Abstract Protection and restoration based resilience in Automatic Switched Optical Networks T. Jakab 1, Zs. Lakatos 2 Budapest University of Technology and Economics, Dept. of Telecommunications 1: Research Fellow 2: Ph.D. Student BME HT Pázmány P. sétány 1/D. Budapest H-1117, Hungary E-mail: jakab@hit.bme.hu Tel.: +36 1 463 1 1 Fax.: +36 1 463 32 63 The introduction of switching capabilities and signalling based distributed intelligence enable to implement advanced and effective resilience schemes in optical transport networks. Automatic switched optical network technology supports both permanent optical channel and switched optical channel based transport services. Based on the achievements of EURESCOM P112 FASHION Project the paper outlines the resilience options supported by the ASON technology and with help of small network examples illustrates and compares the resource needs of the different solutions. Introduction and Motivations Extended deployment of wavelength division multiplexing (WDM) based optical transport technology went on in recent years to meet the increased capacity demands. Since the WDM technology developments have been focused mainly on huge capacity (tens of wavelengths) and extra long haul (hundreds of kilometres) transmission system the lack of complex network functions limits the advanced applications of optical transport networks (OTNs). The automatic switched optical network (ASON) concept and technology is being developed to add distributed intelligence and automatic switching flexibility to OTN. There are three main driving forces motivating the efforts to specify and realise ASON: Advanced network flexibility: Due to the sharp competition and new services dynamically changing traffic should be served by transport networks. Together with the difficulties in modelling and forecasting traffic demands (IP traffic mainly) these problems result in uncertain traffic conditions, and flexible and re-configurable network elements are required to support efficient networking under these uncertain conditions. Fast provisioning of optical channels: Based on advanced management functions and switching flexibility the configuration and reconfiguration of network resources can be speed up significantly, and will become more secure. Advanced resilience techniques: To realise efficient solutions to recover optical channels in case of network element failures the optical networking technology should enable complex node and management functions. The current OTN technology supports dedicated protection based optical resilience mainly, which is fast, but less effective than spare capacity sharing oriented techniques. Based on advanced management functions and automatic switching flexibility more complex resilience techniques like shared protection and restoration can be implemented. Automatic Switched Optical Networks The ASON has to be seen as a successor of the OTN with extended functionality. As a result of the separate control plane, the ASON can perform a set of automatic functions that enhance significantly the network reconfiguration flexibility; save network operation costs and support new OCh services. ASON, in providing multi-client services, is characterised by a control plane that manages network elements in the optical transport network plane (Figure 1). The ASON is intended to allow switching of (optical) network connections within OTN under control of its own signalling network. ASON definition implies the existence of three separate planes in the network: The optical transport plane which provides the functionality required for the transport of the client signals of the ASON; in particular, it provides the capability to cross-connect the characteristic information of the optical channels; The ASON control plane which provides the functionality required for establishing end-to-end connections of client signals with the properties (in terms of protections applied, duration and
time scheduling of the connection, etc.) that are specified by the customer himself during connection set-up phase; The management plane, which performs management functionality, related both to the transport plane and to the control plane. Client equipment (IP router, ATM switch, ) UNI PI OCC CCI Optical switch NNI PI OCC Optical switch Transport plane CCI: Connection Control Interface NMI-A: Network Management Interface for the ASON Control Plane NMI-T: Network Management Interface for the Transport Network NNI: Network to Network Interface Control plane OCC Optical switch Figure 1 Logical view of ASON architecture NMI-A NMI-T Management plane EM/NM OCC: Optical Connection Controller PI: Physical Interface UNI: User to Network Interface The new capabilities implemented in ASON enable different kinds of optical transport services: Permanent OCh service (P-OCh service): provides the customer with an end-to-end OCh between two end-points. The service is provided by the network operator (NO) on the basis of an agreement between NO and customer. Provisioning of a P-OCh service is the responsibility of the NO, who dedicates multiple network resources to the path. It can be realised via manual equipment configuration or TMN-based equipment configuration. Soft-permanent OCh service (SP-OCh service): provides the customer with an end-to-end OCh between two end-points. The service is provided by the NO on the basis of an agreement between NO and customer. Provisioning of a SP-OCh service is the responsibility of the NO, who dedicates multiple network