Optical Cross-connect Nodes Research Articles

Optical networking that exploits massive wavelength/spectrum and spatial parallelisms

As DWDM transmission offers enhanced wavelength/spectrum parallelism, the capacity of optical networks has been substantially increased. Due to the theoretical capacity limit of C-band transmission over single-mode fibers, research into new frequency bands and parallel fibers has become very active. However, the hardware scale of current optical cross-connect nodes will explode with greater wavelength/spectrum and spatial parallelism. Three optical node/network architectures are presented in this paper that take advantage of one or both of these parallelism technologies. These architectures will provide a baseline for cost-effective and bandwidth-abundant future optical networks based on massive parallelism.

SOA Based Photonic Integrated WDM Cross-Connects for Optical Metro-Access Networks

We present a novel optical metro node architecture that exploits the Wavelength Division Multiplexing (WDM) optical cross-connect nodes for interconnecting network elements, as well as computing and storage resources. The photonic WDM cross-connect node based on semiconductor optical amplifiers (SOA) allows switching data signals in wavelength, space, and time for fully exploiting statistical multiplexing. The advantages of using an SOA to realize the WDM cross-connect switch in terms of transparency, switching speed, photonic integrated amplification for loss-less operation, and gain equalization are verified experimentally. The experimental assessment of a 4 × 4 photonic integrated WDM cross-connect confirmed the capability of the cross-connect chip to switch the WDM signal in space and wavelength. Experimental results show lossless operation, low cross-talk <−30 dB, and dynamically switch within few nanoseconds. Moreover, the operation of the cross-connect switch with multiple WDM channels and diverse modulation formats is also investigated and reported. Error-free operation with less than a 2 dB power penalty for a single channel, as well as WDM input operation, has been measured for multiple 10/20/40 Gb/s NRZ-OOK, 20 Gb/s PAM4, and data-rate adaptive DMT traffic. Compensation of the losses indicates that the modular architecture could scale to a larger number of ports.

Energy efficiency in WDM fiber-optical links considering OADM/OXC nodes

In this paper, authors discuss the energy consumption for 1bit transmission as a function of spectral efficiency (SE, bps/Hz), aggregated traffic amount (C, bps), and overall transmission distance (LP2P, km) in wavelength division multiplexing (WDM) systems with optical add-drop multiplexer/ optical cross-connect (OADM/OXC) nodes. The choice of bitrate and modulation formats was limited to three options: 10G non-return-to-zero on-off keying (NRZ-OOK), 40G NRZ differential phase-shift keying (NRZ-DPSK) and 100 dual polarization quadrature phase-shift keying (DP-QPSK). The reasons for that is the legacy in transmission networks and high energy efficiency of 3R (re-amplification, re-timing, re-shaping) regenerators and transponders, respectively. In addition, these formats are commonly used for 10-40-100G mixed-line rate solutions in core networks. It is found out that 100G DP-QPSK is the best solutions among two other considered in terms of both energy consumption per bit and spectral efficiency. Finally and foremost, it is estimated that for the WDM channels added at the OADM/OXC node, energy consumption per 1 transmitted bit could growth more than two times as compared to wavelengths transmitted over point-to-point fiber-optical links and then dropped at the receiving node.

PERFORMANCE ANALYSIS OF WSXC AND WIXC SSM OXC IN WDM OPTICAL NETWORKS

The impact of inband crosstalk on an optical signal passing through optical cross-connect nodes (OXC’s) in wavelength division multiplexing (WDM) optical network, is studied from the equation of electric field with crosstalk and the corresponding current. The analysis has been done for two SSM (space switching matrix) OXC architecture namely WSXC & WIXC where later one has full wavelength conversion capability. Although WIXC attenuates more crosstalk though it is found that depending on the values of optical propagation delay differences, coherent time of lasers and time duration of one bit of the signal, the required power penalty in WIXC may be greater than that of WSXC in some cases. The analysis has been performed on the measures of Bit Error Rate (BER) and Power Penalty.

Ultracompact Waveband Cross-Connect Module Using Waveband Selective Switches: Development and Performance Verification

A compact waveband cross-connect switch module is demonstrated that will be utilized for creating cost-effective multidegree optical cross-connect nodes. Its performance confirms the practical applicability of the developed module.

