Gerard Hoekstra

Optimal File Splitting for Wireless Networks with Concurrent Access

The fundamental limits on channel capacity form a barrier to the sustained growth on the use of wireless networks. To cope with this, multi-path communication solutions provide a promising means to improve reliability and boost Quality of Service (QoS) in areas that are covered by a multitude of wireless access networks. Today, little is known about how to effectively exploit this potential. Motivated by this, we consider N parallel communication networks, each of which is modeled as a processor sharing (PS) queue that handles two types of traffic: foreground and background. We consider a foreground traffic stream of files, each of which is split into N fragments according to a fixed splitting rule (*** 1 ,...,*** N ), where *** *** i = 1 and *** i *** 0 is the fraction of the file that is directed to network i . Upon completion of transmission of all fragments of a file, it is re-assembled at the receiving end. The background streams use dedicated networks without being split. We study the sojourn time tail behavior of the foreground traffic. For the case of light foreground traffic and regularly varying foreground file-size distributions, we obtain a reduced-load approximation (RLA) for the sojourn times, similar to that of a single PS-queue. An important implication of the RLA is that the tail-optimal splitting rule is simply to choose *** i proportional to c i *** ρ i , where c i is the capacity of network i and ρ i is the load offered to network i by the corresponding background stream. This result provides a theoretical foundation for the effectiveness of such a simple splitting rule. Extensive simulations demonstrate that this simple rule indeed performs well, not only with respect to the tail asymptotics, but also with respect to the mean sojourn times. The simulations further support our conjecture that the same splitting rule is also tail-optimal for non-light foreground traffic. Finally, we observe near-insensitivity of the mean sojourn times with respect to the file-size distribution.

Effective load for flow-level performance modelling of file transfers in wireless LANs

Today, a wide range of 802.11-based Wireless LANs (WLANs) have become dominant to provide wireless Internet access for file transfers. For engineering purposes, there is a need for very simple, explicit, yet accurate, models that predict the performance of WLANs under anticipated load conditions. In this context, several detailed packet-level models have been proposed, based on fixed-point equations. Despite the fact that these models generally lead to accurate performance predictions, they do not lead to simple explicit expressions for the performance of WLANs. Motivated by this, we propose a new analytic model that captures the highly complex combined dynamics and protocol overhead of the 802.11 MAC, IP, TCP and application-layer into an explicit expression for a single parameter which will be called the effective service time. Based on the effective service time, we define the effective load to describe the flow-level performance of file transfers over WLANs with an M/G/1 Processor Sharing (PS) model. Using the M/G/1 PS model properties we propose a simple analytic model to obtain WLAN AP buffer content distribution. Despite the fact that PS models are heavily used in modelling flow-level performance in communication networks, an extensive validation of such models has not been published in the field, or context, of WLAN. To this end, our model is validated extensively by comparing the model-based average response times against simulations. The results show that the model leads to highly accurate predictions over a wide range of parameter combinations, including light- and heavy-tailed file-size distributions and light- and heavy-load scenarios. The simplicity and accuracy of the model make the results of high practical relevance and useful for performance engineering purposes.

Optimal Job Splitting in Parallel Processor Sharing Queues

The main barrier to the sustained growth of wireless communications is the Shannon limit that applies to the channel capacity. A promising means to realize high-capacity enhancements is the use of multi-path communication solutions to improve reliability and network performance in areas that are covered by a multitude of overlapping wireless access networks. Despite the enormous potential for capacity enhancements offered by multi-path communication techniques, little is known about how to effectively exploit this. Motivated by this, we study a model where jobs are split and downloaded over N multiple parallel networks, each of which is modeled as a processor sharing (PS) queue. Each job is fragmented, according to a fixed splitting rule and re-assembled at the receiving end. The complex correlation structure between the sojourn times at the PS nodes makes an exact detailed mathematical analysis of the model impossible. Therefore, in this article we propose a simple and fast approximation for the splitting rule that minimizes the expected job-download time. Our approximation is validated extensively by simulations. The results show that the outcomes are extremely accurate over a wide range of parameter combinations.

A simple index rule for efficient traffic splitting over parallel wireless networks with partial information

Multi-path communication solutions provide a promising means to improve the network performance in areas covered by multiple wireless access networks. Today, little is known about how to effectively exploit this potential. We study a model where flows are transferred over multiple parallel networks, each of which is modeled as a processor sharing node. The goal is to minimize the expected transfer time of elastic data traffic by smartly dispatching the flows to the networks, based on partial information about the numbers of foreground and background flows in each of the nodes. In the case of full state information, the optimal policy can be derived via standard MDP-techniques, but for models with partial information an optimal solution is hard to obtain. An important requirement is that the splitting algorithm is efficient, yet simple, easy-to-implement, scalable in the number of parallel networks and robust against changes in the parameter settings. We propose a simple index rule for splitting traffic streams based on partial information, and benchmark the results against the optimal solution in the case of full state information. Extensive simulations with real networks show that this method performs extremely well under practical circumstances for a wide range of realistic parameter settings.

