Based on the reverse asymmetric time synchronization framework, we have proposed several schemes with a major focus on the energy efficiency and computational complexity of a large number of battery-powered, low-cost sensor nodes in wireless sensor networks (WSNs). To address the cumulative end-to-end synchronization error, we have also introduced an idea of compensating for the processing delays at packet-relaying gateways as an energy-efficient way of multi-hop extension of WSN time synchronization schemes. In this paper, we present a comprehensive analysis of the multi-hop extension of WSN time synchronization schemes based on packet-relaying gateways with the per-hop delay compensation and the results of extensive experiments for the energy-efficient time synchronization schemes based on the reverse asymmetric time synchronization framework together with the flooding time synchronization protocol as a representative of existing schemes. Experimental results based on a real testbed demonstrate that the multi-hop extension based on packet-relaying gateways with the per-hop delay compensation greatly improves the performance of time synchronization of all the schemes considered compared to the multi-hop extension based on the conventional time-translating gateways.

Node identification based on unique hardware features like clock skews has been considered an efficient technique in wireless sensor networks (WSNs). Spoofing attacks imitating unique hardware features, however, could significantly impair or break down conventional clock-skew-based node identification due to exposed clock information through broadcasting. To defend against Spoofing attacks, we propose a new node identification scheme callednode identification against Spoofing attack(NISA). It utilizes the reverse time synchronization framework, where sensor nodes’ clock skews are estimated at the head of a WSN, and the spatially-correlated radio link information to achieve simultaneous node identification and attack detection. We further provide centralized and distributed NISA for covering both single-hop and multi-hop scenarios, the former of which employs a single-input and multiple-output convolutional neural network. With a real WSN testbed consisting of TelosB sensor nodes running TinyOS, we investigate the identifiability of clock skews under temperature and voltage variations and evaluate the performance of both centralized and distributed NISA. Experimental results demonstrate that both centralized and distributed NISA could provide accurate node identification and Spoofing attack detection.

The ever-increasing number of WSN deployments based on a large number of battery-powered, low-cost sensor nodes, which are limited in their computing and power resources, puts the focus of WSN time synchronization research on three major aspects of accuracy, energy consumption, and computational complexity. In the literature, the latter two aspects haven't received much attention compared to the accuracy of WSN time synchronization. Especially in multi-hop WSNs, intermediate gateway nodes are overloaded with tasks for not only relaying messages but also a variety of computations for their offspring nodes as well as themselves. Therefore, not only minimizing the energy consumption but also lowering the computational complexity while maintaining the synchronization accuracy is crucial to the design of time synchronization schemes for resource-constrained sensor nodes. In this paper, focusing on the three aspects of WSN time synchronization, we introduce a framework of reverse asymmetric time synchronization for resource-constrained multi-hop WSNs and propose a beaconless energy-efficient time synchronization scheme based on reverse one-way message dissemination. Experimental results with a WSN testbed based on TelosB motes running TinyOS demonstrate that the proposed scheme conserves up to 95% energy consumption compared to the flooding time synchronization protocol while achieving microsecond-level synchronization accuracy.

We consider energy-efficient time synchronization in a wireless sensor network where a head node is equipped with a powerful processor and supplied power from outlet, and sensor nodes are limited in processing and battery-powered. It is thisasymmetrythat our study focuses on; unlike most existing schemes to save the power of all network nodes, we concentrate on battery-powered sensor nodes in minimizing energy consumption for time synchronization. We present a time synchronization scheme based on asynchronous source clock frequency recovery and reverse two-way message exchanges combined with measurement data report messages, where we minimize the number of message transmissions from sensor nodes while achieving sub-microsecond time synchronization accuracy through propagation delay compensation. We carry out the performance analysis of the estimation of both measurement time and clock frequency with lower bounds for the latter. Simulation results verify that the proposed scheme outperforms the schemes based on conventional two-way message exchanges with and without clock frequency recovery in terms of the accuracy of measurement time estimation and the number of message transmissions and receptions at sensor nodes as an indirect measure of energy efficiency.

