Fault tolerance is one of the most desirable properties in the optical network since a large amount of data will be lost if the failure recovery cannot be well achieved. The software-defined network is an innovative paradigm—which decouples the control module from the underlying data forwarding plane—to make fast decisions on detecting and restoring link failures. Therefore, we focus on failure recovery solutions based on software-defined optical networks. The out-of-band control mechanism is utilized for the communication between the controller and the data forwarding elements. We demonstrate the performance of the proposed solutions—including a failure detection scheme, a dynamic all pairs shortest paths algorithm, and a failure recovery application—based on our software-defined optical network platform. Experimental demonstration and numerical evaluation both show its overall feasibility and efficiency.
Free-space optical (FSO) communication system is vulnerable to link disruption due to the propagation loss and atmospheric turbulence. Cooperative FSO system has been proposed to mitigate the adverse effects of link disruption by the cooperative transmission in assistance of relay. However, most of the related works focus on the outage probability analysis and put less attention on the resource utilization. In fact, there exists a waste of bandwidth resource due to time slot waiting and more FSO transceivers are occupied. We mainly focus on the relay selection and link scheduling mechanisms of cooperative FSO system to improve not only network reliability but also resource utilization. We first propose a relay selection algorithm as a solution to combat channel fading. In the relay selection algorithm, two cooperative modes including intrarelay-set and inter-relay-set cooperative modes are proposed to enhance network capacity. Then, a link scheduling algorithm is further proposed to improve the bandwidth utilization and reduce the occupancy rate of transceivers by utilizing more sufficiently the idle FSO links. Simulation results show that the proposed algorithms can provide significant performance gain for cooperative FSO system in reliability and resource utilization and are highly suitable for the growing traffic demands.
As a promising technology for broadband communication, passive optical network (PON) has been deployed to support the last-mile broadband access network. In particular, time-division-multiplexing PON (TDM-PON) has been widely used owing to its mature technology and low cost. To practically implement TDM-PONs, the combination of intensity modulation and direct detection is a very promising technique because it achieves cost reduction in system installation and maintenance. However, the current intensity-modulation and direct-detection TDM-PON still suffers from some problems, which mainly include a high-power penalty, detrimental Brillouin backscattering (BB), and so on. Thus, using directly modulated lasers (DMLs) and colorless optical network units (ONUs), respectively, two intensity-modulation and direct-detection TDM-PON architectures are proposed. Using VPI (an optical simulation software developed by VPIphotonics company) simulators, we first analyze the influences on DML-based intensity-modulation and direct-detection TDM-PON (system 1) performances, which mainly include bit error rate (BER) and power penalty. Next, the BB effect on the BER of the intensity-modulation and direct-detection TDM-PON that uses colorless ONUs (system 2) is also investigated. The simulation results show that: (1) a low-power penalty is achieved without degrading the BER of system 1, and (2) the BB can be effectively reduced using phase modulation of the optical carrier in system 2.
KEYWORDS: Orthogonal frequency division multiplexing, Modulation, Distortion, Optical engineering, Nonlinear optics, Receivers, Data communications, Interference (communication), Data conversion, Signal generators
High peak-to-average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) signal is one of the limitations to the transmission performance of an optical OFDM system. Many PAPR reduction techniques have been proposed in previous works. However, most of them only consider a single scheme while overlooking the complementary features of these techniques. We propose a twofold PAPR reduction technique called Hadamard transform combined with a partial transmit sequence (PTS) (HTCP). The proposed HTCP scheme combines the merits of two complementary techniques, i.e., Hadamard transform and PTS, to improve the performance of the optical OFDM system in terms of PAPR and bit error rate (BER). Furthermore, the side information generated in PTS is transmitted by pilot sequences which increase the utilization of subcarriers. Least square estimation is used to estimate the pilot signal’s phase to recover the side information. The HTCP scheme is theoretically analyzed in a direct-detection optical OFDM system. Simulation results show that the HTCP scheme has a better performance with regards to PAPR and BER compared with the case of applying only the Hadamard transform or PTS technique.
