PREBUFFERING AS A WAY TO EXCEED THE DATA

TRANSFER SPEED LIMITS IN MOBILE CONTROL SYSTEMS

Ondrej Krejcar

Centre for Applied Cybernetics, VSB Technical University of Ostrava, 17. Listopadu 15, Ostrava, Czech Republic

Keywords: Wi-Fi, 802.11b, PDA, Localization, Framework, Predictive, Buffering.

Abstract: The proliferation of mobile computing devices and local-area wireless networks has fostered a growing

interest in location-aware systems and services. Additionally, the ability to let a mobile device determine its

location in an indoor environment supports the creation of a new range of mobile control system

applications. Main area of interest is in model of radio-frequency based system enhancement for locating

and tracking users of our control system inside buildings. The developed framework described here joins the

concepts of location and user tracking as an extension for new control system. The experimental framework

prototype uses a Wi-Fi network infrastructure to let a mobile device determine its indoor position. User

location is used to data pre-buffering and pushing information from server to user’s PDA. All the server

data are saved as artefacts with their position info in building. These technique allow to exceed the data

transfer speed limits in mobile control systems.

1 INTRODUCTION

The usage of various wireless technologies has

increased dramatically and will continue for the

coming years. This will lead to the rise of new

application domains each with their own specific

features and needs. Also, these new domains will

undoubtedly apply and reuse existing (software)

paradigms, components and applications in

information and control systems. Today, this is

easily recognized in the miniaturized applications on

network-connected PDAs that provide more or less

the same functionality as their desktop application

equivalents. It is very likely that these new mobile

application domains adapt new paradigms that

specifically target the mobile environment. We

believe that an important paradigm is context-

awareness. Context is relevant to the mobile user,

because in a mobile environment the context is often

very dynamic and the user interacts differently with

the applications on his mobile device when the

context is changed. While a desktop machine usually

is in a fixed context, a mobile device goes from

work room, cross the building in company area, to

work in-a-meeting, etc. Context is not limited to the

physical world around the user, but also incorporates

the user’s behaviour, and terminal and network

characteristics. Context-awareness concepts can be

found as basic principles in long-term strategic

research for mobile and wireless systems such as

formulated in (WWRF, 2007). The majority of

context-aware computing to date has been restricted

to location-aware computing for mobile applications

(location-based services). Our focus here is on

position determination in an indoor environment.

Location information is used to determine an actual

user position and his future position. We have

performed a number of experiments with the control

system, focusing on position determination, and are

encouraged by the results.

2 BASIC CONCEPTS

The proliferation of mobile computing devices and

local-area wireless networks has fostered a growing

interest in location-aware systems and services. A

key distinguishing feature of such systems is that the

application information and/or interface presented to

the user is, in general, a function of his physical

location. The granularity of location information

needed could vary from one application to another.

For example, locating a book in a library would

require fine-grained information whereas locating a

nearby room in company buildings area requires

fairly coarse-grained location information. While

much research has been focused on development of

111

Krejcar O. (2008).

PREBUFFERING AS A WAY TO EXCEED THE DATA TRANSFER SPEED LIMITS IN MOBILE CONTROL SYSTEMS.

In Proceedings of the Fifth International Conference on Informatics in Control, Automation and Robotics - RA, pages 111-114

DOI: 10.5220/0001494401110114

 SciTePress

services architectures for location-aware systems,

less attention has been paid to the fundamental and

challenging problem of locating and tracking mobile

users, especially in in-building environments. In RF

area we focus mainly on RF wireless networks in

our research. Our goal is to complement the data

networking capabilities of RF wireless LANs with

accurate user location and tracking capabilities for

user needed data pre-buffering. This property we use

as information ground for extension of mobile

control system to exceed the data transfer speed

limits.

2.1 Data Collection

A key step in the proposed research methodology is

the data collection phase. We record information

about the radio signal as a function of a user’s

location. The signal information is used to construct

and validate models for signal propagation. Among

other information, the WaveLAN NIC makes

available the signal strength (SS), which is reported

in units of dBm. A signal strength of Watts is

equivalent to 10*log10(s/0.001) dBm. For example,

signal strength of 1 Watt is equivalent to 30 dBm.

