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Project P709 Planning of Full Optical Network Deliverable 2 Basic factors influencing optical networks Volume 3 of 5: Annex B – Protection in the Optical Layer Suggested readers: PNOs studying potential upgrade possibilities for their SDH networks System engineers and network planners Experts on standard bodies of ITU-T SG13 (Q 19), SG15 (Q 16, 17, 20) and ETSI TM-1WG2/WG3 Researchers engaged in the field of optical transmission networks and technologies For full publication May 1999 EURESCOM PARTICIPANTS in Project P709 are: Finnet Group Swisscom AG Deutsche Telekom AG France Télécom MATÁV Hungarian Telecommunications Company TELECOM ITALIA S.p.a. Portugal Telecom S.A. Telefonica S.A. Sonera Ltd. This document contains material which is the copyright of certain EURESCOM PARTICIPANTS, and may not be reproduced or copied without permission All PARTICIPANTS have agreed to full publication of this document The commercial use of any information contained in this document may require a license from the proprietor of that information. Neither the 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 no liability for loss or damage suffered by any person using this information. This document has been approved by EURESCOM Board of Governors for distribution to all EURESCOM Shareholders. © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer Preface (Prepared by the EURESCOM Permanent Staff) The advances in optical fibre transmission technology over the past years have kept pace with the demand for increased bandwidth. In particular the introduction of the WDM technology enables Telecom Operators to upgrade the capacity of their networks by an order of magnitude. The evolution of photonics makes the development of optical switching and routing structures in the core and metropolitan part of the transport network possible. As a consequence, the development of an optical network infrastructure will enable the flexible, reliable and transparent provision of transport services for any type of traditional and innovative services and applications. Taking into consideration the current trends, the objective of network planning is to find the best possible balance between network implementation cost, network flexibility, network availability and survivability, subject to service requirements and topological constraints. The aim of the P709 EURESCOM Project is to investigate a number of alternative strategies for the planning of the optical transport network - with massive deployment of WDM, OADM, and small size OXC- that will be used in a middle term future. This is the second Deliverable (D2) of P709. D2 summarises the most important factors that have to be taken into account when preparing the planning of optical networks. Restoration and protection techniques implemented in optical networks are assessed in terms of requirements, constraints on network planning and upgrading, as well as their interaction with client layer functionalities. A study of resource allocation and impact on network planning and upgrading is also presented. We should remind the reader that the first P709 Deliverable (D1) provided an overview over network architectures, which potentially may be used in the future and D3 will give an analysis of the existing network planning methods, plus guidelines for planning future optical networks. The present Deliverable (D2) is a very useful study for Optical Network planners & system engineers, and experts on Standard Bodies of ITU-T SG15 and ETSI TM1 (WG2 & WG3). © 1999 EURESCOM Participants in Project P709 page i (vi) Volume 3: Annex B – Protection in the optical layer page ii (vi) Deliverable 2 © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer Table of contents Preface ............................................................................................................................ i Table of contents .......................................................................................................... iii Abbreviations ................................................................................................................. v 1 Introduction ............................................................................................................ 1 2 Optical protection techniques ................................................................................ 2 2.1 Identification of protection techniques ........................................................ 2 2.1.1 Optical channel protection .............................................................. 2 2.1.2 Optical multiplex section protection ............................................... 2 2.1.3 Dedicated protection ....................................................................... 3 2.1.4 Shared protection ............................................................................ 3 2.2 Examples of optically protected rings ......................................................... 3 2.2.1 Optical Channel Dedicated Protection Ring (OC-DPRing) ........... 3 2.2.2 Optical Channel Shared Protection Ring (OC-SPRing) ................. 4 2.2.3 Optical Multiplex Section Dedicated Protection Ring (OMSDPRing) .......................................................................................... 5 2.2.4 Optical Multiplex Section Shared Protection Ring (OMSSPRing) ........................................................................................... 5 3 Protection of interconnected optical network domains.......................................... 7 3.1 Selected reference network configurations.................................................. 7 3.2 Interconnection of sub-networks and protection ......................................... 7 3.3 Protection with optical drop and continue functionality ........................... 10 3.3.1 Two-level OMS-SPRing architecture ........................................... 10 3.3.2 Two-level OMS-SPRing-Optical Mesh architecture .................... 13 3.3.3 Two-level Optical Mesh-OMS-SPRing architecture .................... 13 3.4 Conclusions................................................................................................ 15 4 Wavelength conflicts under protected state in OMS-SPRings ............................ 16 4.1 Protection in OMS-SPRings ...................................................................... 16 4.1.1 Architecture .................................................................................. 16 4.1.2 Single Span Failure in OMS-SPRing............................................ 17 4.1.3 Multiple Span Failure in OMS-SPRing ........................................ 18 4.1.4 Node Failure in OMS-SPRing ...................................................... 19 4.2 Misconnections in OMS-SPRings ............................................................. 20 4.2.1 Introduction ................................................................................... 20 4.2.2 Misconnections without low-priority traffic ................................. 21 4.2.3 Misconnections with low-priority traffic ...................................... 22 4.2.4 Wavelength squelching ................................................................. 24 4.3 Conclusion ................................................................................................. 24 5 Identification of constraints on network planning ............................................... 26 5.1 Protection of individual channels .............................................................. 26 5.1.1 Coloured Section Ring (CSR)....................................................... 26 5.1.2 CSR-CSR hierarchical network .................................................... 28 5.1.3 Optical Multiplex Section Shared Protection Ring (OMSPR) ..... 28 5.2 Protection Priority Classes......................................................................... 29 5.2.1 Priority classes .............................................................................. 30 5.2.2 Other performance priority classes definition .............................. 30 © 1999 EURESCOM Participants in Project P709 page iii (vi) Volume 3: Annex B – Protection in the optical layer 5.3 5.4 5.5 Deliverable 2 Protection against simultaneous failures .................................................... 31 5.3.1 General characteristics of single domain optical protection ......... 31 5.3.2 Protection against simultaneous failures in interconnected optical network domains ............................................................... 32 Existing infrastructures in optical network protection ............................... 34 5.4.1 Existing point-to-point WDM systems.......................................... 34 5.4.2 Existing SDH systems ................................................................... 36 Conclusions ................................................................................................ 36 5.5.1 Protection of individual channels .................................................. 36 5.5.2 Protection priority classes ............................................................. 37 5.5.3 Protection against simultaneous failures ....................................... 37 5.5.4 Existing infrastructures in optical network protection .................. 38 6 Identification of constraints on network upgrading ............................................. 39 6.1 Upgrading Coloured Section Rings ........................................................... 39 6.1.1 Capacity extension ........................................................................ 39 6.1.2 Change in structure ....................................................................... 40 6.1.3 Changing the protection ................................................................ 