resources to the path. It is realised via distributed signalling-based TMN-activated equipment configuration resulting in provisioning speeds faster than for P-OChs. Like for P-OCh, a natural understanding of SP-OCh service is that of a stable in time service, as an enhanced leased line. Automatically switched OCh service (AS-OCh service): provides the customer with an end-to-end, real-time activated OChs between two end nodes. The provisioning process is activated by the customer himself, via a user network interface (UNI) signalling interface, when needed only, and completed by means of node-node interface (NNI) signalling. As soon as the AS-OCh service is not necessary anymore it is torn-down. Based on these point to point related services more complex ones like optical virtual private network service (provides the customer with dedicated network resources (OChs) among two or more customer nodes) and lambda trunking service: (offering a bundle of P-OChs/SP-OChs between two endpoints with same transmission performances) can be implemented, as well. EURESCOM FASHION Project The starting point of the EURESCOM P112 FASHION Flexible, Automatically SwitcHed, client Independent Optical Networks Project [1] is the assumption that there exists the possibility of switching automatically optical channels (OChs) in the optical transport network (OTN). Furthermore, the initial assumption is that this layer is a client-independent layer with its own control plane and management system. All information required by the client layer in order to establish switched connections is exchanged through the signalling at the user-network interface. Both switched and permanent connections are supported in ASON. The client-server (overlay) approach is applied to
ASON, with the consequence that the client network (IP, ATM, etc.) requests resources (connections) from the server network, without any knowledge of its internal structure. FASHION project investigates the applicability of switching approaches to ASON derived from both the circuit switched and the packet/cell switched networks, with a particular attention to the latter that seems to be more promising. FASHION project evaluates and analyses the application of different automatic OCh switching solutions to real networks. Particularly it performs a techno-economic comparison of both traditional OTN (based on wavelength routing of permanent paths) vs. ASON (based on automatic OCh switching) and of potential architectural solutions including different resilience options for ASON [2]. Resilience Options Based on ASON Concept An Overview According to a simplified consideration of ASON functionality (automatic optical switching related functionality only) implementation of resilience schemes based on automatic switching capabilities are overviewed in this section. Resilience options for early stage permanent and soft-permanent optical channel based services, and for more perspective switched optical channel based services are considered. Requirements and conditions to implement resilience schemes in ASON To analyse the applicability of different resilience schemes in ASON the specific requirements and conditions the resilience should be implemented under are summarised first. Service requirements and operators requirements specify the requirements (fast recovery due to high value services, selectivity to support different service classes) to be met by the implemented resilience solutions. According to the granularity of the ASON the protected network entity is the optical channel. However, taking into account the physical limitations of the optical signal transport, and the lack of optical functionality to implement all optical signal transportation, it is not obvious how the protection/restoration of switched channel can be realised end to end in the photonic domain entirely. Further difficulty origins from the segmented network structure. The network is partitioned into domains according to administrative (e.g. multi-operator environment) and technological (e.g. transparent optical islands) reasons. The domain structure, the available interaction capabilities and the visibility may limit the set of applicable resilience schemes. Since ASON is to provide high value wide-band services the protection aspect is not limited to the core part of the network. The targeted users of specific high value services may need protected access to the network; thus protected connections may needed in the access part of the network, as well. Based on the advanced capabilities of ASON different services can be supported by different resilience schemes. Permanent optical channel service resilience Both permanent and soft-permanent optical channel based services are stable in time leased line services, and full service recovery in case of any single failure is required for them. Both protection and restoration oriented solutions can be applied to recover the transport services from any single failure. The applicable solutions can be evaluated according to their complexity, reaction time and extra resource needs for resilience. Protection based resilience schemes due to their point-to-point oriented structure can be applied both in the core and access part of the network. In dedicated 1+1 protection scheme the transmitted signal is split and permanently bridged to both working and protection systems. The decision on which signal to use is made by the receiver end analysing the signals at the receive terminal. A non-revertive