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ABSTRACT

This project embarks on a solution to wireless services in a campus. It proposes a

new approach to support wireless mobile internet working on a large university

campus or similar environment. The architecture of the approach combines

wireless local-area network technology with high-speed switching technology.

The combination provides a wireless communication system with sufficient

aggregate bandwidth to handle massive, synchronized movements of mobile

computers. Furthermore, the approach supports optimal routing to each mobile

computer without requiring modification of the networking software on mobile

computers, non-mobile computers, or routers in the existing Internet. This

architecture describes the design and implementation of a campus size mobile

wireless network. Through a prototype implementation, we have shown that the

approach is feasible.

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CHAPTER ONE

1.0 INTRODUCTION

Recent advances in personal computing and wireless local-area network (LAN)

technologies have resulted in affordable laptop and palmtop computers with

wireless networking capability. A portable computer with a wireless LAN adaptor

can communicate directly with nearby wireless computers. To communicate with

computers that are far away, a wireless mobile computer uses a nearby access

station. Normally, an access station is a stationary computer with a wireless

interface and a connection to conventional network facilities using terrestrial

links. In particular, an access station that connects to the global TCP/IP Internet

can provide a wireless mobile computer with access to other computers at sites

around the world. The wireless interface of an access station can provides

wireless coverage for a small geographical area approximately 50 meters in

diameter. Mobile computers that reside within the area can use radio signals to

communicate with the access station. Because an access station can provide

wireless coverage for only a limited area, multiple access stations are needed to

provide coverage for a large area. Attaching multiple access stations to an

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internet introduces routing problems that result when a mobile computer

migrates from the area of one access station to the area of another.

Consider the example internet illustrated in Figure 1.1.

Figure 1.1 Illustration of an example internet that supports wireless mobile communication.

In the figure, two access stations, A and B, attach to an internet. Mobile computer

M is communicating with computer C via access station A and two routers, R1 and

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R2. To maintain network connectivity when M migrates to the coverage area of

access station B, B must detect that M has arrived and then propagate a routing

update message to allow packets destined for M to be forwarded to itself. To

achieve optimal routing, B must propagate the routing update message to all the

routers and other access stations in the internet because M could be

communicating with an arbitrary set of computers attached to the campus

internet. Note that packets that carry the routing update message compete with

data packets for network bandwidth. The overhead of propagating routing

updates is especially apparent in a large university campus where 50,000 mobile

computers occupy in a small geographic area.

More important, movements of mobiles at a university are massive and

synchronized a large percentage of the population migrates to new locations

during each change of class. Without a careful design, the campus internet may

experience network congestion when most students attempt to use their mobile

computers to communicate from new locations, affecting not only the mobile

computers, but also the non-mobile computers in the campus internet. The

situation becomes worse when congestion causes delay or loss of routing

updates, forcing data packets to follow non-optimum paths.

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Diverse capabilities in the mobile computers chosen by students also complicate

the design. Students are likely to choose mobile computers that use various kinds

of processors to run a variety of operating systems. We seek a design that can

accommodate such diversity.

This dissertation reports research in the area of wireless data communication. The

research investigates how to design a wireless data communication system that is

capable of supporting mobile internetworking in a large university campus. The

system should have the following characteristics.

Can handle a large volume of routing update traffic. Support optimal routing to each wireless mobile computer. Shield the campus internet from mobility management traffic. Provide seamless wireless mobile internetworking without requiring

modifications to the networking software on mobile computers, non-

mobile computers, or routers in the existing Internet.

1.1 STATEMENT OF PROBLEM

With cables connections are only available at pre designated locations - with

wireless they can connect anywhere. Inflexible and expensive - and restrict

students to specific locations where they can study, research and learn.

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A Simply Wireless Local Area Network is worth considering. A wireless lan can be

implemented quickly and cost effectively on your campus. Whether you are

interested in wirelessly enabling a school or an MBA college, Simply Wireless are

the wireless networking professionals to chat about your ideas with. Simply

Wireless has a wealth of experience in the Educational Market, and is currently

working with some of the leading Universities, Post Primary Schools and MBA

colleges.

1.2 OBJECTIVES OF THE STIDY

There are several objectives associated with this project namely

Access everywhere: Laptops are portable, and internet access is becoming more

so with wireless. A wireless network means the laptops are instantly connected

when they walk into class, and even on their way to class.

Accelerate learning: We are all different, some students learn at faster and

slower paces. Using networked laptops and a wireless network - teaching staff

can create assignments so students can work at their own pace.

Flexible classroom layout: Want to shift desks around for a particular class. Do

you need to add more students to the classroom network? With a wireless LAN

from Simply Wireless there are no cables or data ports to to limit your flexibility.

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Moving computers becomes as easy as moving a trolley.

For science teachers: Now science lessons can take place anywhere. The lab is a

locations that's often very difficult to cable, with a wireless network, students can

input data while experiments take place, and as they're observing results.

Web based wireless learning is smart: Wireless makes it easier for students to

work on their online assignments. They can access the school intranet from the

library or cafeteria, being able to learn anywhere. In sum: You get the flexibility,

portability and affordability you need, with the added assurance of Intel reliability

and industry-leading expertise.

Computers on wheels: If you don't have the funding to put a computer in every

classroom - wireless is an easy way to maximize your technology investment. You

can simply wheel your pool of computers into different classrooms as they are

needed. Computers will be used by students more, and rotated hourly if needed.

Students can use the pool of computers, access the school network, and work on

assignments all from the library, or any wirelessly enabled location. Wireless

technology enables computers to roam seamlessly throughout the school - even

to portable classrooms or the playground.

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More Students, less capital expenditure on IT: Your assignment; get your campus

wired to the World Wide Web and other educational resources, but do it within a

limited budget. Can you satisfy community expectations, while adhering to your

budget? The solution is a Simply Wireless LAN. It's modular construction allows

simple network additions as needed.

1.3 Significance of study

In Universities, there is no need to stand in line for a library PC. Students doing

research can record their notes, interact with the Internet, and even access the

library printer on their own wirelessly enabled laptops. Computers and computer

networks are commonplace in education. More and more Educational facilities

are taking advantage of the benefits of wireless networks. Compared to

traditional cable, wireless offers a robust, secure, scalable and economical means

to connect teaching staff and students to the information they need in their day

to day lives. The principal advantages of a campus wireless network are:

Increased flexibility

Students and teaching staff can connect wherever they need access rather

than in designated computer laboratories.

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Scalable

your wireless network can grow as you need. Install an access point in the

hallway and several classrooms are connected to the WLAN instantly. No

cable to lay, need to predetermine where and how many data ports to

install.

Dollars and Sense

A cheaper and less intrusive solution that cable.

1.4 Scope of study

The scope of this project is to create a campus wide wireless network that is

portable, flexible, and easily expandable. A universe of information is accessible

when, and where, it's needed. Schools can provide network connectivity to new

classrooms, without sinking money into space they will temporarily occupy. Using

a Simply Wireless LAN, your educational facility can avoid expensive re-wiring or

messy and often disruptive construction. Students and teachers are connected

immediately.

Students, teaching and administrative staff can move throughout the campus and

maintain continuous network access.

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1.5 Definition of terms

This section defines the terminology used in the remainder of this dissertation.

General Networking Terms

A network is a communication system that allows computers attached to the

system to exchange data. A packet is a block of data transmitted from a computer

across a network. A router is a dedicated computer that attaches to two or more

networks and forwards packets from one network to another. An internet is a

collection of networks physically interconnected by using routers.

A communication channel is a path along which data used for communication

passes. A communication link (or link) is a physical medium over which computers

can send data. A frame is the basic unit of message passed across a

communication link. A frame contains information that allows a network interface

hardware to capture the data contained within. Maximum Transmission Unit

(MTU) is the largest amount of data that can be sent across a communication link

in a single frame.

A host is an end-user computer that attaches to a TCP/IP internet. A datagram (or

IP datagram) is a packet that passes across a TCP/IP internet. A TCP connection is

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an abstraction provided by the TCP protocol software. A TCP connection between

two applications allows each application to deliver data streams to the other

reliably. TCP ensures sequenced, lossless delivery of each byte of data.

