allow users to use any system at anytime and anywhere. Users carrying any integrated terminal can use a wide range of applications provided by multiple wireless networks.4G systems not only provides telecom services, but also a data-rate service when good system reliability is provided. 4G networks face number of challenges in providing service anywhere and anytime which are discussed in Chapter 2.

An event when a mobile station moves from one wireless cell to another is called Handoff. Handoff Criteria, Handoff Strategies, Handoff Methods, Handoff Scenarios and different types of handoffs are discussed in Chapter 3. Chapter 4 discusses the Vertical handoff procedure and algorithm between various networks in different regions. Universal Seamless Handoff Architecture is also discussed in this chapter. Chapter 5 discusses the implementation details, system requirements and results. Chapter 6 discusses the conclusion from the study and implementation. Appendix A and Appendix B contains the source code of main algorithm and test results of simulation respectively. Chapter 7 lists the references used in the thesis.

Chapter 2
WIRELESS NETWORKS – 1G, 2G, 3G & 4G
CONCEPTS AND STAGES OF EVOLUTION

2.History and Evolution of Mobile Services

The History and evolution of mobile services from the 1G (first generation) to fourth generation are discussed in this section. Table1 presents a short history of mobile telephone technologies.

Table 2.0 Short History of Mobile Telephone Technologies

First Generation
Second
Generation

Third Generation
Fourth Generation

Mobile Telephone Service: car phone
Digital Voice +
And Messaging
-Data
Mobile Phone
Fixed Wireless
Loop
Integrated
High Quality
Audio, Video and
Data.
Narrow band and Broadband Multimedia Services + IN/IP
integration
Dynamic
information
access
Telepresence
(virtual
meetings,
education, and
training )
Wearable
devices
Analog Cellular
Technology
Macro cellular
Systems
Digital Cellular
Technology + IN
emergence
Microcellular and
Pico cellular:
Capacity, quality
Enhanced
Cordless
Technology
Broader
Bandwidth
CDMA Radio
Transmission

Information
Compression
Higher Frequency
Spectrum
Utilization

IN + Network
Management
integration ‰ IP
technology
Unified IP and seamless combination of
Broadband hot spots
WLAN/LAN/
PAN
2G/3G + 802.11
Knowledge-
Based
Network
Operations

Table 2.1 Wireless Network and Service Evolution

The history and evolution of mobile service from the 1G (first generation) to fourth generation process began with the designs in the 1970s that have become known as 1G. Refer to table 2.2 for an overview of the evolution of mobile service. The earliest systems were implemented based on analog technology and the basic cellular structure of mobile communication. These early systems solved many fundamental problems. The 2G systems designed in the 1980s were still used mainly for voice applications but were based on digital technology, including digital signal processing techniques. These 2G systems provided circuit-switched data communication services at a low speed.

During 1990s, two organizations worked to define next, or 3G, mobile system, which would eliminate previous incompatibilities and become a truly global system. The 3G system would have higher quality voice channels, as well as broadband data capabilities, up to 2Mbps. An interim step is being taken between 2G and 3G, the 2.5G. It is basically an enhancement of the two major 2G technologies to provide increased capacity on the 2G RF (Radio Frequency) channel and to introduce higher throughput for data service up to 384 kbps. A very important aspect of 2.5G is that the data channels are optimized for packet data, which introduces access to the internet from mobile devices, whether telephone, PDA (Personal digital assistant), or laptop. However, the demand for higher access speed multimedia communication in today’s society, which greatly depends on computer communication in digital format, seems unlimited.

Traditional phone networks (2G cellular networks) such as GSM, used mainly for voice transmission, are essentially circuit-switched. 2.5G networks, such as GPRS, are an extension of 2G networks, in that they use circuit switching for voice and packet switching for data transmission. Circuit switched technology requires that the user be billed by airtime rather than the amount of data transmitted since that bandwidth is reserved for the user. Packet switched technology utilizes bandwidth much more efficiently, allowing each user’s packets to compete for available bandwidth, and billing users for the amount of data transmitted. Thus a move towards using packet-switched, and therefore IP networks, is natural.

3G networks were proposed to eliminate many problems faced by 2G and 2.5 G networks, like low speeds and incompatible technologies (TDMA/CDMA) in different countries. Expectations for 3G included increased bandwidth: 128Kbps in a car and 2 Mbps in fixed applications. In theory, 3G would work over North American as well as European and Asian wireless air interfaces. In reality, the outlook for 3G is neither clear nor certain. Part of the problem is that network providers in Europe and North America currently maintain separate standards’ bodies. The standards’ bodies mirror differences in air interface technologies. In addition there are financial questions as well that cast a doubt over 3G’s desirability. There is a concern that in many countries, 3G will never be deployed. This concern is grounded, in part, in the growing attraction of 4G wireless technologies.