41 6.2 Upgrading CSR-CSR Hierarchical Networks ............................................ 42 6.2.1 Inserting new nodes within a station ............................................. 42 6.2.2 Inserting new nodes ....................................................................... 44 6.2.3 Inserting new rings on the lower level .......................................... 44 6.2.4 Changing protection ...................................................................... 45 6.3 Conclusions ................................................................................................ 45 7 Conclusions .......................................................................................................... 46 References .................................................................................................................... 48 page iv (vi) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 Abbreviations ADM Add Drop Multiplexer AIS Alarm Indication Signal AU-N Administrative Unit (level) N CSR Coloured Section Ring DXC Digital Cross Connect EDFA Erbium Doped Fibre Amplifier ETSI European Telecommunications Standards Institute HO Higher Order HOP Higher Order Path ITU-T Telecommunication standardization sector Telecommunication Union LO Lower Order LOP Lower Order Path MS Multiplex Section MSP Multiplex Section Protection MS-SPR Multiplex Section Shared Protection Ring OA Optical Amplifier OADM Optical Add Drop Multiplexer OC Optical Channel OCH Optical Channel OC-DPR Optical Channel Dedicated Protection Ring OC-SPR Optical Channel Shared Protection Ring OMS Optical Multiplex Section OMS-DPR Optical Multiplex Section Dedicated Protection Ring OMS-SPR Optical Multiplex Section Shared Protection Ring OTM Optical Terminal Multiplexer OTS Optical Transmission Section OXC Optical Cross Connect P Protection RS Regenerator Section SDH Synchronous Digital Hierarchy SDXC Synchronous Digital Cross Connect SNCP Sub Network Connection Protection STM-N Synchronous Transport Module (level) N © 1999 EURESCOM Participants in Project P709 of International page v (vi) Volume 3: Annex B – Protection in the optical layer TM Terminal Multiplexer W Working WDM Wavelength Division Multiplexing page vi (vi) Deliverable 2 © 1999 EURESCOM Participants in Project P709 Deliverable 2 1 Volume 3: Annex B – Protection in the optical layer Introduction Survivability will be of utmost importance in future high capacity all-optical networks. Different strategies can be used to improve the survivability of the transport network. Two main approaches are protection and restoration. Protection uses a preassigned capacity between nodes to replace the failed or degraded transport entities. Restoration can use any capacity available between nodes to find a transport entity that can replace the failed one. Compared to restoration techniques automatic protection is much faster but utilises less efficiently the available network resources. Automatic protection can be applied to different network architectures such as point to point, ring and meshed networks. Automatic protection is widely used in SDH networks. The availability of configurable optical add drop multiplexers will allow network operators to introduce survivable optical ring networks. Several types of optical ring protection can be identified depending on architectural layer where protection is implemented. In meshed networks optical sub network connection protection can be applied. The selected protection strategy has a strong impact on network planning and network upgrading possibilities. Therefore, it is extremely important to understand the characteristics of different optical protection methods when high capacity optical networks are planned. © 1999 EURESCOM Participants in Project P709 page 1 (48) Volume 3: Annex B – Protection in the optical layer 2 Optical protection techniques 2.1 Identification of protection techniques Deliverable 2 Optical protection techniques can be classified according to the architectural layer were protection is implemented, and also in terms of how the capacity is allocated to service and protection channels. The combination of these characteristic results is four possible types of protection. The following table presents examples of optical architectures for these different types of protection: Layer\Capacity Dedicated Shared Optical Channel OC-DPRing OC-SPRing 1+1 link (OC) 1:1 link (OC) OMS-DPRing OMS-SPRing 1+1 link (OMS) 1:1 link (OMS) Optical Multiplex Section Table I – Protection in optical architectures Detailed descriptions of the above optically protected architectures can be found in Deliverable 1 of EURESCOM Project P615. The comparison of performance and requirements of some of these architectures can be found in Deliverable 2 of the same Project. We will now characterise the different types of protection that can be applied in optical networks, from the layer and capacity points of view. 2.1.1 Optical channel protection Optical channel protection is implemented in the Optical Channel (OC) layer. Each optical channel can be protected individually. This allows the selection of which channels to protect in the multiplex. The protection wavelength is transmitted in the protection fibre, in the direction opposite to the service channel (ring case), or over a physically independent path (mesh case). If the optical layer supports SDH client signals, then the OC-protection is functionally equivalent to SDH MS-protection. As such, in this case a proper consideration of advantages and disadvantages should be taken before deciding to replace the SDH protection by the OC-protection. The possibility of keeping protection both in the SDH and optical layers could also be considered, in order to improve the survivability of the network. However, in this case, a proper interworking of the protection mechanisms should be ensured. 2.1.2 Optical multiplex section protection Optical multiplex section protection is implemented in the Optical Multiplex Section (OMS) layer. In this case, all channels in the multiplex are simultaneously protected. This may lead to a great simplification of the client layer electronic equipment, since now there is no need for electronically implemented protection systems for the individual client signals (e.g.: SDH protection), which imply the use of duplicated line interfaces, control electronics and software. Also, the reliability of the network may page 2 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer increase due to the simplification of the electronic equipment and to the more efficient protection. OMS-protection is especially interesting in networks supporting large numbers of optical channels. 2.1.3 Dedicated protection Dedicated protection means that the entity to be protected is protected using dedicated resources. For example, in a ring the OCs are duplicated when inserted, one going in the clockwise direction, while the other goes in the anti-clockwise direction. The receiver selects the best signal by means of a switch. If the duplication and selection of channels is done in the OC layer, we have an OC-DPRing. If the duplication and selection is done in the OMS layer, we have an OMS-DPRing. A 2-fibre ring supports bi-directional communication in an OC-DPRing, since each channel can be transmitted clockwise or anti-clockwise. However, a bi-directional OMS-DPRing needs four fibres, since the channels in the multiplex are processed as a whole. In meshes, made up of point-to-point links, two physically disjoint pairs of fibres must be used to implement bi-directional communications with dedicated protection, one pair supporting the working channels and the other supporting the protection channels. The demands are transferred to the channels in the protection fibre if the working fibre fails. 2.1.4 Shared protection Shared protection means that the working channels share the protection capacity. Under normal conditions all demands are routed over the working channels, and the protection capacity can support low-priority traffic, though this results in a more complex node implementation. Optical rings with shared protection have the capacity of the fibres divided between service and protection channels. As such, a 2-fibre Shared Protection Ring (SPRing) is a bi-directional ring, since one fibre supports clockwise traffic and the other supports anticlockwise traffic. Shared protection for optical meshes (in point-to-point links) would use a similar sharing of the fibre capacity: half the channels in one of the fibre pairs would support half of the demands, and half of the channels in the other fibre pair would support the remaining demands. The remaining half of the fibre capacities would serve as protection capacity. If one fibre pair failed, the working channels would be transferred to the protection channels of the other fibre pair, keeping the link in service. 2.2 Examples of optically protected rings In this section we will describe particular optical ring architectures that implement all possible combinations of optical protection characteristics. 