A local area network (LAN) is a network that uses technologies designed to span a

small geographic region. An Ethernet is an example of a LAN. A wireless LAN is a

local area network that allows wireless communication among hosts that reside in

the network.

Nonstandard Terms

A host is an end-user computer that attaches to a TCP/IP internet. A no mobile

host is a host that attaches to an internet using a terrestrial link. A mobile host (or

mobile) is a portable computer that can migrate from one network to another.

This dissertation describes two types of mobile hosts. One type of mobile host

does not have wireless communication capability. The other type is capable of

wireless communication. This dissertation describes a system that supports the

second type of mobiles.

A base station is a dedicated, non-mobile computer that is capable of wireless

communication with mobiles. A base station provides a group of mobiles with

wireless access to an internet. Each base station supports wireless communication

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for a geographical region called an area. A mobile can communicate directly with

a base station once the mobile is within the area of the base station. An

overlapping area is a geographical region in which a mobile can communicate

with more than one base station using the wireless interface. Each mobile host is

associated with an owner. The base station that is forwarding datagrams for a

mobile is the owner of the mobile or the owning base station of the mobile. The

owning base station of a mobile is also the default router for the mobile.

Handoff refers to the process of transferring the ownership of a mobile from one

base station to another.

1.6 ORGANIZATION OF WORK

In this chapter, we have discussed the personal diary briefly. I also provided

the problem that led to the development of the system. The objectives of the

study, significance of study, scope of study, limitation of study.

In chapter two, I made a literature review of personal diary application,

basically what people have done on the topic.

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In chapter three I have the research methodology, the step followed.

Analysis of the existing system: I stated the things that made the manual diary

unworthy to use; the HLM, DFD.

In chapter four, I showed the design and implementation of the system, the

data dictionary, input-output specification, table format/structure, hardware and

software requirement.

In chapter five I have recommendation and future development; Summary,

conclusion and references.

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CHAPTER TWO

2.0 LITERATURE REVIEW

This chapter explains why supporting mobile computing in a TCP/IP internet is

difficult, presents five approaches that researchers have proposed to overcome

the difficulties, and describes wireless networking systems that are being built at

other research institutions.

2.1 Internet Addressing and Routing

An IP unicast address is a 32-bit integer that has one of the three forms shown

In Figure below

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Figure above IP unicast address structure. Each unicast address consists of a network ID (netid) and a

host ID (hostid).

As the figure illustrates, IP uses a hierarchical addressing scheme: each address

consists of a network ID (netid) and a host ID (hostid); the network ID identifies a

network, and the host ID identifies a host on that network. The addressing

scheme facilitates routing. Conceptually, routing a datagram to a host on a given

network takes two steps. First, IP routers forward the datagram to the network

using the network ID part. Second, when the datagram reaches the destination

network, routers deliver the datagram to the host using the host ID part. The

hierarchical addressing scheme also makes routing information manageable. An

IP router need not maintain routing information on a per-host basis.

Consequently, routers exchange less routing information. Furthermore, since

network topologies do not change frequently, routers can exchange routing

information at longer intervals (e.g., once per 30 seconds, as in RIP.

3.2 The Host Mobility Problem

Despite the advantages, the addressing and routing scheme of IP makes mobile

computing difficult. Consider the example illustrated in Figure below

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Illustration of two base stations, B1 and B2, attached to an internet. A mobile

host, M, is communicating with host H using base station B1.

Figure 3.2 shows two base stations B1 and B2 attached to an internet

interconnected using several routers, denoted using symbol R. Station B1

supports wireless LAN A (with network ID A), and B2 supports wireless LAN B

(with network ID B).

Host H is using TCP/IP [Pos81c] to communicate with mobile host M via station

B1. The network ID part of M's IP address is A (i.e., M is a host in wireless LAN

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A). Suppose M migrates to wireless LAN B. Host H cannot communicate with M

because IP routers continue to forward datagrams destined for M to wireless LAN

A.

Mobile M can acquire a new IP address from wireless LAN B and uses the new

address to communicate with H. Host H sends subsequent datagrams to M using

the new address. IP routers will correctly forward the datagrams to wireless LAN

B.

Thus, H and B reestablish communication. Unfortunately, changing an IP address

breaks the TCP connections that already exist between M and H, because TCP

uses the IP address of each end of a connection to identify the connection. To

summarize the difficulties in supporting host migration in an internet:

Changing an address facilitates routing, but breaks existing TCPconnections.

Maintaining an address keeps existing TCP connections, but creates routingproblems.

In theory, IP routers can treat mobile computers as a special case and maintain a

host-specification route for each mobile. When a mobile migrates to a new

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network, routers propagate a route update for the mobile. In practice,

propagating host-specific routes for mobiles does not work well for two reasons.

First, routers need special protocols to propagate the routes in a timely fashion

because current IP routing protocols are not designed to operate in a rapidly

changing topology. Second, routers need to propagate routes to every router in

the internet because a mobile could be communicating with any host attached to

the internet.

2.3 The Forwarding Concept and Address Insertion Agent

When a mobile switches among base stations, it is important to maintain the TCP

connections that the mobile has established. Reestablishing each TCP connection

every time the mobile changes base station is unacceptably annoying. Thus, all

the approaches that support mobile hosts focus on devising new routing schemes.

All the proposed routing schemes can be explained using a single concept termed

forwarding.

At any time, a mobile host is associated with a forwarder that can reach the

mobile directly. When a mobile migrates to a new network, the mobile acquires a

new forwarder. Routing a datagram to a mobile consists of two steps. First, IP

routers forward the datagram to the forwarder of the mobile. Second, the

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forwarder delivers the datagram to the mobile. The two-step forwarding

procedure indicates that a datagram destined for a mobile needs to carry two IP

addresses: one identifies the mobile, and the other identifies the forwarder for

the mobile. Because IP others only one destination address field, achieving the

two-step forwarding requires an entity referred to as an address insertion agent.

The agent must turn the datagram into another datagram that carries the two

addresses and delivers the resulting datagram to the forwarder, which turns the

received datagram into the original datagram and delivers the original datagram

to the mobile.

3.6 The IBM Approach

Researchers at IBM Corporation also proposed using loose source routing to

support mobile hosts. Unlike the IEN-135 approach, the IBM approach does not

restrict all mobiles to use the same network ID. Each mobile has a home network.

The network ID of a mobile's IP address identifies the home network of the

mobile. Each home network has at least one mobile router (MR) that maintains

the forwarder information and acts as the address insertion agent for the mobiles

in the network.

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To send a datagram to a mobile that is away from its home network, a sender

transmits the datagram, without using the LSRR option. IP routers will forward the

datagram to the home network of the mobile. When the datagram reaches the

home network, an MR intercepts the datagram and forwards the datagram using

the LSRR option to the current forwarder of the mobile. The forwarder then

delivers the datagram to the mobile. When it replies, the mobile uses the LSRR

option to carry the address of the forwarder to the sender. The IBM approach

assumes that the sender will perform a route reversal procedure [PB94] when

responding to the datagram from the mobile.

3.7 The SONY Approach

Researchers at SONY Corporation proposed an approach that incorporates the

forwarder function into each mobile. Thus, each mobile has two IP addresses, one

permanent address, called a virtual IP (VIP) address that is used to identify the

mobile, and one temporary IP (TIP) address for the forwarder. Like the IBM

approach, the SONY approach also uses routers, called primary resolvers

[TUSM94], at home networks to intercept datagrams and serve as address

insertion agents for mobiles. When a mobile migrates to a foreign network, the

mobile uses a mechanism (e.g., Dynamic Host Configuration Protocol to obtain a

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temporary address. The mobile then uses a control packet to carry the VIP-to-TIP

binding back to its primary resolver. Routers along the path are allowed to snoop

the control packet and cache the binding in a table called Address Translation

Table. A no mobile host can also install an Address Translation Table to make

communication with mobiles more efficient. Thus, besides the primary resolvers,

intermediate routers and hosts that have installed the AMT can serve as address

insertion agents for mobiles.

