24-01-2011, 12:38 PM
Sridhar Iyer
K R School of Information Technology
IIT Bombay
Outline
Mobile applications
Wireless networking
Routing in mobile networks
Transport in mobile networks
Application adaptation for mobility
WWW and mobility
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 ...
Mobile Applications - 2
Travelling salesmen
direct access to central customer files
consistent databases for all agents
mobile office
Web access
outdoor Internet access
intelligent travel guide with up-to-datelocation dependent information
Mobile Applications - 3
Location aware services
find services in the local environment, e.g. printer
Information services
push: e.g., stock quotes
pull: e.g., nearest cash ATM
Disconnected operations
mobile agents, e.g., shopping
Entertainment
ad-hoc networks for multi user games
Mobile Applications in Industry
Wireless access: phone.com
Alerting services: myalert.com
Location services: airflash.com
Intranet applications: imedeon.com
Banking services: macalla.com
Web access: wapforum.com
Mobile agents: tryllian.com
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
Effect of Mobility on Protocol Stack
Application
new applications and adaptations
Transport
congestion and flow control
Network
addressing and routing
Link
media access and handoff
Physical
transmission errors and interference
Wireless Networks
Infrastructure-based networks
cellular systems (base station infrastructure)
Ad hoc networks
useful when infrastructure not available, impractical, or expensive
military applications, rescue, home networking
Architecture of the GSM system
GSM is a PLMN (Public Land Mobile Netwk)
Components
MS (mobile station)
BS (base station)
MSC (mobile switching center)
LR (location register)
Subsystems
RSS (radio subsystem): covers all radio aspects
NSS (network and switching subsystem): call forwarding, handover, switching
OSS (operation subsystem): n/w management
Cellular Wireless
Space divided into cells
A base station is responsible to communicate with hosts in its cell
Mobile hosts can change cells while communicating
Hand-off occurs when a mobile host starts communicating via a new base station
Multi-Hop Wireless
May need to traverse multiple links to reach destination
Mobility causes route changes
Hand-Off Procedure
Each base station periodically transmits beacon
Mobile host, on hearing stronger beacon from a new BS, sends it a greeting
changes routing tables to make new BS its default gateway
sends new BS identity of the old BS
New BS acknowledges the greeting and begins to route MH’s packets
Hand-Off Procedure
New BS informs old BS
Old BS changes routing table, to forward any packets for the MH to the new BS
Old BS sends an ack to new BS
New BS sends handoff-completion message to MH
Hand-off Issues
Hand-offs may result in temporary loss of route to MH
with non-overlapping cells, it may be a while before the mobile host receives a beacon from the new BS
While routes are being reestablished during handoff, MH and old BS may attempt to send packets to each other, resulting in loss of packets
Wireless LANs
Infrared (IrDA) or radio links (Wavelan)
Advantages
very flexible within the reception area
Ad-hoc networks possible
(almost) no wiring difficulties
Disadvantages
low bandwidth compared to wired networks (1-10 Mbit/s)
many proprietary solutions
Infrastructure v/s ad-hoc networks (802.11)
Bluetooth
Consortium
Ericsson, Intel, IBM, Nokia, Toshiba - many members
Scenarios
connection of peripheral devices
loudspeaker, joystick, headset
support of ad-hoc networking
small devices, low-cost
bridging of networks
e.g., GSM via mobile phone - Bluetooth - laptop
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
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
Mobile TCP
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
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
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