XML Messaging for Mobile Devices

In recent years, XML has been widely adopted as a universal format for structured data. A variety of XML-based systems have emerged, most prominently SOAP for Web services, XMPP for instant messaging, and RSS and Atom for content syndication. This popularity is helped by the excellent support for XML processing in many programming languages and by the variety of XML-based technologies for more complex needs of applications. Concurrently with this rise of XML, there has also been a qualitative expansion of the Internet's scope. Namely, mobile devices are becoming capable enough to be full-fledged members of various distributed systems. Such devices are battery-powered, their network connections are based on wireless technologies, and their processing capabilities are typically much lower than those of stationary computers. This dissertation presents work performed to try to reconcile these two developments. XML as a highly redundant text-based format is not obviously suitable for mobile devices that need to avoid extraneous processing and communication. Furthermore, the protocols and systems commonly used in XML messaging are often designed for fixed networks and may make assumptions that do not hold in wireless environments. This work identifies four areas of improvement in XML messaging systems: the programming interfaces to the system itself and to XML processing, the serialization format used for the messages, and the protocol used to transmit the messages. We show a complete system that improves the overall performance of XML messaging through consideration of these areas. The work is centered on actually implementing the proposals in a form usable on real mobile devices. The experimentation is performed on actual devices and real networks using the messaging system implemented as a part of this work. The experimentation is extensive and, due to using several different devices, also provides a glimpse of what the performance of these systems may look like in the future.

Performance of Ajax Applications on Mobile Devices

The Ajax approach has outgrown its origin as shorthand for "Asynchronous JavaScript + XML". Three years after its naming, Ajax has become widely adopted by web applications. Therefore, there exists a growing interest in using those applications with mobile devices. This thesis evaluates the presentational capability and measures the performance of five mobile browsers on the Apple iPhone and Nokia models N95 and N800. Performance is benchmarked through user-experienced response times as measured with a stopwatch. 12 Ajax toolkit examples and 8 production-quality applications are targeted, all except one in their real environments. In total, over 1750 observations are analyzed and included in the appendix. Communication delays are not considered; the network connection type is WLAN. Results indicate that the initial loading time of an Ajax application can often exceed 20 seconds. Content reordering may be used to partially overcome this limitation. Proper testing is the key for success: the selected browsers are capable of presenting Ajax applications if their differing implementations are overcome, perhaps using a suitable toolkit.

Service Composition on a Mobile Phone

With the recent increase in interest in service-oriented architectures (SOA) and Web services, developing applications with the Web services paradigm has become feasible. Web services are self-describing, platform-independent computational elements. New applications can be assembled from a set of previously created Web services, which are composed together to make a service that uses its components to perform a certain task. This is the idea of service composition. To bring service composition to a mobile phone, I have created Interactive Service Composer for mobile phones. With Interactive Service Composer, the user is able to build service compositions on his mobile phone, consisting of Web services or services that are available from the mobile phone itself. The service compositions are reusable and can be saved in the phone's memory. Previously saved compositions can also be used in new compositions. While developing applications for mobile phones has been possible for some time, the usability of the solutions is not the same as when developing for desktop computers. When developing for mobile phones, the developer has to more carefully consider the decisions he is going to make with the program he is developing. With the lack of processing power and memory, the applications cannot function as well as on desktop PCs. On the other hand, this does not remove the appeal of developing applications for mobile devices.

