We study the configuration and dual-span failure performance of pre-configured protection cycles (p-cycles) in survivable WDM optical networks with partial wavelength conversion. We formulate the problem as an integer linear program using a non-joint optimization approach. p-cycles and wavelength converters are then optimally determined. The objective is to minimize the total cost of link capacity used by p-cycles and the cost of wavelength converters required to accommodate a set of traffic demands, and to select from the set of optimal cost solutions, the solution which has the best survivability performance against dual failures in terms of the average capacity loss, the average restorability, and the efficiency of the protection capacity. The proposed p-cycle configuration architecture takes full advantage of converter sharing, requiring as few converters as possible. Two different performance improvement cost functions are applied as dual-failure protection performance predictors. One is based on the number of unique p-cycles selected; the other is based on the maximum working capacity protected by any single selected p-cycle. Our numerical results indicate that the latter shows the best performance throughout the simulations. The performance of the methodology and the best prediction function depend on which of the performance metrics is considered to be the most important. The size of the network, the maximum allowable p-cycle length and the number of demands all influence this performance.
We study traffic grooming in synchronous optical network/wavelength-division multiplexing (SONET/WDM) bidirectional line-switched ring (BLSR) networks under the uniform all-to-all traffic model with an objective to reduce total network costs (wavelength and electronic multiplexing costs), in particular, to minimize the number of add-drop multiplexers (ADMs) while using the optimal number of wavelengths. We derive a new tighter lower bound for the number of wavelengths when the number of nodes is a multiple of 4. We show that this lower bound is achievable. We then derive new, more general, and tighter lower bounds for the number of ADMs subject to the constraint that the optimal number of wavelengths is used, and propose heuristic algorithms (the circle construction algorithm and the circle grooming algorithm) that try to minimize the number of ADMs while using the optimal number of wavelengths in BLSR networks. Both the bounds and algorithms are applicable to any value of r and for different wavelength granularity g. All previous ADM lower bounds except perhaps that in (Gerstel et al., 1999) were derived under the assumption that the magnitude of the traffic streams (r) is one unit (r = 1) with respect to the wavelength capacity granularity g. Performance evaluation shows that wherever applicable, our lower bounds are at least as good as existing bounds and are much tighter than existing ones in many cases. Our proposed heuristic grooming algorithms perform very well with traffic streams of larger magnitude. The resulting number of ADMs required is very close to the corresponding lower bounds derived in this paper.
This paper addresses issues on how to support both real-time and non-real-time communication services in a wireless LAN. Unpredictable wireless channel errors may cause applications with real-time traffic to receive degraded quality of services due to packet losses. We propose scheduling algorithms that can take advantages of point coordination function (PCF) of a wireless LAN to support quality of service provisioning for real-time services. Specifically, we
consider two types of service differentiation: (1) absolute delay
differentiated services; and (2) proportional differentiated fair bandwidth services, for real time communication. At the same time, our proposed schemes also try to accommodate best-effort traffic to minimize the delay experienced by best traffic. One challenging issue involved is to consider the packet loss due to channel bit errors. We also establish conditions to admit a new real-time connection. Preliminary performance evaluation of the proposed schemes is
conducted to demonstrate how one of proposed scheme works as well as to study its effectiveness.
In this paper, we have developed an integrated online algorithm for dynamically
routing bandwidth guaranteed label switched paths (LSPs) in IP over WDM optical
networks. Traditionally, routing at upper layer (e.g., IP layer) is independent of
wavelength routing at the optical layer. Wavelength routing at the optical
layer sets up a quasi-static logical topology which is then used at the IP
layer for IP routing. The coarse-grain wavelength channels and the
pre-determined virtual topologies with respect to some {\it a priori} assumed
traffic distribution are barriers to efficient resource use and inflexible to
changing traffic. We take into count the combined knowledge of resource and
topology information in both IP and optical layers. With this added knowledge,
an integrated routing approach can extract better network efficiencies, be more
robust to changing traffic patterns at the IP layer than schemes that either
use dynamic routing information at the IP layer or use a static wavelength
topology only. Our proposed algorithm considers not only the importance of critical links, but
also their relative importance to routing possible future LSP set-up requests
by characterizing their normalized bandwidth contribution to routing future LSP
requests. Moreover, link residual bandwidth information that captures the
link's capability of routing future LSPs is also incorporated into our
algorithm.
