A practical protocol jointly considering PHY and MAC for MIMO based concurrent transmissions in wireless ad hoc networks, called Contrabass, is presented. Concurrent transmissions refer to simultaneous transmissions by multiple nodes over the same carrier frequency within the same interference range. Contrabass is the first-to-date open-loop based concurrent transmission protocol which implements simultaneous channel training for concurrently transmitting links without any control message exchange. Its MAC protocol is designed for each active transmitter to independently decide to transmit with near optimal transmission probability. Contrabass maximizes the number of successful concurrent transmissions, thus achieving very high aggregate throughput, low delays and scalability even under dynamic environments. The design choices of Contrabass are deliberately made to enable practical implementation which is demonstrated through GNURadio implementation and experimentation.

Existing wireless MACs are known to have fairness and QoS problems1. In response, the wireless community have come up with numerous ldquopoint solutionsrdquo, with one-to-one comparison of their solution with existing MACs for the specific fairness model they target. However, with some tradeoff of flexibility and efficiency, many of these MACs can be made relatively free of a a specific fairness model. This decoupling of the MAC from the fairness model is desirable because it allows objective testing of the MAC under different fairness models and also allows independent evolution of fairness policies and research into medium access issues. However, there is no cross-model evaluation of such MACs especially in the context of real multihop wireless testbeds. In this paper we address this issue by testing three MACs - 802.11e, EY-NPMA and DWOP, (which represent different ways of enabling fairness in the MAC) across a number of fairness and QoS models - temporal/rate and proportional fairness, and EDF and static priority scheduling. In the process we also extend EY-NPMA significantly (which we call Siren) for multihop wireless environments. We perform these comparisons on a 30 node multihop wireless sensor testbed comprising of MicaZ devices equipped with the CC2420 radio.

In this demo, we showcase DiffQ - a congestion control protocol inspired by theoretical cross-layer optimization approaches. DiffQ can support congestion control for network flows that use either single-path or opportunistic multi-path routing. Our demo will focus on the performance in single-path routing environments, where contemporary end-point congestion control algorithms like TCP face severe unfairness or even starvation. This is primarily due to the interaction of such protocols with MAC layer unfairness. We demonstrate micro (5 flows) as well as macro-evaluations (60 flows) of such cases. Our demo is conducted on WiSeNet - a 70-node wireless mesh test-bed hosted in the computer science building at NCSU. Distributed over a 100,000 sq ft building, this is one of the largest test-bed installations both in terms of number of nodes and coverage area, hence an ideal testing ground for such scenarios. Experimental results like throughput, MAC-layer statistics, delay, routing path flaps and network buffer overflows are recorded and displayed in real-time and enable a bird's eye-view of the entire network status and allow us to point out various phenomenon as they happen.

In this paper, we focus on the performance of TCP splitting in satellite-terrestrial hybrid networks. By simulation we show that long-time link outage has great negative impact on TCP throughput. To solve the problem, an ACK reservation scheme is proposed to speed up TCP recovery from link outage and analytical estimate of a key parameter of the scheme is provided for achieving high end-to-end TCP throughput. Simulation results validate our analysis, and show that after the end of link failure, the TCP source can be re-started using ACK reservation with virtually no additional lag leading to great improvement on bandwidth utilization and end-to-end throughput.

We study the performance of TCP over a satellite link, which is subject to intermittent blockages. Our simulation-based study characterizes the performance degradation of transferring messages ranging from small web objects to large files over satellite channels with widely varying blockage statistics. Using a split connection gateway, we investigate the impact of trying to preserve TCP end-to-end semantics upon connection close and find that such an approach significantly reduces the ability of small TCP transfers to survive a long duration blockage. We show that by tying the satellite gateway's offered window to the terrestrial TCP sender to the space in the send buffer on the satellite link becomes available, the split-connection gateway can essentially eliminate the delay incurred waiting for the TCP persist timer to expire. Finally, we illustrate how aspects of the gateway's design affect large and small TCP transfers differently.

The host identity protocol (HIP) is a recent protocol proposal for secure host mobility and multihoming using cryptographic-based name space for Internet hosts. This paper reports on our experience with implementing HIP and experimenting with it as a mobility management and host multihoming solution. After first introducing the HIP approach and contrasting it with other solutions, we describe our approach for implementing HIP as an extension to Linux and FreeS/WAN IPsec, including our use and extension of standard APIs. We then characterize the performance of HIP packet exchanges experimentally, and report that the computational overhead is dominated by the DSA signing of the HIP packets. Using 266 MHz Pentium II-based laptops, our HIP implementation took slightly under 1 second on average to complete connection setup, and less than 200 ms to process a mobility-initiated readdress. We also characterize the overhead due to the HIP "cookie challenge" used for stateless connection setup. We conclude by identifying areas for continued HIP development.

This paper describes a method to significantly reduce the complexity of continuous density HMM with only a small degradation in performance. The proposed method is noise-robust and may perform even better than the standard algorithm if training and testing noise conditions are not matched. The method is based on approximating the variance vectors of the Gaussian kernels by a vector quantization (VQ) codebook of a small size. The quantization of the variance vectors is done using an information theoretic distortion measure. Closed form expressions are given for the computation of the VQ codebook and the superiority of the proposed distortion measure over the Euclidean distance is demonstrated. The effectiveness of the proposed method is shown using the connected TI digits database and a noisy version of it. For the connected TI digit database, the proposed method shows that by quantizing the variance to 16 levels we can maintain recognition performance within 1% degradation of the original VR system. In comparison, with Euclidean distortion, a size 256 codebook is needed for a similar error rate.

