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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.
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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.
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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.
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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.
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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