Translucent network design from a CapEx/OpEx perspective

Translucent wdm network design has been widely investigated during the last 10 years. Translucent networks stand halfway between opaque and transparent networks improving the signal budget while reducing the network cost. On one hand, opaque networks provide satisfying quality from source to destination by the use of electrical reg regeneration (Re-amplifying, Re-shaping, and Re-timing) at each network node. In addition to their high cost inherent to numerous 3R regenerations, opaque networks are also constrained by the bit-rate dependence of electrical components. Transparent networks, on the other hand, do not include any electrical regeneration; therefore, the signal quality is degraded due to the accumulation of linear and non-linear effects along the signal’s route. Translucent networks include electrical regeneration at some network nodes. Among the different possible strategies for translucent network design, sparse regeneration inserts regenerators whenever needed to help establish connection requests. In this context the objective of translucent network design is to judiciously choose the regeneration sites in order to guarantee a certain quality of transmission while minimizing the network cost. In this paper, we propose to solve the translucent network design problem by introducing a heuristic for routing, wavelength assignment, and regenerator placement. This heuristic, called COR2P (Cross-Optimization for RWA and Regenerator Placement) aims not only to minimize the number of required regenerators, but also to minimize the number of regeneration sites. In this perspective, we introduce an original cost function that contributes to the optimization of CapEx/OpEx expenditures in translucent network design. In fact, the CapEx-to-OpEx ratio strongly depends on the pricing and management strategy of the carrier. In this respect, COR2P is designed in a way that its parameters can be adjusted according to carriers’ strategies. In order to discuss its different features, we compare COR2P performance with two other algorithms proposed in the literature for translucent network design.     

Benders decomposition algorithms for the fixed-charge relay network design in telecommunications

In translucent networks, signal regenerators that operate based on optical/electrical/optical (O/E/O) conversion are employed to handle the problems of physical impairments and resulting signal quality issues in long-distance transmissions. Regenerators are placed at the relay nodes in a translucent network in such a way that the optical signals are not transmitted farther than a threshold distance without a 3R regeneration (reamplification-reshaping-retiming). In this study, we consider a fixed-charge relay network design (FCRND) problem whose applications are found mainly in long distance translucent optical telecommunication networks. From a modeling perspective, FCRND combines prominent features of the uncapacitated single-assignment hub location (location of relays points) and the fixed-charge network design problems (choice of transmission links in the network and routing of signals) while additionally considering distance-based coverage constraints. We devise Benders decomposition based solution methodologies in which the algorithmic performance is further enhanced by devising strengthened and disaggregated Benders cuts, surrogate constraints for the master problem, and an upper bound heuristic to both obtain and tighten optimality Benders cuts. We obtain solutions within 2.0 % of optimality in very reasonable times as we illustrate in our computational study.     

GMPLS-controlled dynamic translucent optical networks

The evolution of optical technologies has paved the way to the migration from opaque optical networks (i.e., networks in which the optical signal is electronically regenerated at each node) to transparent (i.e., all-optical) networks. Translucent optical networks (i.e., optical networks with sparse opto-electronic regeneration) enable the exploitation of the benefits of both opaque and transparent networks while providing a suitable solution for dynamic connections. Translucent optical networks with dynamic connections can be controlled by the GMPLS protocol suite. This article discusses the enhancements that the GMPLS suite requires for the control of dynamic translucent optical networks with quality of transmission guarantees. Such enhancements concern QoT-awareness and regenerator-awareness and can be achieved by collecting and disseminating the information on QoT and regenerator availability, respectively, and by efficiently leveraging such information for traffic engineering purposes. More specifically, the article proposes two distributed approaches, based on the routing protocol and the signaling protocol, for disseminating regenerator information in the GMPLS control plane. Moreover, three strategies are introduced to efficiently and dynamically designate the regeneration node(s) along the connection route. Routing and signaling approaches are compared in terms of blocking probability, setup time, and control plane load during provisioning and restoration.     