singleended protection switching is performed on the receiver end. No transfer of extra information is required simplifying the procedure considerably. Shared protection schemes (n:m, 1:1) are applied if the protected network entities affected by independent failures. In case of a single failure only one protected entity is failed and based on a dualended revertive switching mechanism one of the shared protection resources is applied to recover the failure. One of the major application of the scheme in the practice the protection of terminal equipment failures. The scheme is applicable to protect optical channels, if the routing mechanism of the network results (able to set-up) disjoint routes between the same source-destination pairs. Protection like solutions provide fast recovery time, in order of protection switching, and require high extra resource needs for resilience. 1+1 dedicated path protection practically duplicates the network load, however shared path protection, where two (or more) disjoint paths between the same sourcedestination pair share the same protection path may decrease the extra load according to the sharing efficiency, which depends on the topology.
Applying restoration with failure-state dependent recovery routes the further decrease of resilience related extra resources can be achieved. The penalty for the higher level of capacity sharing is the increased complexity of recovery processes and the longer recovery time in order of connection set-up time. According to the different networking considerations restoration with different re-routing strategies based on simple routing algorithms supported by ASON can be realised: Recovery via optimal capacity sharing related routes: restoration routes optimised to require minimum extra capacity. To achieve this goal the reuse of restoration related extra capacity in different failure cases should be maximise, and even the working paths and re-routing paths can be optimised together. Recovery via minimal paths: minimum weight (e.g. length) paths are used in each failure case to recover failed optical channels. (Suppose that a simple distributed routing protocol is supported in ASON, and applied to implement restoration.) Recovery via disjoint minimal paths: restoration routes disjoint to the corresponding working ones (supports simple processes to return to the original working path after clearing the failure), are calculated to minimise the l weight (e.g. length). Switched optical channel service resilience Protection Based Solutions In case of protection based resilience of switched optical channels in the access part of the network dual-homing and 1+1 protection can be implemented to connect user endpoints to the network. (Single homing with 1+1 protected user interconnection can be applied, as well.) Dual-homing of users in a line switched network provide high level availability, however, the call set-up supporting the dualhoming becomes more sophisticated. To implement 1+1 channel protection between edge switches a parallel call set-up process is needed to establish two disjoint switched channels. Protection switching can be implemented in the edge switch or at the user endpoint according to the overall structure. Shared path protection is applicable to protect optical channels, if the routing mechanism of the network results (able to set-up) disjoint working routes between the same source-destination pairs. Alternate and adaptive routing mechanisms may fulfil this requirement. Thus, disjoint working routes may share common protection channels, and any of them can be recovered in case of single failures. Restoration like solutions Restoration can be applied in ASON for network level resilience in the core part of the network. The implementation of the restoration can be based on connection set-up, thus in case of a failure an NMS triggered automatic connection re-setup processes can be applied to recover the network. Interrupted connection can be re-routed between the edge switches (global re-routing) or between the switches adjacent to the failed link (local re-routing) according to the general re-routing strategies applied in restoration. The different re-routing schemes imply different operation complexity and recovery performance. The restoration can be applied to the calls interrupted by the failure. The automatic topology discovery based routing mechanism can adopt to the changed network configuration to set-up the connection to serve requests arriving in a failure case. Extra resources can be installed to support such a solution or penalty should be paid in service level degradations (higher blocking probability, increased length of call routes). The two different resilience approaches are with different performances. 1+1 dedicated OCh protection ensures very short service interruption in case of single failures. Duplicated channels result in high resource need, however, the local decision on the receiver end provide fast recovery in order of protection switching. Topology discovery based adaptation of routing tables supports re-setup of interrupted channels in case of restoration. The recovery is not guaranteed in general since interrupted channels and new channel requests are competing for limited resources and channel requests may be blocked. The recovery time is in order of path set-up time for restoration (increased by the automatic topology discovery based updating of routing tables), however, lower amount of resources are needed to implement this solution.
