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Mobile Applications

Cellular Networks Email/Web/Location services

Intranet application access

Wireless application protocol

Wireless LANs Client-Server adaptations

Disconnected operations

Ad-hoc Networks

Vehicular applications Emergencies

Data broadcasting

Mobile agents

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Variability of the Mobile Environment

Connectivityconnected

semi-connected

(asymmetric)weakly connected

disconnected

Mobile Device Capability

form factorWindows based GUI

multimedia

real-time multimedia

Mobilitystationary

nomadic (pedestrian speed)

mobile (vehicular speed)roaming (mobile across networks)

Source: Helal

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Mobile Applications - 1

Vehicles transmission of news, road condition etc

ad-hoc network with near vehicles to prevent accidents

Emergencies early transmission of patient data to the hospital

ad-hoc network in case of earthquakes, cyclones

military ...

Traveling salesmen

direct access to central customer files

consistent databases for all agents

mobile office

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Mobile Applications - 2

Web access outdoor Internet access

intelligent travel guide with up-to-date location dependentinformation

Information services

push: stock quotes; pull: nearest cash ATM

find nearest printer (Jini services)

Disconnected operations file-system caching for off-line work

mobile agents, e.g., shopping

Entertainment

ad-hoc networks for multi user games

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World Wide Web and Mobility

HTTP/HTML have not been designed for mobile applications/devices

HTTP 1.0 characteristics designed for large bandwidth, low delay

stateless, client/server, request/response communication

connection oriented, one connection per request

TCP 3-way handshake, DNS lookup overheads big protocol headers, uncompressed content transfer

primitive caching (often disabled, dynamic objects)

security problems (using SSL/TLS with proxies)

HTML characteristics designed for computers with high performance, color high-

resolution display, mouse, hard disk

typically, web pages optimized for design, not for communication;

ignore end-system characteristics

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System Support for Mobile WWW

Enhanced browsers client-aware support for mobility

Proxies

Client proxy: pre-fetching, caching, off-line use

Network proxy: adaptive content transformation for connections

Client and network proxy

Enhanced servers

server-aware support for mobility serve the content in multiple ways, depending on client capabilities

New protocols/languages

WAP/WML

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Wireless Application Protocol

wapforum.org: co-founded by Ericsson, Motorola, Nokia, Phone.com

Goals deliver Internet services to mobile devices

independence from wireless network standards

GSM, CDMA IS-95, TDMA IS-136, 3G systems (UMTS, W-CDMA)

Browser

Micro browser, similar to existing web browsers Script language

Similar to Javascript, adapted to mobile devices

Gateway

Transition from wireless to wired world

Server Wap/Origin server, similar to existing web servers

Protocol layers

Transport layer, security layer, session layer etc.

Telephony application interface

Access to telephony functions

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WAP: Reference model and protocols

Bearers (GSM, CDPD, ...)

Security Layer (WTLS)

Session Layer (WSP)

Application Layer (WAE)

Transport Layer (WDP)TCP/IP,

UDP/IP,media

SSL/TLS

HTML, Java

HTTP

Internet WAP

WAE comprises WML (Wireless Markup Language), WML Script, WTAI etc.

Source: Schiller

Transaction Layer (WTP)

additional services

and applications

WCMP

A-SAP

S-SAP

TR-SAP

SEC-SAP

T-SAP

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WAP: Stack Overview

WDP (Wireless Datagram Protocol):

Provides transport layer functions

Based on ideas from UDP

WTLS (Wireless Transport Layer Security):

Provides data integrity, privacy, authentication functions

Based on ideas from TLS/SSL

WTP (Wireless Transaction Protocol):

Provides reliable message transfer mechanisms

Based on ideas from TCP

WSP (Wireless Session Protocol):

Provides HTTP 1.1 functionality

Supports session management, security, etc.

WAE (Wireless Application Environment):

Architecture: application model, browser, gateway, server

WML: XML-Syntax, based on card stacks, variables, ...

WTA: telephone services, such as call control, phone book etc.