Optimal regenerator assignment and resource allocation strategies for translucent optical networks

Physical layer impairments in wavelength-routed networks limit the maximum distance, a signal can travel in the optical domain, without significant distortion. Therefore, signal regeneration is required at some intermediate nodes for long-haul lightpaths. In translucent WDM networks, sparsely located regenerators at certain nodes can be used to offset the impact of physical layer impairments. The routing and wavelength assignment (RWA) techniques in such translucent networks need to take into consideration the availability of regenerators and the maximum optical reach of the transparent lightpaths (without any regeneration). Although there has been significant research interest in RWA algorithms for translucent networks, much of the research has focused on dynamic RWA techniques. Only a handful of recent papers have considered the static (offline) case, and they typically propose heuristic algorithms to solve this complex design problem for practical networks. In this paper, we propose a generalized integer linear program (ILP) based formulation for static regenerator assignment and RWA in translucent WDM optical networks, with sparse regenerator placement. To the best of our knowledge, such a formulation that optimally allocates resources for a set of lightpaths for translucent networks, given the physical network, the locations of the regenerators, and the maximum optical reach has not been considered before. The proposed formulation is important for two reasons. First, it can serve as a benchmark for evaluating different heuristic approaches that may be developedin the future. Second, we show that using a novel node representation technique, it is possible to drastically reduce the number of integer variables. This means that unlike existing ILP formulations, our approach can actually be used to generate optimal solutions for practical networks, with hundreds of lightpath demands.     

Dynamic routing in translucent WDM optical networks: the intradomain case

Translucent wavelength-division multiplexing optical networks use sparse placement of regenerators to overcome physical impairments and wavelength contention introduced by fully transparent networks, and achieve a performance close to fully opaque networks at a much less cost. In previous studies, we addressed the placement of regenerators based on static schemes, allowing for only a limited number of regenerators at fixed locations. This paper furthers those studies by proposing a dynamic resource allocation and dynamic routing scheme to operate translucent networks. This scheme is realized through dynamically sharing regeneration resources, including transmitters, receivers, and electronic interfaces, between regeneration and access functions under a multidomain hierarchical translucent network model. An intradomain routing algorithm, which takes into consideration optical-layer constraints as well as dynamic allocation of regeneration resources, is developed to address the problem of translucent dynamic routing in a single routing domain. Network performance in terms of blocking probability, resource utilization, and running times under different resource allocation and routing schemes is measured through simulation experiments.     

Impairment constraint multicasting in translucent WDM networks: architecture, network design and multicasting routing

The popularity of broadband streaming applications requires communication networks to support high-performance multicasting at the optical layer. Suffering from transmission impairments in multi-hop all-optical (transparent) WDM multicasting networks, the signal may be degraded beyond the receivable margin at some multicast destinations. To guarantee the signal quality, we introduce a translucent WDM multicasting network to regenerate the degraded signals at some switching nodes with electronic 3R (reamplification, reshaping and retiming) functionality. The translucent network is built by employing three kinds of multicasting capable switching architectures: (1) all-optical multicasting capable cross connect (oMC-OXC), (2) electronic switch and (3) translucent multicasting capable cross connect (tMC-OXC). Among them both the electronic switch and tMC-OXC are capable of electronic 3R regeneration. Furthermore, we propose a multicast-capable nodes placement algorithm based on regeneration weight, and two multicasting routing algorithms called nearest hub first and nearest on tree hub first to provide signal-quality guaranteed routes for the multicasting requests. The numerical simulation on two typical mesh networks shows that it is sufficient to equip 30% of the nodes or less with signal-regeneration capability to guarantee the signal quality.     

Towards the realistic planning of GMPLS-based optical transport networks

The multi-granular switching concept defined in Generalized Multiprotocol Label Switching (GMPLS) is expected to be a future-proof solution for mitigating the Optical Crossconnet scalability problems associated with the skyrocketing growth of traffic in optical transport networks. In this paper, we address the problem of planning the GMPLS-based (or multi-granular) transport network with color (or label) conversion and signal regeneration capabilities. The objective of the problem is to minimize the total weighted port count in the transport network. The novelty of this problem lies in the incorporation of the following for the first time: (1) considering all traffic granularities defined in GMPLS; (2) allowing wavelength, waveband, and fiber conversions; (3) considering the optical-reach limitation of optical signals; and (4) customizing the optical reach of all-optical paths. Due to the computational complexity of the problem, we propose various efficient heuristics that are capable of solving large-sized problems in a reasonable amount of time. In order to achieve the best possible solution to the planning problem, a comprehensive evaluation of different heuristic variations through illustrative examples and simulations is conducted. The results also provide valuable insights into many issues that can contribute to further research and development in this area.     

混合线路速率的半透明光网络中感知光路传输质量(QoT)的流量疏导、路由与波长分配(GRWA)方法

A Mixed Line Rate (MLR) optical network is a good candidate for a core back-bone network because of its ability to provide diverse line rates to effectively accommodate traffic demands with heterogeneous bandwidth requirements. Because of the deleterious effects of physical impairments, there is a maximum transmission reach for optical signals before they have to be regener-ated. Being expensive devices, regenerators are expected to be sparsely located and used in such a network, called a translucent optical network. In this paper, we consider the Grooming, Routing, and Wavelength Assignment (GRWA) problem so that the Quality of Transmission (QoT) for connections is satisfied, and the network-level performance metric of blocking probability is minimized. Cross-layer heuristics to effectively allocate the sparse regenerators in MLR networks are developed, and extensive simulation results are presented to demonstrate their effectiveness.     

