The impact of SONET digital cross-connect system architecture ondistributed restoration

The viability of distributed control restoration using digital cross-connect systems (DCS) depends on its capability for restoring services within specified time requirements, and its economics for providing restoration compared to other alternatives. The authors report a Bellcore study for the impact of the DCS architecture on distributed restoration. This study concludes that currently proposed distributed control DCS self-healing schemes may not meet the 2 second restoration objective for large metropolitan local exchange carrier's networks, regardless of the distributed algorithm used, if the present DCS system architecture which uses serial message processing and serial path cross-connection remains unchanged. They also discuss several DCS architecture enhancement options, including a parallel processing/cross-connect DCS architecture, which may improve the service restoration time     

Service applications for SONET DCS distributed restoration

This paper determines the scope of network applications and services that could be offered using a SONET DCS-based self-recovering mesh architecture with distributed control. The study includes an outage impact analysis on network services and a determination of how network restoration time objectives will affect the applicability for the distributed controlled DCS network architecture. It is concluded that using SONET DCS distributed control architectures to provide more complete survivability of a network would support numerous applications. Future services will demand a fault-tolerant network with complete survivability; this may only be reached through integration of SONET DCS distributed control architectures with other survivable architectures such as cell relay networks (e.g., supporting SMDS) and self-healing rings     

Survivable network planning methods and tools in Taiwan

The capability of telecommunications networks to restore services affected by failures is a key consideration in designing a broadband network architecture. The authors describe the survivable transport network planning methods and tools of the Telecommunication Laboratories of DGT in Taiwan. The paper consists of three major parts: conventional survivable architecture planning, hubbing architecture based on DCS restoration network planning, and SDH network architecture planning     

Network design sensitivity studies for use of digital cross-connectsystems in survivable network architectures

Provides the results of an economic study on the use of SONET digital cross-connect systems (DCS) to provide survivable transmission network architectures in local exchange networks. Three fundamental survivable transmission technologies are considered: (1) a SONET self-healing ring, (2) a SONET point-to-point fiber system with 1:1 automatic protection switching and diverse routing of protection facilities, and (3) a DCS mesh with automatic DCS restoration (rerouting) protection. These three technologies are used in various combinations to form six survivable network alternatives for evaluation. Two local exchange carrier (LEC) networks are used (a 15 node network and a 53 node network) and demand, network connectivity, and unit equipment cost sensitivities are evaluated on these alternatives. In addition, the survivability of each alternative in the event of a major node failure is calculated. The motivation for the study is to determine the viability of DCS-based survivable network architectures and, in particular, the viability of SONET DCS with integrated optical terminations. The study has two objectives: (1) given a specific survivable network technology, under what conditions is it economical to place a broadband DCS (B-DCS) in a central office as opposed add-drop multiplexers (ADM); and (2) which survivable technologies with B-DCS are economical, and under what conditions. The authors conclude that the most cost-effective networks consist of “hybrids” of SONET point-to-point, ring, and mesh technologies, and that the B-DCS is economically viable for interconnection between these technologies     

IP over optical cross-connect architectures [Topics in Optical Communications]

IP over optical network architectures have been extensively discussed within the research literature over the past few years. However, although signaling protocols between IP routers and optical cross-connect networks have been standardized, large IP backbones are not typically deployed over optical cross-connect networks with automatic reconfigurability features, such as automatic restoration or dynamic establishment of new IP links. One of the most important criteria in determining whether an IP backbone should be carried over such an optical cross-connect network is economic viability. In this article we analyze and explore four architectures for a typical large ISP backbone. In contrast with some other published claims, our results suggest it is more economically attractive to bypass an intermediate cross-connect network, given current equipment and IP backbone network design requirements. However, for ISPs who also provide a large volume of private line services, we propose an integrated architecture for IP over optical cross-connect networks that may provide an attractive alternative for providing rapid and cost-effective restoration from network failures     

Control protocol and its performance analysis for distributed ATMvirtual path self-healing network

A restorable network comprised of a mesh of ATM virtual paths (AVPs) is expected to be superior to distributed SONET digital cross-connect self-healing networks in terms of restoration time. The paper presents a reliable protocol and evaluation of its performance for control information transfer in distributed AVP self-healing networks. Two error-recovery technologies for the data-link level (selective retransmission and go-back-N) in AVP networks are reviewed, and a model for evaluating the delay of the go-back-N method that has been used in existing terrestrial systems is derived analytically. The proposed analytic model is combined with a self-healing simulator for AVP mesh networks to evaluate the restoration time characteristics. Case study results for the worst-case scenario show that the total restoration time, including link-by-link error control for self-healing control messages, is expected to be less than 2 s, which is the time objective for complete restoration for a metropolitan LATA network. The results also show that the estimated average delay penalty for adding the link-by-link error recovery process is 7.5 ms for complete service restoration     