An XML Messaging Service for Mobile Devices

In recent years, XML has been accepted as the format of messages for several applications. Prominent examples include SOAP for Web services, XMPP for instant messaging, and RSS and Atom for content syndication. This XML usage is understandable, as the format itself is a well-accepted standard for structured data, and it has excellent support for many popular programming languages, so inventing an application-specific format no longer seems worth the effort. Simultaneously with this XML's rise to prominence there has been an upsurge in the number and capabilities of various mobile devices. These devices are connected through various wireless technologies to larger networks, and a goal of current research is to integrate them seamlessly into these networks. These two developments seem to be at odds with each other. XML as a fully text-based format takes up more processing power and network bandwidth than binary formats would, whereas the battery-powered nature of mobile devices dictates that energy, both in processing and transmitting, be utilized efficiently. This thesis presents the work we have performed to reconcile these two worlds. We present a message transfer service that we have developed to address what we have identified as the three key issues: XML processing at the application level, a more efficient XML serialization format, and the protocol used to transfer messages. Our presentation includes both a high-level architectural view of the whole message transfer service, as well as detailed descriptions of the three new components. These components consist of an API, and an associated data model, for XML processing designed for messaging applications, a binary serialization format for the data model of the API, and a message transfer protocol providing two-way messaging capability with support for client mobility. We also present relevant performance measurements for the service and its components. As a result of this work, we do not consider XML to be inherently incompatible with mobile devices. As the fixed networking world moves toward XML for interoperable data representation, so should the wireless world also do to provide a better-integrated networking infrastructure. However, the problems that XML adoption has touch all of the higher layers of application programming, so instead of concentrating simply on the serialization format we conclude that improvements need to be made in an integrated fashion in all of these layers.

Community Pharmacies and the Needs of Mobile EU Citizens - A study on Finns living in Spain

Migration within the European Union (EU) has increased since the Union was established. Community pharmacies provide open access to health care services and can be the first, most frequently used or even the only contact with a nation s health care system among mobile community residents. In some of the mass-migration areas in Southern Europe, most of the customers may represent mobile citizens of foreign background. This has not always been taken into consideration in the development of community pharmacy services. Mobile patients have been on the EU's health policy agenda, but they have seldom been mentioned in the context of community pharmacies. In most of the EU member states, governments control the specific legislation concerning community pharmacies and there is no harmonised pharmaceutical policy or consistent minimal standards for community pharmacy services in the EU. The aim of this study was to understand medication use, the role of community pharmacies and the symptom mitigation process of mobile community residents. Finns living in Spain were used as an example to examine how community pharmacies in a EU member state meet the needs of mobile community residents. The data were collected by a survey in 2002 (response rate 53%, n= 533) and by five focus group discussions in 2006 (n=30). A large number (70%) of the respondents had moved to Spain for health reasons and suffered from chronic morbidity. Community pharmacies had an important role in the healthcare of mobile community residents and the respondents were mostly satisfied with these services. However, several medication safety risks related to community pharmacy practices were identified: 1) Availability of prescription medicines without prescription (e.g., antibiotics, sleeping pills, Viagra®, asthma medications, cardiovascular medicines, psoriasis medicines and analgesics); 2) Irrational use of medicines (e.g., 41% of antibiotic users had bought their antibiotics without a prescription, and the most common reasons for antibiotic self-medication were symptomatic common colds and sore throats); 3) Language barriers between patients and pharmacy professionals; 4) Lack of medication counselling; 5) Unqualified pharmacy personnel providing pharmacotherapy. A fifth of the respondents reported experiencing problems during pharmacy visits in Spain, and the lack of a common language was the source of most of these problems. The findings of this study indicate that regulations and their enforcement can play a crucial role in actually assuring the rational and safe use of medicines. These results can be used in the development of pharmaceutical and healthcare policies in the EU. It is important to define consistent minimum standards for community pharmacy services in the EU. Then, the increasing number of mobile community residents could access safe and high quality health care services, including community pharmacy services, in every member state within the EU.