Photonics in future telecommunications

In future telecommunication systems, efforts will be made to exhaust the almost unlimited transmission capacity of optical fibers by applying optical frequency-division-multiplexing (OFDM) techniques. The switching elements of such networks must be able to process OFDM signals as well as time-division-multiplexing (TDM) signals. The extent to which optical signal processing can be brought into use instead of present-day electronic signal processing in such switching elements is examined. Several experiments involving OFDM technologies are described which are intended to underline the significance of those technologies for future telecommunication networks. Various techniques for and experiments on the optical switching of signals in the frequency, space, and time domain are described. Experiments dealing with optical signal regeneration are described     

Distributed Dijkstra sparse placement routing algorithm for translucent optical networks

In this paper we study translucent optical networks as an alternative to fully transparent and fully opaque optical networks. In the former networks, a technique called sparse placement is used to overcome the lightpath blocking caused by the signal quality degradation, using much less regenerators, which must strategically be placed, in contrast to a fully opaque network. In this paper we propose a sparse placement algorithm based on two requirements. The first one is signal regeneration necessary to re-amplify, reshape, and retime the optical signals after some predefined transparent distance in order to successfully receive the signals at the destination node. The other is load balance of the traffic in the network aimed at efficient usage of the network capacity resources. We apply a distributed Dijkstra routing algorithm which dynamically changes weights of links during the process of locating regeneration capable nodes. We compare the performance of the proposed algorithm with commonly used sparse placement algorithms through simulation experiments. The benefits are such that load balancing of the network traffic is fully utilized, and with technological development it will be sufficient to equip up to 30% of nodes in the network with electronic regenerations in order to have the same performance as in an opaque network.

QoT-aware lightpath set-up in GMPLS-controlled WDM networks: A survey

GMPLS-controlled Wavelength Switched Optical Networks (WSON) often undergo upgrade in bit rate or in network element switching capability. In such a scenario many wavelength paths cannot be anymore considered pre-validated from the original network design phase. Thus, it is required to verify the quality of the optical path physical signal during path computation and signaling by considering the optical network impairments (e.g., attenuation, amplified spontaneous emission, etc.).In this paper, the general problem of impairment-aware path computation is presented and then solutions based on GMPLS extensions to be used during path set-up are considered. Specifically, an overview is presented where different approaches for implementing Quality of Transmission (QoT) estimation and QoT measurements in transparent networks are outlined. In addition, techniques for exploiting sparse regeneration in translucent networks are illustrated. Numerical results show that the proposed schemes are effective in finding or designating QoT-validated paths in only few set-up attempts when a fully transparent path is available. If the transparent path becomes unfeasible, the translucent approach is adopted showing how to optimally designate intermediate regenerators to satisfy the end-to-end QoT constraints.     

Distributed optical control plane for dynamic lightpath establishment in translucent optical networks based on reachability graph

In transparent optical networks, physical layer impairments (PLIs) incurred by non-ideal optical transmission media accumulate along an optical path, and the overall effect determines the optical feasibility of the lightpaths. In addition, transparent optical networks suffer from inefficient wavelength utilization, as a connection request may be rejected because of non-availability of a common wavelength on all the links along the chosen route. To increase optical reach, resource utilization, and average call acceptance ratio (and hence revenues), network operators are resort to translucent optical networks. In these networks a limited number of regenerators are placed at a selected set of nodes. In this scenario development of an optical control plane which is aware of PLIs, location and number of regenerators, is of paramount importance for on-demand lightpath provisioning. In this paper, we propose a novel approach of constructing a reachability graph of the physical network considering PLIs and regenerators. If there is no transparent path in the physical network, we route the connections with multiple transparent segments on the reachability graph. We propose efficient mechanisms and corresponding GMPLS protocol extensions for impairment and regenerator aware routing and wavelength assignment (IRA-RWA) in translucent optical networks. The simulation results suggest that our proposed approach together with LSP stitching signaling mechanism is feasible to implement and close to deployment.     