Content encoding, optimized for low-bandwidth channels, simple devices

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WAP: Network elements

wireless networkfixed network

WAP

proxy

WTA

server

filter/WAP

proxyweb

server

filter

PSTN

Internet

Binary WML: binary file format for clients

Binary WML

Binary WML

Binary WML

HTML

HTML

HTML WML

WMLHTML

Source: Schiller

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Origin Servers

WAE: Logical model

web

server

other content

server

Gateway Client

other

WAE

user agents

WMLuser agent

WTA

user agent

encoders

&

decoders

encodedrequest

request

encoded

response

with

content

response

with

content

push

content

encoded

push

content

Source: Schiller

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Wireless Markup Language (WML)

Cards and Decks

Document consists of many cards, cards are grouped to decks

Deck is similar to HTML page, unit of content transmission

WML describes only intent of interaction in an abstract manner

Presentation depends on device capabilities

Features text and images

user interaction

navigation

context management WMLScript

Provides general scripting capabilities

Validity check of user input, local user interaction

Access to device facilities (phone call, address book etc.)

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WML: Example

This is a simple first card!

On the next you can choose ...

... your favorite pizza:

Margherita

Funghi Vulcano

Source: Schiller

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Application Adaptations for Mobility

System-transparent, application-transparent

the conventional, unaware client/server model

System-aware, application-transparent

the client/proxy/server model the disconnected operation model

System-transparent, application-aware

dynamic client/server model

data broadcasting/caching

System-aware, application-aware

the mobile agent model

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The Client/Proxy/Server Model

Proxy functions as a client to the fixed network server

Proxy functions as a mobility-aware server to mobile client

Proxy may be placed in the mobile host (Coda), or the fixed

network, or both (WebExpress)

Enables thin clientdesign:

Resource-poor mobile computers

Application resides on the server

Keyboard and mouse inputs sent from client to server

Display outputs sent from server to client

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Web Proxy in WebExpress

The WebExpress Intercept Model

Source: Helal

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Disconnected Operations: File Systems

Goals

Efficient/transparent access to shared files within a mobile environment Support for disconnected operations while maintaining data consistency

Standard file systems (e.g., NFS) are very inefficient, almost unusable

Approaches

Replication of data (copying, cloning, caching)

Getting data in advance (hoarding, pre-fetching) Main problem: consistency

Typical mechanisms: strong consistency (via atomic updates)

Invalidation of caches through a server

Cannot be used in mobile environments

Mobile computer may not be connected to network

One solution: weak consistency

Tolerate occasional inconsistencies

Apply conflict resolution strategies subsequently

Use version numbering, time-stamps (content independent)

Use dependency graphs (content dependent)

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File Systems - Coda

Hoarding

user can pre-determine a file listwith priorities

contents of the cache determined bythe list and LRU strategy

explicit pre-fetching possible

periodic updating

Consistency asynchronous, background

comparison of files

system weighs speed of updatingagainst minimization of networktraffic

Cache misses function of file size and bandwidth

modeling of user patience: howlong can a user wait for datawithout an error message?

hoarding

write

disconnected

emulating

disconnection

disconnection

connection

strong

connectionweak

connection

States of a client

Source: Schiller

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Mobile Data Management

Maximize query capacity of servers, minimize energy/query at client asymmetric links (high b/w from server to client; low b/w from client to

server)!

Pull data delivery: clients demand, servers respond

clients request (validate) data by sending uplink messages to server

Push data delivery: serversbroadcast data, clients listen

servers push data (and validation reports) through a broadcast channel,to a

community of clients

data are selected based on profiles and registration in each cell

client energy is saved by needing receive mode only

scales to any number of clients

Push and Pull data dissemination: Sharing the channel

Selective Broadcast: Servers broadcast "hot" information only

On-demand Broadcast: Servers choose the next item based on requests

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Organization of Broadcast Data

Flat: cyclically broadcast the union of the requested data

Skewed(Random):

broadcast different items with different frequencies

goal is that the inter-arrival time between two instances of the same

item matches the clients' needs

A B C

A A B C

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Broadcast Disks

Periodic broadcast of one or more disks using a broadcast channel

Disks can of different sizes and can be broadcast at different speed

Frequency of broadcasting each item depends on its access probability

Disk speed can be changed based on client access pattern

Source: Helal

B C

ADisk1

Disk2

A B A C

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inx inx

inxinx inx

inx

inx inx

inx inx

Indexing on Air

Server dynamically adjusts broadcast hotspot

Clients read the index, enters into doze mode, and then performselective

tuning

Query Time: time taken from point a client issues a query until answer is

received

Listening Time: time spent by client listening to the channel

Source: Helal

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Client Caching in Broadcasting

Data are cached at clients to improve access time Lessen dependency on the server's choice of broadcast priority

Traditionally, clients cache "hottest" data to improve hit ratio

Cache data based on PIX:

Probability of access (P)/Broadcast frequency (X).