Wavelength conversion and in-band crosstalk in WDM optical networks

Wavelength-routed all-optical networks have emerged as a popular architectural solution for wide area networks and are being conceived for future broadband communications. Call connection probability in such networks depends on the number of WDM wavelengths employed and on the capability for wavelength conversion at network nodes. Equipping all the network nodes with wavelength conversion capability to reduce the blocking probability is not a cost effective solution due to the high cost of wavelength converters (WC). Hence, networks equipped with converters at only some of the nodes are more practical. However, these nodes should be identified optimally so that network blocking probability is minimized. In this paper, wavelength converters based on four wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is considered. In WDM all-optical networks, in-band crosstalk is regarded as one of the major transmission impairments. In-band crosstalk usually arises when multiple signals at identical or adjacent wavelengths pass through an optical crossconnect node. In-band crosstalk leads to increased receiver BER. This paper considers in-band crosstalk arising in a network which is equipped with wavelength converters. For every dynamically arriving connection request, a route is determined using Dijikstra’s algorithm and a free wavelength is determined using the random wavelength assignment. If no common free wavelength is available, connection is tried using wavelength converters, if available, on the given route. If wavelength assignment does not succeed, the call is blocked. If wavelength assignment succeeds, the BER at the destination node is computed before establishing the lightpath. If the estimated BER exceeds 10−12, the call is blocked. Otherwise, the call is admitted. The results reveal that wavelength converters are not useful in networks that already suffer from crosstalk conditions.

Placements of Shared Wavelength Converter Groups Inside a Cost-Effective Permuted Clos Network

The Clos network is used for optical cross-connect and optical packet switch node, and has a large number of cross-points if the number of wavelengths per fiber is more than that of fibers per node. In this letter, we propose a cost-effective permuted Clos network to reduce the number of crosspoints. We further investigate the placement of fixed wavelength converter groups (WCGs) shared inside the permuted Clos network. On the contrary, a normal Clos network has to be equipped with expensive tunable WCGs. The simulation result implies that equipment of about 50% WCGs achieves similar performance as the full wavelength interchanging node does.

Low crosstalk improved dilated Benes networks for photonic switching

An improved dilated Benes (IDB) network and a generally improved dilated Benes (GIDB) network are introduced to improve the crosstalk characteristic of the dilated Benes (DB) network. The signal-to-noise ratio characteristic of the IDB network and the GIDB network is compared with the DB network. Bit error rate produced by crosstalk is experimentally investigated and compared. Results show that the GIDB network is a low crosstalk photonic switching network that can be used in large capacity optical cross-connect nodes.

Optimal design for survivable backbones with end-to-end and subpath wavebanding

We deal with the optimal design of a survivable optical mesh network supporting end-to-end and subpath waveband switching (WBS) in order to compare various WBS schemes according to a given cost function. An integer linear programming formulation is proposed to minimize the global network cost, which is expressed in terms of port cost, fiber cost, and propagation delay cost. We assume a single-layer multigranularity optical cross-connect (MG-OXC) node with a variable number of incoming fibers and a fixed number of wavelengths per waveband. The results show that on our case-study networks WBS with joint end-to-end and subpath wavebanding guarantees a gain of up to 22% and 85% in port number and global network costs, respectively. © 2006 Optical Society of America OCIS codes: 000.1200, 060.0060.

Numerical Study of Cascadability Performance of Continuous Spectrum Wavelength Blocker/Selective Switch at 10/40/160 Gb/s

Wavelength selective switches (WSS) and wavelength blockers (WB) are crucial elements in reconfigurable optical add-drop multiplexers and cross-connect nodes. For the continuous spectrum WB/WSS (CS-WB/WSS), the overall filtering passband of the WSS device can be adjusted in a quasi-continuous way by grouping two or more adjacent pixels offering enhanced switching flexibility at arbitrary channel spacing or multiline rates. However, loss and group delay ripples appear at each slot boundary causing degradations to the transmitted signal. In this paper, a system perspective analytical model has been developed. Through this model and detailed simulation studies, the relative impact of each imperfection on the overall cascadability performance of the WB/WSS is investigated. Different switching scenarios at various line rates (10, 40, and 160 Gb/s) have been considered and the corresponding specifications for the WSS device have been identified

Multiple attack localization and identification in all-optical networks

The security characteristics of currently emerging all-optical networks display many unique features compared to traditional communication networks. In particular, network transparency raises many security vulnerabilities that differ substantially from conventional failures and should therefore be treated differently. One of the serious problems related to transparency lies in the fact that optical crosstalk is additive and can be exploited to perform service disruption attacks upon the network. Since these attacks can spread rapidly through the network, causing additional problems and triggering multiple alarms, they must be detected and identified at any point in the network where they may occur. However, to monitor all wavelength channels at several detection points into any node is likely to be very expensive. In this paper we provide formal specifications for optical crosstalk that can arise in optical cross-connect nodes. Based on these specifications, we propose an algorithm for localizing the sources of multiple attacks and identifying their nature in all-optical networks.