World Wide Web korpuksena kielitieteellisen tutkimusaineiston haku verkkosivuilta

Tämä tutkielma käsittelee World Wide Webin sisältämien verkkosivujen sisältöjen käyttöä korpusmaisesti kielitieteellisenä tutkimusaineistona. World Wide Web sisältää moninkertaisesti enemmän tekstiä kuin suurimmat olemassa olevat perinteiset tekstikorpukset, joten verkkosivuilta voi todennäköisesti löytää paljon esiintymiä sellaisista sanoista ja rakenteista, jotka ovat perinteisissä korpuksissa harvinaisia. Verkkosivuja voidaan käyttää aineistona kahdella eri tavalla: voidaan kerätä satunnainen otos verkkosivuista ja luoda itsenäinen korpus niiden sisällöistä, tai käyttää koko World Wide Webiä korpuksena verkkohakukoneiden kautta. Verkkosivuja on käytetty tutkimusaineistona monilla eri kielitieteen aloilla, kuten leksikograafisessa tutkimuksessa, syntaktisten rakenteiden tutkimuksessa, pedagogisena materiaalina ja vähemmistökielten tutkimuksessa. Verkkosivuilla on perinteisiin korpuksiin verrattuna useita haitallisia ominaisuuksia, jotka pitää ottaa huomioon, kun niitä käytetään aineistona. Kaikki sivut eivät sisällä kelvollista tekstiä, ja sivut ovat usein esimerkiksi HTML-muotoisia, jolloin ne pitää muuttaa helpommin käsiteltävissä olevaan muotoon. Verkkosivut sisältävät enemmän kielellisiä virheitä kuin perinteiset korpukset, ja niiden tekstityypit ja aihepiirit ovat runsaslukuisempia kuin perinteisten korpusten. Aineiston keräämiseen verkkosivuilta tarvitaan tehokkaita ohjelmatyökaluja. Näistä yleisimpiä ovat kaupalliset verkkohakukoneet, joiden kautta on mahdollista päästä nopeasti käsiksi suureen määrään erilaisia sivuja. Näiden lisäksi voidaan käyttää erityisesti kielitieteellisiin tarpeisiin kehitettyjä työkaluja. Tässä tutkielmassa esitellään ohjelmatyökalut WebCorp, WebAsCorpus.org, BootCaT ja Web as Corpus Toolkit, joiden avulla voi hakea aineistoa verkkosivuilta nimenomaan kielitieteellisiin tarkoituksiin.

Sense of Community in Web Environment

The study of social phenomena in the World Wide Web has been rather fragmentary, andthere is no coherent, reseach-based theory about sense of community in Web environment. Sense of community means part of one's self-concept that has to do with perceiving oneself belonging to, and feeling affinity to a certain social grouping. The present study aimed to find evidence for sense of community in Web environment, and specifically find out what the most critical psychological factors of sense of community would be. Based on known characteristics of real life communities and sense of community, and few occational studies of Web-communities, it was hypothesized that the following factors would be the most critical ones and that they could be grouped as prerequisites, facilitators and consequences of sense of community: awareness and social presence (prerequisites), criteria for membership and borders, common purpose, social interaction and reciprocity, norms and conformity, common history (facilitators), trust and accountability (consequences). In addition to critical factors, the present study aimed to find out if this kind of grouping would be valid. Furthermore, the effect of Web-community members' background variables to sense of community was of interest. In order to answer the questions, an online-questionnaire was created and tested. It included propositions that reflect factors that precede, facilitate and follow the sense of community in Web environment. A factor analysis was calculated to find out the critical factors and analyses of variance were calculated to see if the grouping to prerequisites, facilitators and consequences was right and how the background variables would affect the sense of community in Web environment. The results indicated that the psychological structure of sense of community in Web environment could not be presented with critical variables grouped as prerequisites, facilitators and consequences. Most factors did facilitate the sense of community, but based on this data it could not be argued that some of the factors chronologically precedesense of community and some follow it. Instead, the factor analysis revealed that the most critical factors in sense of community in Web environment are 1) reciprocal involvement, 2) basic trust for others, 3) similarity and common purpose of members, and 4) shared history of members. The most influencing background variables were the member's own participation activity (indicated with reading and writing messages) and the phase in membership lifecycle (from visitor to leader). The more the member participated and the further in membership life cycle he was, the more he felt sense of community. There are many descreptions of sense of community, but the present study was one of the first to actually measure the phenomenon in Web environment, and that gained well documented, valid results based on large data, proving that sense of community in Web environment is possible, and clarifying its psychological structure, thus enhancing the understanding of sense of community in Web environment. Keywords: sense of community, Web-community, psychology of the Internet