In order to achieve multigigabit transmission in deep-space optical communication, our study applies a new modulation mode named orbital angular momentum (OAM) modulation, and uses the encoded OAM states of single photon as data information carriers, thus providing a reliable and high-speed transmission of signals. According to the long link characteristic of deep-space communication, we conduct a reasonable deployment for communication nodes in deep-space environment. First, we present the reliability of deep-space channel and analyze the data rate and spectral efficiency of beams with OAM. Second, we study the characteristics and generations of vortex beams with OAM by simulation. Results show that vortex beams have better spatial multiplexing capability of realizing high capacity data transmission. Finally, we propose an encoding method with OAM states of single photon. The transceiver units are based on spatial light modulators to perform the modulation and demodulation of vortex beams. At the receiver, the charged-coupled device camera is used to detect the signal intensity and decodes the OAM states. Our proposal not only ensures the confidentiality of deep-space optical communication, but also greatly increases the transmission rate.
Through integrating advantages of optical and wireless communications, the Fiber-Wireless (FiWi) has become a promising solution for the “last-mile” broadband access. In particular, greening FiWi has attained extensive attention, because the access network is a main energy contributor in the whole infrastructure. However, prior solutions of greening FiWi shut down or sleep unused/minimally used optical network units for a single segment, where we deploy only one optical linear terminal. We propose a green mechanism referred to as energy-efficient ring (EER) for multisegment FiWi access networks. We utilize an integer linear programming model and a generic algorithm to generate clusters, each having the shortest distance of fully connected segments of its own. Leveraging the backtracking method for each cluster, we then connect segments through fiber links, and the shortest distance fiber ring is constructed. Finally, we sleep low load segments and forward affected traffic to other active segments on the same fiber ring by our sleeping scheme. Experimental results show that our EER mechanism significantly reduces the energy consumption at the slightly additional cost of deploying fiber links.
Due to the number of wavelengths in fibers increasing, the transmission ports consumed in optical cross-connect (OXC) are greatly enhanced. To reduce the complexity and the cost of OXC, waveband switching technology is proposed. At the same time, since an optical channel carries a lot of traffic, its failure may lead to a huge data loss. Therefore, survivability in waveband switching (WBS) optical networks is important. Previous work for survivable WBS optical networks did not consider the holding time of connection request. However, in a practical network, the different connection requests generally have different holding times. This fact affects the network performances. In this paper, we propose a new method, holding time aware differentiated protection (HTADP), to provide the survivability for single-link failure in WBS optical networks. In HTADP, if the holding time of a connection request is smaller than the preset threshold, the protection path will not be assigned for saving network resources. In addition, we design a waveband integrated auxiliary graph for HTADP to well support the routing and waveband assignment. Simulation results demonstrate that HTADP not only saves more transmission ports but also is able to obtain lower blocking probability compared with previous method.
In wavelength-division-multiplexing networks, the fault-tolerance is very important since failures may lead to a significant amount of data loss. At the same time, the waveband switching technique is very interesting since it can save switching ports in optical cross-connections. In addition, the hierarchical routing in multidomain optical networks is very challenging since the interdomain routing is based on aggregative virtual topology. However, previous works did not jointly consider the three issues. We propose two schemes, segment-based protection (SBP) and path-based protection (PBP), to provide the fault-tolerance and waveband grooming in multidomain optical networks. In SBP, for each connection we first compute an interdomain loose route based on virtual topology, and then we compute the intradomain exact path-pairs in each single domain traversed by the loose route based on physical topology. In PBP, for each connection we first compute an interdomain loose primary path and an interdomain loose backup path based on virtual topology, and then we compute the intradomain exact paths in each single domain traversed by the loose primary path and backup path based on the physical topology. To support multidomain waveband grooming, we present the layered auxiliary graph. Simulation results show that PBP performs better than SBP.
