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Power Server: Model 5Assigned sub-task queue Unassigned sub-tasks queue Completed sub-tasks Java application program Sub-HTTP m essage Server program Servlet task Load servlet Sub-HTT

186 13 Power Server: Model 5

Assigned sub-task queue

Unassigned sub-tasks queue Completed

sub-tasks Java application program

Sub-HTTP m essage

Server program

Servlet

task Load servlet

Sub-HTTP message and sub-task

Sub-HTTP message and sub-task

Servlet

Figure13.12 Robust power server model

13.6 Comparison with Model 4

The model in this chapter provides the following advantages:

r The logic of the server program is simpler as the communications are handled

by the web server Thus it is easier to develop and debug It will take a shortertime to develop the whole system

r We do not need to worry about the communication port It will be a consuming exercise to check and change (if necessary) the port number if wehave a large number of servers We will have problems under the followingconditions:

Other Improvements 187

13.7 Further Tests

Five models have been introduced (from Chapters 9 to 12) Each model has its ownadvantages and disadvantages I recommend you to carry out tests with different

models using the same size of problem (e.g., add 1 to 1,000,000 with different

models and configurations) so you can compare the efficiencies of them

13.8 Other Improvements

I have tried to keep the size of all programs as small as possible (in terms ofnumber of coding lines) so they are easy to understand If you develop missioncritical applications in the future, you should consider incorporating some of thefollowing improvements:

r More error-checking routines should be included

r The sub-task queues and answers should be written to disk for applications which

take a long time to complete (e.g., months or even years) If the client computer

crashes for any reason, it can recover from the disk and resume the process

r It is possible that a server computer will crash after it accepts a sub-task from aclient The client computer will wait forever and the job will never be completed.For a time-consuming task, the client should be able to detect this problem Ifthe client does not receive the answer of a sub-task after a reasonable period,the sub-task should be assigned to a different server Instead of maintaining onesub-task, three sub-task files should be maintained as in Fig 13.13

On the other hand, mobile phones allow people to transmit more complex nals over far greater distances In addition to voice communication, nowadaysmany mobile phones are able to access the Internet using the Wireless ApplicationProtocol (WAP) (Beaulieu, 2002; Cook, 2001) technology through the services oftelephone companies.

sig-Personal Digital Assistants (PDAs) are small computers, which can fit into apocket or a purse Such devices provide features such as address books, sched-

ules, to-do lists, e-mail, etc Early model PDAs could communicate with regular

size computers via a special cable connection Some of them employed IR portsfor wireless communication Although IR is more convenient than cable, thereare some drawbacks to this type of communication The communication distance

between devices is very short (i.e., usually within a few inches) and the users

need to align the devices carefully Later version of PDAs could communicatewith networked PCs via a dial-up connection PDAs with built-in modems are stillrelatively new In order to meet the increasing demand for easier communicationbetween mobile devices and wired PCs, there are many new wireless communi-cations standards

All these standards are different, and they all have security weaknesses ever, two of them are beginning to gain wide acceptance among mobile de-vice vendors They are the IEEE802.11b and Bluetooth technologies Many newPDAs and mobile phones come with built-in IEEE802.11b and/or Bluetoothcapabilities

How-188

commu-The standard specifies the communication between wireless client and the cess points It also defines the optional use of encryption However, there are somewell-known security weaknesses in this standard (Park et al., 1998), and the max-imum speed is only 2 Mbps IEEE workgroups have been developing many newIEEE802.11 standards to overcome these problems, and they differentiate thesenewer standards by appending a letter behind the 802.11 Although there are alarge number of new standards, only three standards have products in the marketnow They are IEEE802.11a, IEEE802.11b and IEEE802.11g.

ac-The most popular standard is the IEEE802.11b ac-The data transfer rate is improved

to 11 Mbps and the range to 100 meters while still using the 2.4 GHz radio band.Access points and network cards with this standard are inexpensive Devices fromdifferent vendors are extremely compatible with each other Installation is easyand there is wide-spread use in small offices and homes

On the other hand, IEEE802.11a offers much higher speed (up to 54 Mbps) Ituses the 5 GHz radio band so it avoids interference from electronic devices (such as

microwave ovens, cordless phones, etc.) However, this radio band is not available

for unlicensed use in many countries Another problem is that IEEE802.11a vices are not backward compatible to the slower IEEE802.11b This problem may

de-be barrier for organizations, which already have IEEE802.11b devices to deploy802.11a

