Shaowei Huang

Variable-bandwidth optical paths: comparison between optical code-labelled path and OCDM path

In a conventional wavelength-routed network, the bandwidth of one wavelength is considered as the minimum granularity for a given connection request. Therefore, no multiple connection requests can be accepted by using a single wavelength simultaneously. This may cause inefficiency in the bandwidth utilization in some cases. In this paper, the focus is on the variable-bandwidth approach called an optical code (OC)-based path to improve this bandwidth utilization. The concept of OC-enabling paths is investigated, which shows its potential in resolving the above granularity problem inherent to the wavelength-routed network. First, two optical paths, called the OC-labeled and OC division multiplexing (OCDM) paths, are proposed. The former is based upon label switching and statistical multiplexing, while the latter is based upon OCDM. Next, OC-label and OCDM optical cross connects are described to support OC-labeled and OCDM paths, respectively. In this paper, a coherent time-spread OC is adopted. A two-state flow-fluid traffic model is addressed and regarded as the general analysis model. Finally, the performances between these proposed paths are qualified and compared, and numerical results show that the OC-labeled path outperforms the OCDM path under short burst duration time, whereas the OCDM path, provides higher flexibility than the OC-labeled path, owing to its independence of burst duration time

Architecture design and performance evaluation of multigranularity optical networks based on optical code division multiplexing

Feature Issue on Waveband Switching, Routing, and GroomingIn traditional lambda-based multigranularity optical networks, a lambda is always treated as the basic routing unit, resulting in low wavelength utilization. On the basis of optical code division multiplexing (OCDM) technology, a novel OCDM-based multigranularity optical cross-connect (MG-OXC) is proposed. Compared with the traditional lambda-based MG-OXC, its switching capability has been extended to support fiber switching, waveband switching, lambda switching, and OCDM switching. In a network composed of OCDM-based MG-OXCs, a single wavelength can be shared by distinct label switched paths (LSPs) called OCDM-LSPs, and OCDM-LSP switching can be implemented in the optical domain. To improve the network flexibility for an OCDM-LSP provisioning, two kinds of switches enabling hybrid optical code (OC)-wavelength conversion are designed. Simulation results indicate that a blocking probability reduction of 2 orders can be obtained by deploying only five OCs to a single wavelength. Furthermore, compared with time-division-multiplexing LSP (TDM-LSP), owing to the asynchronous accessibility and the OC conversion, OCDM-LSPs have been shown to permit a simpler switch architecture and achieve better blocking performance than TDM-LSPs.

Management-augmented stateless PCE for Wavelength Routed Optical Networks

Two stateless path computation element (PCE) implementations to be used in GMPLS-based wavelength routed optical networks (WRONs) are discussed (fully stateless PCE, F-SL-PCE, and contention avoidance stateless PCE, CA-SL-PCE). The proposed CA-SL-PCE implementation exploits static management information for avoiding resource contentions among subsequent lightpath requests, while preserving network scalability. Simulation results show the capability of CA-SL-PCE implementation in strongly reducing resource contentions, thus resulting in a significant reduction of the overall blocking probability.

A Study on Cycle Attack by Multiaccess Interference in Multigranularity OCDM-Based Optical Networks

Previously, an optical code-division multiplexing (OCDM)-based network architecture was proposed to improve the wavelength utilization and to provide finer bandwidth granularities to users. By this technology, different channels using distinct optical codes (OCs) can be multiplexed onto the same wavelength, in which an OC is considered as the basic unit in lightpath provisioning. In the ideal case, multiaccess interference (MAI) inherent to the OCDM technology is assumed to be removed completely at intermediate nodes and cannot be propagated or accumulated along the lightpath. However, since no optical-electrical (O/E) or electrical-optical (E/O) conversion is allowed in transparent OCDM-based optical networks, the MAI cannot be removed completely at intermediate nodes with current all-optical regeneration techniques. As a result, the residual MAI may be propagated and accumulated along the lightpath and affect other active lightpaths carried by the same wavelength in the network. The affected active lightpaths may build unintended cycles along which the MAI is accumulated. Furthermore, this MAI keeps increasing when the lightpaths traversed by the cycle are active, which deteriorates the lightpath signal quality. Since this deterioration may eventually result in unacceptable signal quality and service disruption, the phenomenon caused by the MAI is termed as cycle attack in this paper. The explanations of the MAI propagation mechanism and the cycle attack problem are given. A depth-first search (DFS)-based algorithm is proposed to diagnose such cycle attacks under dynamic traffic conditions. The numerical results show that our DFS-based cycle attack diagnostic algorithm enables one to detect cycle attacks effectively, and the two-way resource reservation method associated with heuristic wavelength assignment is shown to mitigate the blocking performance degradation due to cycle attacks greatly with some proper wavelength and OC configuration.

Evaluation of OCDM-Switching and Code Conversion for All-Optical End-to-End Path Provisioning in Multi-granularity Networks

In this paper, an OCDM-based multi- granularity optical cross-connect (MG-OXC) is proposed to enable the optical path switching in bandwidth granularity finer than the wavelength-based path. Such an optical path switched by OCDM-based MG-OXC is called OCDM label-switched path (OCDM-LSP), which is capable of hybrid optical code (OC)/wavelength conversion along the path from a source to destination. With distinct OCs, OCDM-LSPs can share a single wavelength without any synchronization mechanism and be discriminated by ultra- fast optical correlation. The asynchronous ability and OC conversion distinguish the OCDM-LSP from Packet-LSP or TDM-LSP in resolving granularity problem in wavelength- based path. Based upon our proposal, all-optical optical path provisioning including F-LSP, WB-LSP, L-LSP and OCDM- LSP can be realized in the transport layer. Simulation is conducted and the results show the proposed OCDM-switching has gained a 2 order of blocking probability reduction even though only 5 OCs are employed. Furthermore, OC conversion in OCDM-LSP is proved to be another benefit for high flexibility in optical path provisioning and blocking performance improvement comparing to TDM-LSP.