2.2.1 Optical Channel Dedicated Protection Ring (OC-DPRing) The OC-DPRing is an example of ring architecture with optical routing and optical protection. Optical routing allows a virtual mesh topology by allocation of different wavelengths to each possible link connecting pairs of nodes. The OC-DPRing with 4fibres allows bi-directional traffic with 1+1 protection. It offers greater flexibility than its 2-fibre counterpart, and requires fewer wavelengths. The total number of © 1999 EURESCOM Participants in Project P709 page 3 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 wavelengths needed can be reduced if wavelengths are re-used. The following figure represents an OC-DPRing with wavelength re-use (e.g.: adjacent spans use the same wavelength). Protection in this architecture is implemented in the OC layer. Optical switches are used to connect traffic to the working or protection fibres, according to the state of the ring (normal or failure conditions, respectively). Care should be taken when wavelengths are re-used: to avoid wavelength conflicts in the protection fibres under failure conditions, dual-ended protection switching should be implemented. Figure 1. OC-DPRing with wavelength re-use 2.2.2 Optical Channel Shared Protection Ring (OC-SPRing) This ring uses two fibres for bi-directional communication between the nodes. Under normal working conditions each fibre carries a different wavelength: 1 clockwise, 2 anti-clockwise. If a section or node fails, the adjacent nodes will re-route the wavelengths to the complementary arc of the ring, sharing the capacity of the fibres between the two optical channels. Therefore, we have shared protection at the OC level (Figure 2). SDH ADM SDH ADM A B C A B C F E D F E D Figure 2. The OC-SPRing under normal and section failure conditions Regarding traffic routing, this is done electrically by the SDH equipment, as the optical channels only exist between adjacent nodes, corresponding to physical nodeto-node sections. page 4 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 2.2.3 Optical Multiplex Section Dedicated Protection Ring (OMS-DPRing) An OMS-DPRing can be a 2-fibre or 4-fibre ring. The traffic routing is implemented at the OC level, by means of allocating different wavelengths to the different node-tonode links. The protection is implemented at the OMS layer hence all channels are simultaneously protected in case of failure. Since we have dedicated protection, in the 2-fibre ring one of the fibres is dedicated to protection of the traffic circulating in the opposite direction in the service fibre. When a section or node failure occurs, the nodes adjacent to the failure will re-route the traffic from the service fibre to the protection fibre, along the complimentary arc of the ring. 2.2.4 Optical Multiplex Section Shared Protection Ring (OMS-SPRing) This architecture is a 2- or 4-fibre ring, where routing is done at the OC level, as direct node-to-node logical links can be established using different wavelengths. This allows the implementation of full logical meshes, connecting every node to every other node in the ring (Figure 3). A A B D C D B A B D C C Figure 3. The OMS-SPRing under normal and section failure conditions. Central scheme represents node connectivity based on wavelength routing Protection is implemented at the OMS layer, the capacity of each fibre being shared between service and protection wavelengths. For example, in the clockwise fibre the first half of the wavelength spectrum (e.g.: channels 1 to 8 in a 16-channel ring) is used for service traffic, while the second half is used for protection of the service channels in the anti-clockwise fibre. In the anti-clockwise fibre things are inverted: the first half of the spectrum is used for protection of the service channels in the clockwise fibre, while the second half is used for service traffic (Figure 4). Failure conditions Normal conditions service channels protection channels service channels protection channels protection channels service channels protection channels service channels Figure 4. Shared protection of optical channels in the OMS-SPRing © 1999 EURESCOM Participants in Project P709 page 5 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 If a section or node fails, the adjacent nodes re-route the traffic to the complementary arc of the ring. The service wavelengths of the clockwise fibre are re-routed to the protection wavelengths of the anti-clockwise fibre, and the service wavelengths of the anti-clockwise fibre are re-routed to the protection wavelengths of the clockwise fibre. page 6 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 3 Protection of interconnected optical network domains This chapter describes the general principles for protection of interconnected optical network domains. Only the selected optical network architectures defined in Deliverable 1 of P709 will be studied. The main characteristics of the protection of these interconnected optical domains will be described. 3.1 Selected reference network configurations Deliverable 1 of P709 selected the following as reference network configurations: Two-level CS-Ring with network SDH connectivity Two-level OMS-SPRing network with limited optical connectivity Two-level OMS-SPRing-Optical Mesh architecture Two-level Optical Mesh-OMS-SPRing architecture with full optical connectivity All the selected reference network configurations use dual node interconnection because in large capacity core networks disjoint alternative routing is an essential requirement. 3.2 Interconnection of sub-networks and protection This section studies the characteristics of the dual node interconnection of optical domains, with special focus in the case of interconnected rings. Only two fibre architectures will be analysed. As a starting point, it is important to clarify the concepts of physical interconnection and logical interconnection between two rings. Two rings that “touch” each other in two (one) nodes are physically interconnected in a dual-node (single-node) way: we have a dual-node (single-node) physical interconnection. A wavelength that is routed through two single-node physically interconnected rings (Figure 5 for both OCHDPRings and OMS-SPRings) is protected within each ring, dropped from the first ring by the OADM I1 and added into the second ring by the OADM I3. In this case, the scheme applied at the interconnection is a single-homing interconnection. On the contrary, a wavelength that is routed through two dual-node physically interconnected rings, and uses both interconnection nodes to cross the border between rings, performs a “dual-homing interconnection”. Node A Node A I1 I1 I3 I3 Node B a) Node B b) Figure 5. Single-homing interconnection a) OC-DPRings, b) OMS-SPRings © 1999 EURESCOM Participants in Project P709 page 7 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 Trying to apply to optical rings the concepts proposed by the ETSI document DTR/TM-03041 (that deal with SDH network protection interworking), two kinds of dual-homing architectures can be applied at the interface between two dual-node physically interconnected rings: Virtual ring architecture Drop and continue architecture (see section 0) Node A Node A I1 I2 I1 I2 I3 I4 I3 I4 Node B a) Node B b) Figure 6. Dual-homing interconnection based on virtual ring architecture, a) OC-DPRings, b) tentative application to OMS-SPRings The Virtual Ring Architecture (VRA) can only be applied to OC-DPRings. The routing and protection scheme is shown in Figure 6a). As it can be seen the same wavelength connection is protected as if the two rings were a single one and both interconnection nodes are used to cross the border between rings (i.e. dual-homing interconnection). A possible application of the virtual ring architecture concept to OMS-SPRings is shown in Figure 6b). This proposal duplicates the wavelength from A to B, routing the wavelengths in a route disjoint way. In practice, the protection at the OMS layer (within the OMS-SPRings) is combined with route diversity at the OCH layer. The protection mechanism recovers single link or node failures within each ring, while the route diversity strategy improves the connection survivability in case of failure of an interconnection link between the two rings. However, if both wavelengths carry the same client layer connections (i.e. they are a copy of the other) this solution is worse than the VRA over OCH-DPRings, because it requires twice the bandwith without improving availability (in a first approximation). In fact both VRA over OCHDPRings and the solution of Figure 6b) can survive against a single link/node failure between A and B. However, in general, when route diversity is applied, each route between a pair of nodes carries only half of the client layer connections between these two nodes. That has two consequences: the number of wavelengths to be carried in the optical layer, in principle, halves or, otherwise stated, no protection redundancy is applied on client layer page 8 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 connections (although this can rise the problem of a low utilisation of the OCHs when the client layer demands are not high enough); the client connection resilience against failures should be evaluated in a twolayer perspective. In practice, under these conditions, the architecture of Figure 6b) carries half the client layer connections between nodes A and B over the 1, and the other half over 2. So it recovers: all the client connections in case of single node/link failure within each ring only half the client connections in case of failure of an interconnection link This improves the performance in terms of resource requirements, which is now as good as the VRA over OCH-DPRings, but we decrease the interconnection resilience. It should be noticed that the scheme in Figure 6b) cannot be considered a dualhoming interconnection, as each wavelength crosses the border between rings through a single node. There are other architectures that can be used to improve the performances of VRA over OCH-DPRings without introducing drop-and-continue: they use a single-homing solution over dual-node physically interconnected rings. Figure 7 shows these architectures in the case of OCH-DPRings (uni-directional, but it works for bidirectional OCH-DPRings as well) and OMS-SPRings. The logical interconnection is a single-homing one in both cases because each wavelength is dropped from the ring by one and only one OADM. Node A Node A I1 I2 I1 I2 I3 I4 I3 I4 Node B a) Node B b) Figure 7. Improved virtual ring architectures a) OC-DPRings, b) OMS-SPRings The proposal here is