The WaveLAN driver extracts the SS information

from the WaveLAN firmware each time a broadcast

packet is received. Then the information is make

available to user-level applications via system calls.

It uses the wlconfig utility, which provides a

wrapper around the calls, to extract the signal

information.

2.2 Localization Methodology

The general principle is that if a Wi-Fi-enabled

mobile device is close to such a stationary device –

Access Point (AP), it can “ask” the location

provider’s position by setting up a Wi-Fi connection.

If the mobile device knows the position of the

stationary device, it also knows that its own position

is within a range of this location provider (100

meters app.). Granularity of location can be

improved by triangulation of two or several visible

Wi-Fi APs. The PDA client will support the

application in automatically retrieving location

information from nearby location providers, and in

interacting with the server. Naturally, this principle

can be applied to other wireless technologies. The

application (locator) is now implemented in C#

using the MS Visual Studio .NET 2005 with .NET

compact framework and a special OpenNETCF

library enhancement (OpenNETCF, 2007). Schema

on figure 1 describes a runtime localization process.

The stars points are exactly measured and computed

points of suppose user position. The real track on

figure presents real movement of user during the

time. The exact track mean computed track from

measured Wi-Fi intensity level.

Figure 1: Localization principle - triangulation.

2.3 Predictive Data Push Technology

This part of project is based on a model of location-

aware enhancement, which we used in debug control

system. These info-data are used in developed

framework to increase real dataflow from wireless

access point (server side) to PDA (client side).

Primary dataflow is enlarged by data pre-buffering.

These techniques form the basis for predictive data

push technology (PDPT). PDPT push data from

information server to clients PDA to be on hand

when user comes at desired location. The benefit of

PDPT consists in reduction of time needed to

display desired information requested by a user

command on PDA. Time delay may vary from a few

seconds to number of minutes. It depends on two

aspects. First one is the quality of wireless Wi-Fi

connection used by client PDA. A theoretic speed of

Wi-Fi connection is max 687 kB/s. However, the

test of transfer rate from server to client’s PDA,

which we have carried out within our Wi-Fi

infrastructure provided the result speed from 43 to

160 KB/s on three various type of PDA (HTC

Roadster, Blueangel and Universal). The second

aspect is the size of copied data. We advice to use

partitioned blocks from original data files or blocks.

2.4 Data Artefact Management

The PDPT Server SQL database manages the

information (for example data about Ethernet

hardware such as Ethernet switch, UTP socket,

CAT5 cable lead, etc.) in the context of their

location in building environment. This context

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information is same as location information about

user track. The PDPT core controls the data which

are copied from server to PDA client by context

information (position info). Each database artefact

must be saved in database along the position info

belongs to. The data artefact manager is used to

manage these information.

Figure 2: System architecture – UML design.

2.5 Framework Design

PDPT framework design is based on most

commonly used server-client architecture. To

process data the server has online connection to the

control system. Data from technology are

continually saved to SQL Server database (Tiffany,

2003) and (Reynolds, 2003). The part of this

database (desired by user location or his demand) is

replicated online to client’s PDA where it is

visualized on the screen. User PDA has location

sensor component which continuously sends to the

framework kernel the information about nearby

AP’s intensity. The kernel processes this information

and makes a decision if and how a part of SQL

Server database will be replicated to client’s SQL

Server CE database. The kernel decisions constitute

the most important part of whole framework because

the kernel must continually compute the position of

the user and track and make a prediction of his

future movement. After doing this prediction the

appropriate data (part of SQL Server database) are

pre-buffered to client’s database for future possible

requirements. The PDPT framework server is

created as Microsoft web services to handle as

bridge between SQL Server and PDPT PDA Clients.

2.6 PDPT Client

For testing and tuning of PDPT Core the PDPT

Client application was created. This client realizes

classical system and extension by PDPT and Locator

module. Figure 3 show classical view of data

presentation form SQL CE database to user (in this

case the image of Ethernet network in company area.

Figure 3: PDPT Client – Windows Mobile application.