Restoration message transfer mechanism and restorationcharacteristics of double-search self-healing ATM network

The self-healing network is particularly interesting with regard to ATM networks, because the restoration time can be shortened by using the advantages of the ATM network. This paper studies a self-healing ATM network based on virtual path (VP) protection switching. First, a novel self-healing algorithm-the double-search self-healing algorithm-is proposed. It is shown that this algorithm can restore failed bidirectional VPs faster and find alternate VPs more effectively than existing self-healing algorithms. Second, it is shown that the restoration information for self-healing control (SHC) messages must be transferred by specific cells carrying the control and OAM information (Ic&o). Message parameters and a cell format are proposed. Third, evaluation of the restoration characteristics using the proposed self-healing algorithm by computer simulation indicates that good performance against a transmission link failure is obtained even in a large-scale network model with 110 nodes. The results also indicate that the VP group (VPG) method can improve the restoration time without reducing the restoration ratio     

Future transport network architectures

The architecture of today's long distance transmission networks, which we call the baseline architecture, is a complex and multilayered hierarchy of TDM circuits. One premise of the baseline architecture is that restoration from network failures is provided mostly by SONET/SDH rings. This article presents an alternative architecture that uses ATM and optical layer cross-connect technology for TDM services. Using a sophisticated set of network design tools developed at AT&T Labs-Research, we show that the alternative architecture offers dramatic capital savings and improved network efficiency over the baseline architecture. Most of this savings can be attributed to use of OLXC mesh network restoration, which makes more efficient use of capacity than SONET/SDH rings, and use of ATM switching for transport of TDM circuits, which consolidates the numerous TDM equipment layers inherent to the baseline architecture. In addition, motivated by the rapid growth of IP services, we analyze in the alternative architecture whether to provide restoration for IP services in the IP layer itself, by rerouting packets over precalculated restoration paths with MPLS, or to alternatively provide restoration of failed IP layer links in the OLXC layer. One potential advantage of IP layer restoration is that network operators may choose to only restore a fraction of the services, in particular the “priority” services, affected by a network failure. This article gives some methodology of how to determine this fraction at which IP layer restoration is cost-competitive with OLXC restoration     

Fast restoration of ATM networks

Asynchronous transfer mode (ATM) is now well recognized as the fundamental switching and multiplexing technique for future broadband ISDN. As these networks will be increasingly relied upon for providing a multitude of integrated voice, data, and video services, network reliability is a key concern. There are several intrinsic features of ATM networks that could potentially be exploited to provide improved restoration techniques, beyond those established for synchronous transfer mode (STM) networks, such as digital cross-connect restoration or self-healing rings. These features include ATM cell level error detection, inherent rate adaptation and nonhierarchical multiplexing. The authors explore the use of these features in developing fast restoration strategies for ATM networks. In particular, they address: (1) ATM error detection capabilities for enhanced failure detection, (2) network rerouting strategies, (3) spare capacity allocation, and (4) network control architecture and related implementation aspects. Their findings suggest that fast network span failure detection and bandwidth-efficient rerouting capabilities can be combined to develop restoration strategies for ATM networks with significantly greater performance-cost ratios when compared to existing STM network restoration strategies     

Precision time-transfer in transport networks using digitalcross-connect systems

A technique for precise synchronization of the time-of-day clocks in networks of digital cross-connect systems (DCSs) is described. This method is intended to enhance the performance of reconfigurable transport networks and is specifically devised to exploit the fine time resolution of the carrier signals to which a DCS has direct access. The resulting scheme is a master-slave, multisite, implicitly delay compensated, nonhierarchical time-transfer method with a theoretical precision of one bit time at the carrier rate. In practice, precision is limited by transmission span delay asymmetries but residual time errors of under 1 μs are predicted. The method is intended for n/ n space-switching DCS, but other DCS properties can be accommodated. Requirements for DCS equipment design are given, including a generic circuit module for DCS hardware support of the time transfer function. The proposed method applies to DS-3 networks or synchronous optical networks and requires no change in the standards. Measurement or improved estimation of characteristic span delay asymmetries is recommended to refine the preliminary performance estimate     