to carry in each wavelength half the client layer connections, so that the resource requirements can be, in principle, the same of the VRA over OCHDPRings (or better, in the case of OMS-SPRings). But here each Survivable SubNetwork (SSN, i.e. ring) can recover a single node/link failure, allowing multiple failures (one per ring) between node A and B. But both these architectures are weaker than the VRA over OCH-DPRings at the interconnection nodes: only half the client layer connections survive against an interconnection failure. If protecting locally the © 1999 EURESCOM Participants in Project P709 page 9 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 interconnection OCHs between I1 and I3, and between I2 and I4, with a 1:N protection, in order to overcome this weakness, is feasible and cost-effective, these solutions are generally the best ones without d&c. 3.3 Protection with optical drop and continue functionality This section studies the characteristics of the dual node interconnection of optical domains when optical drop and continue functionality is used. Only four-fibre architectures with optical connectivity will be analysed. Four-fibre OMS-SPRings require four fibres for each span of the ring. Working and protection channels are carried over different fibres: two multiplex sections transmitting in opposite directions carry the working channels while two multiplex sections, also transmitting in opposite directions, carry the protection channels. This permits the bidirectional transport of working traffic. The difference between OMSSPRings and OMS-DPRings is that in OMS-SPRings no dedicated resources (e.g. duplication of OCs) are used. 3.3.1 Two-level OMS-SPRing architecture The two-level OMS-SPRing architecture with dual node interconnection is shown in Figure 8. It consists of one upper level OMS-SPRing with termination node A (ring 1) and two lower level OMS-SPRings with termination nodes B and C (rings 2 and 3). Interconnection and routing are done at the optical layer. For the interconnection of termination nodes B and C the signal is sent using wavelength 1 from node B to the primary interconnection node L1. Drop and continue functionality is used at node L1 to drop the signal towards the primary interconnection node U1 in ring 1 and to continue the same signal to the secondary interconnection node L2. In node L2 the signal is dropped to the secondary interconnection node U2 in ring 1. From node U2 the signal is fed to node U1 where a service selector is used to select between the signals coming from nodes L1and U2. The selection is then fed through termination node A to the primary interconnection node U3. Drop and continue functionality is used at node U3 to drop the signal towards the primary interconnection node L3 in ring 3 and to continue the same signal to the secondary interconnection node U4. In node U4 the signal is dropped to the secondary interconnection node L4 in ring 3. From node L4 the signal is fed to node L3 where a service selector is used to select between the signals coming from nodes L 4 and U3. The selection is then fed to termination node C. In the other direction of transmission the signal is sent using the same wavelength 1 from node C to the primary interconnection node L3 where drop and continue functionality is used to send the signal to both node U3 and node L4. In node L4 the signal is dropped to the secondary interconnection node U4 from which it is fed to the primary interconnection node U3 where a service selector is used to make a selection between the signals coming from nodes L3 and U4. The selected signal is then fed through node A to the primary interconnection node U1. Drop and continue functionality is used at node U1 to drop the signal towards the primary interconnection node L1 in ring 2 and to continue the same signal to the secondary interconnection node U2. In node U2 the signal is dropped to the secondary interconnection node L2 in ring 2. From node L2 the signal is fed to node L1 where a service selector is used to select between the signals coming from nodes U1 and L2. The selection is then fed to termination node B. page 10 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 The drop-and-continue function can be simply implemented with a an optical splitter. The simplest way to implement the service-selector function is to use an optical switch activated by power monitoring. This interconnection architecture provides protection against the failure of one or both interconnecting nodes (each on different ring, but on the same interconnect), or the connection between the two interconnecting nodes. The architecture can also protect against a single failure in each of the rings provided that these failures are not terminating node failures, or these failures do not combine to affect both interconnections between the rings. Within the OMS-SPRings, any failure outside the termination, primary or secondary nodes is handled in a standard fashion. The nature of the failure (cable cut or node failure) has no impact on the configuration of the termination, primary or secondary nodes. A cable cut failure is shown in Figure 9. A failure of the primary node in the OMS-SPRing results in a secondary connection of the signal. This is also shown in Figure 9. During a failure of the secondary node the signal at the termination node is unaffected, since it receives its signal from the primary node. A Ring 1 U1 U3 U2 U4 Upper level network Lower level network L1 L2 Ring 2 1 1 B L4 L3 Ring 3 1 1 C Working Protection Optical add-drop multiplexer Service selector Drop and continue Figure 8. Two-level OMS-SPRing architecture with dual node interconnection © 1999 EURESCOM Participants in Project P709 page 11 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 For the interconnection of termination nodes A and B or A and C other wavelengths (2, 3) have to be used. One wavelength is needed for each traffic demand in this architecture. The same wavelength can be used for both directions of transmission. In some cases the same wavelength might also be used for other connections which do not overlap with the ones where the wavelength is already used. In failure situations this may, however, cause wavelength conflicts in OMS-SPRings when the normal ring protection procedure is used. A Ring 1 U1 U3 U2 U4 Upper level network Lower level network L1 L2 Ring 2 1 1 B L4 L3 Ring 3 1 1 C Working Protection Optical add-drop multiplexer Service selector Drop and continue Figure 9. Two-level OMS-SPRing architecture: response to complete primary node failure in Ring 1 and cable cut in Ring 3 page 12 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 3.3.2 Volume 3: Annex B – Protection in the optical layer Two-level OMS-SPRing-Optical Mesh architecture The two-level OMS-SPRing-Optical Mesh architecture is shown in Figure 10. It consists of two Optical Meshes with termination nodes B and C and one upper level OMS-SPRing with termination node A. Interconnection and routing are done at the optical layer. For the interconnection of termination nodes A and B the signal is sent using wavelength 1 from node A to the primary interconnection node U1. Drop and continue functionality is used at node U1 to drop the signal towards the primary interconnection node L1 and to continue the same signal to the secondary interconnection node U2. In node U2 the signal is dropped to the secondary interconnection node L2. From nodes L1 and L2 the signal is fed using separate routes to termination node B where a selection is made between the two signal paths. In the other direction of transmission the signal is sent using the same wavelength 1 from node B on separate routes to the primary interconnection node L1 and to the secondary interconnection node L2. From nodes L1 and L2 the signal is sent to nodes U1 and U2 respectively. From node U2 the signal is fed to the primary interconnection node U1 where a service selector is used to make a selection between the signals. The selected signal is then fed to termination node A. For the interconnection of termination nodes A and C wavelength 2 is used in a similar way as shown in Figure 10. The architecture can protect against any single failure between the termination nodes. Within the OMS-SPRing, any failure outside the termination, primary or secondary nodes is handled in a standard fashion. In this architecture one wavelength is needed for each traffic demand. The same wavelength may also be used in other parts of the network when there is no possibility of wavelength conflicts in the normal or protected state. In order to improve the survivability of the meshed networks against simultaneous failures more wavelengths should be used. 3.3.3 Two-level Optical Mesh-OMS-SPRing architecture The two-level Optical Mesh-OMS-SPRing architecture with dual node interconnection is presented in Figure 11. It consists of two lower level OMS-SPRings (rings 1 and 2) with dual node interconnection to an upper level Optical Mesh. Interconnection and routing are done at the optical layer. For the interconnection of termination nodes B and C the signal is sent using wavelength 1 from node B to the primary interconnection node L1. Drop and continue functionality is used at node L1 to drop the signal to the primary interconnection node U1 and to continue the same signal to the secondary interconnection node L2. From node U1 the signal is sent to node U3 and further to node L3 in ring 2. In node L2 the signal is dropped to the secondary interconnection node U2. From node U2 the signal is fed to node U4 and further to node L4 in ring 2. From node L4 the signal is sent to node L3 where a service selector is used to select between the signals coming from nodes L4 and U3. The selection is then fed to termination node C. In the other transmission direction the signal is sent using the same wavelength 1 from node C to node B in a similar way. The architecture can protect against any single failure between nodes B and C. This architecture can also protect against a single failure in each of the rings provided that these failures are not terminating node failures, or these failures do not combine to affect both interconnections between the rings. For this level of protection only one © 1999 EURESCOM Participants in Project P709 page 13 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 wavelength is needed for each traffic demand. Survivability in the meshed network can be improved by using more wavelengths. 