Domain Knowledge Mobile Applications Part 2

Posted on Dec 6 2007 - 1:19pm by Raj

 Mobility and Routing

Finding a path from a source to destination
Issues

  • Frequent route changes:  amount of data transferred between route changes may be much smaller than traditional networks
  • Route changes related to host movement

 Goal of routing protocols

  • decrease routing-related overhead
  • find short routes
  • find “stable” routes


 

  • Mobile IP would need to modify the previous hand-off procedure to inform the home agent the identity of the new foreign agent
  • Triangular optimization can reduce the routing delay
    route directly to foreign agent, instead of via home agent

Mobility and Transport

  • Transport protocols typically designed for fixed end-systems,  wired networks.
    Issues
  • packet loss due to wireless characteristics
  • packet loss due to mobility
  • TCP assumes congestion if packet dropped
  • acks, retransmissions and performance
  • TCP cannot be changed fundamentally

Mobile TCP:

§

I-TCP segments the connection:

  • no changes to the TCP protocol for hosts connected to the wired Internet
  • optimized TCP protocol for mobile hosts
  • splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no real end-to-end connection any longer
  • hosts in the fixed part of the net do not notice the characteristics of the wireless part

Advantages:

  • no changes in the fixed network necessary
  • transmission errors on the wireless link do not propagate into the fixed network
  • simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and mobile host

Disadvantages:

  • loss of end-to-end semantics
  • higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

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

System-aware, application-aware

  • the mobile agent model

The Client/Proxy/Server Model :

  • Proxy functions as a client to the fixed network server, and as a mobility-aware server to the mobile client
  • Proxy may be placed in the mobile host (Coda), or the fixed network, or both (WebExpress)
  • Enables thin client design for resource-poor  mobile computers

The Mobile Agent Model

  • Mobile agent receives client request and 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 Data Management

  • Pull data delivery: clients request data by sending uplink msgs to server
  • Push data delivery: servers push data (and validation reports) through a broadcast channel,to a community of clients
  • Client caching strategies and cache invalidation algorithms are critical

World Wide Web and Mobility:

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

HTTP Characteristics

–stateless, client/server, request/response

–connection oriented, one connection per request

–primitive caching and security

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

HTTP and Mobility:

§HTTP:

–designed for large bandwidth and low delay

–big protocol headers (stateless, ASCII)

–uncompressed content transfer

–TCP 3-way handshake, DNS lookup overheads

Caching:

–often disabled by information providers

–dynamic objects, customized pages, generated on request via CGI

Security problems
–how to use SSL/TLS together with proxies?

System Support for Mobile WWW:

Enhanced browsers

Client proxy

–pre-fetching, caching, off-line use

Network proxy a

-adaptive content transformation  for connections

Client and network proxy

Enhanced servers

HDML (handheld device markup language)
HDTP (handheld device transport protocol)

WAP – Wireless Application Protocol:

§Forum: wapforum.org

–co-founded by Ericsson, Motorola, Nokia, Unwired Planet

Goals

–deliver Internet services to mobile devices

–independence from wireless network standards

Platforms

–e.g., GSM (900, 1800, 1900), CDMA IS-95, TDMA IS-136, 3rd generation systems (IMT-2000, UMTS, W-CDMA)

WAP Overview:

Browser

–“micro browser”, similar to existing web browsers

Script language

–similar to Java script, adapted to mobile devices

Gateway

–transition from wireless to wired world

Server

–“wap server”, similar to existing web servers

Protocol layers

–transport layer, security layer, session layer etc.

Wireless Markup Language (WML):

§Cards and Decks

–WML document consists of many cards, cards are grouped to decks

–a deck is similar to an 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

Limitations of Mobile Environment:

  • ·Limitations of the Wireless Network
  • ·heterogeneity of fragmented networks
  • ·frequent disconnections
  • ·limited communication bandwidth
  • ·Limitations Imposed by Mobility
  • ·lack of mobility awareness by system/applications
  • ·Limitations of the Mobile Computer
  • ·short battery lifetime
    ·limited capacities

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