Queueing Networks for Vertical Handover

PhDIt is widely expected that next-generation wireless communication systems will be heterogeneous, integrating a wide variety of wireless access networks. Of particular interest recently is a mix of cellular networks (GSM/GPRS and WCDMA) and wireless local area networks (WLANs) to provide complementary features in terms of coverage, capacity and mobility support. If cellular/ WLAN interworking is to be the basis for a heterogeneous network then the analysis of complex handover traffic rates in the system (especially vertical handover) is one of the most essential issues to be considered. This thesis describes the application of queueing-network theory to the modelling of this heterogeneous wireless overlay system. A network of queues (or queueing network) is a powerful mathematical tool in the performance evaluation of many large-scale engineering systems. It has been used in the modelling of hierarchically structured cellular wireless networks with much success, including queueing network modelling in the study of cellular/ WLAN interworking systems. In the process of queueing network modelling, obtaining the network topology of a system is usually the first step in the construction of a good model, but this topology analysis has never before been used in the handover traffic study in heterogeneous overlay wireless networks. In this thesis, a new topology scheme to facilitate the analysis of handover traffic is proposed. The structural similarity between hierarchical cellular structure and heterogeneous wireless overlay networks is also compared. By replacing the microcells with WLANs in a hierarchical structure, the interworking system is modelled as an open network of Erlang loss systems and with the new topology, the performance measures of blocking probabilities and dropping probabilities can be determined. Both homogeneous and non-homogeneous traffic have been considered, circuit switched and packet-switched. Example scenarios have been used to validate the models, the numerical results showing clear agreement with the known validation scenarios

Modelling and performance evaluation of wireless and mobile communication systems in heterogeneous environments

It is widely expected that next generation wireless communication systems will be heterogeneous, integrating a wide variety of wireless access networks. Of particular interest recently is the integration of cellular networks (GSM, GPRS, UMTS, EDGE and LTE) and wireless local area networks (WLANs) to provide complementary features in terms of coverage, capacity and mobility support. These different networks will work together using vertical handover techniques and hence understanding how well these mechanisms perform is a significant issue. In this thesis, these networks are modelled to yield performance results such as mean queue lengths and blocking probabilities over a range of different conditions. The results are then analysed using network constraints to yield operational graphs based on handover probabilities to different networks. Firstly, individual networks with horizontal handover are analysed using performability techniques. The thesis moves on to look at vertical handovers between cellular networks using pure performance models. Then the integration of cellular networks and WLAN is considered. While analysing these results it became clear that the common models that were being used were subjected to handover hysteresis resulting from feedback loops in the model. A new analytical model was developed which addressed this issue but was shown to be problematic in developing state probabilities for more complicated scenarios. Guard channels analysis, which is normally used to give priority to handover traffic in mobile networks, was employed as a practical solution to the observed handover hysteresis. Overall, using different analytical techniques as well as simulation, the results of this work form an important part in the design and development of future mobile systems

On the design, implementation and experimental evaluation of a novel gateway architecture for the GSM Short Message Service

Congestion and capacity pi'ohIens of the existing mobile communication networks of the late eighties resulted in the demand for a brand new mobile telephony standard. Competition amongst the various existing standards implementation was fierce and lead to the availability of a plethora of incompatible networks and very little hope for the establishment of a global technology geared up to the expectancy of the users. Paging network were widely used as a cheaper alternative to voice enabled networks and were Offering users the ability to receive textual information while on the move. Bridging the gap between the paging world and the mobile communication world was essential. The Global System for Mobile communications was designed as the European answer. The European alternative was promising a feature rich, secure and truly global system with the added ability to handle two-way paging like functionalities with the Short Message Service (SMS). The success of a new service such as SMS relies heavily on its adoption by a majority of users. which in turn is mainly a consequence of the availability of software application gateways. In the mid nineties a Sheffield based software company realising the market needs teamed up with the University's research department to produce the first commercially available gateway architecture for the SMS. The resulting work is described in this thesis. The first part of the analysis examines the architecture of' a (ISM network and the building blocks of the SMS. The technical implementation is described and the fundamental properties examined such as roaming. routing, protocol limitations, usability and interoperahility problems. The specification and design of the gateway architecture is then addressed with an emphasis on character set conversion, routing and queLicing issues. The implementation details are then examined with a description of each of' the modules. The performance of the gateway is examined with the implenientation of a test bed fed by traffic generated by customers. The issues examined were: identification of bottlenecks, protocol efficiencies and an analysis of the chosen queueing model implementation. The second part of the analysis presents the results obtained from the measurements taken for a period of a year. Arm analytical model was formulated to validate the results from the measurements. The comparison revealed the ability of the model to simulate the behaviour of the gateway under medium to heavy loads and highlighted the areas that would be most affected by optimisation. The important factors limiting throughput and quality of service were di scoverecl in the capacity of the connections to network operators and policy chosen for the message queues. An alternative queueing discipline is proposed that would lead to increased fairness offered to the wide variety of applications connected to the gateway and the network operators through a single link of' known capacity. Interactive conversations such as quizzes and gaines based are offered low latency while more bandwidth demanding ones such as mass voting applications benefit from very high throughput. The overall gateway,architecture described was the first one of its kind and consequently each of its module was designed and implemented from scratch. As a result Dialogue (Tommnunicatmons benefited l'i'omn the novelty and head start needed in a fierce competitive market to position itself as a market leader ]it mobile applications and services, competing with global companies such as Logica. Ericcson or Microsoft

Mobile Networks

The growth in the use of mobile networks has come mainly with the third generation systems and voice traffic. With the current third generation and the arrival of the 4G, the number of mobile users in the world will exceed the number of landlines users. Audio and video streaming have had a significant increase, parallel to the requirements of bandwidth and quality of service demanded by those applications. Mobile networks require that the applications and protocols that have worked successfully in fixed networks can be used with the same level of quality in mobile scenarios. Until the third generation of mobile networks, the need to ensure reliable handovers was still an important issue. On the eve of a new generation of access networks (4G) and increased connectivity between networks of different characteristics commonly called hybrid (satellite, ad-hoc, sensors, wired, WIMAX, LAN, etc.), it is necessary to transfer mechanisms of mobility to future generations of networks. In order to achieve this, it is essential to carry out a comprehensive evaluation of the performance of current protocols and the diverse topologies to suit the new mobility conditions