The advantage of the SONY approach is self-sufficiency: a mobile need not rely on

a forwarder to be able to communicate when visiting a foreign network. However,

the mobile still needs to obtain a temporary address from the foreign network.

The extra address requirement makes the scheme unappealing in an environment

where IP addresses are a scarce resource. For the next version of IP, IPv6, where

IP addresses are abundant, researchers have proposed using schemes similar to

the SONY approach to support mobile hosts.

3.10 Related Wireless Data Network Systems

A number of institutions are building wireless data network system for mobile

computing research. Carnegie Mellon University is building a wireless networking

infrastructure called Wireless Andrew [HJ96]. Currently, Wireless Andrew consists

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of two types of wireless systems: a wide area system using 19.2 Kbits/second

Cellular Digital Packet Data (CDPD) service and a local area system using 2

Mbits/second NCR WaveLAN technology. A wireless access station called Wave

POINT provides wireless access for mobiles in a small geographical area. All

WavePOINT stations are connected to a dedicated backbone network consisting

of 10 Mbits/second Ethernet [MB76] hubs. Routers interconnect the backbone

network to the campus internet. WaveLAN uses proprietary link layer protocols

to allow a mobile to roam from the area of one WavePOINT station to another

without losing network connectivity.

At the University of California at Berkeley, researchers are building a wireless

network system, InfoNet [LBSR95], for supporting the InfoPad project [N+96]. A

mobile computer in InfoPad is a portable multimedia terminal called Pad. Each

Pad uses two wireless links to communicate with a Gateway, which provides Pads

with access to a backbone network. The link from a Pad to a Gateway is a Proxim

radio link with a capacity of 244 Kbits/second; the link from a Gateway to aPad is

a Plessey radio link with a capacity of 700 Kbits/second [LBSR95]. Each Gateway

supports a small geographical region, called picocell, with a typical radius of 10

meters. Server processes use protocols to support seamless migration of a Pad

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from one picocell to another. The servers make the Pad appear to be a stationary

terminal attached to the backbone network.

The Mosquito Net project at Stanford University is investigating operating system

and application issues in mobile and wireless computing. Researchers have built a

test bed that consists of a wireless network and a collection of wired networks.

The wireless network uses the Ricochet micro-cellular data network service

provided by Metricom. The service uses pole-top radio units to provide wireless

access for mobiles. Each radio unit offers a raw data rate of 100 Kbits/second.

Ricochet uses Metricom proprietary routing protocols to provide roaming service

to mobiles. Mosquito Net uses a scheme similar to the IETF Mobile IP scheme to

support transparent host migration among the wireless network and the wired

networks, with emphasis on not using foreign agents.

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CHAPTER THREE

3.0 Research methodology

Research methodology is a collective term for the structured process of

conducting research. There are many different methodologies used in various

types of research and the term is usually considered to include research design,

data gathering and data analysis.

Research methodologies can be quantitative (for example, measuring the number

of times someone does something under certain conditions) or qualitative (for

example, asking people how they feel about a certain situation). Ideally,

comprehensive research should try to incorporate both qualitative and

quantitative methodologies but this is not always possible, usually due to time

and financial constraints.

Research methodologies are generally used in academic research to test

hypotheses or theories. A good design should ensure the research is valid, i.e. it

clearly tests the hypothesis and not extraneous variables, and that the research is

reliable, i.e. it yields consistent results every time. The approach used here is

SSADM.

http://www.blurtit.com/q462869.htmlhttp://www.blurtit.com/q462869.html

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What is SSADM?

Structured Systems Analysis and Design Method (SSADM) is a systems approach

to the analysis and design of information systems. SSADM was produced for a UK

government office concerned with the use of technology in government, from

1980 onwards. The names "Structured Systems Analysis and Design Method" and

"SSADM" are now Registered Trade Marks of the Office of Government

Commerce (OGC), which is an Office of the United Kingdom's Treasury.

Introduction

System design methods are a discipline within the software development industry

which seek to provide a framework for activity and the capture, storage,

transformation and dissemination of information so as to enable the economic

development of computer systems that are fit for purpose.

SSADM is a waterfall method by which an Information System design can be

arrived at; SSADM can be thought to represent a pinnacle of the rigorous

document-led approach to system design, and contrasts with more contemporary

Rapid Application Development methods such as DSDM.

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3.1 The existing system

The present system is a system of communication where students, personnel and

lecturers have no mobile communication network. They communicate either

meeting each other or by using the network of popular service providers like

MTN.

3.2 The proposed system

The proposed system is a system that will provide a campus wide wireless

network that is portable, flexible, and easily expandable. A universe of

information is accessible when, and where, it's needed. Schools can provide

network connectivity to new classrooms, without sinking money into space they

will temporarily occupy. Using a Simply Wireless LAN, your educational facility can

avoid expensive re-wiring or messy and often disruptive construction. Students

and teachers are connected immediately.

Students, teaching and administrative staff can move throughout the campus and

maintain continuous network access.

3.3 The Cross point approach to the new system

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This chapter proposes a new approach to support wireless mobile

internetworking called cross point, the proposed approach combines wireless LAN

technology with asynchronous Transfer Mode (ATM) switching technology. The

combination provides a wireless communication system with sufficient aggregate

bandwidth to handle both data transfer and routing updates.

The chapter describes the architectural design, the addressing and routing

scheme, design issues, and analyses of the bandwidth requirements. It provides

an overview of how the design can be used to support mobile internetworking in

a large university campus.

3.4 Architectural Design

The design uses a scalable, high-speed communication fabric to interconnect all

base stations and special purpose routers called cross point routers. Base stations

provide wireless access for mobile hosts (or mobiles). Cross point routers

interconnect the cross point network to the campus internet, allowing mobiles to

communicate with hosts outside cross point. The high-speed communication

fabric provides high-bandwidth, low-latency communication channels among the

attached cross point processors (i.e., base stations and cross point routers).

3.5 An Overview

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We use an example to provide an overview of how the design can be used to

support mobile computing. Figure 4.2 shows a mobile host, M, is communicating

Illustration of the architectural design of a cross point wireless mobile network. A high-speed

communication fabric interconnects all base stations and cross point routers.

with a host, S, that attaches to the campus internet. Base station B1 and cross

point router R are forwarding IP datagrams for M. The datagrams pass through

the data channel d1, which is the channel that B1 and R used to transport

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datagrams to each other. When mobile M migrates to the area of base station B2,

B2 uses the control channel to exchange control messages with B1. Once B1

allows B2 to capture M, base station B2 will inform router R to forward the

datagrams destined for M over data channel d2, allowing seamless ommunication

between mobile M and host S. Both M and S are unaware of the routing change.

The cross point protocol software handles all the details to support seamless

mobile communication.

3.6 Special Interface for High-Speed Processing

To process control and data messages at high speed, each cross point processor

includes a special interface. The interfaces handle routing within cross point and

control functions that allow seamless mobile communication. In particular, the

interface implements an address-to-circuit binding for selecting an outgoing

virtual circuit using a destination IP address (e.g., a mobile's IP address). When a

mobile migrates to the area of a new base station, the interfaces handle all route

changes. When routing information arrives at the interface, the interface

automatically updates its address-to-circuit binding and begins using the new

binding.

3.7 Analysis of Bandwidth Requirement

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Note that when a handoff occurs, the new owner base station uses the control

VCs to deliver a route update to all the other cross point processors. The update

traffic can be significant when many mobiles move in synchronized fashion. This

section provides two analyses of the bandwidth required to handle routing

updates that result from massive, synchronized movement of mobiles. The first

analysis considers the aggregate bandwidth requirement on the ATM switching

fabric; the second investigates the individual link bandwidth requirement of a

base station. The analyses assume that there are N cross point processors

attached to the ATM network, a base station uses point-to-point circuits to deliver

each route update to all the other processors, and each update is carried in an

ATM cell. Thus, a base station transmits (N - 1) cells per route update. Also, the

analyses assume that ATM links use Synchronous Digital Hierarchy (SDH) framing

scheme. Because of the framing overhead, the available bandwidth at the ATM

layer is 149.760 Mb/s on a 155.520 Mb/s link and 599.040 Mb/s on a 622.080

Mb/s link.