A novel passive protected SONET bidirectional self-healing ringarchitecture

Among SONET ring alternatives, the four-fiber bidirectional self-healing ring architecture (BSHR/4) has the highest available capacity and can work with today's systems with minimum change. A cost-effective implementation for a BSHR/4 which uses a totally passive ring as the protection ring of the BSHR/4 is presented. The ring uses a SONET ring to carry working traffic and a passive ring to carry protection traffic in case of network component failures. The passive ring, which is composed of optical switches and optical amplifiers, is essentially an optical add-drop protection ring. The optical signal add-drop is controlled by SONET add-drop multiplexers using existing SONET self-healing protocols. A SONET control scheme for this implementation is also discussed. The proposed BSHR/4 not only has a significant cost advantage over conventional implementations, but also can easily evolve from any two-fiber ring, with a minimum capital investment and offering twice the capacity     

Control algorithms of SONET integrated self-healing networks

As the deployment of high-speed fiber transmission systems has been accelerated, they are widely recognized as a firm infrastructure of the information society. In SONET, the self-healing networks have been highlighted as one of the most advanced mechanisms used to realize SONET survivable networks. Several schemes have been proposed and studied due to the rapid progress in the development of highly intelligent network elements (NEs). A DCS based distributed self-healing network is discussed from the viewpoint of its control algorithms. The self-healing algorithm, called TRANS, is explained in detail. It possesses such desirable features as providing fast and flexible restoration with line and path level restoration applied to an individual STS-1 channel, the capability of handling multiple and even node failures, and so on. Both software simulation and hardware experiment verify that TRANS works properly in a real distributed environment, the result of which is shown in the paper. In addition, the combined use of TRANS and ring restoration control is proposed taking into account a practical SONET application     

Optical digital cross-connect system using photonic switch matricesand optical amplifiers

This paper presents, for potential application to network failure restoration, an optical digital cross-connect system (DCS) which uses both a photonic switching network and an electric DCS. It is shown that a system constructed of LiNbO₃ 8×8 switch matrices and semiconductor traveling-wave optical amplifiers (TWA's) could be applied to metropolitan area networks. An experimental optical DCS system has been designed and fabricated which incorporates both LiNbO₃ switch matrices and TWA's, and with it, line-failure restoration, a fundamental operation of optical DCS, has been successfully demonstrated     

Optical path cross-connect system scale evaluation using pathaccommodation design for restricted wavelength multiplexing

The optical path (OP) technology, which employs both wavelength-division multiplexing and wavelength routing, will be the key to enhanced network integrity and an ubiquitous broadband integrated services digital network (B-ISDN) in the future. To construct the OP network, path accommodation design that can solve simultaneously the problems of path routing and wavelength assignment must be established. Since optical wavelengths are scarce resources, even with state-of-the-art technologies, the available number of wavelengths that can be multiplexed into a fiber is restricted to a relatively small number. This entails space division multiplexing with multiple fibers in the link in order to accommodate the large number of paths within a link. This paper proposes novel OP accommodation design algorithms that can heuristically establish wavelength paths (WPs) or virtual wavelength paths (VWPs) in the network, where each link is composed of multiple fibers. These algorithms minimize the average number of fibers (in other words, cross-connect ports) handled at the WP/VWP cross-connect nodes and enable us to obtain the required OP cross-connect (OPXC) system scale at each node in WP/VWP networks. Algorithms that consider failure restoration are also proposed. Some WP/VWP accommodation designs over a polygrid network are simulated using the proposed algorithms. The difference between the WP and VWP schemes in terms of the required OPXC system scale with and without considering failure restoration is quantitatively evaluated for the first time     

The DCS as a Universal Digital Cross-Connect System

Existing DCS (digital cross-connect system) technology performs a subset of the digital cross-connect functions required in telecommunications networks. It must be used in tandem with other cross-connection systems such as DSX's (digital signal cross-connect systems) and DF's (distributing frames). It is these two technologies which provide most of the interconnection functions for digital facilities and equipment from the interoffice, subscriber loop, and central office parts of a network. Hence, if the DCS is to serve as a more universal digital cross-connect system, it must be enhanced to provide functions now performed by DSX's and DF's. This paper proposes expansion of DCS functions to incorporate some (or all) of the capabilities of the DSX and DF at discrete bit rates of the digital transmission hierarchy. Functional requirements, technology alternatives, planning/engineering issues, and operations/maintenance considerations are discussed.     