IEEE802.11g provides a higher speed (up to 54 Mbps), and it is backwardcompatible to 802.11b Organizations do not need to replace existing 802.11bdevices when they add new 802.11g technologies The first product is available inJanuary 2003, and it still uses the 2.4 GHz radio band

14.3 Bluetooth

Bluetooth is named after the King Harald Bluetooth who united Denmark andNorway The Bluetooth specification is a low-cost, low-power, wireless solution,which covers 10 m The maximum speed is 700 Kbps Bluetooth uses the 2.4 GHzradio frequency range, and it avoids interference from other electronic devicesusing frequency-hopping technologies

This technology can be used to replace the inconvenient IR It is easy to nize devices and transfer files The inexpensive Bluetooth module can be built intoprinters, fax machines and other peripherals It is not a competitor of IEEE802.11

synchro-190 14 Wireless P2P System

standards as the low-power Bluetooth can operate only within a relatively shortdistance It simply enables users to eliminate or minimize the number of cablesbetween computers and peripherals Devices with Bluetooth technologies can dis-cover and talk to each other easily (Lamm et al., 2001)

14.4 Wireless LAN

Wireless LAN technologies provide the following advantages:

r You can move the computer to any place within the wireless signal coverage.Thus, a wireless network is more flexible in the long run

r You can avoid the cost of installing network cables through walls or under floors

r In many cases, a wireless network is easier and cheaper to extend than a wirednetwork

14.5 Wireless Routers

In addition to wireless applications, many people have installed wireless routers athome Wireless routers became available several years ago, are very easy to installand mostly cost less than $100 They are convenient in the following ways:

r It is quite common that families have several computers, usually in differentrooms The installation of broadband cables to network those computers would

be a time-consuming and expensive task Wireless routers solve this problemeasily

r Users can move their computers easily around the home, as long as they remainwithin the coverage of the wireless signal They do not need to rearrange networkcables For example, a notebook computer can be moved from the bedroom tothe living room with ease at any time, always retaining its Internet connection

14.6 Internet-Enabled Devices

The number of mobile phones has grown significantly in the past few years Thereare 1.2 billion mobile phones, and these ubiquitous devices are changing theways in which we conduct our daily lives Mobile phones are becoming morepowerful, and manufacturers are incorporating more functions into them For ex-ample, many mobile phones have MP3 functions Many can also take photos,play video, send e-mail messages and display and allow the editing of simpledocuments

In addition to voice communication, many mobile phones are able to accessthe Internet using WAP technology The owners of such phones can communicatewith other users who are connected to the Internet via either wireless devices orwired connections, as shown in Fig 14.1

Problems of Mobile Phones 191

User,s

notebook computer

Access point

Internet User,s PDA

User,s

smart phone

Other users with wired environment

Other user,s notebook computer

Access point

Other user,s PDA

Other user,s smart phone

Figure14.1 Internet with mobile devices

14.7 Problems of Mobile Phones

We can consider high-end mobile phones as mobile computers because they have

r a processor,

r memory in the phone,

r a keyboard and

r the ability to connect to the Internet with one of the following services:

However, there are several problems in designing and developing software formobile devices These problems prevent mobile phones from participating witheither clients or servers in our P2P models Now let us discuss these:

r Speed of the CPU The CPU speed on smart phones is much lower than that

in regular PCs However, the owners of these mobile devices operate in realtime, and they expect quick responses It is not possible to run a computationallyintensive application on a mobile phone

r Limited memory Mobile phones have small amounts of memory, but manycomputer packages consume large amounts of memory Hence, software forthese devices must be carefully designed to reduce memory requirements

r Screen size The screen size on mobile phones is much smaller than regularmonitors Developers need to design the screen output carefully Only essentialinformation should be displayed

r Time-consuming input method Average phones have only 15 to 25 keys whileregular computer keyboards have more than 102 keys The keys on phones are

192 14 Wireless P2P System

much smaller than those on the computers One solution is to use a ‘touch screen’

as on the Sony Ericsson P910 However, this technology is still expensive, soonly top model smart phones use it Typing data into a phone is always unpleasantand time consuming To make an application popular, designers need to reducethe amount of input needed

r Lack of standards We do not have common standards for mobile phones Theyhave different numbers of keys and functions and different capacities such asscreen size They also have different kinds of operating systems, such as Sym-

bian, Windows CE, Palm, etc Different manufacturers support different kinds

of development tools It is difficult to develop software that can be implemented

on all platforms

r Lack of standard Java support Although many high-end mobile phones supportJava, some do not As mobile phones have only a small amount of memory, Javaprograms for normal computers are usually too large and must be modified Aspecial development kit, J2ME, must be used to compile the programs