3.8 Aggregate Bandwidth Analysis

Suppose that there are M mobiles that change base stations every second.

Because each change results in (N 1) cells transferred through the ATM fabric,

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the aggregate bandwidth (BW) required to support the updates can be

represented using the following equation:

BW = M * (N - 1) *53 *8 bits=second (4.1)

3.9 Link Bandwidth Analysis

Another approach to analyzing the bandwidth requirement takes into account the

maximum number of mobiles a base station can handle. If the wireless LAN

hardware allows a base station to handle at most M mobiles, the base state can

generate at most M*(N-1) route update cells that result from M new mobiles

entering its area. Suppose that all M mobiles migrate to the base station's area

within one second. In this case, Equation 4.1 is still valid in deriving the needed

bandwidth.

3.10 Comparison to Other Approaches (why my proposed approach is the best)

This section compares the cross point approach to the Columbia and the IETF

Mobile IP approaches. Both the Columbia and the Mobile IP approaches are

capable of supporting mobile internetworking in a campus environment, without

requiring modification to the no mobile hosts and IP routers. The cross point

approach takes a step further, without requiring modification or addition to the

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networking software on each mobile. The Columbia approach requires each

mobile host to install an additional software module that processes beaconing

messages from base stations and determines with which base station to

associate. The Mobile IP approach requires each mobile to install an additional

software module that implements the Mobile IP protocol.

Cross point versus the Columbia Approach

Both the cross point and the Columbia approaches focus on building a campus

sized wireless mobile network. Both approaches reserve a single IP network ID for

the mobiles. A mobile host uses the same IP address regardless of which base

station the mobile is using. Base stations cooperate to support seamless mobile

communication as mobiles roam the campus. In essence, both approaches use

protocols to make each mobile appear to be a stationary computer that attaches

to a virtual wireless subnet of the campus internet.

The Columbia approach attaches base stations to the campus internet.

Conceptually, the wireless subnet is embedded in the campus internet. As a

result, base stations can use IP tunnels to transport datagrams that are destined

for remote mobiles. No additional cables or equipment are needed to

interconnect base stations.

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However, embedding the wireless subnet in the campus internet increases the

load on the campus internet. Three types of traffic compete with each other for

the available network resources on the campus internet: the traffic originated

from the no mobile hosts attached to the campus internet, the traffic of the

control messages that are needed to support seamless mobile communication,

and the traffic of the tunneled datagrams. When network congestion occurs, each

type of traffic has equal chance of being discarded by the IP routers on campus.

Cross point takes a different approach: base stations are attached to a high-speed

interconnect, which then connects to the campus internet using cross point

routers. In other words, the cross point network is a parallel network of the

campus internet. The advantage of using a parallel network is that all the traffic to

support seamless mobile communication is concerned within the high-speed

interconnects. The non-mobile hosts on the campus internet are not affected by

the mobility management traffic. However, building a parallel network costs more

because new equipment needs to be purchased, and each base station needs to

be connected to the high-speed interconnect.

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CHAPTER FOUR

4.0 SYSTEM DESIGN

PROTOCOLS AND ROUTING: DESIGN

As described in the previous chapter, multiple base stations are needed to

provide wireless coverage for the entire campus. As a mobile roams the campus,

base stations must cooperate to transfer the ownership of the mobile from one

base station to another.

This chapter describes how cross point processors use protocols to determine the

ownership of a mobile and to transfer the ownership of a mobile from one base

station to the next. The ownership information is the routing information that

cross point processors use to forward datagrams for mobiles. This chapter

describes routing within cross point and protocol design. The next chapter will

describe implementation details.

4.1 Routing

Each cross point processor maintains a routing table for forwarding datagrams.

The table contains one entry for each mobile. A routing entry for a mobile

contains both the ownership and reachability information of the mobile. To

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illustrate how cross point processors cooperate to forward a datagram to a

mobile, consider the example illustrated in Figure 4.1.

In the figure, a cross point processor, P, receives a datagram destined for a

mobile, M. Processor P consults the routing entry that corresponds to M. The

routing entry indicates mobile M is reachable via base station B, so processor P

forwards the datagram across the cross point interconnect to base station B.

When it receives the datagram, base station B retrieves the routing entry for M

and learns that M is

Figure 5.1 Routing a datagram across Crosspoint to a mobile. The routing table on each Crosspoint

processor maintains a routing entry for each mobile host.

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reachable locally; base station B forwards the datagram over the wireless

interface to M.

Processor P is either a base station or a cross point router. If P is a base station,

the sender of the datagram must be another mobile; if P is a cross point router,

the sender must be a host outside cross point. When mobile M answers the

sender with a reply, base station B receives the reply. Since it owns M, base

station B forwards the reply. If the reply is for another mobile, base station B

forwards the reply using its routing table, as described earlier. If the reply is for a

host outside cross point, B immediately forwards the reply to a pre-assigned cross

point router without consulting the routing table.

4.2 Routing Between Two Mobiles

Routing between two mobiles is more complicated. Two mobiles can be in-range

or out-of-range of each other. If two mobiles are in-range of each other, the two

can communicate directly, without the support from base stations. If two mobiles

are out-of-range of each other, base stations must provide routing support to

allow the two mobiles to communicate. Because a mobile assumes direct

communication with the other mobile is possible, base stations must provide

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support to allow two mobiles that are out-of-range of each other to

communicate.

5.2 Protocol Overview

The routing process described in the previous section assumes each Crosspoint

processor has a correct routing entry for each mobile. In a wireless network

environ-

ment where hosts are mobile, routing protocols that are designed for wired

networks

(e.g., Routing Information Protocol (RIP) [Hed88] or Open SPF Protocol (OSPF)

[Moy94]) are inadequate. Thus, Crosspoint uses four protocols to maintain the

rout-

ing table on each Crosspoint processor. The goal of the protocol design is to

ensure

the following invariant:

At any time, exactly one base station handles a mobile host's communi-

cation requests.

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Overlapping areas present a challenge to achieving the design goal. When a

mobile

emits a frame, one or more base stations may receive the frame. Furthermore, a

base

station that receives the frame has no knowledge of which other base stations

have

received the same frame. The initial capture protocol ensures that only one base

captures the mobile when the mobile initiates communication the _rst time. As

the

mobile roams, the hando_ protocol ensures that the ownership of the mobile is

passed

from one base station to the next. The owner of the mobile uses the revalidation

protocol to determine whether the mobile is still reachable. Finally, the recapture

protocol ensures that only one base station captures the mobile when the mobile

was

previously determined unreachable and initiates communication.

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The rest of this chapter describes the design of each of the protocols.

45

5.3 The Hando_ Protocol

A base station uses the hando_ protocol to negotiate and transfer the ownership

of a mobile. As Figure 5.2 illustrates, the protocol follows a request-reply

interaction

between a base station that tries to capture a mobile and the mobile's owner. To

capture the mobile, the non-owner base station sends a hando_ request to the

owner

and awaits a reply. The owner processes the request and responds with either a

positive acknowledgment (i.e., an ACK) or a negative acknowledgment (i.e., a

NAK)1.

An ACK reply permits the non-owner to capture the mobile; a NAK reply denies

the

request2. If anACK arrives, the non-owner captures the mobile and informs the

other

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Crosspoint processors about the ownership change by propagating a routing

update.

handoff ACK/NAK

handoff request

Owner Non-Owner

Time Time

ACK

propagate route update

Figure 5.2 Hando_ protocol interaction between a base station that tries to obtain

the ownership of a mobile and the owner base station of the mobile. The non-

owner

propagates a route update for the mobile when receiving a hando_ ACK message

from the owner.

1The owner base station uses a hando_ algorithm to process the request. Chapter

8 will describe

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the hando_ algorithm in details.

2Chapter 8 will describe how the hando_ protocol handles loss of protocol

messages.