A time-shift scheduling-enabled optical flow switched network architecture and its performance.

Optical flow switching (OFS) has been proposed as a simple and cost-effective transport technology for users with large transactions (>1 second). In previous studies, a fast wavelength reservation method was deployed for flow transmission in OFS-based networks. However, reserving a single wavelength for users with small transactions encounters a very common problem: inefficient wavelength utilization. In this paper, a flow transmission cycle is introduced to each wavelength, and each cycle consists of multiple slots, so that flows of different transactions can be multiplexed onto a single wavelength. It is assumed that inter-metropolitan area network (MAN) traffic is transported over wide area network (WAN). A global time-shift scheduling methodology taking into account propagation delays in MAN is designed to avoid potential contentions occurring among different flows which are carried by the same wavelength in WAN. The contributions of this paper are, first it provides a new OFS network architecture which can achieve better throughput and average wavelength utilization performance without losing the feature of simple transport structure provided by OFS; second, it is the first time that issues of how OFS networks are managed and controlled are addressed from a system point of view.

Logical Topology Design for Eliminating Cycle Attacks in Optical Code Path Networks

In optical code path networks a cycle attack phenomenon has been reported. Cycle attack is caused by multiaccess interference (MAI) among different optical code (OC) paths carried by the same wavelength. A previous study proposed a heuristic wavelength assignment scheme to avoid this unintended cycle attack problem. However, the wavelength assignment retry in the previous proposal may impose an overhead on OC path establishment latency, and furthermore it was not a complete solution to eliminate cycle attacks. In this paper, we propose a cycle-attack-free logical topology design to eliminate the unintended cycle attack problem in the network planning phase instead of the wavelength assignment. The basic idea is to convert the physical network topology into some tree-based logical topologies in which OC paths are routed. Simulation results show that (a) using the tree-based topologies with smaller average hop count helps to obtain better blocking performance and (b) in distributed optical path establishment our proposed approach performs better in terms of blocking and delay performance under the relative dynamic traffic condition.

Impact of MAI Noise Cycle Attack on OCDM-Based Optical Networks and its Diagnostic/Mitigation Algorithm

In our previous studies, an optical-code-division multiplexing (OCDM)-based network architecture has been proposed to improve the wavelength utilization and to provide finer bandwidth granularities to users. However, since up-to-date all-optical regeneration technologies are still immature, signal quality degradation at intermediate nodes is considered as a crucial problem in transparent optical networks. Because the remaining noises may propagate, and be accumulated along the light-path, which eventually result in unacceptable signal quality. In this paper, we first define a cycle attack problem due to multi-access interference (MAI) propagation in transparent OCDM-based networks, which may cause severe service disruption. A depth-first-search (DFS)-based algorithm is proposed to diagnose such cycle attacks and a heuristic wavelength assignment called advanced first-fit (AFF) scheme taking cycle attacks into account is found to be a simple but effective approach mitigating the cycle attack impacts by computer simulations.

An experimental analysis on OSPF-TE convergence time

Open shortest path first (OSPF) protocol is commonly used as an interior gateway protocol (IGP) in MPLS and generalized MPLS (GMPLS) networks to determine the topology over which label-switched paths (LSPs) can be established. Traffic-engineering extensions (network states such as link bandwidth information, available wavelengths, signal quality, etc) have been recently enabled in OSPF (henceforth, called OSPF-TE) to support shortest path first (SPF) tree calculation upon different purposes, thus possibly achieving optimal path computation and helping improve resource utilization efficiency. Adding these features into routing phase can exploit the OSPF robustness, and no additional network component is required to manage the traffic-engineering information. However, this traffic-engineering enhancement also complicates OSPF behavior. Since network states change frequently upon the dynamic trafficengineered LSP setup and release, the network is easily driven from a stable state to unstable operating regimes. In this paper, we focus on studying the OSPF-TE stability in terms of convergence time. Convergence time is referred to the time spent by the network to go back to steady states upon any network state change. An external observation method (based on black-box method) is employed to estimate the convergence time. Several experimental test-beds are developed to emulate dynamic LSP setup/release, re-routing upon single-link failure. The experimental results show that with OSPF-TE the network requires more time to converge compared to the conventional OSPF protocol without TE extension. Especially, in case of wavelength-routed optical network (WRON), introducing per wavelength availability and wavelength continuity constraint to OSPF-TE suffers severe convergence time and a large number of advertised link state advertisements (LSAs). Our study implies that long convergence time and large number of LSAs flooded in the network might cause scalability problems in OSPF-TE and impose limitations on OSPF-TE applications. New solutions to mitigate the s convergence time and to reduce the amount of state information are desired in the future.

Fine Multi-granularity Optical Paths by Novel OCDM-based Switch Architecture

Optical code division multiplexing (OCDM) path enabling to establish finer granularity path than wavelength path and its hierarchical architecture for multi-granularity optical path switching have been investigated. Significant improvement of blocking probability is achieved, compared to the conventional multi-granularity OXC based upon wavelength.