18-08-2012, 04:42 PM
Services for Mobile Users
Introduction to Mobile Computing
Various definitions of mobile computing:
not the same as wireless computing
nomadic (or location-independent) computing
Our interest is in supporting users who work from multiple locations, and whose means of “connection” to their home system may take different forms at different times.
Emphasis to date has been on functionality, with little attention to performance.
What Mobile Users Want
Seamless mobility
“connect” from any location, at any time
convenience of use (no extra setup, “plug and play”)
same computing environment, same services, consistent interfaces, regardless of location
Mobile users may be willing to sacrifice some performance for mobility, but only some.
Technical Challenges
Networking Challenges
communications issues: protocols (old and new), technologies (old and new)
accommodating host relocations
network services to mobile users (e.g., mobile multicast)
Operating System Challenges
OS support for mobility oriented devices (e.g. intermittently powered hard drives, limited resources)
OS services for mobile clients (e.g., to ensure data availability, data integrity)
Accommodating Mobile Host Relocations
The problem:
IP routing is based on the network component of a host’s IP address, which is bound inextricably with its location.
Moving to a new location means acquiring a new IP address and then informing all “correspondents”.
Roaming must be handled on an ad hoc case-by-case basis (by individual users, system administrators, or both).
Mobile IP aims to provide for seamless relocation by providing services to mobile users as if they were at their home network.
Wireless Computing
Existing wireless technologies (such as infrared, radio or cellular) can be employed for signal propagation
Can provide for tetherless computing
Wireless links are characterized by
higher error rates, more lost packets, longer delays
For wireless links to integrate seamlessly into the internet, TCP must work well over wireless connections since TCP/IP is the basis for many current network applications
TCP in a Wireless Environment
Problems with TCP in a wireless environment:
TCP congestion management: uses loss as congestion indicator
TCP timers: use delays for timeouts and retransmissions
Proposed solution:
sender manages end-to-end packet transmission
a (transparent) proxy looks after loss on the wireless link
caches packets from sender for transmission over wireless link
performs retransmissions of dropped packets
ACKS from receiver flow through to sender
sender retransmissions reduced
TCP semantics preserved