46

5.3.1 Handling Multiple Requests

Because a mobile may stay in an overlapping area, the mobile's owner may

receive

hando_ requests from multiple base stations. Because it does not know in

advance

how many hando_ requests will arrive, the owner does not wait for all requests to

arrive then process them. Instead, the owner processes each request as the

request

arrives. Once it has permitted a base station to capture the mobile, the owner

denies

subsequent hando_ requests to capture the mobile. The owner includes the new

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owner's ID in each hando_ NAK message to inform the requesting base station

that

subsequent hando_ requests should direct to the new owner.

5.3.2 Matching a Request and a Reply

Each hando_ request carries a sequence number that allows the requesting base

station to match a reply. A hando_ reply in response to a hando_ request carries

the

sequence number of the request. A base station only accepts a reply with a

matched

sequence number and from the correct sender.

5.3.3 Reducing the Frequency of Sending Hando_ Requests

If every frame emitted from a mobile causes a hando_ request sent to the

mobile's

owner, the owner may be overwhelmed when the mobile is situated in an

overlapping

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area and emits many frames in a short interval. A base station uses two schemes

to

reduce the frequency of sending a hando_ request.

First, if the signal strength to a mobile is weak (e.g., less than a threshold), a base

station does not send a hando_ request for the mobile, because the probability of

receiving an ACK is low, and even if the base station receives an ACK,

communication

with the mobile may be impossible. We have observed that a base station's

antenna

can be more sensitive than a mobile's. As a result, the situation where a base

station

can receive a frame from a mobile, but the mobile cannot receive a frame from

the

47

base station can occur. In such circumstances, a base station can use the signal

threshold to minimize the e_ect of asymmetric reception.

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Second, a base station imposes a _xed time interval _Thando_ between two suc-

cessive hando_ requests. Thus, the rate at which a base station sends hando_

requests

is bounded.

5.4 The Initial Capture Protocol

When all the Crosspoint processors initialize the _rst time, no processor has a

valid routing table3. When a mobile initiates communication, all the base stations

that detect the mobile must use the initial capture protocol to ensure that only

one

base station captures the mobile.

We describe two designs of the protocol below. In the _rst design, all base

stations

that detect the mobile submit a bid for the mobile to the others. The base station

that has the highest bid captures the mobile. The second design uses the hando_

protocol for the initial capture4.

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5.4.1 Bidding for a Mobile

Conceptually, the bidding process consists of three steps. First, all the base sta-

tions that detect the mobile form a bid. Second, each of the base stations starts a

timer and sends the bid to the other participants of the bidding process. Third, a

participant compares incoming bids with its own bid; if an incoming bid is greater

than the local bid, the participant ceases its attempt to capture the mobile by

cancel-

ing its timer. Eventually, the base station with the highest bid captures the mobile

after its timer expires.

To form a bid, a base station encodes the measured signal strength to the mobile

in the high-order bits and a random number in the low-order bits. Thus, a base

3Chapter 8 will describe the initialization procedure a Crosspoint processor takes

before become

operational.

4The current implementation uses the second design.

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48

station that has a better signal to the mobile has a higher bid, and base stations

that

have the same signal strength to the mobile have equal chance to capture the

mobile.

Finally, base stations compare host IDs to break a tie.

Because a base station that detects the mobile does not know which other base

stations have also detected the mobile, the base station cannot send its bid

directly

to the participants of the bidding process. However, the base station knows that

the

participants must belong to the set of neighboring base stations, whose areas

overlap

with the base station's area. Thus, the base station sends its bid to all the

neighboring

base stations. A base station that is not a participant discards the incoming bids.

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The set of neighboring base stations can be con_gured statically during initial-

ization. A base station can also determine the set dynamically by checking

incoming

hando_ requests. Because a hando_ request is triggered by a frame emitted from

a

mobile, if a base station receives a frame from a mobile followed shortly by a

hando_

request for the mobile (e.g., 10 ms later), the base station can add the sender of

the

request in the set of neighbors.

5.4.2 Using Hando_ for Initial Capture

Alternatively, base stations can use the hando_ protocol for initial capturing a

mobile. The idea is simple: each Crosspoint processor uses a prede_ned formula

to initialize its routing table. The formula ensures that the routing entry of each

mobile is consistent across all Crosspoint processors. In particular, the owner of a

given mobile is set to the same Crosspoint router. Thus, when a mobile initiates

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communication, each of the base stations that detect the mobile will send a

hando_

request to the same Crosspoint router. The Crosspoint router allows the sender of

the _rst hando_ request to capture the mobile.

Unlike the bidding process, the Crosspoint router does not use a timer to wait

for all the hando_ requests to arrive. That is because the Crosspoint router does

not

know a priori how many hando_ requests will arrive. When only one base station

49

detects the mobile, the latency is unnecessary. The owner trades a possible non-

optimal hando_ decision for a reduced latency in hando_ processing.

5.5 The Recapture Protocol

Recall that a routing entry contains both the ownership and reachability informa-

tion about a mobile. If a mobile becomes unreachable, Crosspoint processors

change

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the reachability information in the routing entry for the mobile, but maintain the

ownership information. When a previously unreachable mobile initiates

communica-

tion, each of the base station that detect the mobile looks up the routing entry

that

corresponds to the mobile and sends a hando_ request to the owner of the

mobile.

If it also detects the mobile, the owner captures the mobile and then processes

each

incoming hando_ request. If it does not detect the mobile, the owner permits the

sender of the _rst arriving hando_ request to capture the mobile.

5.6 The Revalidation Protocol

Once a base station has captured a mobile, the base station needs to revalidate

the

mobile periodically for two reasons. First, the mobile may be turned o_ by its

user.

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Without a mechanism for determining the mobile's status, subsequent datagrams

to

the mobile may end up wasting resources. Once it has determined that the

mobile

is no longer reachable, the base station can propagate the information to the

other

Crosspoint processors. For example, when a datagram for the mobile from a

Cross-

point router arrives, the owner can send a control message to inform the router

that

the mobile is no longer reachable. The router then discards subsequent

datagrams

that are destined for the mobile, thereby limiting the use of resource on useless

tra_c.

Second, the mobilemay not emit a frame when it roams into the area of a new

base

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station. Consequently, the new base station cannot detect the mobile.

Meanwhile,

datagrams are being forwarded to the mobile's owner, which no longer has a

wireless

link to the mobile. Note that the mobile may emit frames (e.g., ACKs) if it were

50

able to receive the datagrams forwarded by the owner. And, the emitted frames

are exactly what the new base station needed to capture the mobile. When such

a

situation occurs, the owner needs a way to detect the mobile's absence and

request

the assistance of the other base stations to locate the mobile.

5.6.1 The Two Stages of Revalidation

A base station uses the revalidation protocol to determine whether a mobile that

it

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owns is still reachable. The owner base station initiates the protocol when it

receives

a datagram destined for the mobile, and the mobile has not emitted a frame for a

prede_ned period. The owner uses ICMP echo requests [Pos81b] to elicit a

response

from the mobile. A response from the mobile indicates the mobile is still

reachable.

The revalidation protocol consists of two stages. Each stage corresponds to a

search range. The _rst stage corresponds to a a local search. The owner starts a

timer and continues to forward datagrams for the mobile. If the mobile emits a

frame

(e.g., a datagram in response to the received datagrams), the base station infers

that

the mobile is still reachable and stops the search by canceling the timer. When

the

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timer expires, the base station sends an ICMP echo request to the mobile and

starts

another timer. After sending a _xed number of ICMP echo requests without

receiving

a reply, the base station proceeds to the second stage.

The second stage corresponds to a neighborhood search. The owner requests the

assistance of the neighboring base stations to locate the mobile. As in the _rst

stage,

the owner starts a timer to send an echo request. Unlike the _rst stage, after the

timer expires, the owner sends an echo request to the mobile and a control

message

to the neighboring base stations. Upon receiving the control message, each of the

neighboring base stations sends an echo request to the mobile. A neighbor that

receives an echo reply from the mobile will send a hando_ request to the owner,

indicating the mobile is still reachable. The owner keeps trying until either the

mobile

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responds or the number of retries exceeds a threshold. In the later case, the

owner

declares the mobile unreachable.