On determining the optimal optical reach for a long-haul network

Systems that enable optical bypass, where traffic transiting a node can remain in the optical domain as opposed to undergoing costly optical-electronic-optical (O/E/O) conversion, are gradually being accepted in carrier networks. An important factor in determining the cost effectiveness of such systems is the optical reach, the distance an optical signal can travel before needing to be regenerated. Longer optical reach results in a smaller number of required regenerations and hence less equipment and lower operating costs. In order to achieve longer reach, however, more expensive equipment such as amplifiers and transponders is typically needed. As the optical reach continues to increase, the cost benefit provided by reduced regeneration is eventually offset by the more expensive system equipment, leading to a concave curve of total network cost versus optical reach. This paper describes the analysis of four representative North American long-haul networks over a range of assumptions to determine the optimal optical reach from a cost perspective. In such networks, an optical reach in the range of 2500-3500 km yields the minimum, or close to the minimum, total capital cost over a wide range of assumptions, while representing a good tradeoff between decreased operating costs and increased initial network cost.     

On the improvement of network resource utilization efficiency for the establishment of static dependable connections in SRLG-constrained WDM translucent networks

In this paper, we investigate the problem of establishing static connections with fault-tolerant requirements, also known as dependable connections, taking into account quality of transmission constraints. To the best of our knowledge, this is the first study that tackles the aforementioned problem under shared risk link group (SRLG) constraints in translucent WDM optical mesh networks where typically a set of strategically localized network nodes are equipped with regeneration capability to overcome physical-layer impairment effects. A novel cross-layer heuristic approach is introduced to solve the problem for an heterogeneous networked scenario relying on a cost-effective two-stage protection procedure which combines the well-known path protection and partial path protection schemes in order to ensure instantaneous recovery from any SRLG-failure event. The proposed heuristic integrates a generic auxiliary graph model that incorporates various network heterogeneity factors such as the number of transceivers at each network node, the number of wavelengths on each fiber link, and the regeneration capability of each node, represented by different edges in the constructed graph. Moreover, the integrated auxiliary graph can be applied efficiently to model either single- or mixed-line-rate translucent WDM optical networks wherein different modulation formats are employed in order to support the transmission at different line rates. Our solution approach aims at maximizing the total number of accommodated requests by reducing network resource consumption through the simultaneous use of the backup–backup and primary–backup multiplexing techniques. We, here, present extended versions of these two techniques that generalize the sharing concept to some other important node resources—specifically, regeneration equipments which constitute the major cost factor in optical transport networks—in addition to link resources (i.e., wavelength channels). As far as we know, this is the first attempt to deploy simultaneously generalized versions of the backup–backup and primary–backup multiplexing techniques when considering static traffic patterns without compromising the 100 % fault-recoverability guarantee. The performances of the proposed heuristic are evaluated and discussed through extensive numerical experiments carried out on different network topologies. Significant improvements are demonstrated, either in terms of network blocking performance or in terms of resource utilization efficiency, in comparison with previously proposed approaches.     

Provisioning of dynamic traffic in mixed-line-rate optical networks with launch power determination

In mixed-line-rate (MLR) networks, different line rates on different wavelengths can coexist on the same fiber. MLR architectures can be built over transparent optical networks, where the transmitted signals remain in the optical domain along the entire path. Along the transparent optical path, a signal experiences various physical layer impairments (PLIs), and its quality degrades as it travels through each optical component. One of the major factors that affect the transmission quality is the launch power of the optical signal. The power must be large enough to ensure noise resiliency at the receiver, but it must be below the limit where fiber nonlinearities distort the signal. Moreover, high launch power is disruptive not only for the actual lightpath itself but also for neighboring lightpaths, and this effect is particularly critical in MLR networks since advanced modulation techniques used for high line rates are highly susceptible to PLIs. In this study, we investigate the problem of determining the appropriate launch power for provisioning of dynamic connection requests in MLR networks. By setting the appropriate launch power for each connection, we aim to maximize the number of established connections. We propose two different heuristics to determine the appropriate launch power of a lightpath. Worst-case best-case average (WBA) is based on optical reach of signal in a transparent optical network. In impairment-aware launch power determination (I-ALPD), current state of the network and impairments are evaluated to determine the launch power. The proposed approaches are practical and can adapt to the needs of network operators. Simulation results show that the performances of the proposed approaches show better results than the existing schemes in terms of blocking probability and bandwidth blocking ratio.     