Highly-Reliable and Fast M:N End-to-End Restoration Scheme for Photonic IP Networks

The concept of an optical path layer has become increasingly attractive with the growth of traffic in the backbone network. The recent advances in optical switching technology support the deployment of optical cross-connect (OXC) nodes and the construction of large-scale optical path networks. This paper proposes a highly-reliable and fast preassigned restoration scheme for optical path networks. To achieve the preassigned restoration scheme, this paper investigates the extension of the Generalized Multi-Protocol Label Switching (GMPLS) protocol functionality considering the interoperability with GMPLS capable IP routers in the future. This paper also proposes a new network control architecture called the partition model through discussion of network architecture. We clarify that the M:N end-to-end restoration scheme achieves efficient resource usage and management of the network especially in the partitioned model network. With the finite design of the GMPLS protocol extension based on the M:N end-to-end restoration scheme, we successfully achieve an intelligent protocol that guarantees 100% recovery against single link failure and is capable of protection grade fast restoration of the optical path less than 50 msec. To our knowledge, this is the first demonstration of GMPLS-controlled protection grade fast optical path restoration.

Semi-Dynamic Protection Path Configuration for Hybrid Optical Cross-Connect Networks

In this paper, a semi-dynamic protection path configuration method is proposed for WDM optical networks. In the method, the protection path is established by connecting several sub-lightpaths from the source node to the destination node of the original working lightpath, as opposed to conventional path restoration method where a single protection lightpath between the source-destination pair performs restoration. The proposed method provides enhanced flexibility in protection path configuration and relieves the cost of spare capacity reservation. This paper also studies the effects of wavelength conversion capability of intermediate optical cross-connect nodes on protection path routing and spare capacity utilization. In terms of spare capacity utilization, the proposed method shows substantial reduction of spare capacity overhead compared with dedicated path restoration in all optical networks without wavelength conversion, and shows similar capacity efficiency compared with shared path restoration in opaque networks with full wavelength conversion capability. In terms of robustness, the proposed method shows nearly the same restoration ratio for double-link failure as that of dynamic restoration method.

QoS-Friendly Wavelength Assignment in Dynamic Wavelength-Routed Optical Networks

In-band crosstalk has been widely considered as a major transmission impairment that significantly impacts the bit error rate (BER) performance of lightpaths in circuit-switched all-optical wavelength-routed networks. Such crosstalk usually occurs when multiple wavelengths pass through an optical crossconnect node, and the magnitude of the crosstalk is largely dependent on the wavelengths assigned to the lightpaths. Traditional wavelength assignment (WA) schemes pay little regard to the physical layer quality of service (QoS), and hence cannot provide optimized network performance in practical networks with imperfect physical transmission media. In this paper, we first present our categorization of in-band crosstalk based on the location of crosstalk generation, then we propose two QoS-friendly WA approaches, one of which selects the wavelength based on the estimated BERs, and the other based on the weighted crosstalk number counting. The two approaches have different computation complexities. Numerical results show that both approaches can significantly improve the BER blocking rate by suppressing the created in-band crosstalk, but the BER-based approach generally gives the better performance at the price of more extensive computation.

Differentiated Wavelength Assignment with QoS Recovery for DWDM Next Generation Internet Backbone Networks

This paper describes a differentiated wavelength assignment strategy for the appropriate allocation of wavelengths on wavelength-routed quality of service (QoS) routes with differentiated QoS class and recovery capabilities. The paper is based on the differentiated MPλS services (DMS) model as a framework for optical bandwidth management and the real time provisioning of optical channels in re-configurable transport networks. MPλS stands for multi protocol lambda switching which is a functional part of GMPLS (generalized MPLS). We discuss here a QoS recovery scheme at the optical layer, at the MPλS layer, and at the IP layer, as related to QoS failure caused by device failures or attack-induced faults in dense-wavelength division multiplexing (DWDM) transport networks. According to the QoS characteristics of wavelength in optical links and the type of used optical cross-connect (OXC) nodes, a differentiated wavelength assignment strategy that considers QoS recovery capability is also described. We point out important problems that should be resolved in the DMS model for optical QoS routing with differentiated wavelength assignment.