A 4G or 4th generation network, a new generation of wireless is intended to complement and replace the 3G systems. Accessing information anywhere, anytime, with a seamless connection to a wide range of information and services, and receiving a large volume of information, data, pictures, video, and so on as shown is Figure 2.2 are the keys of the 4G infrastructures.

Figure 2.2 4G Visions

The future 4G infrastructure [3] will consist of a set of various networks using IP as a common protocol so that users are in control because they will be able to choose every application and environment. A 4G or 4th generation network is the name given to an IP based mobile system that provides access through a collection of radio interfaces. A 4G network promises seamless roaming/handover and best connected service, combining multiple radio access interfaces (such as WLAN, Bluetooth, GPRS) into a single network that subscribers may use [12]. With this feature, users will have access to different services, increased coverage, the convenience of a single device, one bill with reduced total access cost, and more reliable wireless access even with the failure or loss of one or more networks.

4G was simply an initiative by R & D labs to move beyond the limitations, and address the problems of 3G which was having trouble meeting its promised performance and throughput. In the most general level, 4G architecture includes three basic areas of connectivity: Personal Area Networking (such as Bluetooth), local high-speed access points on the network including wireless LAN technologies, and cellular connectivity.

4G calls for a wide range of mobile devices that support global roaming. Each device will be able to interact with Internet-based information that will be modified on the fly for the network being used by the device at that moment. The roots of 4G lie in the idea of pervasive computing [20]. The glue for all this is likely to be software defined radio (SDR) [13]. SDR enables devices such as cell phones, PDAs, PCs and a whole range of other devices to scan the airwaves for the best possible method of connectivity, at the best price. In an SDR environment, functions that are formerly carried out solely in hardware – such as the generation of the transmitted radio signal and the tuning of the received radio signal – are performed by software [7]. Thus, the radio is programmable and able to transmit and receive over a wide range of frequencies while emulating virtually any desired transmission format. As the number of wireless subscribers rapidly increases guaranteeing the quality of services anytime, anywhere, and by any-media becomes indispensable.

These services require various networks to be integrated into IP-based networks, which further require a seamless vertical handoff to 4th generation wireless networks. And as one of the next generation mobile communications, the 4th generation mobile communications provides various services, such as high-speed data services and IP-based access to Radio Access Network, etc. Various interface techniques such as WLAN, Bluetooth, UTMS, and CDMA2000 are integrated into the IP-based networks as an overlay structure. In this structure, the optimum services are provided to mobile hosts. Mobile hosts in this structure can be connected to the network through various access points. Moreover, a seamless handoff should also be supported between different air interface techniques during inter network movement.

2.1.Features of 4G Networks

High Speed – 4G systems should offer a peak speed of more than 100Mbits per second in stationary mode with an average of 20Mbits per second when traveling. High Network Capacity – Should be at least 10 times that of 3G systems. This will quicken the download time of a 10-Mbyte file to one second on 4G, from 200 seconds on 3G, enabling high-definition video to stream to phones and create a virtual reality experience on high-resolution handset screens. Fast/Seamless handover across multiple networks – 4G wireless networks should support global roaming across multiple wireless and mobile networks. Next-generation multimedia support – The underlying network for 4G must be able to support fast speed volume data transmission at a lower cost than today.

The goal of 4G [4] is to replace the current proliferation of core mobile networks with a single worldwide core network standard, based on IP for control, video, packet data, and voice. This will provide uniform video, voice, and data services to the mobile host, based entirely in IP. The objective is to offer seamless multimedia services to users accessing an all IP based infrastructure through heterogeneous access technologies. IP is assumed to act as an adhesive for providing global connectivity and mobility among networks. An all IP-based 4G wireless network has inherent advantages over its predecessors. It is compatible with, and independent of the underlying radio access technology [4].

An IP wireless network replaces the old Signaling System 7 (SS7) [6] telecommunications protocol, which is considered massively redundant. This is because SS7 signal transmission consumes a larger part of network bandwidth even when there is no signaling traffic for the simple reason that it uses a call setup mechanism to reserve bandwidth, rather time/frequency slots in the radio waves. IP networks, on the other hand, are connectionless and use the slots only when they have data to send. Hence there is optimum usage of the available bandwidth. Today, wireless communications are heavily biased toward voice, even though studies indicate that growth in wireless data traffic is rising exponentially relative to demand for voice traffic. Because an all IP core layer is easily scalable, it is ideally suited to meet this challenge. The goal was a merged data/voice/multimedia network.