Analysis of signal distortion and crosstalk penalties induced by optical filters in optical networks

The design of optical communication networks with network switching elements operating in the optical domain requires careful system analysis and potentially stringent component requirements. We consider here network elements such as transparent optical cross-connects that demultiplex WDM signals, optically switch individual channels, and then multiplex the wavelengths together again before transmission into the next span. Network element optical impairments that can significantly degrade signal quality are in-band (same wavelength) crosstalk and signal distortion from filter concatenation effects. We examine tradeoffs between accumulated crosstalk and filter distortion in the context of the optical filters used in the network elements and demonstrate the balance that must be struck in the design of the filters and network system. As an example, we study a 10-Gb/s network with 50-GHz channel spacing, examining both nonreturn-to-zero (NRZ) and return-to-zero (RZ) modulation formats. In both cases, we find optimal filter bandwidths that minimize the total signal degradation measured in terms of Q penalty, including filter misalignment statistics and signal laser frequency offset. A model is developed to treat the statistical nature of filter misalignment and its effect on filter-generated in-band crosstalk. The optical node penalties suffered by RZ signals can be significantly higher than that of NRZ signals and must be considered when estimating overall system reach.     

Network economics of optical transport networks with soft decision forward error correction (SD-FEC) technology

The goal of long haul DWDM transmission is to deliver error-free digital information at extremely high data rates and over very long distances, ideally without the need for regeneration of the signal. Forward error correction (FEC) is a method of encoding a signal with additional overhead information so that optical receivers can detect and correct errors that occur in the transmission path. The latest enhancement for FEC is the use of a soft decision algorithm that significantly improves the optical reach. This study evaluates the impact of soft decision forward error correction (SD-FEC) technology upon network design and economics in a long haul optical transport network. The network study shows that the SD-FEC technology not only reduces the total cost of ownership, but also simplifies the network design. Real-world network models are utilized to quantify and compare results.     

Optical networks research at UCL

New dynamic optical switching technologies and adaptive transmission techniques will allow increased capacity and flexibility in future core and metro networks. The Optical Networks Group at UCL has carried out research focused on these areas, in collaboration with industry and other university research groups, for over a decade. In this paper, the technical challenges to be overcome in implementing dynamic, high-capacity networks are discussed, and recent results of the group’s work are presented. New network architectures based on wavelength-routing and optical burst switching have been proposed and analysed, and the experimental demonstration of burst switching using fast wavelength-tunable lasers is described. High-capacity dense wavelength division multiplexed transmission systems, using advanced regeneration and equalisation techniques, have been studied, both theoretically, and experimentally using a dynamically reconfigurable recirculating fibre loop. Research results on advanced optical signal formats, all-optical 3R regeneration at 40 Gbit/s channel rates, adaptive analogue and digital electronic processing techniques and optical finite impulse response equalisation are presented. Finally, the development of optical performance monitoring technology, a key component of future all-optical networks, is described. The paper concludes with a prediction of future trends in optical communications research.     

The potential of photonic networks

Simple and flexible photonic networks will provide the basic technologies for the telecommunication networks of the 21st century, broadband-ISDN, and fiber-to-the-home. It is recognized that SDH and ATM systems have much difficulty increasing the throughput by electrical signal processing technologies, such as signal processing speed, latency, and so on. Photonic networks have the potential to solve these problems and to construct ultra-high-capacity, simple, and flexible transport networks. This becomes very promising with the development of optical fiber amplifier and other new optical device technologies. This article reviews trends in optical transmission technologies and discusses the possibilities of the photonic network. Conceptual photonic network examples for the transport network, access network, and customer premises network considering the present network are shown. Future research items for constructing the photonic network are also discussed     

Optical Network Design With Mixed Line Rates and Multiple Modulation Formats

With the growth of traffic volume and the emergence of various new applications, future telecom networks are expected to be increasingly heterogeneous with respect to applications supported and underlying technologies employed. To address this heterogeneity, it may be most cost effective to set up different lightpaths at different bit rates in such a backbone telecom mesh network employing optical wavelength-division multiplexing. This approach can be cost effective because low-bit-rate services will need less grooming (i.e., less multiplexing with other low-bit-rate services onto high-capacity wavelengths), while a high-bit-rate service can be accommodated directly on a wavelength itself. Optical networks with mixed line rates (MLRs), e.g., 10/40/100 Gb/s over different wavelength channels, are a new networking paradigm. The unregenerated reach of a lightpath depends on its line rate. So, the assignment of a line rate to a lightpath is a tradeoff between its capacity and transparent reach. Thus, based on their signal-quality constraints (threshold bit error rate), intelligent assignment of line rates to lightpaths can minimize the need for signal regeneration. This constraint on the transparent reach based on threshold signal quality can be relaxed by employing more advanced modulation formats, but with more investment. We propose a design method for MLR optical networks with transceivers employing different modulation formats. Our results demonstrate the tradeoff between a transceiver's cost and its optical reach in overall network design.