Cache data replacement

cost-based is not practical

requires perfect knowledge of access probabilities

comparison of PIX values with all resident pages

Alternative: LIX, LRU with broadcast frequency pages are placed on lists based on their frequency (X)

lists are ordered based on L, the running avg. of interaccess times

page with lowest LIX= L/X is replaced

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Client Cache Invalidation

Why?

Value of data may have changed since caching by client

When? Synchronous: send invalidation reports periodically

Asynchronous: send invalidation information for an item, as soon

as its value changes To whom?

Stateful server: to affected clients

Stateless server: broadcast to everyone

What to send?

invalidation: only which items were updated

propagation: the values of updated items are sent

aggregated information/ materialized views

Another issue

Commit of transactions involving read and writeby clients

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The Mobile Agent Model

Mobile agent receives client request and

Mobile agent moves into fixed network

Mobile agent acts as a client to the server

Mobile agent performs transformations and filtering

Mobile agent returns back to mobile platform, when theclient is connected

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Mobile Code

Definition: Capability to dynamically change the bindings between code

fragments and the location where they are executed

Approaches: (Not a totally new concept)

Remote batch job submission & use of PostScript to control

printers

Distributed OS led to more structured approach

Process Migration

Object Migration (Mobile Objects) Mobile Code Systems (Mobile Agents)

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Process Migration

Transfer of OS process from one m/c to other

Migration mechanisms handle bindings between

process and execution environment (e.g. open fds, env variables)

Provide for load balancing

Most of these facilities provide transparentprocess

migration

Other like Locus provide for some control

like external signal or migrate( ) system call

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Object Migration

Makes possible to move objects among address spaces finer grained mobility with respect to processes

e..g Emerald system : Different granularity levels - small to

complex objects

does not provide complete transparency

COOL (oo extension of Chorus OS) allows total transparent

migration

Process and Object migration address issues when

code and state are moved among hosts of loosely coupled, small

scale distributed systems insufficient when applied to large scale settings

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Mobile Code Systems

Code mobility is exploited on Internet Scale Large scale, heterogeneous hosts, technologies Strong v/s weak mobility

Mobility is location aware

Programming language

provides mechanisms and abstractions that enable shipping/fetching of code to/from nodes

Underlying run-time

supports marshalling, code, check in , security etc

no knowledge of migration policies

Applications

Not just for load balancing

E-commerce, distributed information retrieval, workflow

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Mobile Code Systems: Design

Several Paradigms:

Client Server Remote Evaluation

Code on Demand

Mobile Agent

Example:

Two friends Sita and Gita

interact to make a cake (results of service)

recipe is needed (know-how about service)

also ingredients (movable resources) oven to bake (hard to move resource)

a person to mix ingredients as per recipe (computational componentresponsible for execution of code)

prepare the cake (execute the service)

where the cake is prepared (site of execution)

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Client-Server: CS

Sita would like to have chocolate cake:

she does not know the recipe

she does not have the required ingredients nor an oven

Sita knows that Gita (who likes baking cakes)

knows the recipe

has a well supplied kitchen

Sita calls Gita asking: Can you make me a chocolate cake please?

Gita makes the cake and delivers it back to Sita

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Remote Evaluation: REV

Sita would like to have chocolate cake: she knows the recipe

she does not have the required ingredients nor an oven

Sita knows that Gita (who likes baking cakes) has a well supplied kitchen

does not know the recipe

Sita calls Gita asking: Can you make me a chocolate cake please? Here is the recipe

Gita makes the cake and delivers it back to Sita

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Code on Demand : COD

Sita would like to have chocolate cake: she has the required ingredients and an oven

she does not know the recipe

Sita knows that Gita knows the recipe (and is willing to share it)

Sita calls Gita asking:

Can you tell me the recipe for making chocolate cake please?

Gita tells her the recipe and Sita makes the cake

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Mobile Agent: MA

Sita would like to have chocolate cake: she has the recipe and the required ingredients

she does not have an oven

Sita knows that Gita has an oven (and is willing to share it)

Sita could

prepare the batter

go to Gitas place bake the cake

Several other variations are also possible

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Mobile Agents: Example

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