Cory Beard

Prioritized resource allocation for stressed networks

Overloads that occur during times of network stress result in blocked access to all users, independent of importance. These overloads can occur because of degraded resource availability or abnormally high demand. Public broadband networks must dynamically recognize some multimedia connections as having greater importance than others and allocate resources accordingly. A new approach to connection admission control is proposed that uses an upper limit policy to optimize the admission of connections based on the weighted sum of blocking across traffic classes. This results in a simple algorithm suitable for multimedia and packet networks. This work is also the first to demonstrate that the use of an upper limit policy is superior to traditional approaches of adding extra capacity or partitioning capacity, both in terms of the amount of resources required and sensitivity to load variations. An upper limit policy can also be deployed much faster when a large overload occurs from a disaster event.

Analytical models for understanding space, backoff, and flow correlation in CSMA wireless networks

In wireless networks employing carrier-sense multiple-access with collision avoidance (CSMA/CA), correlations between the service processes of different nodes arise as a result of competition for common wireless channels and the dependencies between upstream and downstream traffic flows. These dependencies make the development of tractable performance models extremely difficult. To address this purpose, we present a new continuous-time model for CSMA wireless networks where we combine a node model and a channel model in order to capture correlation. Simplification methods are presented that make our models computationally tractable for large networks with minimal loss of accuracy. The model can be used for both single and multi-hop wireless networks and takes into account non-saturated queues, backoff-stage dependence of collision probabilities, and the correlation between departure processes and arrival processes of adjacent nodes. The model can be used to compute probabilistic quality of service guarantees to optimize end-to-end throughput and end-to-end delay by adjusting arrival and backoff rates along various paths.

Preemptive and delay-based mechanisms to provide preference to emergency traffic

Preemptive and delay-based policies are derived here to provide preferential treatment to emergency users as motivated by the need to respond to natural or man-made disasters. These are applied in circuit switched networks or IP-based networks in conjunction with reservation mechanisms at single links or routers where congestion occurs. A unique optimization formulation is provided where policies are constrained by blocking requirements for emergency traffic, then optimized to provide lowest preemption probabilities. An iterative optimization approach is derived for a multi-dimensional Markov chain to provide an approximate solution for a non-linear integer program. Markov state space concerns are mitigated by introducing and analyzing algorithmic simplifications. Polices are found for hard preemption where sessions are interrupted or soft preemption where session quality is reduced. In addition, delay-based approaches are also examined where emergency users are allowed to wait for network resources if they are first blocked. All of the above approaches are compared with each other and with traditional approaches that hold back resources for emergency traffic. The results demonstrate that in general no approach is superior based on network performance characteristics alone. Qualitative factors must also be considered, most notably the nuisance to non-emergency users from preemption.

Design and analysis of multi-level active queue management mechanisms for emergency traffic

Multiple average-multiple threshold (MAMT) active queue management (AQM) is proposed as a solution for providing available and dependable service to traffic from emergency users after disasters. MAMT is a simple but effective approach that can be applied at strategic network locations, where heavy congestion is anticipated. It can provide low loss to emergency packets while dropping non-emergency packets only as much as necessary. Fluid flow analysis and simulation is conducted to provide guidelines for proper MAMT design, especially regarding the queue size and averaging parameters that are most important. This work considers non-responsive traffic exclusively, since non-responsive traffic types are currently getting the most attention from emergency management organizations. Plus, very little work has been performed regarding AQM and non-responsive traffic. It demonstrates queue oscillation problems that previously may have been attributed to the interactions between TCP and AQM, but which are actually inherent to AQM and can be greatly reduced with proper parameter settings. MAMT is shown to perform well over a range of loads and can effectively protect emergency traffic from surges in non-emergency traffic.

Evaluating geographic vulnerabilities in networks

In wireless ad-hoc and wireline networks used for search and rescue, military operations, and emergency communications; many failure modes are geographic in nature. They include jammers, explosions, enemy attacks, terrain issues, and natural causes like floods, storms, and fires. This paper proposes two methods to gain valuable insights into the physical topography and geographic vulnerabilities of networks. The 2-Terminal method and All-Terminal method find areas that given a threat of a certain radius can disconnect either the source and destination pair or any component of the network respectively. We believe that these methods could be used to optimize network node selection, placement and design. To be tractable, both methods incorporate innovative search techniques to use the size of the threat to reduce the complexity of the search.

Tunable traffic control for multihop CSMA networks

A new analytical model for multihop CSMA networks is presented, using matrix exponential methods that model node queueing and backoff processes. This model also captures in a new way the collisions between nodes and then provides performance analysis for not just throughput, but also packet dropping, end-to-end delay, and jitter. All performance metrics are shown to be very accurate compared to simulation results. The model is then used to propose methods for controlling performance of CSMA multihop networks by using different traffic rates and backoff rates for different nodes or paths, depending on their network location.

A Controlled Preemption Scheme for Emergency Applications in Cellular Networks

In this paper, we introduce a threshold-based preemption strategy for supporting emergency traffic in cellular networks. Compared with current commercially used policies, our scheme can not only guarantee a certain amount of resources to public customers but can also provide immediate access for emergency users and flexibility for providers to adapt to different requirements and operating scenarios. In addition, under the combined preemption and queuing framework, interesting analytical relationships among channel occupancy, gross service time, and success probability for public traffic are revealed. Based on this, guidelines to further improve the satisfaction of public customers can be provided.

Comparison of combined preemption and queuing schemes for admission control in a cellular emergency network

Preemption is a possible way to ensure emergency traffics priority in wireless network. But a pure preemption policy can be especially harsh to low priority calls. A way to improve this is introducing a queuing mechanism, which means putting preempted calls into a queue so that they can resume when channels become available, as long as they do not give up due to waiting too long in the queue. Also, we bring out a single preemption scheme, to avoid calls being preempted multiple times. The analytical results for performance analysis of these methods are given, and based on them the performance of different schemes are evaluated and compared

Using Multi-Topology Routing to improve routing during geographically correlated failures

During large geographic events in networks, the routing churn that occurs has been shown to cause significant impacts in routing stabilization following the event. This work proposes a set of algorithms that is based on Multi-Topology Routing (MTR) for pre-planning for geographically correlated failures. Thus, in the event of a failure, our approach, Geographic MTR, switches to virtual topologies that reduce the impact of routing changes that can result in dropped connections until a new link state and shortest path trees can be established. We propose two algorithms to generate virtual topologies, Geographic Coverage MTR (gcMTR) and Geographic Targeted MTR (gtMTR). The first method, gcMTR, is to create virtual topologies taking a network wide coverage approach, while gtMTR is a targeted approach that can be used in anticipation of a specific event where knowledge of that event exists. A third algorithm proposed in this work specifies a way to detect a geographic event and select a topology to use. We evaluated our approach on two network topologies and observed that the number of connections that are dropped during a geographic event can be reduced significantly, thereby reducing the impact to the non-affected part of the network.

Tunable Preemption Controls for a Cellular Emergency Network

In this paper we introduce the preemption threshold based strategy to cellular networks for supporting emergency traffic. With the new strategy, in addition to providing immediate access for emergency traffic, a certain amount of resources can be protected for public traffic by tuning the preemption threshold. Also, with the assumption that preempted users will restart the session after resumption, we find that a conservation law holds for the average duration of calls that succeed or fail to finish. Numerical results about the evaluation of this strategy are presented and show an obvious improvement over related strategies.