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Services for Mobile Users. Mobility was the requirement of the 90’s, first in communications and then in computing. rapidly growing demand by users many interested players: equipment manufacturers, infrastructure and service providers

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Services for Mobile Users

  • Mobility was the requirement of the 90’s, first in communications and then in computing.

    • rapidly growing demand by users

    • many interested players:

      • equipment manufacturers, infrastructure and service providers

  • Current technology (devices, access) makes mobile computing feasible, but present support for it is limited.

Introduction to Mobile Computing

  • Various definitions of mobile computing:

    • not the same as wireless computing

    • nomadic (orlocation-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.


System Services

Network Services

Transport Medium

The Mobile Computing Stack

Mobile User

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)

Technical Challenges (continued)

  • Other Challenges

    • device design: size and weight, usability

    • energy conservation

    • security, authentication, authorization

    • application development

    • . . .

Recent Research Projects

  • Accommodating mobile host relocations

    • with Carey Williamson, Vineet Chikarmane, Wayne Mackrell

  • Multicast support for mobile hosts

    • with Carey Williamson , Tim Harrison, Wayne Mackrell

  • TCP over wireless links

    • with Venkat Josyula

  • File system support for mobile users

    • with Kevin Froese

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.

Mobile IP: An Emerging Standard

  • Features of Mobile IP:

    • Separates “location” from “address”.

    • No new IP addresses or address formats required.

    • Only “mobile aware” routers and mobile units need new software. Other routers and hosts use current IP.

  • Impact of Mobile IP on users:

    • Can take any computer to any location; routing of communications from correspondents is done “automatically”.

    • Services provided as if at home network.

Mobile IP: How It Works

  • Mobile unit registers with the foreign network upon arrival.

  • Home Agent and Foreign Agent cooperate to deliver IP datagrams to the mobile unit.

    • forwarding caches at both agents

    • IP-in-IP encapsulation

  • Mobile unit deregisters (explicitly or implicitly) upon leaving foreign network.

IP header

To: care-of address

IP header

To: mobile node

datagram payload

Datagram forwarding

  • HA tells local nodes and routers to send MN’s datagrams to it

  • HA intercepts datagrams intended for MN, then encapsulates and forwards them to MN’s care-of address

  • FA receives encapsulated datagrams, then decapsulates them and delivers them to MN

Mobile IP: Routing

Integrating Wireless Access

  • What are the implications of integrating wireless connections into the internetworking fabric?

  • Our focus was TCP, with emphasis on short range connections:

    • tests of functionality and performance by experiment and simulation.

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

Sample Measurement Results

  • Retransmission Time-out Behaviour

The Proxy Model




Sample Simulation Results

  • Impact of proxy on end-to-end throughput

Proxy ON

Proxy OFF

Summary of Findings

  • Design decisions within TCP present problems when applications run over wireless (lossy) links. These problems have a profound impact on end-to-end performance of the application.

  • While proxy solutions cannot affect the loss, they can control TCP’s response to it and thus improve end-to-end performance.

File System Support for Mobile Clients

  • Location-independent computing characterised by

    • disconnection, movement to a new working location, reconnection

    • type and quality of connection (to home file server) varies

  • Mobile users want access to remotely stored files, regardless of current type of connection.

    • this research is focused on maintaining acceptable file access performance across different forms of connection.

  • The ultimate distributed file system

  • File caching at the client is fundamental to any solution.

File Caching for Mobile Computing

  • Goal is to provide effective file system service to mobile clients.

  • Optimistic caching of file replicas at the client is a key to availability.

  • Project considered impact on performance of

    • configuration issues, at the client and on the network

    • cache management strategies

    • demand characteristics

File System Operation

Strongly Connected Operation

  • a high-bandwidth connection is available, over which read and write operations are serviced

  • file caching can improve performance (by reducing latency)

  • the conventional distributed file system

Disconnected Operation(CODA file system)

  • no connection to home file server

    • users optimisticallyhoard replicas of desired files prior to disconnection

    • all file operations processed in the cache

  • read misses are fatal

  • updates to file system are logged at the client

  • upon reconnection, replay of logged events reintegrates changes with home file system

Weakly Connected Operation

  • a low-bandwidth connection is available

  • read misses are no longer fatal

  • asynchronous write backs provide for reintegration of logged changes with home file system, but must share the bandwidth available with reads

  • reads should have priority

Project Objectives

  • To investigate performance issues relating to mobility-aware file caching using trace-driven simulations.

    • configuration parameters:

      • cache unit, cache size, bandwidth available

    • policy parameters:

      • what to write, when to write, read/write scheduling

    • performance measures



Effect of write-back policy

(10 MB cache)

Resource tradeoff

Sample Results

Summary of Findings

  • It is possible to provide quite acceptable remote file service to weakly connected mobile clients, even when very little bandwidth is available.

    • Reads can be serviced in a timely manner.

    • Even very simple write-back policies can provide timely reintegration.

    • Requires only reasonably sized caches at the mobile client.

The Future: Wearable Computers

A whole new meaning to the term “smarty pants”

Concluding Remarks

  • Mobile functionality is available now, but performance remains an issue.

  • What the future holds:

    • Better devices for mobile users.

    • Seamless and transparent mobility.

    • Better mobility infrastructure.

      • Mobile IP everywhere: foreign agent capabilities at conference sites, hotels, airports, ...

      • widespread support for wireless access: base stations on many networks

  • But, there’s still much work to be done to get us there.

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