3 EXPERIMENTS

We have executed a number of indoor experiments

with the PDPT framework, using the PDPT PDA

application. Wi-Fi access points are placed at

different locations in buildings, where the access

point cells partly overlap. We have used

triangulation principle of AP intensity to get better

granularity. It has been found that the location

determination mechanism selects the access point

that is closest to the mobile user as the best location

provider. This technique partially uses a special

Radius server (Radius, 2007) to realize “roaming”

known in cell networks. Currently, the usability of

the PDPT PDA application is somewhat limited due

to the fact that the device has to be continuously

powered. If not, the Wi-Fi interface and the

application cannot execute the location

determination algorithm, and the PDPT server does

not receive location updates from the PDA client.

3.1 Data Transfer Tests using PDPT

The result of utilization of PDPT framework is

mainly at data transfer speed reducing. The second

test is focused on real usage of developed PDPT

Framework and his main issue at increased data

transfer. At table 1 are summary of eighteen tests

PREBUFFERING AS A WAY TO EXCEED THE DATA TRANSFER SPEED LIMITS IN MOBILE CONTROL

SYSTEMS

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with three type of PDA and three type of data

transfer mode. Each of these eighteen tests is

fivefold reiterated for better accuracy. The data in

table are average values from each iterations.

Table 1: Data transfer tests results.

no Type Mode Data Time Speed

1 Sql CE 257 0.4 643

2 Sql CE 891 0.4 2228

3 Sql 257 5 51

4 Sql 891 13 69

5 PDPT 257 1.1 234

HTC

Blue-

angel

PDPT 891 3.2 278

7 Sql CE 257 0.5 514

8 Sql CE 891 0.5 1782

9 Sql 257 5 51

10 Sql 891 14 64

11 PDPT 257 1.2 214

iPAQ

h4150

PDPT 891 3.7 241

The data mode column has three data transfer mode.

The SQL CE mode represents the data saved at

mobile device memory (SQL Server CE) and the

data transfer time is very high. The second mode

SQL means data which are stored at server (SQL

Server 2005). Primary the data are loaded over

Ethernet / Internet to SQL Server CE of mobile

device and secondary the data are shown to user.

The data transfers time consumption of this method

is generally very high and the waiting time for user

is very large. The third data mode PDPT is

combination of previous two methods. The PDPT

mode has very good results in form of data transfer

acceleration. Realization of this test consists at user

movement from location A to B at different way

direction. Location B was a destination with

requested data which are not contained at SQL CE

buffer in mobile device before test.

4 CONCLUSIONS

The main objective of this paper is in the

enhancement of control system for locating and

tracking of users inside a building. It is possible to

locate and track the users with high degree of

accuracy. In this paper, we have presented the

control system framework that uses and handles

location information and control system

functionality. The indoor location of a mobile user is

obtained through an infrastructure of Wi-Fi access

points. This mechanism measures the quality of the

link of nearby location provider access points to

determine actual user position. User location is used

in the core of server application of PDPT framework

to data pre-buffering and pushing information from

server to user PDA. Data pre-buffering is most

important technique to reduce time from user request

to system response. The experiments show that the

location determination mechanism provides a good

indication of the actual location of the user in most

cases. The median resolution of the system is

approximately five meters. Some inaccuracy does

not influence the way of how the localization is

derived from the Wi-Fi infrastructure. For the PDPT

framework application this was not found to be a big

limitation as it can be found at chapter Experiments.

The experiments also show that the current state of

the basic technology used for the framework (mobile

device hardware, PDA operating system, wireless

network technology) is now at the level of a high

usability of the PDPT application.

ACKNOWLEDGEMENTS

This work was supported by the Ministry of

Education of the Czech Republic under Project

1M0587

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planning and building an 802.11 Network, CMP

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Wigley, A., Roxburgs, P., 2003. ASP.NET applications for

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Tiffany, R., 2003. SQL Server CE Database Development

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Radius, 2007, http://www.ietf.org/, The Internet

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Moore, R., Lopes, J., 1999. Paper templates. In

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WWRF, 2007, http://www.wireless-world-research.org/

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