14.8 Extending the Power of Mobile Phones

Fortunately, there is a standard for connecting mobile phones to the Internet: WAP.WAP1.0 uses the wireless markup language (WML), but WAP2.0 can support bothextensible markup language (XML) and html pages A mobile phone accesses aWAP file by the following process (Fig 14.2):

1 The mobile phone sends a GPRS signal

2 The computer of the telephone company transforms the GPRS signal to a HTTPmessage and sends it to a remote server

WAP files

Server computer Internet

14.2 Accessing the Internet with WAP

Wireless P2P Systems with Mobile Phones 193

e l v r e S m a r g r p

Web server

t s e q r P T H e s n p s e R

GPRS signal

e s n p s e R

t s e q r P T H

e s n p s e R

Internet

s k s a T

Figure14.3 Invoking servlet with mobile phone

3 The server retrieves the corresponding WAP file and sends it back to the phone company’s computer

tele-4 The telephone company’s computer transforms the WAP file into a telephonesignal and sends it to the phone

It is possible to invoke a servlet with a GPRS signal This ability enables thephone to handle some computationally intensive jobs Instead of performing cal-culations with the phone’s processor, the phone sends HTTP messages to invokeservlets in a remote computer The servlets perform the calculations, assembleWAP files and send the answers back to the phone The procedures are described

in Fig 14.3

Some computationally intensive jobs can take several hours to complete, but wecannot expect a user to hold a phone for several hours As an alternative, we cansend the answer to the phone with SMS message, as in Fig 14.4 If the answer

is too long to fit on a single SMS message, then a server can store the answer AURL address can then be embedded in the SMS message, and the user can click

on it to access the answer on the server

14.9 Wireless P2P Systems with Mobile Phones

The model in Figs 14.3 and 14.4 can be further modified so that a mobile phonecan access the power of a large P2P system A mobile phone can only be a client(but not a server as it lacks computing power) The new model is presented inFig 14.5 The owner of a mobile phone requests a web page from a remote site bytyping the URL (or using a bookmark in the memory of the phone) After obtainingthe web page, the user can then type in the necessary data and send them to the

phone

GPRS signal

Telephone company,s computer

SMS

Telephone company,s computer

Request to generate SMS

Web server

Tasks

Servlet program

Sub-files

Servlet program

Web server

Servlet program

Server 2

Servlet program

Server 1

Servlet program

Sub-task

Answer o

f sub-task

.

Sub-task

Answer of sub-task

Figure14.5 Wireless model

194

Wireless P2P Systems with Mobile Phones 195

tier 1 web server The tier 1 web server will invoke a servlet that will divide a singletask into many small sub-tasks These sub-tasks will then be stored as queues onthe hard disk of the system

The tier 1 server will send an HTTP message to a tier 2 server, which will theninvoke another servlet The tier 1 server will then transfer a sub-task to the tier

2 server for further computation Note that there are many tier 2 servers in thesystem but only one tier 1 server as shown in Fig 14.5

After the computation, the tier 2 server will send the results to the tier 1 server.The tier 1 server will then send another sub-task to the tier 2 server if the sub-taskqueue is not empty The tier 1 server computer will collect answers of sub-tasksfrom all tier 2 servers

r It is easier to carry a mobile phone than a notebook computer.

r No training is required as almost everyone can use a phone

r It can access computers behind a firewall

r There is no need to buy additional equipment because most users have their ownphones

15.2 Client—Mobile Phone

As discussed in Chapter 14, the memory of a mobile phone is small compared with

a desktop computer Instead of running a program, users download a WAP pagefrom the tier-1 server This page allows the user to type in data for the operation

As it is more difficult to type in data with a mobile phone, the volume of typing isminimized in our model Users need to type in only two items:

r The range of numbers to be added

r The number of sub-tasks

The source codes of the web page are presented in Fig 15.1

15.3 Tier 1 Server Program—phoneServlet.java

The structure of this program is presented in Fig 15.2 There are three methods inthe program:

r Init()

r doGet()

r destroy()

196

Tier 1 Server Program—phoneServlet.java 197