Simulation results show that our proposed algorithm performs better than both
routing algorithms do in terms of LSP set-up request rejection rate and
available bandwidths.
In this work, we devise and study the performance of a new active queue
management mechanism for Web traffic to more intelligently select packets to
drop incipient of network congestion. The proposed mechanism specifically
targets short-lived or fragile flows (e.g., most HTTP flows) to keep link
utilization high while reducing the HTTP response time. The goal of the
proposed active queue management scheme is to protect new TCP flows and TCP
flows that have packet dropped recently from potential network congestion, thus
achieving better response times. Our simulation studies have compared the
performance of RED and the proposed AQM for a network with only HTTP traffic at
loads less, close to, and more than the network capacity. Simulations show that
a subsidy given to a flow that that is in its initial stage provides
significantly better performance in terms of HTTP request-reply delays without
sacrificing the link utilization. The new scheme is very simple to implement.
It contains one additional control parameter to RED, and the tuning of this
parameter is simple. Since most HTTP flows are short-lived, only the state
information for a subset of active flows needs to be maintained for a very
short period of time, and then all the resources used for keeping the state
information can be reclaimed.
In this work, we present online algorithms for dynamic routing bandwidth guaranteed label switched paths (LSPs) where LSP set-up requests (in terms of a pair of ingress and egress routers as well as its bandwidth requirement) arrive one by one and there is no a priori knowledge regarding future LSP set-up requests. In addition, we consider rerouting of LSPs in this work. Rerouting of LSPs has not been well studied in previous work on LSP routing. The need of LSP rerouting arises in a number of ways: occurrence of faults (link and/or node failures), re-optimization of existing LSPs' routes to accommodate traffic fluctuation, requests with higher priorities, and so on. We formulate the bandwidth guaranteed LSP routing with rerouting capability as a multi-commodity flow problem. The solution to this problem is used as the benchmark for comparing other computationally less costly algorithms studied in this paper. Furthermore, to more efficiently utilize the network resources, we propose online routing algorithms which route bandwidth demands over multiple paths at the ingress router to satisfy the customer requests while providing better service survivability. Traffic splitting and distribution over the multiple paths are carefully handled using table-based hashing schemes while the order of packets within a flow is preserved. Preliminary simulations are conducted to show the performance of different design choices and the effectiveness of the rerouting and multi-path routing algorithms in terms of LSP set-up request rejection probability and bandwidth blocking probability.
Liquid crystals have been shown to align on polarized ultra- violet (PUV) exposed polyimide films with the liquid crystal director oriented perpendicular to the polarization direction. Uniform pretilt is generated by a second oblique PUV exposure. Our results indicate that the liquid crystal has uniform pre-tilt with no preferred alignment direction on unexposed polyimide films. Normal exposure aligns the director but the pre-tilt is degenerate, pointing either to the plus or minus direction. Oblique exposure breaks the degeneracy producing a unique pre-tilt direction. The pre- tilt angle is related to the degree of imidization of the polyimide film. By adjusting the chemistry of polyimide, the pre-tilt angle can be varied continuously. Varying the structure of polyimide results in liquid crystal aligning either parallel or perpendicular to the PUV direction. Uniform alignment and pre-tilt can be produced with a single oblique exposure if the liquid crystal aligns parallel to the polarization direction. The photoalignment of liquid crystal on thin films of the azo dye methyl red spin coated on indium tin oxide glass surface is observed for the first time. Methyl red molecules are believed to first adsorb on the substrate with an isotropic orientation. After exposure with polarized visible light, the elongated structure of the molecule results in dichroic absorption of the light by molecules oriented parallel to the polarization direction. Our study shows that photo induced cis/trans isomerization process produces the preferred alignment of methyl red and therefore the liquid crystal perpendicular to the polarization direction.
In this paper, a number of spatial/spatial-frequency image representations are reviewed. Wavelets have recently generated much interest, both in applied areas as well as in more theoretical ones. Wavelet transform relative to some basic wavelets provides a flexible time- frequency window which automatically narrows when observing high frequency phenomena and widens when studying low frequency environments. As a result, it is suitable for visual information representation. Applications in computer vision such as image compression and image enhancement are examined.
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