Compressionless routing (CR) is an adaptive routing framework which provides a unified framework for efficient deadlock free adaptive routing and fault tolerance. CR exploits the tight coupling between wormhole routers for flow control to detect and recover from potential deadlock situations. Fault tolerant compressionless routing (FCR) extends CR to support end to end fault tolerant delivery. Detailed routing algorithms, implementation complexity, and performance simulation results for CR and FCR are presented. These results show that the hardware for CR and FCR networks is modest. Further, CR and FCR networks can achieve superior performance to alternatives such as dimension order routing. Compressionless routing has several key advantages: deadlock free adaptive routing in toroidal networks with no virtual channels, simple router designs, order preserving message transmission, applicability to a wide variety of network topologies, and elimination of the need for buffer allocation messages. Fault tolerant compressionless routing has several additional advantages: data integrity in the presence of transient faults (nonstop fault tolerance), permanent fault tolerance, and elimination of the need for software buffering and retry for reliability. The advantages of CR and FCR not only simplify hardware support for adaptive routing and fault tolerance, they also can simplify software communication layers.

Network service guarantees not only provide significant performance benefits to distributed computing systems (more balanced resource utilization, fast fault recovery, and fair network access), but they are also essential for many new applications requiring real-time communications with continuous data types (audio/video). Most existing algorithms which provide network service guarantees are too complicated to be feasible in high-speed, low-cost switches for multicomputer networks. The simpler algorithms proposed provide only limited service guarantees or waste significant network resources.We present a novel, cost-effective queueing and scheduling algorithm, called Rotating Combined Queueing (RCQ), which can efficient]y support a range of service guarantees including deterministic delay bounds and bandwidth guarantees in multicomputer networks. By allowing bursty traffic to utilize unused network resources efficiently, RCQ also can provide competitive performance to best-effort data communications. Such cost-effective service guarantees not only provide substantial benefits to the overall system performance, but can also further expand the domain of multicomputer applications to encompass distributed multimedia applications requiring isochronous communications.

Network throughput can be increased by allowing multipath, adaptive routing. Adaptive routing allows more freedom in the paths taken by messages, spreading load over physical channels more evenly. The flexibility of adaptive routing introduces new possibilities of deadlock. Previous deadlock avoidance schemes in k -ary n -cubes require an exponential number of virtual channels. We describe a family of deadlock-free routing algorithms, called planar-adaptive routing algorithms, that require only a constant number of virtual channels, independent of networks size and dimension. Planar-adaptive routing algorithms reduce the complexity of deadlock prevention by reducing the number of choices at each routing step. In the fault-free case, planar-adaptive networks are guaranteed to be deadlock-free. In the presence of network faults, the planar-adaptive router can be extended with misrouting to produce a working network which remains provably deadlock free and is provably livelock free. In addition, planar-adaptive networks can simultaneously support both in-order and adaptive, out-of-order packet delivery. Planar-adaptive routing is of practical significance. It provides the simplest known support for deadlock-free adaptive routing in k -ary n -cubes of more than two dimensions (with k >2). Restricting adaptivity reduces the hardware complexity, improving router speed or allowing additional performance-enhancing network features. The structure of planar-adaptive routers is amenable to efficient implementation. Simulation studies show that planar-adaptive routers can increase the robustness of network throughput for nonuniform communication patterns. Planar-adaptive routers outperform deterministic routers with equal hardware resources. Further, adding virtual lanes to planar-adaptive routers increases this advantage. Comparisons with fully adaptive routers show that planar-adaptive routers, limited adaptive routers, can give superior performance. These results indicate the best way to allocate router resources to combine adaptivity and virtual lanes. Planar-adaptive routers are a special case of limited adaptivity routers. We define a class of adaptive routers with f degrees of routing freedom. This class, termed f-flat adaptive routers , allows a direct cost-performance tradeoff between implementation cost (speed and silicon area) and routing freedom (channel utilization). For a network of a particular dimension, the cost of adaptivity grows linearly with the routing freedom. However, the rate of growth is a much larger constant for high-dimensional networks. All of the properties proven for planar-adaptive routers, such as deadlock and livelock freedom, also apply to f -flat adaptive routers.

Compressionless Routing (CR) is a new adaptive routing framework which provides a unified framework for efficient deadlock-free adaptive routing and fault-tolerance. CR exploits the tight-coupling between wormhole routers for flow control to detect potential deadlock situations and recover from them. Fault-tolerant Compressionless Routing (FCR) extends Compressionless Routing to support end-to-end fault-tolerant delivery. Detailed routing algorithms, implementation complexity and performance simulation results for CR and FCR are presented. CR has the following advantages: deadlock-free adaptive routing in torus networks with no virtual channels, simple router designs, order-preserving message transmission, applicability to a wide variety of network topologies, and elimination of the need for buffer allocation messages. FCR has the following advantages: tolerates transient faults while maintaining data integrity (nonstop fault-tolerance), tolerates permanent faults, can be applied to a wide variety of network topologies, and eliminates the need for software buffering and retry for reliability. These advantages of CR and FCR not only simplify hardware support for adaptive routing and fault-tolerance, they also can simplify communication software layers.>

Network throughput can be increased by allowing multipath, adaptive routing. Adaptive routing allows more freedom in the paths taken by messages, spreading load over physical channels more evenly. The flexibility of adaptive routing introduces new possibilities of deadlock. Previous deadlock avoidance schemes in k-ary n-cubes require an exponential number of virtual channels, independent of network size and dimension. Planar adaptive routing algorithms reduce the complexity of deadlock prevention by reducing the number of choices at each routing step. In the fault-free case, planar-adaptive networks are guaranteed to be deadlock-free. In the presence of network faults, the planar-adaptive router can be extended with misrouting to produce a working network which remains provably deadlock free and is provably livelock free. In addition, planar adaptive networks can simultaneously support both in-order and adaptive, out-of-order packet delivery.