As a special case of p cycles, a Hamiltonian cycle protection scheme is proposed to achieve fast failure restoration and simple management in fault-tolerant networks. We extend the idea of a Hamiltonian cycle protection scheme to fault-tolerant wavelength-division-multiplexing (WDM) optical fiber networks, and propose a new Heuristic Hamiltonian cycle protection algorithm (HHCPA) to tolerate the single-fiber failure. In the HHCPA, we consider the idea of differentiated protection for different-level demands, i.e., high-level demands with protection requirements and low-level demands without protection requirements. We also develop the link-cost function to achieve the load balancing and proper link selection in computing the light paths for each demand to effectively reduce the backup wavelength consumption. Simulation results show that, compared to conventional algorithm, the HHCPA can obtain significant performance improvement in resource utilization ratio and blocking probability.
We propose a novel survivable algorithm, called backup-shared with trap avoidance (BSTA), to protect against single failure with shared-risk link groups (SRLGs) in wavelength-division multiplexing optical networks. In BSTA, each connection request will be assigned one primary path and one SRLG-disjoint backup, or one primary path and two SRLG-disjoint segment-backup paths. In contrast with previous algorithms, BSTA not only can effectively avoid the trap problem, but also can obtain better resource utilization ratio and lower blocking probability. Simulation results are shown to be promising.
In wavelength-division-multiplexing optical networks, the fiber links may share some common physical resources (e.g., cables, conduits), and the consequence is that they have a correlated link failure probability (CLFP), which means that the probability that link l will fail depends on whether link f has failed. Based on CLFP, we propose a new heuristic survivable algorithm, called differentiated path-shared protection (DPSP), to protect against double-link failures in WDM optical networks. In DPSP, each connection request can be assigned one working path and additional backup paths according to the differentiated reliability requirements of users. Compared to previous work, DPSP can obtain better performances in resource utilization ratio and blocking probability.
This paper investigates the dynamic survivable routing problem and proposes an effective path-based protection algorithm, called the dynamic joint routing approach (DJRA), to completely protect against double-link failures in meshed wavelength division multiplexing optical networks. In DJRA, each connection is assigned a primary path and two link-disjoint backup paths to tolerate the simultaneous failures of two arbitrary fiber links. In order to improve the algorithm performance, we consider three key issues: (1) sharing spare backup resources, (2) failure recovery time, and (3) trap-path avoidance. Therefore, compared to previous algorithms, DJRA obtains better performance and is also able to yield flexible trade-off solutions.
In this paper, we describe and investigate the protection design for survivable WDM mesh networks, and propose our heuristic shared-path protection algorithm, called dual-link failures protection (DLFP). We also present a backup resources assignment scheme to completely protect the dual-link failures, and suggest a joint selection method, which consider the integrated performance of resource utilization and protection recovery time, to compute an optimal working and backup path pair. The simulation results show that DLFP can not only perform better than previous three-step algorithm (TSA) for dual-link failures but also determine the appropriate tradeoffs between the resource utilization ratio (or blocking ratio) and the protection recovery time.
Under the constraints of the number of transceivers per node and wavelength continuity, the fairness issue of connections with different bandwidth granularities in survivable traffic-grooming WDM mesh networks is investigated. An extended wavelength-plane graph (EWG) model and two dynamic grooming path protection algorithms with considering blocking fairness are proposed. Based on dynamic traffic with different load, the performance of our schemes has been evaluated via simulations. The results show that they provide good blocking fairness while at the same time ensure a small increase in the overall traffic blocking probability.
In this paper, we propose a new shared-path protection approach, called joint routing algorithm (JRA), under shared-risk link group (SRLG) constraints for survivable WDM mesh networks. JRA differs from previous algorithms that are socalled separated routing algorithm (SRA), and can find K path pairs and select an optimal path pair as the result, while SRA can only find a path pair that may be not an optimal routing pair. So, JRA can perform better than SRA. We also study the relationship between the protection switching time and the resource utilization, and suggest a new joint cost function to compute the least-cost path pairs. Under dynamic traffics with different load, the simulation results show that JRA not only has better performances than SRA but also can determine the appropriate tradeoffs between the resource utilization ratio (or blocking ratio) and the protection switching time.
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