On the Processor Sharing Properties of File Transfers in a WLAN Testbed

802.11-based WLAN deployments have become a commodity to provide today’s wireless Internet access. In this paper, we conduct a practical study on the performance of FTP file transfers over real WLAN equipment. To this end, we propose a new analytic model that translates the highly complex dynamics of the FTP/TCP/IP/MAC-stack, and their interactions, into a single parameter, which will be called the effective load. The effective load is used to describe the flow-level behavior of FTP-based file transfers over WLANs without admission control as a Processor-Sharing (PS)-model. Next, despite the fact that PS models are heavily used in modeling flow-level performance in WLAN networks, an extensive validation of such models with real equipment has not been conducted. Motivated by this, we validate in the present paper our analytic model by comparing the model-based response times against the outcomes obtained from a testbed environment. The results show (a) that the obtained mean download times are fairly insensitive to the file-size distribution, as suggested by the PS-model, and (b) that the model leads to accurate predictions over a broad range of parameters combinations, including different file-size distributions and light- and heavy-load scenarios.

On Comparing the Performance of Dynamic Multi-Network Optimizations

With a large variety of wireless access technologies available, multi-homed devices may strongly improve the performance and reliability of communication when using multiple networks simultaneously. A key question for the practical application of multi-path strategies is the granularity at which the traffic streams should be dispersed among the available networks. This level of granularity may be expected to have a major impact on both the efficiency and complexity of practical realizations. Motivated by this, we compare two dynamic strategies that operate at different levels of granularity. The first strategy, which we call network selection, requires little operational complexity and dynamically assigns an arriving application data transfer to the network that delivers the highest expected performance. Our second strategy, which we call traffic-splitting, is of higher complexity and aims to optimally split individual data transfers among the available networks. To this end, we (1) develop quantitative models that describe the performance of both strategies, (2) determine the (near-)optimal algorithms for both strategies, and (3) validate the efficiency and practical usefulness of the algorithms via extensive network simulations and experiments in a real-life testbed environment. These experimental results show that the optimal strategies obtained from the theoretical models lead to extremely well-performing solutions in practical circumstances. Moreover, the results show that the splitting of data transfers, which is easy to embed in the network requiring no information on the number of flows in the system, leads to a much better performance compared to dynamic network selection.

Throughput Modeling of the IEEE MAC for Sensor Networks

In this paper we provide a model for analyzing the saturation throughput of the ieee 802.15.4 mac protocol, which is the de-facto standard for wireless sensor networks, ensuring fair access to the channel. To this end, we introduce the concept of a natural layer, which reflects the time that a sensor node typically has to wait prior to sending a packet. The model is simple and provides new insight how the throughput depends on the protocol parameters and the number of nodes in the network. Validation experiments with simulations demonstrate that the model is highly accurate for a wide range of parameter settings of the mac protocol, and applicable to both large and small networks. As a byproduct, we discuss fundamental differences in the protocol stack and corresponding throughput models of the popular 802.11 standard.

Cost-Efficient Allocation of Additional Resources for the Service Placement Problem in Next-Generation Internet

One of the major challenges in next-generation Internet is to allocate services to nodes in the network. This problem, known as the service placement problem, can be solved by layered graph approach. However, due to the existence of resource bottleneck, the requests are rejected from the beginning in the resource constrained network. In this paper we propose two iterative algorithms for efficient allocation of additional resources in order to improve the ratio of accepted service placement requests. To this end, we (1) introduce a new concept of sensitivity for each service node to locate the bottleneck node, (2) state the problem of allocating additional resources, and (3) use sensitivity to propose a simple iterative algorithm and an utilization-based iterative algorithm for efficient resource allocation. The performance of these two algorithms is evaluated by simulation experiments in a variety of parameter settings. The results show that the proposed algorithms increase request acceptance ratio significantly by allocating additional resources into the bottleneck node and links. The utilization-based iterative algorithm also decreases the long-term cost by making efficient use of additional resources.

Traffic splitting policies in parallel queues with concurrent access: A comparison

Surrounded by a multitude of wireless networks, users can nowadays experience significant performance improvements when smartly combining multiple networks concurrently (e.g., for transferring files). This phenomenon is called Concurrent Access (CA). Some users, which are referred to as foreground (FG) users, are able to access and utilize multiple networks simultaneously. The traffic streams from the FG users are optimized over multiple networks, in the presence of background (BG) users that can use only one network. In the literature a variety of traffic splitting algorithms have been proposed, with a focus on improving the performance of the FG users, whereas the influence of smart traffic splitting on the performance experienced by the BG users, as well as the resulting splitting ratios over the different networks, have received hardly attention. In this paper, we evaluate and compare the performance of these algorithms with respect to three quality metrics: (1) the file transfer performance of the FG traffic, (2) the file transfer performance of the BG traffic, and (3) the traffic splitting ratios, i.e. the fractions of traffic that is sent over each of the access networks. Our simulations-based results provide a number of valuable insights in the pros and cons of the different job splitting and assignment algorithms.

Poster: optimal dispatching policies for parallel processor sharing nodes with partial information

Many of today’s wireless networks have already closely approached the Shannon limit on channel capacity. A powerful alternative to increase the overall data rate then becomes one in which multiple, likely different, networks are used concurrently. The concurrent use of multiple networks simultaneously has opened up enormous possibilities for increasing bandwidth, improving reliability, and enhancing Quality of Service (QoS) in areas that are covered by multiple wireless access networks.