51

5.6.2 Discussion

The two stages of revalidation reect the owner's inability to distinguish the fol-

lowing three cases: 1) the mobile is active but does not emit a frame; 2) the

mobile

is active but no longer in-range of its owner; 3) the mobile has been deactivated.

All

the above cases have the same outcome: the owner does not receive a frame

from the

mobile.

In the _rst case, the mobile is active and still in-range of the owner. The mobile is

likely to respond to either the incoming datagrams or the echo requests. If the

mobile

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is using TCP [Pos81c], incoming datagrams normally arrive in groups (i.e., the

packet

train model described in [Jai86]). Moreover, a TCP receiver normally transmits an

ACK segment for every two data segments received [Bra89]. Thus, the mobile is

likely

to emit a frame in response to the incoming datagrams before the _rst echo

request

is sent.

In the second case, the mobile is active but out-of-range of the owner. The owner

relies on neighboring base stations to detect the mobile. Using the neighboring

base

stations exploits the locality property of movement.

In the third case, the mobile is no longer reachable. The revalidation protocol

will run to the completion. Although resources are consumed to determine the

status

of the mobile, once the mobile is determined unreachable, subsequent datagrams

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destined for the mobile will be discarded and do not trigger another revalidation.

As

long as the mobile does not attract incoming datagrams (e.g., a TCP client [CS93]),

the owner will not invoke revalidation, even if the mobile is unreachable.

5.7 Summary

Crosspoint uses host speci_c routing. Each Crosspoint processor maintains a

routing entry for each mobile. The routing entry that corresponds to a mobile

contains

the ownership and reachability information for the mobile. This chapter describes

four

protocols that Crosspoint processors use to determine the ownership and

reachability

52

information for a mobile. The initial capture protocol ensures that only one base

captures a mobile that initiates communication the _rst time. The hando_

protocol

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ensures that the ownership of a mobile is passed from one base station to the

next.

The revalidation protocol is used by the owner of a mobile to determine whether

the

mobile is still reachable. Finally, the recapture protocol ensures that only one

base

station captures a mobile that was previously determined unreachable and

initiates

communication.

53

6. PROTOCOLS AND ROUTING: IMPLEMENTATION

The previous chapter describes routing concepts and the design of four protocols:

initial capture, hando_, revalidation, and recapture. This chapter focuses on the

implementation details. It uses a _nite state machine [ASU88] model to explain

protocol processing. The model provides a concise and precise description of how

the

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Crosspoint protocol software handles events such as timeouts and incoming

messages,

and hence is well suited for guiding protocol implementation.

6.1 Finite State Machine, Events, and Routing Table

As the name implies, a _nite state machine has a _nite set of states. One of the

states in the set is designated as the initial state. A _nite state machine begins in

the

initial state and makes state transitions in response to input events.

Conceptually, each Crosspoint processor maintains a _nite state machine for each

mobile. Each _nite state machine is augmented with memory. A Crosspoint

processor

uses the memory to store data needed for processing input events. An input

event

is either a message or a timeout. A message event is either a frame emitted from

a mobile, a datagram destined for a mobile, or a protocol message (e.g., a hando_

request). A Crosspoint processor can invoke zero or more protocol actions when

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processing an input event. A protocol action includes emitting a protocol

message,

setting/resetting a timer, or modifying the memory of a state machine.

Each Crosspoint processor uses a table to store routing entries. There is one entry

for each mobile. The memory of a mobile's state machine corresponds to the

routing

entry for the mobile. For e_cient access, the table entries are indexed by mobile

host

ID (i.e., the host ID portion of the IP address of a mobile). To access the routing

54

entry for a mobile given the mobile's IP address, a Crosspoint processor extracts

the

host ID from the IP address, uses the ID as an index to the routing table, and

accesses

the entry in constant time.

6.1.1 The Seven States

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Each state machine has 7 states. Based on reachability information, the 7 states

are categorized into 3 groups:

_ Group 1: Reachable Locally

{ LOCALLY OWNED

{ HANDOFF ACKED

{ REVALIDATE

_ Group 2: Reachable Remotely

{ REMOTELY OWNED

{ HANDOFF REQUESTED

_ Group 3: Unreachable

{ UNREACHABLE LOCAL

{ UNREACHABLE REMOTE

At any time, a mobile's state machine is in one of the 7 states. The routing entry

for the mobile stores the current state information. We briey describe each state

below; later sections will explain the role of each state in detail.

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If a mobile's state1 on a base station is LOCALLY OWNED, the base station is the

owner of the mobile. State HANDOFF ACKED indicates that the owner has

permitted

another base station to capture the mobile by sending a hando_ ACK message.

State

REVALIDATE indicates that the owner is verifying whether the mobile is still

reachable.

1We use the current state of a mobile's state machine to denote the mobile's

state.

55

If a mobile's state on a Crosspoint processor is REMOTELY OWNED, the Crosspoint

processor is not the owner of the mobile. State HANDOFF REQUESTED indicates

that

the processor has sent a hando_ request to the mobile's owner and is waiting for

a

hando_ reply from the owner.

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Finally, if a mobile's state on a Crosspoint processor is UNREACHABLE REMOTE

or UNREACHABLE LOCAL, the Crosspoint processor cannot reach the mobile. State

UNREACHABLE REMOTE indicates that the processor does not own the mobile.

State

UNREACHABLE LOCAL indicates that the processor is the owner of the mobile.

6.1.2 The Owner ID

Each Crosspoint processor has an identi_er (ID). Unrelated to the ID of a mobile,

the ID of a Crosspoint processor is a 16-bit integer that uniquely identi_es the pro-

cessor. To identify the owner of a mobile, the routing entry for the mobile

contains

an owner ID _eld. A Crosspoint processor can use the owner ID _eld to derive the

data channel and the control channel with which to communicate with the owner

of

the mobile.

6.2 Protocol Processing Using Finite State Machines

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We use the state machines illustrated in Figures 6.1 and 6.2 to explain the

protocol

processing on a base station and on a Crosspoint router, respectively. As the

_gures

show, the state machine on a Crosspoint router is much simpler than that on a

base

station. Because it does not have a wireless interface, a Crosspoint router does

not

try to capture a mobile. The state machine on a Crosspoint router moves between

the REMOTELY OWNED state and the UNREACHABLE REMOTE state. In contrast,

the state

machine on a base station can move among all 7 possible states.

Although each base station (or Crosspoint router) uses the same state machine

to process events for all mobiles, each mobile has its own current state. The

current

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state of a mobile determines how a Crosspoint processor processes an input

event for

the mobile. Each input event is tagged with the ID of a mobile. When an event for

56

handoff NAK

handoff req

timeout

echo request

(start timer)

handoff NAK

handoff req

LOCALLY

UNREACHABLE UNREACHABLE

HANDOFF

REQUESTED

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REMOTELY

OWNED OWNED

HANDOFF

ACKED

HANDOFF REMOTELY

ACKED OWNED

REVALIDATE

(owner) (non-owner)

ACKED

* timestamp has not been updated for a preset time period

(1) forwarded if carries IP (2) buffered if carries IP (3) discarded by rate limit (4)

discarded

HANDOFF

handoff req

timeout (> k retries)

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unreachable

frame (1)

bcast route update

route update

frame (3)

frame (4)

datagram*

frame (2)

route update

frame (2)

handoff ACK

handoff req

handoff ACK

frame (1)

handoff req

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handoff ACK

handoff req

handoff NAK

(start timer)

handoff req

handoff ACK

bcast route update

handoff req

handoff NAK

frame (2)

frame (1)

(cancel timer)

LOCAL REMOTE

begin

Figure 6.1 A _nite state machine used to explain the protocol processing on a base

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station. The state machine begins in the UNREACHABLE REMOTE state.