Ongelmana yhteistyökyvyttömyys? : Teoreettisen ymmärryksen etsintää web-avusteiselle tiedontuottamisyhteistyölle

Tutkimuksessa analysoidaan teoriatasolla web-avusteisen tiedontuottamisyhteistyön kokeilutoiminnassa esiin tulleita ongelmia ja kootaan niiden viitoittamana teoriaperustaa tulevaisuuden työskentelytavalle, tietämyksen web-avusteiselle ryhmäprosessoinnille. Keskiössä on ihminen kognitiivisena tiedonkäsittelijänä ja elinikäisenä oppijana. Organisaatiossa vallitsevat toimintastrategiat, ryhmätyöskentelyn organisointi ja ryhmässä toteutuvat käytännöt muodostavat yksilön toimintaa ohjaavan sosiaalisen toimintaympäristön, joka säätelee yhteistyöprosessin onnistumista. Yhteistyötä tarkastellaan sosiaalisen tiedonkäsittelyn ryhmäilmiönä, jossa käsite yhteistyökyky kyseenalaistuu. Tuotetussa teoriaperustassa ihminen nähdään monitasoisten oppimishaasteiden ristitulessa. Koulutuksessa vallitseva absoluuttinen tietokäsitys on pystyttävä muuttamaan konstruktivistiseksi tietokäsitykseksi: yhteistä tietämystä ryhmänä tuotettaessa yksilöiden tietämys on rakennusaineena. Koulussa saatu sosiaalisen tiedonkäsittelyn malli, itsekeskeisen tiedonkäsittelyn malli, ei sovellu tietämyksen ryhmäprosessointiin. Malliin liittyviä oppeja on poisopittava samalla kun ryhmäjäsenenä on suunniteltava ja yhdessä opeteltava ryhmäkeskeisiä ja web-avusteisia tiedonkäsittelymalleja. Näin yhteistyöprosessissa toteutuu rinnakkain asiaoppimista ja toimintaoppimista. Näitä molempia on tarpeen tukea kehitettäessä uudenlaisia web-avusteisia työskentelymenetelmiä. Osassa I esitellään tutkimuksen kokemusperäisiä virikkeitä. Suhteellisen tietokäsityksen mukaisesti tämä tutkimus on kiinteästi kytkeytynyt tutkijan elinikäiseen oppimiseen ja tutkimus kuvataan tutkijan yksilöllisen tietämyksen prosessointina. Tutkimusaiheen löytymiseen johtaneet käytännön havainnot aloittavat sen takia raportoinnin. Erityisesti koulun sosiaalisen tiedonkäsittelyn organisointi on tarkastelun kohteena. Osassa II hahmotetaan tutkimusnäkökulmaa. Tutkimuskysymykset ovat lähinnä viitoittamassa tietämyksen etsintää. Tutkimusmenetelmä, kokeilutoiminnan analysointi, on saanut vahvasti vaikutteita toimintatutkimuksesta. Tutkimusasetelman perustana on toimintaan kytkeytyvä ihmiskäsitys. Kirjallisuuden avulla etsitään mahdollisia kytkeytymiä ihmisen käyttäytymisen ja sosiaalisen toiminnan organisoinnin välillä. Osa III kuvailee tutkimusaihetta koskevan ymmärryksen prosessointia. Esiymmärrys yhteistyöstä ja yhteistyökäsitteen määrittely mahdollistavat kokeilutoimintaan ryhtymisen. Uusi viestintämahdollisuus (www) nähdään yhteisessä tiedonkäsittelyssä apuvälineenä. Käytännön kokeilujaksot tarjoavat aineksia kognitiivisen tiedonkäsittelyn syvällisten periaatteiden löytymiselle. Inhimillinen toiminta ja sosiaalinen tiedonkäsittely saavat teoriatason analysoinnissa oppimiseen perustuvia tulkintoja. Osassa IV esitellään tutkimuksen tavoitteena ollutta teoriaperustaa tietämyksen web-avusteiselle ryhmäprosessoinnille. Teoriatiivistelmässä keskiöön nousee ihmisen kognitiivinen tiedonkäsittely sekä konstruktivistisen asiaoppimisen että selviytymispainotteisen toimintaoppimisen muodossa. Sosiaalinen toimintaympäristö nähdään osallisena kognitiivisessa tiedonkäsittelyssä ja selityksenä sille, että yhteinen tiedonkäsittely ei onnistu pelkästään tietoverkkoympäristössä vaan tarvitaan myös sosiaalisia tapaamisia. Kokeilutoiminnan opetuksia tuodaan esille yhteistyöprosessin keskeisten tapahtumien tarkastelussa. Tuotettua teoriaperustaa koetellaan tieteen kentällä vertailemalla sitä muiden tutkijoiden julkaisemiin käsityksiin. Tietoverkkoteknologian suhdetta inhimilliseen tiedonkäsittelyyn verrataan muualla saatuihin ja julkaistuihin käsityksiin. Avainsanat: tietointensiivinen yhteistyö, tiedontuottamisyhteistyö, yhteistyöprosessi, yhteistyökyky, yhteistyökyvyttömyys, virtuaalinen yhteistyöorganisaatio, yksilöllinen tietämys, yhteisen ymmärryksen etsintä, yhteinen tietämys, kognitiivinen tiedonkäsittely, asiaoppiminen, toimintaoppiminen, sosiaalinen tiedonkäsittely, sosiaalinen toimintaympäristö, automatisoitunut toimintamalli, yksilökeskeinen toimintamalli, itsekeskeinen toimintamalli, ryhmäkeskeinen toimintamalli, sosiaalisen tiedonkäsittelyn toimintastrategia, työympäristössä oppiminen.

