With the mass availability of smartphones, mobile access to e-mail is gaining importance. Over the years, the IMAP protocol has been extended with many features ranging from extensions adding new functionality to those improving efficiency over an unreliable network. This thesis evaluates the available extensions based on their suitability for use in the context of a mobile client. Three new extensions have been developed, each improving the protocol in a distinct way. The thesis also discusses how most of these extensions were implemented in Trojitá, the author's free software open source IMAP e-mail client.

<p>Unlicensed Mobile Access (UMA) provides transparent access to 2G and 3G networks for Mobile Stations over the unlicensed radio interface. Unlicensed radio tehnologies such as Bluetooth or WLAN technology connects the Mobile Station to the fixed IP network of the home or office and delivers high bandwith to the Mobile Station.</p><p>The purpose of this report is to examine if subscribers can feel as secure using UMA as they do when using any of the alternetive mobile technologies that UMA supports. The security evaluation is performed by first investigating the current security level of GSM, GPRS and UMTS and then compares them to the security mechanisms of UMA.</p><p>This evaluation noticed two distinct flaws in UMA that could compromise security. The conclusion is that these flaws do not have a major inpact and UMA does provide a general higher level of security than the 2G and 3G counterparts.</p>

Wireless networks increasingly utilize diverse spectral bands, which exhibit vast differences in transmission range, bandwidth and available airtime. While tremendous efforts have been devoted to enable efficient mobile access of single-band networks and increase their throughput, e.g., via network coding, such single-band solutions are unfortunately oblivious to the diversity and abundance of the available spectral bands. In this thesis, I present and evaluate novel schemes for mobile access and for throughput increase using network coding, schemes that are designed for diverse-band wireless networks, i.e., networks operating in multiple diverse bands. Specifically, I introduce the first scheme designed for mobile clients to evaluate and select both APs and spectral bands in diverse-band networks. The fundamental problem is that the potentially vast number of spectrum and AP options may render scanning prohibitive. Thus, my key technique is for clients to infer the critical metrics of channel quality and available airtime for their current location and bands using limited measurements collected in other bands and at other locations. I evaluate my scheme via experiments and emulations, which are enabled by a four-band testbed that I deploy. A key finding is that under a diverse set of operating conditions, mobile clients can accurately predict their performance without a direct measurement at their current location and spectral bands. Moreover, I introduce the first band selection schemes designed for diverse-band networks exploiting overheard packets to enable network coding. The main problem is that band selections in such networks are challenged by conflicting factors affecting throughput: while the number of overhearing nodes generally increases with decreasing frequency, channel width and spatial reuse unfortunately decrease. Thus, the key technique of the proposed schemes is to jointly incorporate coding gains, channel width and spatial reuse in band selections. I evaluate these schemes via simulations employing a physical-layer model driven by measurements collected using the deployed four-band testbed. An important finding is that the proposed schemes can outperform coding-oblivious spectrum access in terms of throughput, as their band selection enables more coding opportunities. My work has two key implications. First, it can significantly improve throughput performance in networks enabled by today’s unlicensed spectrum and by the billion-dollar industry of white-space networking. Second, I anticipate that this thesis will highly impact future research, as I open new research areas in a domain that has attracted such tremendous commercial and research interest.

Unlicensed Mobile Access (UMA) provides transparent access to 2G and 3G networks for Mobile Stations over the unlicensed radio interface. Unlicensed radio tehnologies such as Bluetooth or WLAN technology connects the Mobile Station to the fixed IP network of the home or office and delivers high bandwith to the Mobile Station. The purpose of this report is to examine if subscribers can feel as secure using UMA as they do when using any of the alternetive mobile technologies that UMA supports. The security evaluation is performed by first investigating the current security level of GSM, GPRS and UMTS and then compares them to the security mechanisms of UMA. This evaluation noticed two distinct flaws in UMA that could compromise security. The conclusion is that these flaws do not have a major inpact and UMA does provide a general higher level of security than the 2G and 3G counterparts.

The lack of interoperability between Pay‐TV service providers and a horizontally integrated business transaction model have compromised the competition in the Pay‐TV market. In addition, the lack of interactivity with customers has resulted in high churn rate and improper security measures have contributed into considerable business loss. These issues are the main cause of high operational costs and subscription fees in the Pay‐TV systems. This paper presents a novel end‐to‐end system architecture for Pay‐TV systems cooperating mobile and broadcasting technologies. It provides a cost‐effective, scalable, dynamic and secure access control mechanism supporting converged services and new business opportunities in Pay‐TV systems. It enhances interactivity, security and potentially reduces customer attrition and operational cost. In this platform, service providers can effectively interact with their customers, personalise their services and adopt appropriate security measures. It breaks up the rigid relationship between a viewer and set‐top box as imposed by traditional conditional access systems, thus, a viewer can fully enjoy his entitlements via an arbitrary set‐top box. Having thoroughly considered state‐of‐the‐art technologies currently being used across the world, the thesis highlights novel use cases and presents the full design and implementation aspects of the system. The design section is enriched by providing possible security structures supported thereby. A business collaboration structure is proposed, followed by a reference model for implementing the system. Finally, the security architectures are analysed to propose the best architecture on the basis of security, complexity and set‐top box production cost criteria.