Illustrative Examples to Highlight the Advantages of ASON Based Resilience Techniques Based on the models and methodology developed in frames of EURESCOM FASHION project the applicability of different ASON based resilience schemes to a small size network (hypothetical Hungarian core network with 7 nodes and 13 edges) is studied. In the studies resilience of both semipermanent and switched optical channel based services is considered. Concerning permanent optical channel based services the saving on path based restoration in comparison 1+1 dedicated path protection, and the penalty on non capacity-optimal resilience strategies are analysed. Studying savings on restoration in comparison with dedicated protection a simple approach limited to savings on line capacities, and an extended one taking into account savings in switching capacities are considered. Relative Hop*OCh 3% 25% 2% 15% 1% 5% % notprot Resilience Cases Extra for Resilience Working optimal path rest. min. path rest. disjoint min. path rest. Figure 2a Total link capacities in different resilience cases 1+1 Relative #Switch Ports 3% 25% 2% 15% 1% 5% % Extra for Resilience Working not prot. full flex. 1+1 end switch. Resilience Cases 1+1 full flex. Figure 2b Total switch capacities (#ports) capacities in different resilience cases optimal path rest. full flex. Figures 2a and 2b illustrates the results of protection/restoration comparisons. Concerning the line capacities (Figure 2a), as it is expected extra capacity for resilience is the lowest for capacity-optimal restoration (less then 5%) and the highest for 1+1 dedicated protection (about 15%). The minimal path restoration requires 25% more in comparison with the capacity-optimal one (it is 75% extra in total), and disjoint minimal path restoration is resulted in 75% more in comparison with the capacityoptimal one (it is 1% extra in total). Despite of the small size of the network the restoration performs well, however, in larger size networks the sharing efficiency could be better. Instead of path based restoration the partitioning of the paths into commonly recovered sections further saving can be gained, and the reaction times can be shortened. The amount of savings in switching capacity (Figure 2b) achieved by the restoration is very similar to that for the line capacity. A full flexible network is assumed, i.e. each line capacity unit terminates on switches, and the switching capacity is expressed in number of ports. (As a reference, the amount of switches for protection switching only is given, as well.) To complete the permanent optical channel based services related resilience studies, the savings on shared protection in comparison with dedicated protection for networks of different connectivity are analyzed. In a distributed automatically switched environment a simple alternate or adaptive routing can be applied to distribute the traffic load around the network. As a result of such a routing different routes between the same source-destination pairs can be obtained, and if these routes are disjoint ones they may share the same protection resources supposing dominant single failures in the network. The sharing of spare resource may decrease the extra capacity needs for resilience and results in a more effective network. An at least three-connective network topology is required to apply shared path protection. Increasing the network connectivity enables higher number of disjoint routes between source-destination pairs and improves the sharing factor of protection resources. Ten-node three-, four-, five- and six-connective Harary-graphs are applied to study the impact on network connectivity on sharing efficiency in case of shared path protection. (Harary-graphs are simple regular graphs with given connectivity for each node pairs, and with minimum number of edges.) Figure 3 depicts the results of relative difference in total optical channel between the resource needs of dedicated and shared path protection. More then 7% savings can be achieved in case of six-connective
graph, however, the more then 2% saving in case of three-connective graph - which graph connectivity is more realistic in practical cases is significant, as well. 8% 7% Relative difference 6% 5% 4% 3% 2% 1% % H1_3 H1_4 H1_5 H1_6 Network examples Figure 3 Savings on shared path protection in comparison with dedicated one for networks with different connectivity without wavelength conversion function Besides the resilience of permanent optical channel based services, the impact of failures and the resilience options in case of switched optical channles are studied, as well. To analyze the impact of single link failures on switched optical channel based services, the network is roughly dimensioned for an assumed traffic pattern under specified blocking probability (Poison traffic is