57

UNREACHABLE

REMOTELY

OWNED

(Crosspoint Router)

unreachable

handoff req

handoff ACK

route update

route update

handoff req

handoff NAK

REMOTE

begin

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Figure 6.2 A _nite state machine used to explain the protocol processing on a

Cross-

point router. The state machine begins in the UNREACHABLE REMOTE state.

the mobile occurs, the Crosspoint processor retrieves the routing entry of the

mobile

and processes the event.

6.3 Initial Capture

After system initialization, the ownership of every mobile is unknown. Each Cross-

point processor sets the state of every mobile to UNREACHABLE REMOTE and the

owner

ID of every mobile to a predetermined Crosspoint router. That is, for a given

mobile,

the state and owner ID of the mobile on all Crosspoint processors are identical.

Be-

cause the state of each mobile is set to UNREACHABLE REMOTE, Crosspoint

processors

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discard incoming datagrams destined for mobiles. Crosspoint processors use

frames

emitted from a mobile to detect the presence of the mobile and establish the

correct

routing state for the mobile. To illustrate how Crosspoint processors cooperate to

establish the routing state for a mobile, consider the following example.

58

Assume that a mobile, M, emits a frame, and the frame is received by one or

more nearby base stations. A base station that receives the frame uses the source

IP

address carried in the frame to retrieve routing entry that corresponds to M.

Because

M's state is UNREACHABLE REMOTE, as Figure 6.1 shows, the base station bu_ers

the

frame, sends a hando_ request to the mobile's owner, changes the mobile's state

to

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HANDOFF REQUESTED, and awaits a reply from the owner. While waiting (i.e., in

the

HANDOFF REQUESTED state), the base station bu_ers other IP frames received

from M.

Because M's owner is set to a Crosspoint router, and M's state on the router is set

to UNREACHABLE REMOTE, as Figure 6.2 illustrates, the router answers the _rst

incom-

ing hando_ request with a hando_ ACK and changes M's state to REMOTELY

OWNED.

In the REMOTELY OWNED state, the owner denies subsequent hando_ requests.

That is,

the owner allows the sender of the _rst hando_ request to capture M.

The base station that receives the hando_ ACK reply from the router changes M's

state to LOCALLY OWNED, broadcasts a routing update to the other Crosspoint

proces-

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sors, forwards the bu_ered frames, and starts handling M's communication

requests.

That is, the base station captures M and becomes the owner of M. A Crosspoint

processor that receives the route update changes M's state to REMOTELY OWNED

and

records the new owner's ID in the routing entry. Thus, the routing state for mobile

M is established.

6.4 Hando_

After initial capture, mobile M has an owner base station. As M communicates,

M emits frames. A base station that receives a frame from M uses the state in-

formation about M to process the frame. If M's state is LOCALLY OWNED, the

base

station is the owner of M; the base station forwards the frame. If M's state is

REMOTELY OWNED, the base station sends a hando_ request to M's owner and

changes

M's state to HANDOFF REQUESTED.

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59

The owner of mobileM processes the hando_ request using the hando_

algorithm2.

If the algorithm denies the request, the owner sends a hando_ NAK message back

to

the requesting base station, without changing M's state. If the algorithm accepts

the

request, the owner answers the request with a hando_ ACK message and changes

M's

state to HANDOFF ACKED. In the HANDOFF ACKED state, the owner denies

subsequent

hando_ requests and awaits a route update message from the new owner.

When the hando_ ACK message arrives, the new owner changes M's state to

LOCALLY OWNED and broadcasts a route update message for M. When it receives

the

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route update message, the previous owner changes M's state from HANDOFF

ACKED to

REMOTELY OWNED, completing the ownership transfer.

6.4.1 Limiting Hando_ Request Rate

As described in section 5.3.3 of the previous chapter, a base station imposes a

minimum delay _Thando_ between two successive hando_ requests. To

implement

the policy, a base station maintains a timestamp, Tnext hando_ , for each mobile.

When

a base station sends a hando_ request for a mobile, the base station adds

_Thando_ to

the time at which the hando_ request is sent, and stores the result in the Tnext

hando_ .

Tnext hando_ indicates the time beyond which the base station can send the next

hando_ request for the mobile.

6.4.2 Datagram Forwarding During Hando_

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Because transferring the ownership of a mobile from one base station to another

requires exchanging protocol messages across a network, the time needed to

complete

the ownership transfer is nonnegligible. Figure 6.3 illustrates the latency needed

to

complete the ownership transfer of a mobile, M.

In the _gure, base station A is the owner of M. Another base station, B, uses

the hando_ protocol to obtain the ownership of M from A. The ownership

transfer

requires three messages exchanged between the two base stations. At time T1,

base

2Chapter 8 will describe the hando_ algorithm.

60

T2

T1 T1

T2

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handoff ACK

route update

datagrams from M buffered

datagrams from M buffered

datagrams for M forwarded to M

datagrams for M forwarded to M

Base Station B

(New Owner)

handoff request

datagrams from M forwarded (for mobile M)

datagrams for M forwarded to M

datagrams from M discarded

datagrams for M forwarded to B

(Owner of M)

Base Station A

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(LOCALLY_OWNED)

(HANDOFF_ACKED) (HANDOFF_REQUESTED)

(HANDOFF_REQUESTED)

(HANDOFF_ACKED) (LOCALLY_OWNED)

T4

T3

(REMOTELY_OWNED) T4 (LOCALLY_OWNED)

datagrams for M forwarded to M

datagrams from M discarded datagrams from M forwarded

datagrams for M forwarded to B

T3

Figure 6.3 Illustration of the latency needed to complete an ownership transfer,

the

handling of datagrams at various time intervals, and the change of states on the

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participating base stations. Vertical lines down the _gure represent increasing

time

and diagonal lines across the middle represent network packet transmission.

station B sends a hando_ request for M to base station A and changes M's state

to HANDOFF REQUESTED. At time T2, A processes the request, answers with a

hando_

ACK, and changes M's state to HANDOFF ACKED. After processing the hando_

ACK,

B broadcasts a route update and changes M's state to LOCALLY OWNED at T3. At

time T4, A receives the route update and changes M's state to REMOTELY

OWNED,

completing the ownership transfer. The latency needed to complete the

ownership

transfer, referred to as hando_ latency, is T4 T1.

While the hando_ processing is taking place, datagrams destined for M as well as

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emitted from M can arrive at both base stations. The next two subsections

describe

how the protocol is designed to avoid packet lost and minimize duplication of

packets

between T1 and T4.

61

6.4.2.1 Handling Datagrams from Mobiles During Hando_

When base station B obtains the ownership of mobile M from base station A, M

must be in-range of B. Furthermore, B must have a better wireless link to M. Two

cases are possible. First, if M is in-range of A, then the signal strength between A

and M must be weaker than that between B and M. Second, M is out-of-range of

A, so A transfers the ownership to B. The two cases are illustrated in Figures 6.4

and 6.5, respectively.

mobile

handoff ACK

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handoff req

M

owner

A B

Figure 6.4 Illustration of a hando_ occurs while mobile M is situated in an overlap-

ping area of two base stations, A and B. Base station A is the owner of M. Base

station B is using the hando_ protocol to obtain the ownership of M from A. B has

a better wireless link to M than A.

In the _rst case, M is in-range of base stations A and B. Thus, during the

ownership transfer, both base stations can receive datagrams emitted from M.

The

state of mobile M determines how each base station processes a datagram

received

from M.

As Figure 6.3 illustrates, between T1 and T3, base station B bu_ers datagrams

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from M because M's state during the interval is HANDOFF REQUESTED. After

receiving

62

mobile

handoff ACK

M

B

owner

A handoff req

Figure 6.5 Illustration of a hando_ occurs while mobile M is away from the area of

its owner, base station A. Base station A allows base station B to capture M

because

M is out-of-range of A.

the hando_ ACK at T3, B delivers the bu_ered datagrams and starts forwarding

datagrams emitted from M. Thus, from T1 to T4, B delivers all the datagrams

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emitted from M, without a loss.

Base station A also receives the datagrams that B receives between T1 and T4.