In this paper we study IEEE 802.11s-based wireless local area network (WLAN) mesh interworking with external LANs. Interworking with multiple mesh portals is necessary not only for network reliability but also for load balancing and increased throughput. In case, however, two or more mesh portals connect the mesh to one external LAN segment, broadcast loops may occur and the IEEE 802.1D bridging protocol may cause the LAN ports of mesh portals to be closed, which results in interworking with the LAN through only one mesh portal. To enable multiple portals in WLAN mesh interworking, we propose a new architecture for mesh portals to implement all but the port blocking function of IEEE 802.1D bridge; thus allowing bridge protocol data units (BPDUs) to be transparently forwarded inside the mesh network. Our proposal is to eliminate the formation of broadcast loops while enabling multiple mesh portals to interwork with the same external LAN segment. This can be achieved by filtering frames at mesh portals making use of new fields we introduce in the portal announcement information element and mesh header so that no frame can be transmitted from a mesh portal to another portal more than once when both mesh portals are connected to the same LAN segment.

The fast retrial random access (FRRA) algorithm has been recently proposed for orthogonal frequency-division multiple access (OFDMA) uplink channels to improve the performance by taking advantage of multiple channels in the frequency domain. In this paper, we study the impact of channel outage on the throughput of FRRA algorithm through extended performance analysis taking into account the effects of both channel outage and packet collisions. Numerical examples show that the channel outage can provide a constructive effect on the throughput of FRRA algorithm.

The last mile continues to be a major bottleneck in the Internet. Its low bandwidth and flexibility prevents the deployment of new services and the development of new applications. In this paper we present a summary of current efforts in access networks research, focusing in particular on fiber optic solutions. We present the Stanford University aCCESS (SUCCESS) initiative within the Photonics & Networking Research Laboratory (PNRL). As part of this initiative, two novel network architectures have been developed, SUCCESS-HPON and SUCCESS-DWA, which propose a smooth migration path from current TDM-PONs to future higher bandwidth, cost-efficient, scalable WDM-PONs. In addition, we present SUCCESS-LCO, a spectral-shaping line coding technique that enables a cost-effective shorter-term capacity upgrade of existing TDM-PONs. We discuss as well what we believe are the main open research areas in optical access networks.

In this paper we study the tunable resources scheduling problem for WDM-PON under the Stanford University aCCESS - hybrid WDM/TDM PON (SUCCESS-HPON) architecture, a next generation optical access network. In SUCCESS-HPON a few tunable transmitters and receivers at the OLT are shared by all the users for both downstream and upstream transmission using a centralized light sources approach. Each ONU is assigned a single wavelength for both downstream and upstream data transmission; for the latter, a continuous wave is provided by the OLT for the ONUs to amplitude-modulate upstream data onto it and send it back to the OLT. Through the use of novel scheduling algorithms, it is possible to provide service to all users in the network with just a few tunable transmitters and receivers, making the network very cost-efficient. The performance of these algorithms has been extensively studied through simulations. In this paper we attempt to prove 100% throughput guarantee on a particular scheduling algorithm. We show that the maximum weight matching (MWM) algorithm for admissible traffic with the strong law of large numbers property is stable and can guarantee 100% throughput in a simplified model of SUCCESS-HPON. We derive this result by converting the scheduling problem under consideration to a generalization of the well-known crossbar input-queued switch scheduling problem, and then use a fluid model of a discrete time switch together with the extended Birkhoff-von Neumann (BvN) decomposition theorem. The MWM algorithm can be easily implemented on the SUCCESS-HPON architecture since the number of wavelengths, which determines the complexity of implementation of MWM algorithm, is usually small. This proof of MWM in SUCCESS-HPON is meaningful in that it suggests a practical scheduling algorithm with 100% throughput guarantee as well as determines a theoretical bound. As a byproduct of this research, we prove the extended BvN decomposition theorem, which may be useful in proving 100% throughput guarantee of MWM algorithm in resource scheduling (allocation) problems where certain number of users share the same kind of resources, for example, as in the case that K users are sharing Ntype-A resources and Mtype-B resources.