GORA: an algorithm for designing optical cross-connect nodes with improved dependability

This paper considers the problem of enhancing the dependability (availability and its influencing factors) of optical cross connect nodes. These nodes are expected to be a crucial part of future optical communication networks. The genetic optimum redundancy allocation (GORA) algorithm proposed here is based on a hybrid genetic algorithm and solves the dependability enhancement problem by optimizing the allocation of redundant modules in the nodes. Three redundancy schemes are considered: (1) global duplicate (2) local standby and (3) k-out-of- n:G redundancy. After applying the GORA algorithm to four example nodes, the optimized third redundancy scheme provided the overall best dependability performance. This suggests that redundancy is more effective when provided at the OXN subsystem level. Further capabilities of the GORA algorithm as a network planning tool also presented and some improvements are proposed.

Optical hop number limits imposed by various 2 x 2 cross-connect node designs

The success of transparent optical transport networks depends on the availability of optical cross-connect nodes (OXNs) that induce minimal impairments on the signals they cross-connect. This should extend the possible coverage and flexibility of path restoration within a meshed network topology by raising the upper bound on the achievable optical hop (traversable OXNs) number. We provide a brief survey and categorization of the currently proposed OXNs. Furthermore, the possible limits they impose on the number of hops are established by a series of transmission performance simulations. Microoptic and all-fiber OXNs are identified to be suitable for networks with a low connectivity and channel count. In case larger OXNs are needed, then microoptic and integrated OXNs provide a better option. The results obtained are applicable as guidelines for the deployment of future optical ring topologies.

An Improved All-Fiber Cross-Connect Node for Future Optical Transport Networks

In this paper, we briefly review the current state-of-the-art in fiber devices for communication systems and propose some modifications to a previously reported all-fiber optical cross-connect node. These modifications seek to improve the node's spectral characteristics, modularity, and optical spectral efficiency. Similar recommendations are also made for optical add–drop multiplexers to be used in optical ring topologies. Using the added features, the transmission performance of the modified 2×2 node is analyzed and comparisons are made with the previously reported nodes of the same dimensions. The results indicate a significant improvement in node cascadability performance over the latter case. Further comparisons were carried out to include other types of all-fiber cross-connect nodes, with the modified node again proving to be an overall competitive solution.

Crosstalk Limitation in Optical Cross-Connect Node with Filters

Crosstalk is an important limiting factor in optical wavelength division multiplexing (WDM) networks with optical cross-connect (OXC) nodes. The impact of coherent and incoherent crosstalk on an optical signal passing through a typical OXC architecture is studied in this paper. We give the analytical power penalty expressions for both coherent and incoherent cases. Through simulation calculation, the coherent crosstalk is found to be the most important limitation in the OXC. In order to relax the total crosstalk specification requirement for components, the use of filters is proposed. On the other hand, the power penalty depends less on the number of input fibers N compared with the number of multiplexing wavelengths m for the ideal performance of mechanical switch. If the number of links must to be increased due to traffic demand, we can gracefully adapt to the demand by adding modular units one by one to the design maximum of N.

Coherent and incoherent crosstalk in WDM optical networks

The impact of coherent and incoherent crosstalk on an optical signal passing through optical cross-connect nodes (OXC's) in wavelength division multiplexing (WDM) optical networks is studied, and the analytical expressions are given. Such crosstalk will be generated when the optical propagation delay differences of optical paths in an OXC do not exceed the coherent time of the lasers. While causing fluctuation of signal power, coherent crosstalk may cause noise or not, depending on the relationship between the optical propagation delay differences and the time duration of one bit of the signal. Incoherent crosstalk may cause very high noise power, because it can be a coherent combination of crosstalk contributions. The statistical impact of all crosstalk contributions on signal is studied by simulation, and the concept of quantile is proposed to relax the crosstalk specification requirement for components. The crosstalk specification requirements are then obtained for components used in WDM optical networks with different scales.