Coskun Cetinkaya

Opportunistic traffic scheduling over multiple network paths

Multipath routing enables a networks traffic to be split among two or more possibly disjoint paths in order to reduce latency, improve throughput, and balance traffic loads. Yet, once the control plane establishes multiple routes, a policy is needed for efficiently splitting traffic among the selected paths. In this paper, we introduce opportunistic multipath scheduling (OMS), a technique for exploiting short term variations in path quality to minimize delay, while simultaneously ensuring that the splitting rules dictated by the routing protocol are satisfied. In particular, OMS uses measured path conditions on time scales of up to several seconds to opportunistically favor low-latency high-throughput paths. Consequently, OMS ensures that over longer time scales relevant for traffic management policies, traffic is split according to the ratios determined by the routing protocol. We develop a model of OMS and derive an asymptotic lower bound on the performance of OMS as a function of path conditions (mean, variance, and Hurst parameter) for self-similar traffic. An example finding from the model is that long-time-scale traffic fluctuations represented by a larger Hurst parameter improve the performance gain of OMS vs. round-robin scheduling, even under paths that are statistically identical. Finally, we use an extensive simulation-based performance study to evaluate the accuracy of the analytical model, explore the impact of OMS on TCP throughput, and study the impact of factors such as delayed measurements

Improving the efficiency of multipath traffic via opportunistic traffic scheduling

Multipath routing, as defined by OSPF extensions and other protocols, enables a networks traffic to be split among two, or more, possibly disjoint paths. Advantages of multipath vs. unipath routing include load balancing, reduced latency, and improved throughput. However, once the control plane establishes multiple routes, a policy is needed for efficiently splitting traffic among the selected paths. In this paper, we introduce opportunistic multipath scheduling (OMS), a technique for exploiting short-term variations in path quality to minimize delay, while simultaneously ensuring that the splitting rules dictated by the routing protocol are satisfied. We develop a performance model of OMS and derive an asymptotic lower bound on the performance of OMS as a function of path conditions (mean, variance, and Hurst parameter) for self-similar traffic. Finally, we use an extensive simulation-based performance study to evaluate the accuracy of the analytical model, explore the impact of OMS on TCP throughput and performance of real-time traffic, and study the impact of factors such as delayed measurements.

Service differentiation mechanisms for WLANs

Designing a medium access control (MAC) protocol that simultaneously provides service differentiation and high throughput, and allows individual users to share limited spectrum resources fairly is a challenging problem for wireless LANs when applications have diverse performance requirements, such as high throughput, low delay, and delay jitter. In this paper, we propose efficient flow-based and class-based weighted fair queueing (WFQ) mechanisms with very simple-state information that considers only collisions, like the standard IEEE 802.11e MAC protocol. We utilize an analytical throughput model to obtain the optimal parameter settings for both mechanisms. Simulation results show that both mechanisms provide weighted throughput within 1% of the given throughput ratio. Additionally, we propose a novel and efficient priority mechanism. Our key technique involves each node changing its backoff counter based on both its own packets priority level and the priority level of the transmitted packet. Specifically, a node will increase its backoff counter linearly with a higher-priority packet transmission and decrease it exponentially with a lower-priority packet transmission. Simulation results show that our mechanism always protects high-priority traffic while still providing high throughput.

A short-term fair MAC protocol for WLANs

Designing a medium access control (MAC) protocol that simultaneously provides high throughput and allows individual users to share limited spectrum resources fairly, especially in the short-term time horizon, is a challenging problem for wireless LANs. In this paper, we propose an efficient cooperative MAC protocol with very simple state information that considers only collisions, like the standard IEEE 802.11 MAC protocol. However, contrary to the IEEE 802.11 MAC, the cooperative MAC gives collided users priority to access the channel by assigning them shorter backoff counters and interframe-spaces than users who did not participate in the collision event. In other words, collided users are the only ones allowed to transmit in the following contention period. For the cooperative MAC protocol, we utilize an analytical throughput model to obtain the optimal parameter settings. Simulation results show that the cooperative MAC provides significant improvement in short-term fairness and access delay, while still providing high network throughput.

Using Rerouting to Improve Aggregate Based Resource Allocation

This paper studies the effect of rerouting for augmenting aggregate based resource allocation in the trade-off between overhead and utilization. Aggregation is a common approach used to address the scalability issue in resource allocation. However, resources committed in bulk may be under utilized while other resource requests are being turned down for lack of resources in some shared links. The aim of rerouting is to free up committed resources for better utilization by reusing resources vacated by terminated flows and by moving existing flows to alternative paths. Our results show that rerouting improves performance over a wide range of network loads on two different network topologies.

Aggregate based resource allocation with rerouting

This paper studies the effect of rerouting for augmenting aggregate based resource allocation in the trade-off between overhead and utilization. Aggregation is a common approach to address the scalability issue in resource allocation. However, resources committed in bulk may be under utilized while other resource requests are being turned down for lack of resources in some shared links. The aim of rerouting is to free up committed resources for better utilization by reusing resources vacated by terminated flows and by moving existing flows to alternative paths. Our results show that rerouting improves performance over a wide range of network loads on two different network topologies. In particular, we show that depending on the network load and topology, it is possible to reduce both blocking rate and routing cost.

Service Differentiation Mechanism Via Cooperative Medium Access Control Protocol

Providing differentiated Quality of Service (QoS) levels is an important challenge for wireless ad hoc networks and wireless LANs when applications have diverse performance requirements. The IEEE 802.11e MAC protocol can provide a Dynamic MAC by assigning different AIFSs, contention window expansion factors (PFs), and (CW min , CW max ) pairs for different classes and can provide a Static MAC by adjusting the durations of AIFSs based on priority levels [Aad01]. In this paper, we propose a novel and efficient service differentiation mechanism via the C-MAC. In our protocol, each node will change its backoff counter based on both its own packet’s priority level and the priority level of the transmitted packet. The simulation results indicate that the Static MAC provides a service differentiation at the expense of significant goodput degradation when the amount of high priority class traffic is low. On the other hand, the Dynamic MAC fails to prevent low priority classes accessing the channel resulting in significant high priority class goodput degradation when the network load is high. However, our mechanism always provides an efficient service differentiation mechanism and high goodput with a small goodput degradation.

Asynchronous Multi-Channel MAC Protocol

Traditional IEEE 802.11-based networks use only a single channel to ensure connectivity; however, multi-channel capability is available to provide higher network throughput by allowing simultaneous multiple transmissions that do not interfere with each other. The use of multiple channels raises several new challenges: mainly finding the receiver, scheduling the next transmission, balancing the load among channels, and dealing with multi-channel hidden terminals. This paper proposes a novel and efficient, asynchronous multi-channel medium-access control (MAC) protocol using a single transceiver. The key technique is that the proposed protocol separates searching and scheduling processes where searching users switch channels faster than regular users. Simulation results indicate that the proposed protocol achieves up to 14.10 Mbps network throughput, whereas the IEEE 802.11 MAC protocol, with a single channel, provides only 4.35 Mbps in the best case when three orthogonal channels with an 11- Mbps data rate are available

Allocation of Component Types to Machines in the Automated Assembly of Printed Circuit Boards

Although the use of electronic component placement machines has brought reliability and speed to the printed circuit board (PCB) assembly process, to get higher utilization, one needs to solve the resulting complex operations research problems efficiently. In this study, the problem of distributing the assembly workload to two machines deployed on an assembly line with two identical component placement machines to minimize the line idle time is considered. This problem is NP-Complete even in its simplest form. A mathematical model and several heuristics have been proposed to solve this problem efficiently.