In this paper we study the problem of scheduling variable-length frames in WDM-PON under Stanford University access (SUCCESS), a next-generation hybrid WDM/TDM optical access network architecture. The SUCCESS WDM-PON architecture has unique features that have direct impact on the design of scheduling algorithms: first, tunable transmitters and receivers at OLT are shared by ONU to reduce transceiver counts; second, the tunable transmitters not only generate downstream data traffic but also provide ONU with optical continuous wave (CW) bursts for upstream transmissions. To provide efficient bidirectional transmissions between OLT and ONU, we propose a batch scheduling algorithm based on the sequential scheduling algorithm previously studied. The key idea is to provide room for optimization and priority queueing by scheduling over more than one frame. In the batch scheduling, frames arriving at OLT during a batch period are stored in virtual output queues (VOQ) and scheduled at the end of the batch period. Through simulation with various configurations, we demonstrate that the proposed batch scheduling algorithm, compared to the original sequential scheduling algorithm, provides higher throughput, especially when the system load is high, and better fairness between upstream and downstream transmissions.

Optical carrier sense multiple access with collision avoidance (CSMA/CA) is a media access control (MAC) protocol proposed for future metropolitan area wavelength division multiplexing (WDM) ring networks with a fixed receiver and a tunable transmitter at access nodes. In this paper, we focus on the unslotted version of the optical CSMA/CA MAC which is a fully-distributed and asynchronous protocol. We present the results of design and performance evaluation of fairness control schemes based on longest queue first (LQF) scheduling and two random routing algorithms - full random routing (FRR) and partial random routing (PRR). Through extensive network-level simulation of a WDM ring network with 10 nodes and 10 wavelengths on a 100 km ring at 10 Gbit/s line rate, we demonstrate a combination of the LQF scheduling and the PRR with a retransmission counter provides good fairness (fairness index of 0.9995) with high bandwidth efficiency and small delay spread, under highly unbalanced traffic conditions.

We consider the issue of source clock frequency recovery in packet networks and propose a new adaptive clock method based on the Kalman filter (KF) with low-pass prefiltering-KALP (Kalman filter-based Adaptive clock method with Low-pass Prefiltering) in short. Noting that because of the difficulty in modeling as well as the nonwhite Gaussian nature of the packet jitter most existing adaptive clock methods could not successfully adopt the Kalman filter, we take a new approach to packet jitter modeling for the KALP. We model the packet jitter not directly but after shaping its characteristics by low-pass prefiltering. This low-pass prefiltering is an important arrangement as it helps to convert the packet jitter into a low-pass signal regardless of its original characteristics, thus enabling to model the prefiltered packet jitter using a simple first-order autoregressive [AR(1)] process. The low-pass prefilter should be carefully selected not to lose the timing information while prefiltering, and the moving averager employed in this paper satisfies this requirement. The AR(1)-modeled jitter component is amenable to the KF-based processing, which in this case becomes an optimal estimator. The design parameters including the initial conditions of the KF and AR(1) parameters can be determined based on the service clock specification and packet interarrival times during the delay smoothing process. We carry out various simulations to compare the performance of the KALP with the existing buffer-based adaptive clock method and demonstrate that the KALP can significantly reduce the fluctuation in the level of receiving buffer as well as the time to recover the source clock frequency.

We propose the multi-channel deflection crossbar (MCDC) as a promising architecture for broadband ATM switching under the realistic constraints imposed by VLSI technology. MCDC is an internally nonblocking, buffer efficient, crossbar based architecture. MCDC is also capable of multi-channel switching with cell sequence integrity preserved. Through a comparative study against a well-known ATM switch architecture, we show that the proposed MCDC architecture is able to incorporate a greater number of input channels and buffers for a given IC die size and requires a lower clocking rate.

Studies the traffic shaping issues for VBR video services in ATM networks with stress put on the source clock frequency recovery aspect. The authors propose a "three-level traffic shaper" (TLTS) which is able to limit the minimum as well as the maximum cell rate of input VBR traffic. For a VBR video source modeled after a two-state Markov modulated Poisson process, they determine, as performance measures, the cell loss probability, the underflow probability, and the maximum delay that a cell can experience in the proposed TLTS. Then they apply the TLTS for source clock frequency recovery in VBR video services. In extending the existing CBR-based synchronous residual time stamp (SRTS) method for VBR video services, it is necessary to restrict the minimum, as well as the maximum, number of cells generated in a fixed period of time to ensure the transmission of timing information, and the proposed TLTS is tailored to meet this necessity by adopting an optimal dimensioning procedure based on the genetic algorithm (GA). Finally the authors provide some numerical examples to demonstrate the optimal dimensioning procedure of TLTS and the related performances of statistical multiplexing and source clock frequency recovery.>