J.L. van den Berg

A feedback fluid queue with two congestion control thresholds

Feedback fluid queues play an important role in modeling congestion control mechanisms for packet networks. In this paper we present and analyze a fluid queue with a feedback-based traffic rate adaptation scheme which uses two thresholds. The higher threshold B1 is used to signal the beginning of congestion while the lower threshold B2 signals the end of congestion. These two parameters together allow to make the trade-off between maximizing throughput performance and minimizing delay. The difference between the two thresholds helps to control the amount of feedback signals sent to the traffic source. In our model the input source can behave like either of two Markov fluid processes. The first applies as long as the upper threshold B1 has not been hit from below. As soon as that happens, the traffic source adapts and switches to the second process, until B2 (smaller than B1) is hit from above. We analyze the model by setting up the Kolmogorov forward equations, then solving the corresponding balance equations using a spectral expansion, and finally identifying sufficient constraints to solve for the unknowns in the solution. In particular, our analysis yields expressions for the stationary distribution of the buffer occupancy, the buffer delay distribution, and the throughput.

Response times in a two-node queueing network with feedback

The study presented in this paper is motivated by the performance analysis of response times in distributed information systems, where transactions are handled by iterative server and database actions. We model system response times as sojourn times in a two-node open queueing network with a processor sharing (PS) node and a first-come-first-served (FCFS) node. External customers arrive at the PS node according to a Poisson process. After departing from the PS node a customer proceeds to the FCFS node with probability p, and with probability 1 - p the customer departs from the system. After a visit to the FCFS node, customers are fed back to the PS node. The service requirements at both nodes are exponentially distributed. The model is a Jackson network, admitting a product-from solution for the joint number of customers at the nodes, immediately leading to a closed-form expression for the mean sojourn times in steady-state. The variance of the sojourn times, however, does not admit an exact expression-the complexity is caused by the possibility of overtaking. In this paper we propose a methodology for deriving simple, explicit and fast-to-evaluate approximations for the variance of the sojourn times. Numerical results demonstrate that the approximations are very accurate in most model instances.

LTE uplink scheduling - flow level analysis

Long Term Evolution (LTE) is a cellular technology foreseen to extend the capacity and improve the performance of current 3G cellular networks. A key mechanism in the LTE traffic handling is the packet scheduler, which is in charge of allocating resources to active flows in both the frequency and time dimension. In this paper we present a performance comparison of three distinct scheduling schemes for LTE uplink with main focus on the impact of flow-level dynamics resulting from the random user behaviour. We apply a combined analytical/simulation approach which enables fast evaluation of flow-level performance measures. The results show that by considering flow-level dynamics we are able to observe performance trends that would otherwise stay hidden if only packet-level analysis is performed.

Dynamic Profit Optimization of Composite Web Services with SLAs

In this paper we investigate sequential decision mechanisms for composite web services. After executing each sub-service within a sequential workflow, decisions are made whether to terminate or continue the execution of the workflow. These decisions are based on observed response times, expected rewards, and typical Service Level Agreement parameters such as costs, penalties, and agreed response-time objectives. We propose a model for the sequential decision-making process within which we explore a couple of decision algorithms. We benchmarked these algorithms against the profit made when executing the workflow without decision-making. We show that algorithm based on backward recursion principle of dynamic programming is optimal with respect to profit. Next, we analyse the structure of erroneous decisions for both algorithms and show that significant profit gains can be obtained by sequential decision making.

Impact of relay station positioning on LTE uplink performance at flow level

Long Term Evolution (LTE) is the latest cellular system that is being standardized by the 3rd Generation Partnership Project (3GPP) and is expected to substantially improve end-user throughput and reduce user plane latency, while at the same time significantly improve user experience with full mobility. To realize these expectations, 3GPP is investigating the relay station (RS) usage in LTE networks, which is an intermediate station placed between a Mobile Station (MS) and evolved Node-Bs (eNode-Bs). The RS positioning is an important factor that severely influences the LTE uplink performance. This paper analyses the impact of RS positioning on the LTE uplink performance at flow level by using numerical and simulation experiments.

