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communication media

Michinari Ueda

Scho ol of Information Science,

Japan Advanced Institute of Science and Technology

February 15, 1999

Keywords: network,mobile computing,Mobile IP, network architecture, operating

system.

The progress ofcommunication mediaand the miniaturization of computer hardware

realizemobile computing environmentthat we can access services and resources oncom-

puter networks without depending the time and the situation. We can use wired-LAN,

wireless-LAN, ISDN, PHS and so on as a communication medium. And we can keep

a connection to a network carring portable computers and selecting a communication

medium at the place where weare staying. However,evenif wecan keepa physicalcon-

nectiontothenetwork,itisundesirabletointerruptour workand tomustsetupnetwork

congurations.

Many p ortable computers have suspend and resume functions. Using the functions

we can restore the state b efore cutting the power supply. But the same medium before

suspendingcan not always beused. And whenwe use wireless-LAN asa communication

medium, wehavetoassume thataradio wavemaynot reachedtoour machinesforsome

reason or other. In this situation if we do not want to suspend our work, we would

like to keep a connection to a network with p ortable phone and so on. In these cases if

communicationmediahavechangedrunningapplicationscannotkeepconnectionstotheir

partners. Hence, when we switch communication media in existing systems, we always

haveto need to recongurate networksettings,to restart applicationsand torestore the

state b efore disconnection. These situations are inconvenientfor us.

On the other hand there is the systems that let hosts move keeping connections of

network applicationsonthe assumption ofusing a singlecommunicationmedium. These

systems are an implementation of IETF Mobile IP that constructs mobile computing

environment on the network using TCP/IP, original protocols that can let hosts move

seamlessly by making full use of the merit of wireless media, and so on. However, once

Copyright c

1999byMichinariUeda

cation medium are lack of exibility. Hence we need the system that let hosts move

switchingcommunicationmedia,can keep connectionsb etween networkapplicationsand

can corresp ond tovariouscharacteristicsof communicationmedia.

The JAIST Mobile IP system satises these demands and provides the framework to

realize more exible mobile computing environment. The JAIST Mobile IP system can

solovethe following foreprobrems.

tolet hosts movesmoothlykeeping connectionsbetween networkapplications.

toswitch toa suitablecommunication medium automatically

tocontrol timing of switching media

toadapt variouscharacteristicsof communicationmedia

Usual systemus have corresponded to these probrems one by one, but the JAIST

Mobile IP system providesthe framework tosolve them totally. And the JAIST Mobile

IP system is based on IETF Mobile IP, hence it can be built on the existing TCP/IP

networks. It's a large advantage. In this paper, we focus on the fourth probrem "to

adapt various characteristics of communication media". And we discuss the design and

implementationofthemechanismthatcanadaptthechangesofcharacteristics(error-rate,

band-width, delay, and soon) that switchingmedia and hosts movementcould cause.

Various optimizing metho ds suited to some characteristic of communication media

havebeen studied foralong time. But ifwe use thesexed methodsonthe environment

that let communication media switch, their performance can not be guaranteed. And

using their methods may cause performance down depending on the situation. These

optimizing methodsspcializedtoone communication medium work entirely withinusual

systemsthat don'tletmediaswitch. Butitisundesirabletoinsertusual xedoptimizing

method withinthe JAIST MobileIP system becauseit assumesthat one communication

medium switches dinamically to anotherdierentin its characteristics.

In this pap er we propose the mechanism that changes existing optimizing methods

dependingon the situation. The mechanismcontainsmodules specialized tocharacteris-

tics of media. The modules can be changeddinamically ascharacteristics varry and can

improveeciencyof packetforwardingtothe fullonthe mobilecomputingenvironment.

It is easy to add a new module and to modify modules because of common interfaces

provided to modules. And usual methods have often had to change network comp osi-

tion entirely includingprotocols. Butour mechanismdoesoptimizing processesbased on

snoopingpacketstomobilehosts,andrealizesperformanceimprovementwithoutaecting

existing networks.

And toevaluate ourmechanism weimplementsthe JAIST Mobile IP system intothe

Real-Time Machop erating system. Inevaluation webuilda packetscompressingmodule

and a error recoverymodule as test cases of optimizingmodules. A packets compressing

moduleisselectedtocompressforwardingpacketsfromandtomobilehostswhenanarrow

band-widthmediumisused. Aerrorrecoverymo duleisselectedtoretransmitlostpackets

anddemonstratethatourmechanismiseectivetorealizethemobilenetworkthatadapts

characteristicsof communicationmedia.