1 2 A U1 U3 U2 U4 Upper level network Lower level network L1 L2 L4 B 1 1 1 2 L3 C 2 2 Working Protection Optical add-drop multiplexer Optical cross connect Service selector Drop and continue Figure 10. Two-level OMS-SPRing-Optical Mesh architecture with dual node interconnection page 14 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 A U1 U3 U2 U4 Upper level network Lower level network L1 L2 Ring 1 1 1 B L4 L3 Ring 2 1 1 C Working Protection Optical add-drop multiplexer Optical cross connect Service selector Drop and continue Figure 11. Two-level Optical Mesh-OMS-SPRing architecture with dual node interconnection 3.4 Conclusions To improve the survivability of interconnected optical network domains the number of used wavelengths has to be increased. The increase of the number of wavelengths could be avoided by introduction of optical drop and continue functionality and optical switching functionality in the interconnection nodes. In two fibre OMSSPRings two wavelengths, one for each direction of transmission, are needed for each traffic demand between domains in order to provide protection against any single failure between connected nodes. In four fibre OMS-SPRings one wavelength is needed for each traffic demand between domains in order to provide protection against any single failure between connected nodes. The same wavelength can be used in both directions of transmission. To further improve the protection of the interconnections against simultaneous failures the number of wavelengths has to be increased. © 1999 EURESCOM Participants in Project P709 page 15 (48) Volume 3: Annex B – Protection in the optical layer 4 Deliverable 2 Wavelength conflicts under protected state in OMSSPRings This chapter identifies and analyses wavelength conflicts under protected state in OMS-SPRings. This study is based on the analysis of the conditions leading to "wrong connections" in SDH MS-SPRings under protection, translating these conditions to the frequency domain (wavelengths). 4.1 Protection in OMS-SPRings This section overviews the two-fibre OMS-SPRing architecture. The main characteristics of the architecture are briefly described. Then, the behaviour of the architecture under the different types of failures that can be protected against is studied. The types of failures considered are: single span, multiple span and node failures. 4.1.1 Architecture In the two-fibre OMS-SPRing, the capacity on each of the two fibres is shared by working and protection traffic. For example, a sixteen-wavelength WDM system would have eight channels allocated for working traffic and eight for protection traffic on each of the two fibres. The working channels in one fibre are protected by the protection channels, travelling in the opposite direction around the ring, in the other fibre. In the event of a failure, working traffic on one fibre would be switched over to the protection capacity on the other fibre. Therefore the eight working channels on one fibre have the same wavelengths as the eight protection channels on the other fibre and vice versa. In summary, the main characteristics of the OMS-SPRing architecture are: fibres: 2 physical topology: ring logical topology: mesh routing: optical channel and SDH path layer protection: optical multiplex section shared protection (dual-ended switching) span failure: protected multiple span failure: protected node failure: protected page 16 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 4.1.2 Single Span Failure in OMS-SPRing a) (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection B " C (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) b) " service " F E D A B C F E D Service Fibre failure Protection NOTE: A two-fibre ring only uses ring switches to restore traffic. Figure 12. Two-fibre OMS-shared protection ring architecture. a) working state, b) protection state under single span failure In the example considered here the working traffic is bidirectionally transported between nodes A and D under normal operating conditions (Figure 12a). If a failure occurs in the span between nodes B and C, both nodes adjacent to the failed span loop (bridge and switch) the working traffic to the protection channels in the other fibre. The working traffic is, then, transported on the long path, to bypass the failed span (Figure 12b). © 1999 EURESCOM Participants in Project P709 page 17 (48) Volume 3: Annex B – Protection in the optical layer 4.1.3 Deliverable 2 Multiple Span Failure in OMS-SPRing a) (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection B " C (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) " service " F E D A B C F E b) D S e r v i c e F i b r e f a i l u r e P r o t e c t i o n Figure 13. Two-fibre OMS-shared protection ring architecture. a) working state, b) protection state under multiple span failures In the previous figure, a multiple span failure (in spans A-B and B-C) is illustrated. In this situation, the traffic to node B is lost. All the remaining traffic is recovered. page 18 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 4.1.4 Node Failure in OMS-SPRing a) (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection B " C (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) " service " F E D A B C F E b) D S e r v i c e N o d e f a i l u r e P r o t e c t i o n Figure 14. Two-fibre OMS-shared protection ring architecture. a) working state, b) protection state under node failure The consequences of a node failure, are quite similar to the situation presented in the previous point. As before, the traffic to node B is lost. All the remaining traffic is recovered. © 1999 EURESCOM Participants in Project P709 page 19 (48) Volume 3: Annex B – Protection in the optical layer 4.2 Misconnections in OMS-SPRings 4.2.1 Introduction Deliverable 2 As it was said before, this study is based on the analysis of the conditions leading to “wrong connections” in SDH MS-SPRings under protection, translating these conditions to the frequency domain (wavelengths). So, it is now appropriate to present a summary about misconnections in SDH MS-SPRings. In an SDH MS-SPRing, in order to perform a ring switch, the protection channels are essentially shared among each span of the ring. Also, extra traffic may reside in the protection channels when the protection channels are not currently being used to restore working traffic transported on the working channels. Thus, each protection channel time slot is subject to use by multiple services (services on the same time slot but on different spans, and service from extra traffic). With no extra traffic in the ring, under certain multiple span failures, such as those that cause node(s) isolation, services (on the same time slot but on different spans) may contend for access to the same protection channel time slot. This yields a potential for misconnected traffic. With extra traffic in the ring, even under single point failures, a service on the working channels may contend for access to the same protection channel time slot that carries the extra traffic. This also yields a potential for misconnected traffic. So, how can a potential misconnection be identified? A potential misconnection is determined by identifying the nodes that will act as the switching nodes for a bridge request, and by examining the traffic that will be affected by the switch. The switching nodes can be determined from the node addresses in the K1 and K2 bytes. The switching nodes determine the traffic affected by the protection switch from the information contained in their ring maps and from the identification of the switching nodes. Each of the ring maps, then, shall contain at minimum: 1. information regarding the order in which the nodes appear on the ring, 2. the AU-4 time slot assignments for traffic that is both terminated at that node and passed-through that node, 3. for each of these AU-4 time slots, the node addresses at which the traffic enters and exits the ring, 4. and an optional indication of whether the AU is being accessed at the lower order VC level somewhere on the ring. Inserting the appropriate AU-AIS (an “all-ones” loading) in those time slots where misconnected traffic could occur shall squelch potential misconnections. For rings operating at an AU-4 level, this squelching occurs at the switching nodes. For rings using lower order VC access, squelching locations are under study. So, a misconnection exists when a node extracts a channel, which must be delivered to another node. In the two-fibre OMS-SPRing case, an analogy between wavelengths and time slots (AU-4 of the SDH case) will be used. As such, a misconnection can occur in two distinct situations. The first situation is characterised by a multiple span failure page 20 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 without low-priority traffic in the protection channels. The second situation presents a single span failure with low-priority traffic in the protection channels. The use of extra low-priority traffic in the protection channels of an OMS-SPRing is still under discussion, since it brings additional complexity to the node equipment. However, the discussion in this document will not take these implementation matters into account, as it is kept at a functional level. 