2.2.Possible Architectures for 4G Networks

Accessing different mobile and wireless networks is one of the most challenging problems to be faced in the deployment of 4G technology [22]. Figure 2.3 shows three possible architectures: Using a multimode device

An overlay network
A common access protocol

Figure 2.3 Possible Architectures for 4G Networks

2.2.1. Multimode Devices

To access services on different wireless networks, one single physical terminal with multiple interfaces is used. Existing advanced mobile phone system on code division multiple access dual function cell phone, dual function satellite cell phone and global system for mobile telecommunications are examples of Multimode Device architecture. Call completion can be improved and coverage area is expanded effectively using this architecture. When there is network, link or switch failure, reliable wireless coverage should be provided. The handoff between networks can be initiated by user, device or network. There is no requirement of wireless network modification or employment of inter working devices as the device itself incorporates most of the additional complexity. A database can be deployed by each network which stores the information to keep track of user location, device capabilities, network conditions and user preferences.

2.2.2.Overlay Network

There are several universal access points in overlay network with which a user can access. A wireless network is selected by each universal access points based on availability, quality of service specifications and user defined choices [8]. Protocol and frequency translation, content adaptation is performed by universal access point on behalf of users. As the user moves from one universal access point to another, rather than the user or the device, Hand-offs are performed by overlay networks. User, network, device information, capabilities and preferences are stored by the universal access point. Single billing and subscription is supported as universal access points keep track of the various resources a caller uses.

2.2.3.Common Access Protocol

Supporting one or two standard access protocols by wireless networks allows this protocol to become viable. Using wireless asynchronous mode requires networking between different networks as one possible solution. Transmission of ATM cells with additional headers or wireless ATM cells requiring changes in the wireless networks must be allowed by every wireless network to implement wireless ATM. One protocol might be used by one or more types of satellite based networks while another protocol is used by one or more terrestrial wireless networks.

2.3.Mobility Management Issues in 4G Networks

A critical aspect of 4G is Mobility [2]. The three main issues regarding mobility management [16] in 4G networks are as follows: 1)The optimal choice of access technology or how to be best connected is the first issue dealt with in the mobility of 4G. Considering how the terminal and an overlay network choose the radio access technology is necessary when a user is given connectivity from more than one technology at any one time.

Application Layer
TCP/UDP
IP

Figure 2.4 Network Technologies

There are several network technologies available today, which can be viewed as complementary. For high data rate indoor coverage, WLAN is best suited. GPRS or UMTS are best suited for nation wide coverage and can be regarded as wide area networks, providing a higher degree of mobility. An optimal choice of radio access technology among all those available should be made by the user of the mobile terminal or the network. The network to be connected and when to perform a handover between different networks are determined by a handover algorithm. Ideally, the handover algorithm would assure that the best overall wireless link is chosen. The type of application being run by the user at the time of handover should be taken into consideration during the network selection strategy. This ensures stability as well as optimal bandwidth for interactive and background services.

2) The second issue regards the design of a mobility enabled IP networking architecture, which contains the functionality to deal with mobility between access technologies. This includes fast, seamless vertical (between heterogeneous technologies) handovers (IP micromobility), quality of service (QoS), security and accounting. Real-time applications in the future will require fast/seamless handovers for smooth operation. Mobility in IPv6 [15] is not optimized to take advantage of specific mechanisms that may be deployed in different administrative domains. Instead, IPv6 provides mobility in a manner that resembles only simple portability. To enhance mobility in IPv6, ‘micro-mobility’ protocols, Cellular IP and Hierarchical Mobile IPv6 [5] have been developed for seamless handovers i.e.; handover that result minimal handover delay, minimal packet loss, and minimal loss of communication state.

3) The adaptation of multimedia transmission across 4G networks is the third and the last issue. As multimedia is the main service feature of 4G networks, and changing radio access networks may in particular result in drastic changes in the network changes. Thus the framework for multimedia transmission must be adaptive. In cellular networks such as UMTS, users compete for scarce and expensive bandwidth. Variable bit rate services provide a way to ensure service provisioning at lower costs. In addition the radio environment has dynamics that renders it difficult to provide a guaranteed network service. This required that the services are adaptive and robust against varying radio conditions. High variations in the network Quality of Service [18] leads to significant variations of the multimedia quality. The result could sometimes be unacceptable to the users. Avoiding this requires choosing an adaptive encoding framework for multimedia transmission. The network should signal QoS variations to allow the application to be aware in real time of the network conditions. User interactions will help to ensure personalized adaptation of the multimedia presentation.

