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

Variability of the Mobile Environment

Connectivity• connected• semi-connected (asymmetric)• weakly connected• disconnected

Mobile Device Capability• form factor• Windows based GUI• multimedia• real-time multimedia

Mobility• stationary• nomadic (pedestrian speed)• mobile (vehicular speed)• roaming (mobile across networks)

Source: Helal

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

Mobile Applications - 2

Web access– outdoor Internet access – intelligent travel guide with up-to-date location dependent

information

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

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

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

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

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

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

WAP: Network elements

wireless networkfixed network

WAPproxy

WTAserver

filter/WAPproxyweb

server

filter

PSTN

Internet

Binary WML: binary file format for clients

Binary WML

Binary WML

Binary WML

HTML

HTML

HTML WML

WMLHTML

Source: Schiller

Origin Servers

WAE: Logical model

webserver

other contentserver

Gateway Client

otherWAE

user agents

WMLuser agent

WTAuser agent

encoders&

decoders

encodedrequest

request

encodedresponsewithcontent

responsewithcontent

pushcontent

encodedpushcontent

Source: Schiller

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.)

WML: Example <WML> <CARD> <DO TYPE="ACCEPT"> <GO URL="#card_two"/> </DO> This is a simple first card! On the next you can choose ... </CARD> <CARD NAME="card_two"> ... your favorite pizza: <SELECT KEY="PIZZA"> <OPTION

VALUE=”M”>Margherita</OPTION> <OPTION VALUE=”F”>Funghi</OPTION> <OPTION VALUE=”V”>Vulcano</OPTION> </SELECT> </CARD> </WML>

Source: Schiller

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

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 client design:– 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

Web Proxy in WebExpress

The WebExpress Intercept Model

Source: Helal

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)

File Systems: Coda

Application transparent extensions of client and server– changes in the cache manager of a client

– applications use cache replicates of files

– extensive, transparent hoarding

Consistency– system keeps record of changes; compares files upon reconnection

– if different users have changed the same file, manual reintegration of the file into the system is necessary

– optimistic approach, coarse grained (file size)

mobile client

cacheapplication server

Source: Schiller

File Systems - Coda

Hoarding– user can pre-determine a file list

with priorities– contents of the cache determined by

the list and LRU strategy – explicit pre-fetching possible– periodic updating

Consistency– asynchronous, background

comparison of files– system weighs speed of updating

against minimization of network traffic

Cache misses– function of file size and bandwidth – modeling of user patience: how

long can a user wait for data without an error message?

hoarding

writedisconnected

emulating

disconnection

disconnection

connection

strongconnection

weakconnection

States of a client

Source: Schiller

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: servers broadcast 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

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

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

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 perform selective

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

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

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 write by clients

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 the client is connected

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)

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 transparent process

migration Other like Locus provide for some control

– like external signal or migrate( ) system call

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

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…

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 component

responsible for execution of code)– prepare the cake (execute the service)– where the cake is prepared (site of execution)

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

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

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

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 Gita’s place – bake the cake

Several other variations are also possible

Mobile Agents: Example

Mobile Applications in Industry

Wireless access: phone.com Alerting services: myalert.com Location services: airflash.com Intranet applications: imedeon.com Banking services: macalla.com Mobile agents: tryllian.com ….

wapforum.org palowireless.com