Methicillin Resistance In Staphylococci : Horizontal Transfer of Mobile Genetic Element (SCCmec) Between Staphylococcal Species

Regardless of the existence of antibiotics, infectious diseases are the leading causes of death in the world. Staphylococci cause many infections of varying severity, although they can also exist peacefully in many parts of the human body. Most often Staphylococcus aureus colonises the nose, and that colonisation is considered to be a risk factor for spread of this bacterium. S. aureus is considered to be the most important Staphylococcus species. It poses a challenge to the field of medicine, and one of the most problematic aspects is the drastic increase of the methicillin-resistant S. aureus (MRSA) strains in hospitals and community world-wide, including Finland. In addition, most of the clinical coagulase-negative staphylococcus (CNS) isolates express resistance to methicillin. Methicillin-resistance in S. aureus is caused by the mecA gene that encodes an extra penicillin-binding protein (PBP) 2a. The mecA gene is found in a mobile genomic island called staphylococcal chromosome cassette mec (SCCmec). The SCCmec consists of the mec gene and cassette chromosome recombinase (ccr)gene complexes. The areas of the SCCmec element outside the ccr and mec complex are known as the junkyard J regions. So far, eight types of SCCmec(SCCmec I- SCCmec VIII) and a number of variants have been described. The SCCmec island is an acquired element in S. aureus. Lately, it appears that CNS might be the storage place of the SCCmec that aid the S. aureus by providing it with the resistant elements. The SCCmec is known to exist only in the staphylococci. The aim of the present study was to investigate the horizontal transfer of SCCmec between the S. aureus and CNS. One specific aim was to study whether or not some methicillin-sensitive S. aureus (MSSA) strains are more inclined to receive the SCCmec than others. This was done by comparing the genetic background of clinical MSSA isolates in the health care facilities of the Helsinki and Uusimaa Hospital District in 2001 to the representatives of the epidemic MRSA (EMRSA) genotypes, which have been encountered in Finland during 1992-2004. Majority of the clinical MSSA strains were related to the EMRSA strains. This finding suggests that horizontal transfer of SCCmec from unknown donor(s) to several MSSA background genotypes has occurred in Finland. The molecular characteristics of representative clinical methicillin-resistant S. epidermidis (MRSE) isolates recovered in Finnish hospitals between 1990 and 1998 were also studied, examining their genetic relation to each other and to the internationally recognised MRSE clones as well, so as to ascertain the common traits between the SCCmec elements in MRSE and MRSA. The clinical MRSE strains were genetically related to each other; eleven PFGE types were associated with sequence type ST2 that has been identified world-wide. A single MRSE strain may possess two SCCmec types III and IV, which were recognised among the MRSA strains. Moreover, six months after the onset of an outbreak of MRSA possessing a SCCmec type V in a long-term care facility in Northern Finland (LTCF) in 2003, the SCCmec element of nasally carried methicillin-resistant staphylococci was studied. Among the residents of a LTCF, nasal carriage of MR-CNS was common with extreme diversity of SCCmec types. MRSE was the most prevalent CNS species. Horizontal transfer of SCCmec elements is