assumed). A link failure results in a decrease of available network resources, automatic topology discovery based adaptation of routing tables is the network s reaction for the failure, and the increase of blocking probability is the penalty if there are no extra resources to substitute the failed ones..7.7.6.6 Blocking.5.4.3.2 Blocking.5.4.3.2.1.1 Figure 3a Blocking probabilities in the not protected case with fixed routing in case of different single link failures Figure 3b Blocking probabilities in the not protected case with alternate routing in case of different single link failures.12.12.1.1 Blocking.8.6.4.2 Blocking.8.6.4.2 Figure 4a Blocking probabilities in the 1+1 protected case with fixed routing in case of different single link failure scenarios Figure 4b Blocking probabilities in the 1+1 protected case with alternate routing in case of different single link failure scenarios
The network is dimensioned under fixed shortest path routing for 1% network level blocking, and analysed in case of single link failures with the same routing and with fixed alternate routing. In case of link failure the adaptation of routing tables are assumed, and the connection requests are routed via the modified routes. In case of fixed shortest path routing, if the fixed route is blocked the request is not served. As Figures 3a depicts, due to the single link failures the blocking is increased for 2% to 7% according to the different topological positions of the failed links. Applying fix alternate routing (a few alternatives are allowed in case of blocked routes) the network level blocking can be decreased below 1% in almost each failure case. Thus, replacing fixed routing by alternate one the original failure free blocking can be restored without installing extra resources (Figures 3b). Figures 4a and 4b give the results of a similar small study with shortest pair fixed routing. According to this routing two disjoint paths are set-up to serve each connection request realising 1+1 protection of switched optical channels. The impact of single link failures are more significant in this case (Figures 4a), since besides the resource needs there are strong structural requirements due to the required disjoint paths. The increased network level blocking exceed 1% in some failure cases, and the improvement of blocking applying alternate routing instead of fixed (Figures 4b) is small in the critical failure cases. To protect network from the increased blocking in case on single link failures, extra resources should be installed. Increased link capacities are required to guarantee the specified service level (blocking) both in failure free and single link failure cases. Network dimensioning can be extended for the failure cases, as well, and extra link capacities can be allocated to keep the specified blocking probability in case of single link failures. 35 35 total maximum link capacity 3 3 Total link capacity 25 2 15 1 Total maximum link capacity fail_free 25 2 15 1 fail_free 5 5 Figure 5a Increased link capacities in not protected case with fixed routing in case of different single link failure scenarios to restore the original service quality Figure 5b Increased link capacities in 1+1 protected case with fixed routing in case of different single link failure scenarios to restore the original service quality Total network capacity (the sum of link capacities) required to ensure specified blocking in failure cases is given on Figures 5a and 5b for each single link failure in case of the different routing algorithms (fixed shortest path, fixed shortest disjoint pair), respectively. Similarly to permanent optical channel based services related resilience studies, the savings on shared protection in comparison with dedicated protection for networks of different connectivity are analyzed for switched optical channel based services, as well. The same Harary-graphs are studied under uniform traffic requirements (Poisson traffic). As it can be depicted on Figure 6 the similar impact of graph connectivity on shared path protection is identified in case of dynamic traffic as for semi-permanent connections. The only difference is that the saving for the four -connective and five-connective networks is nearly the same in the analyzed example. Summary and Conclusions Based on automatic switching capabilities and distributed intelligence in automatic switched optical networks restoration can be apply to recover permanent optical channel based services from different network failures. In comparison with dedicated protection significant resource savings can be achieved due to the effective spare capacity sharing, however, the recovery time is increased from order of protection switching to order of connection set-up time (Table 1). The high extra resource needs of protection based resilience solutions can be decreased applying shared path protection, however, a
