Summary of the thesis

The aim of the Ph.D. project presented in this thesis, was to propose and analyze a set of solutions to address the problem of quality of service provisioning and management of multimedia sessions composed of diverse multiple flows with different quality of service requirements in incumbent mobile wireless networks. The solutions proposed were based on a novel queuing system that provides customized preferential treatment to the classes of flows according to their different quality of service requirements. The novel queuing system provided a base for development of buffer management schemes for quality of service control and optimization at the air interface bottleneck of the state-of- the-art High-Speed Downlink Packet Access System (HSDPA), a 3.5G mobile system standardized by 3GPP as an enhancement to the widely deployed 3G Universal Mobile Telecommunications Systems (UMTS). The main findings and contributions of the research project can be summarized as follows.

8.1.1 Definition of a novel Time-Space Priority queuing system (TSP)

In most multi-flow queuing situations, the diverse flows possess different characteristics and can usually be classed into real-time class for delay sensitive traffic with partial loss tolerance, and non-real-time class for traffic that is loss sensitive and delay tolerant. In order to jointly optimize both requirements, a priority queuing mechanism that can provide delay prioritization for the real-time class, whilst allowing the non-real-time class to have loss prioritization is essential. In the literature survey (chapter 2), it was

found that most existing priority queuing solutions were based on single priority; i.e. either loss prioritization, in which the queue capacity is dimensioned to allow a higher loss priority class to gain preferential access in order to minimize the loss rate at the expense of the other class(es), or delay prioritization, where the service discipline in the queue allows preferential transmission of one class to minimize its delay/jitter of the priority class at the expense of that of other class(es). Investigation of these priority queuing disciplines in previous research were driven by the needs of the systems at the time; for example in ATM, most priority queuing focused on loss prioritization schemes such as partial buffer sharing (PBS) or pushout schemes because the low transmission delay (relative to queuing delay) eliminated the need for delay prioritization in the queuing.

The proposed Time-Space Priority (TSP) queuing incorporates delay prioritization by attaching time (service) priority to the real-time class; but it also restricts real-time class admission into the queue in order to provide loss prioritization to the non-real-time class which is allowed unlimited access to the queue (i.e. space priority). The real-time class packets are queued ahead of the non-real-time packets, but the threshold restricting their admission enables the loss tolerance of the real-time flow to be exploited to further minimize non-real-time loss and acts also to some extent as a (non-real-time class) starvation mitigation mechanism. Furthermore, the TSP queuing minimizes jitter in the real-time class. In a saturated (full) queue, a displacement policy can allow real-time packets to drop non-real-time packets (up to the real-time class admission threshold limit) from the TSP queue in order to curb excessive real-time packet losses.

In chapter 4, TSP is presented and analyzed by stochastic-analytic models along with validation of the models using discrete event simulation. The analyses provided insight into TSP behaviour under a range of multi-class traffic and queue configurations indicating that through careful selection of the queue configuration (real-time flow admission threshold), TSP queuing can provide optimized joint QoS control of the loss/delay requirements of both classes of flows. Further comparative analyses with the conventional priority queuing schemes showed that TSP queuing combined the advan- tages of high buffer utilization, effectiveness in achieving optimum trade-off between

the real-time and non-real-time flow QoS requirement, as well as simplicity of imple- mentation.

8.1.2 Definition of adaptive QoS control strategy based on buffer threshold optimization engine

The analyses of TSP in chapter 4 allowed a cost function to be derived in such a way that a combined quality of service optimization is achievable to enable QoS control of the classes in a multimedia session. The Weighted Grade of Service (WGoS) cost function derived, takes into account traffic intensities of the real-time and non-real-time class as well as the performance metrics that characterize the QoS of both traffic classes. In the cost function, each of the considered performance metrics are weighted according to their relative importance allowing for service class differentiation. Results of experi- ments provided in chapter 4, showed that for different real-time and non-real-time traffic mixes, the WGoS cost function enabled us to determine the optimum buffer threshold i.e. the value of the TSP threshold that minimizes the WGoS for the given traffic confi- guration.