Flexible synchronous broad-band subscriber loop system: opticalshuttle bus

A flexible, synchronous, broadband subscriber loop system called the optical shuttle bus is discussed. The emphasis is on the proposed design architecture and performance of a 1.8-Gb/s prototype system. The optical shuttle bus is constructed of multigigabit/second intelligent optical shuttle nodes (OSNs). The software-defined OSNs allow the bus to be configured as a synchronous transmission network with any topology and provides flexible channel allocation by means of a drop/insert/cross-connect function. The OSN can be configured as a highly reliable network by virtue of its self-healing functions such as line switch, bypass, and loop back. The proposed bus architecture has been successfully applied in a 1.8-Gb/s prototype in which three OSNs are connected by one 10-km and two 5-dm single-mode fibers in a ring configuration. Very compact and inexpensive equipment is realized by using high-speed ICs. Experimental results show the proposed architecture to be a powerful one for the realization of synchronous broadband subscriber loop systems     

An ATM VP-based self-healing ring

This paper proposes an advanced ATM VP-based self-healing ring. This ring system offers better recovery control and path management than the conventional STM-based self-healing ring. This self-healing ring has two main characteristics. First, protection VP's are established in advance which have the same termination points as the working VP's regardless of the types of path connection. Recovery is accomplished by connecting the working VP's to their protection VP's at nodes adjacent to the failure point. This restores a failed path without internode negotiation. This network, therefore, realizes a simple recovery scheme. Second, recovery is executed in the ATM VP layer. A function to detect and remove the endless looping cells is not needed. The proposed architecture permits the flexibility of VP's to be applied to path management and recovery. The bandwidth of the protection VP's can be shared and effective VPI assignment is achieved. The proposed ring network reliably supports various broadband services in a cost-effective manner     

All-optical four-fiber bidirectional line-switched ring

An all-optical four-fiber bidirectional line-switched ring (O-4F/BLSR) architecture is proposed. This new physical layer networking protocol uses wavelengths as tributaries and an optical supervisory channel to carry overhead information. Optical channels can be added and dropped from the ring, and virtual wavelength paths can be provisioned. Both node and link failures of a network can be protected through a two layer protection scheme. Protection switching within the optical multiplex section layer (OMS) restores failure caused by loss of optical continuity in a way similar to the SONET 4F/BLSR line switching. Protection switching within the optical channel layer restores single channel failure using 1:N protection. Test results show that the O-4F/BLSR can restore traffic in less than 50 ms. A self-healing, bandwidth efficient, and scalable all-optical transport network evolving from this O-4F/BLSR architecture is possible     

Network performance and integrity enhancement with optical pathlayer technologies

Path layer technologies will play a key role in the development of a powerful and failure resilient B-ISDN. So far, they have been based on electrical technologies. This paper highlights WDM/FDM techniques and demonstrates that optical paths can greatly enhance the path layer capability and, therefore, the network performance. It is also shown that effective network failure restoration can be achieved with optical paths. The applicability of the wavelength path (WP) technique to global area networks is revealed by comparing different optical path realization techniques. WPs are applied to the national backbone network example to evaluate the required number of wavelengths, and to identify optical cross-connect node requirements. It then proposes a new optical path concept: the virtual wavelength path (VWP). In the VWP scheme, wavelengths are assigned on a link-by-link basis. In other words, the wavelength assigned to a wavelength path has only local significance. Significant benefits of the VWP such as the simplified path accommodation design within a transmission facility network and the reduced number of wavelengths needed, are elucidated. An optical cross-connect node architecture that enables the VWPs is also proposed. The architecture allows the VWP concept to be realized with commercially available optical technologies. The optical path layer concept proposed exploits and consolidates the layered transport network architecture and optical technologies, and will open up new opportunities for creating a B-ISDN that is bandwidth abundant and has a high degree of integrity     

Reliable architecture for high-capacity fiber-radio systems

In this paper, we present a reliable architecture for high-capacity fiber-radio system and demonstrate its feasibility. The main trunk of the proposed architecture is a star network with branches comprising a series of concatenated ring subnets. The conventional drawback, the lack of reliability, of star networks is overcome by the ring subnets with a self-healing function based on optical switches. To compensate the extra loss of the system, we adopt compact erbium-doped waveguide amplifiers to construct our proposed architecture. The architecture can provide highly reliable, flexible, and robust performance for fiber-radio systems.