Between T1 and T2, A forwards datagrams from M because A owns M. Between

T2

and T4, A has allowed B to capture M, so A discards datagrams from M.

The way A and B handle the datagrams received from M between T1 and T2

possible duplicate delivery of datagrams. Because A has already forwarded

datagrams

from M received between T1 and T2, when B delivers the bu_ered datagrams at

T3,

the datagrams received between T1 and T2 are duplicates. The number of

duplicates

is at least one if the frame that triggers the hando_ request carries an IP

datagram3.

The total number of duplicates that can result depends on how often M emits IP

datagrams between T1 and T2. No duplicate results if the frame that trigger the

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3Other than emitting IP frames, a mobile can emit ARP frames as well.

63

hando_ request is not an IP frame, and M does not emit datagrams between T1 to

T2.

If B does not bu_er the datagrams received between T1 and T2, the probability

of duplicating datagrams during hando_ is eliminated. However, as the second

case

illustrates, bu_ering datagrams between T1 and T2 is necessary to avoid datagram

loss.

In the second case, illustrated in Figure 6.5, mobile M is away from its owner's

area (i.e., A's area) while the ownership transfer occurs. Because base station A

cannot receive datagrams emitted from M, base station B must bu_er the

datagrams

received from M between T1 and T2 to avoid loss of datagrams.

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These two cases illustrate that bu_ering in one case can cause possible

duplication

of datagrams, while in the other case can prevent loss of datagrams. Because a

base

station cannot distinguish between the two cases, the base station always bu_ers

datagrams emitted from a mobile when the mobile's state is HANDOFF

REQUESTED.

6.4.2.2 Handling Datagrams Destined for Mobiles During Hando_

Besides handling datagrams from mobile M, base stations A and B also handle

incoming datagrams that are destined for M. Figure 6.3 shows how each base

station

handles a datagram destined for M at various intervals between T1 and T4.

From time T1 to T2, base station A is the owner of M, so A forwards the datagram

directly to M over the wireless interface. Between T2 and T4, A has allowed B to

captureM and is waiting for a route update from B (i.e.,M's state is HANDOFF

ACKED);

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A forwards the datagram to B because B has a better wireless link to M.

From time T1 to T3, base station B is not the owner of M. However, mobile M is

in-range of B, so B forwards the datagram directly to M. Between T3 and T4, B is

the owner of M, so B forwards the datagram directly to M.

64

6.4.3 Avoiding Redundant Redirect Messages During Hando_

Completing the ownership transfer of a mobile requires the new owner of the

mobile to propagate a route update message to the previous owner and the other

Crosspoint processors. Because the route update message takes a _nite amount

of

time to reach a destination Crosspoint processor, a datagram destined for the

mobile

can arrive at the destination processor before the route update arrives. When

such a

datagram arrives, the processor will forward the datagram to the previous owner

of

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the mobile. Forwarding such a datagram can result in a redundant redirect

message

from the previous owner, as the example illustrated in Figure 6.6 explains.

Tb Tc

redirect

redundant information

Ta

incoming datagram

new owner

previous owner

Crosspoint processor P

datagram

route update

Figure 6.6 Illustration of how the relative order in receiving a route update can

cause

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the previous owner to generate a redundant redirect message. The horizontal

lines

from left to right represent increasing time.

In the _gure, a base station (denoted as new owner) captures a mobile and prop-

agates a route update at Ta. The route update reaches the previous owner of the

mobile at Tb and a Crosspoint processor, P, at Tc. On receipt of the route update,

the previous owner and P immediately make an ownership change for the mobile

65

(shown as thick lines). Between Tb and Tc, a datagram destined for the mobile ar-

rives at processor P. P forwards the datagram to the previous owner because P

has

not received the route update yet. When the datagram from P arrives, the

previous

owner forwards the datagram to the new owner and sends a redirect message

back to

P. The redirect message informs P that future datagrams to the mobile should be

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directed to the new owner. The redirect message is redundant because P has

already

learned the ownership change at Tc.

If a point-to-point virtual circuit is used to deliver the route update, the new

owner can propagate the route update in the following order to reduce the

possibility

of generating redundant redirect messages4. First, the update is sent to the

Crosspoint

routers. Second, the update is sent to the other base stations excluding the

previous

owner. Finally, the update is sent to the previous owner.

Because mobile hosts tend to access stationary server computers outside Cross-

point, mobiles are more likely to communicate with hosts outside Crosspoint.

Thus,

datagrams destined for a given mobile are more likely to arrive at a Crosspoint

router

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than at a base station. By propagating a route update _rst to the Crosspoint

routers,

the new owner allows the Crosspoint routers to make the routing change as soon

as

possible, thus reducing the probability of datagrams forwarded by Crosspoint

routers

arriving at the previous owner.

Sending a route update to the previous owner last also helps in reducing

redundant

redirect messages from the previous owner. For example, in Figure 6.7, processor

P

receives the route update earlier than the previous owner. Thus, P can forward

datagrams that arrive between Tb and Tc directly to the new owner. However, if a

datagram arrives between Ta and Tb, P will forward the datagram to the previous

owner. If the previous owner receives the forwarded datagram before Tc, the

previous

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owner forwards the datagram to the new owner without generating a redirect,

because

the previous owner handles the datagram in the HANDOFF ACKED state (see

Figure 6.3).

4If a point-to-multipoint virtual circuit is used, the new owner only delivers a

single copy of a

route update and hence cannot manipulate the order of sending the update.

66

Ta Tb Tc

new owner

previous owner

datagram

incoming datagram

datagram

Crosspoint processor P

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state is HANDOFF_ACKED

route update

route update

Figure 6.7 Avoiding redundant redirect messages by sending route update to the

previous owner last. The horizontal lines from left to right represent increasing

time.

Manipulating the order of propagating a route update cannot eliminate

redundant

redirect messages completely. For example, in Figure 6.7, if the datagram that P

forwards between Ta and Tb arrives at the previous owner after Tc, the previous

owner

will send a redundant redirect message back to P. The probability of generating

such a redirect message decreases as the interval between Ta and Tb decreases

and

the interval between Tb and Tc increases. Allowing the previous owner to receive

a

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route update last decreases the interval between Ta and Tb and increases the

interval

between Tb and Tc, thus reducing the probability of generating redundant

redirect

messages.

6.5 Processing Datagrams Destined for Mobiles

Subsection 6.4.2 has described how a base station handles an incoming datagram

that is destined for a mobile during hando_. This subsection de_nes precisely how

a Crosspoint processor uses a mobile's state to process datagrams destined for

the

mobile.

67

6.5.1 Processing on a Crosspoint Router

Each Crosspoint router has two network interfaces: one attaches to the cam-

pus internet, and the other attaches to the Crosspoint interconnect. A Crosspoint

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router forwards datagrams between the two interfaces. Figure 6.8 illustrates how

a

Crosspoint router processes a datagram destined for a mobile using the state of

the

mobile5.

REMOTELY

OWNED

UNREACHABLE

REMOTE

send redirect to sender if from Crosspoint

send ICMP host unreachable if from campus

(discard datagram)

(forward datagram to owner)

send redirect to sender if from Crosspoint

datagram destined for a mobile

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datagram destined for a mobile

Figure 6.8 Illustration of how a Crosspoint router uses a mobile's state to process

a

datagram destined for the mobile. Redirect is a control message.

In the UNREACHABLE REMOTE state, the router discards the datagram because

the

mobile is unreachable. In addition, if the datagram comes from the campus

internet,

the router sends an ICMP host unreachable [Pos81b] message back to the sender;

if

the datagram comes from the Crosspoint interconnect, the router does not send

an

unreachable control message to the sender because only the owner of the mobile

can

issue an unreachable control message. Instead, the router sends a redirect control

message to the sender to correct the routing entry on the sender.

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5Recall that a mobile's state on a Crosspoint router only has two possible values.

68

In the REMOTELY OWNED state, the router forwards the datagram to the owner

of

the mobile. If the datagram arrives from the Crosspoint interface, the datagram is

a

misrouted datagram, so the router sends a redirect message back to the s