4.2.2 Misconnections without low-priority traffic a) (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection B " C (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) F " service " E D b) A F B C E D S e r v i c e N o d e f a i l u r e P r o t e c t i o n Figure 15. Two-fibre OMS-shared protection ring architecture. a) working state, with wavelength reuse b) misconnection under protected state This example shows a bidirectional communication between nodes C and D and nodes D and F, with wavelength reuse (Figure 15a). In the case of failure of node D, for example, nodes E and C loop (bridge and switch) the traffic. So, node F extracts the traffic inserted by node C, and node C extracts the traffic inserted by node F, resulting in a misconnection (Figure 15b). A possible solution, is similar to the one adopted in SDH MS-SPRing. In SDH MSSPRings inserting the appropriate AU-AIS (an “all-ones” loading) in those time slots where misconnected traffic could occur shall squelch potential misconnections. For example, for rings operating at an AU-4 level, this squelching occurs at the switching nodes. © 1999 EURESCOM Participants in Project P709 page 21 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 So, in this particular case, nodes E and C must perform the squelching of the traffic that will be misconnected, (“a wavelength squelching”) before the ring switch set-up (Figure 16). A discussion of how to perform this “wavelength squelching” is presented in section 4.2.4. (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection B " C (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) " service " Squelching F E Squelching D S e r v i c e N o d e f a i l u r e P r o t e c t i o n Figure 16.Two-fibre OMS-shared protection ring architecture: example of misconnection 4.2.3 Misconnections with low-priority traffic To study this subject, two bidirectional connections, between nodes D and F, and nodes A and C, are considered. Between nodes D and F, the communication is supported by bidirectional working traffic in service wavelengths 1 and 2. Between nodes A and C, the communication is supported by bidirectional extra traffic (lowpriority) in protection wavelengths 1 and 2 (Figure 17a). When a span failure occurs between nodes E and D, how does the protection scheme work? Nodes D and E loop (bridge+switch) the working traffic (Figure 17b). So, when node E bridges the working traffic to the protection channels, the consequence is that node A extracts the traffic inserted by node F. When, node E performs the switch, node F extracts the low-priority traffic inserted by node A. A similar situation occurs in node D. When node D bridges the working traffic to the protection channel, the node C extracts the traffic inserted by node D. When node D performs the switch, it extracts the traffic inserted by node C. To avoid this problem, nodes A and C must perform the squelching of the extra traffic (“a wavelength squelching”), before the ring switch set-up (Figure 18). page 22 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 a) (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection " B C 1 (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) F b) F " service " E 1 A B E D C D S e r v i c e F i b r e f a i l u r e P r o t e c t i o n Figure 17. Two-fibre OMS-shared protection ring architecture. a) working state, with extra traffic in the protection channels b) misconnection under protected state (1,3, 5,...) clockwise service wavelengths A (,, ,...) " protection " B C Squelching (1,3, 5,...) anti-clockwise protection wavelengths (,, ,...) F " service " Squelching E D Service Fibre failure Protection Figure 18. Two-fibre OMS-shared protection ring architecture: example of misconnection © 1999 EURESCOM Participants in Project P709 page 23 (48) Volume 3: Annex B – Protection in the optical layer 4.2.4 Deliverable 2 Wavelength squelching In the previous two sub-sections, it was shown that the optical channels subject to misconnections under protected state in an OMS-SPRing should be “squelched”. This operation is the frequency domain equivalent of the AU-4 (time domain) squelching performed in SDH MS-SPRings. In the SDH case the squelching is done by loading the misconnected AU-4 with an AIS signal (an “all-ones” loading). In the optical architecture case this operation cannot be done directly on the optical channel because there is no access, in the optical channel layer, to the client layer signal. A possible way of performing the wavelength squelching is by associating an overhead channel to each optical channel, and reserving some capacity in this overhead channel for an AIS-like indication. Initial studies regarding the types of alarm signals needed by the optical layers have already been performed in EURESCOM Project P615, for example, where such a signal has been considered [see P615 Deliverable 1, Annex B]. The overhead channels associated to the optical channels could be transported by a specific wavelength, perhaps outside the gain band of the optical amplifiers. When a failure occurs that could lead to misconnected wavelengths, the nodes initiating the switching procedure should check which wavelengths can be affected. As in the SDH, this is done by looking at the ring maps which now contain the following information: 1. logical ordering of nodes in the ring 2. identification of the optical channels being terminated at each node 3. identification of the optical channels passing through each node After identifying the optical channels that can be misconnected, the switching nodes should insert the AIS-like indication in the proper place in the associated overhead channels, so that the affected receiving nodes can avoid using the misconnected optical channels. Of course, all overhead channels should be accessible to all nodes in the ring. This would be possible if the wavelength supporting the overhead information is electrically terminated at all nodes. It should be noted that the optical channels are still kept in the optical domain, being only terminated in the nodes they interconnect. 4.3 Conclusion In this chapter the problem of wavelength conflicts, or misconnections, under protected state in OMS-SPRings has been addressed. The approach followed in this study consisted of establishing an analogy with the SDH MS-SPRing case. In the SDH case, misconnections occur at the AU-4 level under certain failure conditions: node or multiple span failure without low-priority traffic in the protection AUs, and single span failure with low-priority traffic in the protection AUs. To avoid the misconnections, the nodes squelch the AUs that will be misconnected by inserting an AIS signal in these AUs. The decision on which AUs to squelch is done taking into account which nodes are performing the protection switching and also the node connectivity defined in the ring maps. To study the OMS-SPRing case, wavelengths become the analogous of the time slots in SDH. As such, misconnections will now occur at the optical channel level under page 24 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer conditions similar to the SDH case: node or multiple span failures without lowpriority traffic in the protection wavelengths, and single span failures with lowpriority traffic in the protection wavelengths. The proposed solution for the misconnections is based on wavelength squelching, using the overhead channel associated to the optical channels to support an Optical Channel AIS signal. This solution would maintain the independence of the optical layers from the client layers, as the client signals would not be directly manipulated. An issue left for further study is the proper way of linking the AIS alarm at the Optical Channel Layer to the management system of the client signal (typically SDH), in order that the client would be able to reject the misconnected signals. © 1999 EURESCOM Participants in Project P709 page 25 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 5 Identification of items with impact on network planning 5.1 Protection of individual channels Certain channels of an hierarchical network may have higher importance requiring very high availability that otherwise cannot be ensured. On the other hand, the structure itself may have weak points making the network in certain failure constellations vulnerable. The purpose of this section is to investigate how individual channels can be protected on optical networks. 