speculated to be based on the sharing of SCCmec type V between MRSA and MRSE in the same person. Additionally, the SCCmec element of the clinical human S. sciuri isolates was studied. Some of the SCCmec regions were present in S. sciuri and the pls gene was common in it. This finding supports the hypothesis of genetic exchange happening between staphylococcal species. Evaluation of the epidemiology of methicillin-resistant staphylococcal colonisation is necessary in order to understand the apparent emergence of these strains and to develop appropriate control strategies. SCCmec typing is essential for understanding the emergence of MRSA strains from CNS, considering that the MR-CNS may represent the gene pool for the continuous creation of new SCCmec types from which MRSA might originate.

Cross-layer Assisted TCP Algorithms for Vertical Handoff

The ever expanding growth of the wireless access to the Internet in recent years has led to the proliferation of wireless and mobile devices to connect to the Internet. This has created the possibility of mobile devices equipped with multiple radio interfaces to connect to the Internet using any of several wireless access network technologies such as GPRS, WLAN and WiMAX in order to get the connectivity best suited for the application. These access networks are highly heterogeneous and they vary widely in their characteristics such as bandwidth, propagation delay and geographical coverage. The mechanism by which a mobile device switches between these access networks during an ongoing connection is referred to as vertical handoff and it often results in an abrupt and significant change in the access link characteristics. The most common Internet applications such as Web browsing and e-mail make use of the Transmission Control Protocol (TCP) as their transport protocol and the behaviour of TCP depends on the end-to-end path characteristics such as bandwidth and round-trip time (RTT). As the wireless access link is most likely the bottleneck of a TCP end-to-end path, the abrupt changes in the link characteristics due to a vertical handoff may affect TCP behaviour adversely degrading the performance of the application. The focus of this thesis is to study the effect of a vertical handoff on TCP behaviour and to propose algorithms that improve the handoff behaviour of TCP using cross-layer information about the changes in the access link characteristics. We begin this study by identifying the various problems of TCP due to a vertical handoff based on extensive simulation experiments. We use this study as a basis to develop cross-layer assisted TCP algorithms in handoff scenarios involving GPRS and WLAN access networks. We then extend the scope of the study by developing cross-layer assisted TCP algorithms in a broader context applicable to a wide range of bandwidth and delay changes during a handoff. And finally, the algorithms developed here are shown to be easily extendable to the multiple-TCP flow scenario. We evaluate the proposed algorithms by comparison with standard TCP (TCP SACK) and show that the proposed algorithms are effective in improving TCP behavior in vertical handoff involving a wide range of bandwidth and delay of the access networks. Our algorithms are easy to implement in real systems and they involve modifications to the TCP sender algorithm only. The proposed algorithms are conservative in nature and they do not adversely affect the performance of TCP in the absence of cross-layer information.