higher connectivity (at lest three-connective) network topology should be available to realise the optional savings. 7% 6% Relative difference 5% 4% 3% 2% 1% % H1_3 H1_4 H1_5 H1_6 Network examples Figure 6 Savings on shared protection in comparison with dedicated protection for networks of different connectivity Different simple restoration routing strategies may offer benefits in networking (supported implementation, simple return to working path after clearing a failure, etc.), however, increased resources needs are resulted from these simple routing strategies in comparison with the capacity optimal path based restoration. Summarising the evaluation of the resilience options for switched optical channel based services the topology discovery based restoration (re-setup) of channels in use can be applied to protect services not sensitive for short interruptions, and dedicated channel protection can be offered for more interruption time sensitive applications (Table 2). In case of topology discovery based restoration extra resources are needed (in comparison with the failure free case) to ensure specified service quality (blocking), or adaptive routing algorithm is required to distribute the same traffic on the network with reduced resources. Applying 1+1 protection for channels in use and topology discovery based adaptation of routing to serve new connection requests a higher amount of extra resources are required to maintain the specified service quality (blocking). Furthermore, the network topology should provide two disjoint paths in case of any single. Table 1 Summary of soft-permanent service resilience related results (not protected case =1%) Extra Resilience scheme total link capacity Dedicated 1+1 path protection 152% Shared path protection 131% Disjoint path restoration 13% Minimal path restoration 72% Optimal sharing path restoration 39% The automatic optical channel switching capability of ASON supports the implementation of different potential resilience schemes. A proper functional description and a detailed techno-economical analysis are needed to evaluate the available options. Besides other important strategic issues EURESCOM P112 FASHION Project has been focused to perform ASON resilience related analysis and comparisons the achieved results are available on EURESCOM web site [1]. Acknowledgement This document is based on work done within the EURESCOM Project P112 FASHION. The authors gratefully acknowledge the support of EURESCOM for carrying out this work. The authors wish to express special thanks to all P112 participants from Telecom Italia Labs, France Telecom, MATAV Telecom, OTE, Swisscom, Tele Portugal, and Telenor.
Table 2 Summary of switched optical channel service resilience related results Resilience scheme Total link capacity Max. blocking in single failure cases Recovery time Restoration without extra resources 1%.7 /.14 * Topology discovery with update of routing tables + path set-up Restoration with extra resources to maintain original blocking 1+1 protection without extra resources 1+1 protection and restoration with extra resources to maintain original blocking Note 149%.5 Topology discovery with update of routing tables + path set-up 195%.11/.1 Protection switching 248%.12* Protection switching Recovery in case of single link failure Not guaranteed Not guaranteed Guaranteed Guaranteed This document is based on results achieved in a EURESCOM Project. It is not a document approved by EURESCOM, and may not reflect the technical position of all the EURESCOM Shareholders. The contents and the specifications given in this document may be subject to further changes without prior notification. Neither the Project participants nor EURESCOM warrant that the information contained in the report is capable of use, or that use of the information is free from risk, and accept neither liability for loss or damage suffered by any person using this information nor for any damage which may be caused by the modification of a specification. This document contains material, which is the copyright of some EURESCOM Project Participants and may not be reproduced or copied without permission. The commercial use of any information contained in this document may require a license from the proprietor of that information. Acknowledgement This work was partially supported by the research grant OTKA T3685. References [1] EURESCOM P112 FASHION Project, www.eurescom.de [2] R. Clemente, J. Robadey, L. Jereb, Z. Ioannidis, J. Santos: Network operator perspective on optical networks - Evolution towards ASON, accepted paper,! th International Telecommunication Network Strategy and Planning Synposium, NETWORKS22, June 22, Munich, Germany * Network level blocking applying corresponding alternate routing