Scheduler-dependent inter-cell interference and its impact on LTE uplink performance at flow level

The Long Term Evolution (LTE) cellular technology is expected to extend the capacity and improve the performance of current 3G cellular networks. Among the key mechanisms in LTE responsible for traffic management is the packet scheduler, which handles the allocation of resources to active flows in both the frequency and time dimension. This paper investigates for various scheduling scheme how they affect the inter-cell interference characteristics and how the interference in turn affects the users performance. A special focus in the analysis is on the impact of flow-level dynamics resulting from the random user behaviour. For this we use a hybrid analytical/simulation approach which enables fast evaluation of flow-level performance measures. Most interestingly, our findings show that the scheduling policy significantly affects the inter-cell interference pattern but that the scheduler specific pattern has little impact on the flow-level performance.

Scheduler Dependent Modeling of Inter-cell Interference in UMTS EUL

In this paper we analyze the performance of the UMTS Enhanced Uplink (EUL) in a network scenario, under various packet scheduling schemes. Besides the impact of the intrinsic differences of the scheduling schemes on EUL performance (which we studied in a previous paper for a single cell scenario), we are particularly interested in how the different scheduling schemes influence EUL performance through their impact on the characteristics of the intercell interference. For our analysis we use a hybrid analytical/ simulation approach, originally developed for the single cell situation, and extend it to our multi cell scenario. We show that the mutual influence between neighbouring cells due to inter-cell interference is largely determined by only one or two power iteration steps, which considerably speeds up computations. Our approach takes into account both the packet-level characteristics and the flow-level dynamics due to the random user behaviour. For the considered schedulers we evaluate and compare performance measures such as the mean flow transfer time and throughput.

Sojourn-time approximations for a multi-server processor sharing system with priorities

We study mean sojourn times in a multi-server processor sharing system with two priority classes and with general service-time distributions. For high-priority customers, the mean sojourn time follows directly from classical results on symmetric queues. For low-priority customers, in the absence of exact results, we propose a simple and explicit approximation for the mean sojourn time. Extensive numerical experiments demonstrate that the approximations are highly accurate for a wide range of parameter settings.

A hybrid procedure for efficient link dimensioning

An important task for network operators is to properly dimension the capacity of their links. Often, this is done by simple rules of thumb based on coarse traffic measurements provided, e.g., by SNMP. More accurate estimations of the required link capacity typically require packet-level measurements, which are hard to implement in today’s high-speed networks. The challenge is, therefore, to accurately estimate the traffic statistics needed for estimating the required link capacity with minimal traffic measurement effort. This paper proposes a novel, hybrid procedure for link dimensioning that combines flow-level measurements, minimal efforts on packet captures, and an analytical traffic model. The result is an efficient and robust method to estimate required link capacities. Although the idea of estimating required capacities from flows is not new, the novelty of this paper is that it proposes a complete, efficient and deployable procedure. The proposed procedure has been extensively validated using real-world traffic captures dating from 2011 to 2012. Results show that, with minimal measurement effort, we are able to efficiently estimate the required bandwidth at timescales as low as 1 ms.

Sensitivity Analysis of the Optimal Parameter Settings of an LTE Packet Scheduler

Advanced packet scheduling schemes in 3G/3G+ mobile networks provide one or more parameters to optimise the trade-off between QoS and resource efficiency. In this paper we study the sensitivity of the optimal parameter setting for packet scheduling in LTE radio networks with respect to various traffic and environment aspects. For our investigations we consider a reference packet scheduling algorithm containing elements of proportional fairness and packet urgency to support mixes of real-time and non-real-time traffic. We present extensive simulation results showing the impact of traffic characteristics (like file size distribution, traffic mix) and environment conditions (regarding e.g. multipath fading and shadowing) on the optimal parameter setting. Although, in some cases, efficiency gains of about twenty percent can be achieved by proper tuning of the scheduling parameters, the overall view from our investigations is that a single, robust setting of the parameters can be determined which provides near optimal trade-offs under almost all practically relevant conditions.