Middleware for Mobile Sensing Applications in Urban Environments

Sensor networks represent an attractive tool to observe the physical world. Networks of tiny sensors can be used to detect a fire in a forest, to monitor the level of pollution in a river, or to check on the structural integrity of a bridge. Application-specific deployments of static-sensor networks have been widely investigated. Commonly, these networks involve a centralized data-collection point and no sharing of data outside the organization that owns it. Although this approach can accommodate many application scenarios, it significantly deviates from the pervasive computing vision of ubiquitous sensing where user applications seamlessly access anytime, anywhere data produced by sensors embedded in the surroundings. With the ubiquity and ever-increasing capabilities of mobile devices, urban environments can help give substance to the ubiquitous sensing vision through Urbanets, spontaneously created urban networks. Urbanets consist of mobile multi-sensor devices, such as smart phones and vehicular systems, public sensor networks deployed by municipalities, and individual sensors incorporated in buildings, roads, or daily artifacts. My thesis is that "multi-sensor mobile devices can be successfully programmed to become the underpinning elements of an open, infrastructure-less, distributed sensing platform that can bring sensor data out of their traditional close-loop networks into everyday urban applications". Urbanets can support a variety of services ranging from emergency and surveillance to tourist guidance and entertainment. For instance, cars can be used to provide traffic information services to alert drivers to upcoming traffic jams, and phones to provide shopping recommender services to inform users of special offers at the mall. Urbanets cannot be programmed using traditional distributed computing models, which assume underlying networks with functionally homogeneous nodes, stable configurations, and known delays. Conversely, Urbanets have functionally heterogeneous nodes, volatile configurations, and unknown delays. Instead, solutions developed for sensor networks and mobile ad hoc networks can be leveraged to provide novel architectures that address Urbanet-specific requirements, while providing useful abstractions that hide the network complexity from the programmer. This dissertation presents two middleware architectures that can support mobile sensing applications in Urbanets. Contory offers a declarative programming model that views Urbanets as a distributed sensor database and exposes an SQL-like interface to developers. Context-aware Migratory Services provides a client-server paradigm, where services are capable of migrating to different nodes in the network in order to maintain a continuous and semantically correct interaction with clients. Compared to previous approaches to supporting mobile sensing urban applications, our architectures are entirely distributed and do not assume constant availability of Internet connectivity. In addition, they allow on-demand collection of sensor data with the accuracy and at the frequency required by every application. These architectures have been implemented in Java and tested on smart phones. They have proved successful in supporting several prototype applications and experimental results obtained in ad hoc networks of phones have demonstrated their feasibility with reasonable performance in terms of latency, memory, and energy consumption.

TCP Performance in Heterogeneous Wireless Networks

The TCP protocol is used by most Internet applications today, including the recent mobile wireless terminals that use TCP for their World-Wide Web, E-mail and other traffic. The recent wireless network technologies, such as GPRS, are known to cause delay spikes in packet transfer. This causes unnecessary TCP retransmission timeouts. This dissertation proposes a mechanism, Forward RTO-Recovery (F-RTO) for detecting the unnecessary TCP retransmission timeouts and thus allow TCP to take appropriate follow-up actions. We analyze a Linux F-RTO implementation in various network scenarios and investigate different alternatives to the basic algorithm. The second part of this dissertation is focused on quickly adapting the TCP's transmission rate when the underlying link characteristics change suddenly. This can happen, for example, due to vertical hand-offs between GPRS and WLAN wireless technologies. We investigate the Quick-Start algorithm that, in collaboration with the network routers, aims to quickly probe the available bandwidth on a network path, and allow TCP's congestion control algorithms to use that information. By extensive simulations we study the different router algorithms and parameters for Quick-Start, and discuss the challenges Quick-Start faces in the current Internet. We also study the performance of Quick-Start when applied to vertical hand-offs between different wireless link technologies.