Wireless Mobile Networks has Mobility Management as an integral function. Mobility Management influences the type and quality of Wireless Network service offerings. Each generation of Wireless Mobile Network has different mechanisms for Mobility Management. Network support of subscriber mobility requires registration, authentication, paging, roaming, radio resource management and excess channel capacity. Mobility Management focuses on the network’s ability to allocate radio access network resources.

2.4.Challenges in 4G Networks

4G Networks are all IP based heterogeneous networks that allow users to use any system at anytime and anywhere. Users carrying any integrated terminal can use a wide range of applications provided by multiple wireless networks. 4G systems provide not only telecommunications services, but also a data-rate service when good system reliability is provided. At the same time, a low per-bit transmission cost is maintained. Users can use multiple services from any provider at the same time. Imagine a 4G mobile user who is looking for information on movies shown in nearby cinemas. The mobile may simultaneously connect to different wireless systems. These wireless systems may include Global Positioning System (GPS) (for tracking users current location), a wireless LAN (for receiving previews of the movies in nearby cinemas), and a code-division multiple access (for making a telephone call to one of the cinemas). In this example, the user is actually using multiple wireless services that differ in quality of service (QoS) levels [18], security policies, device settings, charging methods, and applications. There are number of challenges faced by 4G networks in integrating all the services.

An overview of challenges in Integrating Heterogeneous Systems The challenges mentioned in the above table are grouped into three different aspects: Mobile Station
System
Service

2.4.1.Mobile Station

To use large variety of services and wireless networks in 4G systems, multimode user terminals are essential as they can adapt to different wireless networks by recon Figuring them. The need to use multiple terminals is eliminated. Adapting software radio approach is the most promising way of implementing multimode user terminals [14]. The analog part of the receiver consists of an antenna, a band pass filter, and a low noise amplifier. The received analog signal is digitized by the analog/digital converter immediately after the analog processing. The processing in the next stage is then performed by a reprogrammable baseband digital signal processor. The digital signal processor will process the digitized signal in accordance with the wireless environment. Unfortunately, the current software radio technology is not completely feasible for all the different wireless networks due to the following technological problems. It is impossible to have one antenna and one low noise amplifier to serve the wide range of frequency bands. Using multiple analog parts to work in different frequency bands is the only solution. The design complexity and physical size of a terminal are increased. And existing analog/digital converters are not fast enough.

2.4.2.System

For 4G infrastructure to provide wireless services at any time and anywhere, terminal mobility is a must. Terminal mobility allows mobile clients to roam across geographic boundaries of wireless networks. The two main issues in terminal mobility are location management and handoff management. The system tracks and locates a mobile terminal for possible connection. Location management involves handling all the information about the roaming terminals, such as original and current located cells, authentication information, and QoS capabilities. Handoff Management maintains ongoing communication when the terminal roams. Mobile IPv6 is a standardized IP-based mobility protocol for IPv6 wireless systems.

Each terminal has an IPv6 home address. Whenever the terminal moves outside the local network, the home address becomes invalid, and the terminal obtains a new IPv6 address called care-of address in the visited network. A binding between the terminal’s home address and care-of address is updated to its home agent in order to support continuous communications. This kind of handoff process causes an increase in system load, high handover latency, and packet losses. It is hard to decide the correct handoff time because measuring handoffs among different wireless systems is very complicated. The uncertain handoff completion time adds to the complexity in designing good handoff mechanisms.

2.4.3. Services

More comprehensive billing and accounting systems are needed, with the increase of service varieties in 4G systems. Customers may subscribe to many services from a number of service providers at the same time rather than only one operator. Dealing with multiple service providers might be inconvenient for customers. Operators need to design new business architecture, accounting processes, and accounting data maintenance. It is challenging to formulate one single billing method that covers all the billing schemes involved. 4G networks support multimedia communications, which consists of different media components with possibly different charging units. This adds difficulty to the task of designing a good charging scheme for all customers. The media components may have different QoS requirements. To decide a good tariff for all possible components is very complicated. To build a structural billing system for 4G, several frameworks have been studied.

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