Based upon the analyses, a strategy for adaptive configuration of the buffer thre- shold by utilizing the TSP model as analytic engine for optimizing the threshold via the WGoS cost function is proposed. This is motivated by the fact that analytical models are well suited as kernels in optimization systems because they allow for fast processing; and also because in a system like HSDPA where the strategy is applicable, changes in traffic arrival rates and highly variable service (transmission) rates will necessitate adaptive configuration of the TSP buffer threshold. The scheme involves measuring the input traffic rates and service rates at the air interface which are then fed into an analytic engine which utilizes the analytic model of the TSP buffer to determine the optimum threshold that minimizes the cost function. Thus, at periodic intervals, or triggered by other criteria, the parameters can be sampled again and a new optimum buffer value is calculated. This allows for a coarse or semi-real-time adaptation of the buffer configura- tion to changing QoS requirements to provide adaptive QoS control.

8.1.3 Definition of flow control algorithm for HSDPA multimedia traffic

From the literature survey presented in chapter 2, it is clear that there is a growing trend towards utilizing buffer management in mobile wireless networks as a means to facilitate effective sharing of bottleneck air interface resources for enhanced end-to-end QoS support. However, existing proposals have not addressed QoS support of multimedia sessions with concurrent real-time and non-real-time classes/flows; whereas one of the specified objectives of 3G systems and beyond is the requirement to support not only traditional voice only or data only services but also multimedia services comprising multiplexed flows in a single user session.

This thesis proposed a buffer management scheme based on TSP queuing system in chapter 6 (Enhanced Time-Space priority, E-TSP buffer management), for HSDPA multimedia sessions with real-time and non-real-time flows. The E-TSP buffer manage- ment scheme incorporates additional flow control mechanism that employs a novel credit-based flow control algorithm. The credit-based flow control algorithm is designed to optimize the queuing at the air interface buffer (Node B) in response to time-varying radio link quality of the mobile station receiving the multimedia traffic, as well as the shared downlink channel load variation. Chapter 6 presented a performance evaluation of E-TSP which showed that the flow control algorithm meets its objectives of efficient utilization of buffer and radio link transmission resources resulting in improved higher layer protocol performance and consequent end-to-end QoS enhancement. Thus, it can be concluded from the investigation in chapter 6 that effective per session buffer man- agement at the air interface of shared channels in a mobile system, can significantly improve end-to-end traffic performance during multimedia sessions.

8.1.4 Definition of dynamic QoS optimization scheme for HSDPA multimedia traffic

In chapter 7, a dynamic buffer management scheme (D-TSP) for HSDPA multimedia session is proposed. D-TSP is based on the idea that the allocation of transmission resources at the air interface can be optimized in real-time by dynamically switching the time (transmission) priority between the real-time and non-real-time flows in the Time-

Space Priority buffer. Given a transmission opportunity allocated to the mobile station with multiplexed real-time and non-real-time flows, D-TSP can ensure that the real-time flow gets just as much bandwidth as it requires to guarantee its QoS requirements, while any unused spare capacity is automatically accorded to the non-real-time flow. That way, the transmission resources allocated to the mobile station is optimized between the two flows whilst also alleviating potential starvation of the non-real-time flow. D-TSP relies on the estimation of the real-time flow delay budget in the base station buffer, which is then used to control the transmission priority switching. Note that due to statistical multiplexing in the HSDPA shared channel and stochastic nature of the service process, the high variability necessitates a real-time optimization of the QoS control between the real-time and non-real-time flows. As such, the adaptive buffer configura- tion provided by the semi-real-time WGoS optimization engine can enable a coarse QoS control which will be fine-tuned by the real-time dynamic scheme, D-TSP. Furthermore, the dynamic priority switching of D-TSP will optimize the performance of the inter-user packet scheduling algorithm employed in allocating the transmission time to the existing mobile stations in the HSDPA cell.

In chapter 7, D-TSP is evaluated via extensive HSDPA simulations and the results demonstrate the effectiveness of the system even when a greedy source constant bit-rate real-time streaming flow is multiplexed with non-real-time data in an end-user multime- dia session.