5.1.1 Coloured Section Ring (CSR) CSRs have self-healing features, i.e. in case of a single failure, automatic protection is performed. The routing and protection is performed entirely on SDH MS layer. The structure provides protection for span failure. In CSRs one wavelength is assigned to each span. If the traffic goes via nonneighbouring nodes, o/e/o conversion is made at the intermediate nodes, and the traffic is passed through by the SDH ADMs on electrical layer. This makes the CSRs vulnerable against the failure of these intermediate nodes since the ring cannot perform protection. Figure 19 depicts the protection architecture according to ETSI TS 101 010. It can be seen that neighbouring protection “blocks” are chained in case of a connection going through several MSs (which means several wavelengths). The gaps between protection “blocks” indicate the weak points of the system. network layer service LOP HOP MS RS OCH OMS OTS k-1 k k+1 k+2 geographical location Figure 19. Protection layer interconnections of CSR: Chained intra-layer network protection This is the reason why the CSR in preferably designed by choosing the right logical node order. It means that the wavelengths are assigned to the main traffic flows between nodes to minimise the need for transiting at nodes electrically. This results that the logical order may differ from the physical order. If a high priority connection is established via such transiting nodes, the ring does not protect against the failure of this node. Therefore additional protection shall be page 26 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 provided. This additional protection can be made in SDH layer on the HO or LO path level (1+1 path protection) (Figure 20). ADM ADM APS on VC layer passing through on VC layer OADM OADM CS-Ring OADM OADM working protection ADM ADM Optical add-drop multiplexer Figure 20. CSR - Path protection on VC level This situation is shown in Figure 21 by protection architecture. The whole MS chain is covered by an upper protection “block” in the HOP (or LOP) layer. This protection structure is called hybrid interlayer protection. network layer service LOP HOP MS RS OCH OMS OTS k-1 k k+1 k+2 geographical location Figure 21. Protection layer interconnections of CSR using 1+1 HO path protection: Hybrid inter-layer network protection © 1999 EURESCOM Participants in Project P709 page 27 (48) Volume 3: Annex B – Protection in the optical layer 5.1.2 Deliverable 2 CSR-CSR hierarchical network The situation is slightly different when CSRs are interconnected forming a CSR-CSR hierarchical network. In this structure, two interconnection nodes serve as gateways to the upper/lower network level. On the lower level, the main traffic flows are pointing towards these hub nodes, therefore a unique wavelength can be assigned toward each hub node avoiding the intermediate o/e/o conversions. If this cannot be made, SDH 1+1 path protection can be established. On the higher level ring it is very difficult to choose a node order that eliminates the o/e/o conversions, hence the application of extra protection seems to be more frequently necessary. 5.1.3 Optical Multiplex Section Shared Protection Ring (OMSPR) In OMSPRings the protection is entirely performed on optical layer. The structure gives protection not only against span and node failures, but against double span failures as well. In other words, OMSPR exhibits higher availability, having no weak point as CSR has. network layer service LOP HOP MS RS OCH OMS OTS k-1 k k+1 k+2 geographical location Figure 22. Protection layer interconnections of OMSPR: Single intra-layer network protection page 28 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 network layer service LOP HOP MS RS OCH OMS OTS k-1 k k+1 k+2 geographical location Figure 23. Protection layer interconnections of OMSPR using 1+1 MSP: Hybrid inter-layer network protection As the protection architecture shows it in Figure 22, the entire connection is covered by a continuous block indicating that there is no such weak point in the system as in CSRs. Therefore, additional protection does not seem to be necessary. Yet, the possibility is given: extra protection can be made on electrical layer (1+1 MSP or HO/LO path protection). (See Figure 23) 5.2 Protection Priority Classes Implementing a network survivability policy should consider the possibility of providing different grades of service in what concerns network recovery performance, and create the concept of survivability priority classes. These would be defined by different efficiencies (100%, 50%, 25%, 0%) and recovery times (50ms, 200ms, 1min, no guaranteed time), and be charged accordingly. More variants can be offered combining these two characteristics. The user could also be given the possibility of defining their own subclasses. The guaranty of a certain survivability performance means that in the worst case that is the performance available to the user, and that better performance maybe available. Users contracting the optical transport service without any guaranteed network recovering performance, could simple rely on their own electrical recovery strategies, e.g. SDH protection techniques. Recovery time is directly related to the chosen technique for implementing survivability, and depends on the failure detection speed, on the efficiency of used protocol and algorithms. When protection is used the limit is 50ms (SDH imposition), but for lower efficiency classes, longer time could be tolerated; if restoration is used recovery time for 100% efficiency is around 200ms. Efficiency is dependent of available resources for network recovery and efficiency of used protocols (protection) and algorithms (restoration). © 1999 EURESCOM Participants in Project P709 page 29 (48) Volume 3: Annex B – Protection in the optical layer 5.2.1 Deliverable 2 Priority classes The obvious classes to provide in terms of recovery efficiency are the following: Highest priority class I: (100%, 50ms) protection for path or link failures in ring architectures. The user would have protection of all its traffic, within the SDH time limit, for path failures: this would cover multiple link and node failures within the same path, but not terminal node failure. In order to cover this type of failure node equipment duplication could be necessary. This could be implemented by 1+1 architectures (the most expensive option), but also by 1:1 and 1:n and n:m architectures. Highest priority class II: (100%, 50ms) protection for path or single link failure in meshed architectures. 100% protection efficiency, within SDH time limit, assured for path or single link failures. This could be implemented by 1+1 architectures (the most expensive option), but also by 1:1 and 1:n and n:m architectures. In meshed architectures multiple link failure coverage with 100% single link protection depends on the type of resource allocation and on the actual failure scenario. One way to assure this feature is to consider and dimension rings overlayed with the meshed structure for protection purposes, subject to the restriction of path hop count limit and path length limit. Less than 100% protection classes make use of protection capacity reserved, dedicated or shared (i.e., excluded the 1+1 case). Medium priority class: (50%, 50ms) One simple way to provide this is, use 1:1 protection, and split the user traffic between working and protection channels. In this case, 50% of traffic is fully protected and the other 50% makes use of protection capacity, therefore being unprotected. In a 1:1 scheme a user can therefore contract two channels with 50% protection. When a failure occurs, the unprotected traffic is discarded in order to assure protection to the protected traffic. The option of defining what traffic to protect and what traffic to discard can be given to the user (user subclass definition). The use of shared protection schemes n:m also allows offering such a service class. However, the optical channels cannot be fractioned. Therefore, this percentage performance for optical protection in such schemes has to take that particularity into account. For instance with a 1:2, scheme one can have one channel in class 100% and two channels unprotected; two channels in class 50% and 1 channel unprotected; three channels in class 30% of protection. Unprotected traffic: This traffic will be the one carried in the protection channels, being discarded in the case of failure, in order to allow the protection of working channels. 5.2.2 Other performance priority classes definition Architectures with n:m protection schemes provide a good basis for establishing protection priority classes. Fixed a network architecture, the population of each protection class has a limit. Completely filling the 100% performance class will prohibit the offering of lower performance class services, if the network was first dimensioned to assure 100% protection. The number of priority classes to offer simultaneously, their grading and their limit population should be parameter for network planning. page 30 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer Complex architectures mesh-ring, ring-mesh or ring-ring, are potential promising for the provision of diversified protection grades, as was already referred for the case of meshed with overlayed rings. In these architectures an additional parameter influencing the protection scheme will be the interconnection of the two levels of the architecture, and in particular, what traffic, how much, in what failure scenarios and how will be protected by the other level of the architecture. Mixed survivability techniques can also be possible for these networks, including protection and restoration; this will hopefully allow the optimisation of spare resources use at the cost, additional management complexity, more intelligence in the network and lower recovery speed. However, this last factor cannot be very penalising as for many users an interruption of about 200ms can be tolerable, and therefore this can result on a variant of offered survivability priority classes. 5.3 Protection against simultaneous failures Protection mechanisms are usually designed to protect against single failures. The network should be planned in such a way that the impact of simultaneous failures is minimised. However, protection against simultaneous failures may be necessary in certain parts of the network and should also be taken into account in network planning. This section studies the impact of protection against simultaneous failures on network planning. The focus is in the protection of interconnected optical domains described in Deliverable 1 of P709. 5.3.1 General characteristics of single domain optical protection 5.3.1.1 Optical ring protection Unidirectional link failure and bi-directional link failure are single point failures. Section or link failure and an optical component failure are examples of this kind of failures. As is well known single domain CS-Ring and OMS-SPRing architectures protect against any single point failure. Multiple failures are single point failures occurring at more than one physical location in a ring. These failures are either link failures, nodal failure or combination of them. A node failure can be considered as a bi-directional link failure occurring on both sides of the node. In some cases, depending on the physical location of the failures, the traffic can be protected against multiple failures in both CS-Ring and OMSSPRing. However, the ring architecture cannot protect against terminating node failures and usually part of the total traffic will be lost also under other multiple failure conditions. Multiple failures may also result in ring segmentation. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in optical ring architectures. 5.3.1.2 Optical sub-network connection protection Optical SNCP architecture protects against any single point failure. The traffic can also be fully protected in some multiple failure cases, depending on the physical location of the failures. However, the optical SNCP cannot protect against terminating node failures and also in many other multiple failure cases the traffic is lost. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in optical SNCP architecture. © 1999 EURESCOM Participants in Project P709 page 31 (48) Volume 3: Annex B – Protection in the optical layer 5.3.2 Deliverable 2 Protection against simultaneous failures in interconnected optical network domains This section examines if additional resources are needed for protection against multiple failures in interconnected optical network domains. Only the selected dual node interconnection architectures defined in Deliverable 1 of P709 will be studied. Each individual optical sub-network of the interconnected optical network domains protects against multiple failures according to the description given in the previous section. Therefore, only the protection of interconnected traffic against simultaneous failures will be discussed in this section. 5.3.2.1 Two-level CS-Ring architecture The interconnection of two nodes located in different CS-Rings can be provided with one wavelength by using optical drop and continue functionality and optical selectors in the interconnection nodes. This architecture provides protection against several multiple failure cases. However, the architecture cannot protect against terminating node failures, or failures affecting simultaneously on both interconnections between the rings. In the case where drop and continue functionality is not available two wavelengths are needed to provide the interconnection of two nodes located in different CS-Rings. This architecture can protect against any single point failure and also against some failures occurring simultaneously in the network. The architecture cannot protect against terminating node failures, or failures affecting simultaneously on both interconnections between any two rings, or failures affecting simultaneously on certain combinations of interconnections between the two lower level rings and the upper level ring. The survivability of the network can be improved by using four wavelengths to provide the interconnection of two nodes located in different CSRings. This architecture provides protection against several multiple failure cases. However, the architecture cannot protect against terminating node failures, or failures affecting simultaneously on both interconnections between the rings. The described dual node interconnected two-level CS-Ring architectures cannot provide protection against all multiple failure cases. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. 5.3.2.2 Two-level OMS-SPRing architecture The interconnection of two nodes located in different OMS-SPRings can be provided with two wavelengths by using optical drop and continue functionality and optical selectors in the interconnection nodes. This architecture protects against several multiple failure cases. However, the architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between the rings. In the case where drop and continue functionality is not available two wavelengths are needed to provide the interconnection of the two nodes located in different OMSSPRings. This architecture can protect against any single point failure and also against some failures occurring simultaneously in the network. The architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between any two rings, or failures affecting page 32 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer simultaneously on certain combinations of interconnections between the two lower level rings and the upper level ring. The survivability of the network can be improved by using four wavelengths to provide the interconnection of the two nodes located in different OMS-SPRings. This architecture protects against several multiple failure cases. However, the architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between the rings. The described dual node interconnected two-level OMS-SPRing architectures cannot provide protection against all multiple failure cases. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. 5.3.2.3 Two-level OMS-SPRing-Optical Mesh architecture The interconnection of two nodes located in different optical SNCP meshes can be provided with two wavelengths by using optical drop and continue functionality and optical selectors in the interconnection nodes. This architecture protects against several multiple failure cases. However, the architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between the ring and mesh, or certain combinations of failures occurring simultaneously in one interconnection node of the OMS-SPRing and in the links or nodes of the optical SNCP mesh. In the case where drop and continue functionality is not available two wavelengths are needed to provide the interconnection of the two nodes located in different optical SNCP meshes. This architecture can protect against any single point failure and also against some failures occurring simultaneously in the network. The architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between the ring and the mesh. In addition, the architecture cannot protect against failures affecting simultaneously on certain combinations of interconnections between the two lower level meshes and the upper level ring, or certain simultaneous failures occurring in the optical SNCP meshes. The survivability of the network can be improved by using four wavelengths to provide the interconnection of the two nodes located in different optical SNCP meshes. This architecture protects against several multiple failure cases. However, the architecture cannot provide protection against terminating node failures, or failures affecting simultaneously on both interconnections between the ring and mesh. In addition, the architecture cannot protect against certain combinations of failures occurring simultaneously in one interconnection node of the OMS-SPRing and in the links or nodes of the optical SNCP mesh. The described dual node interconnected two-level OMS-SPRing-Optical Mesh architectures cannot provide protection against all multiple failure cases. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. 5.3.2.4 Two-level Optical Mesh-OMS-SPRing architecture The interconnection of two nodes located in different OMS-SPRings can be provided with two wavelengths by using optical drop and continue functionality and optical selectors in the interconnection nodes. This architecture provides protection against several multiple failure cases. However, the architecture cannot protect against © 1999 EURESCOM Participants in Project P709 page 33 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 terminating node failures, or failures affecting simultaneously on both interconnections between the ring and mesh, or certain combinations of failures occurring simultaneously in one interconnection node of the OMS-SPRing and in the links or nodes of the optical SNCP mesh. In the case where drop and continue functionality is not available two wavelengths are needed to provide the interconnection of the two nodes located in different OMSSPRings. This architecture can provide protection against any single point failure and also against some failures occurring simultaneously in the network. The architecture cannot protect against terminating node failures, or failures affecting simultaneously on both interconnections between the ring and the mesh. In addition, the architecture cannot provide protection against failures affecting simultaneously on certain combinations of interconnections between the two lower level rings and the upper level mesh, or certain simultaneous failures occurring in the optical SNCP mesh. The survivability of the network can be improved by using four wavelengths to provide the interconnection of the two nodes located in different OMS-SPRings. This architecture provides protection against several multiple failure cases. However, the architecture cannot protect against terminating node failures, or failures affecting simultaneously on both interconnections between the ring and mesh. In addition, the architecture cannot provide protection against certain combinations of failures occurring simultaneously in one interconnection node of the OMS-SPRing and in the links or nodes of the optical SNCP mesh. The described dual node interconnected two-level Optical Mesh-OMS-SPRing architectures cannot provide protection against all multiple failure cases. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. 5.4 Existing infrastructures in optical network protection Existing infrastructures should be in some way utilised when choosing the protection mechanisms for the optical network. One possibility is to use existing point-to-point WDM systems for the interconnection of network domains. In some cases also SDH connections and SDH protection may be used. This section presents some possible solutions for using existing systems for protection in the optical network. 5.4.1 Existing point-to-point WDM systems Point-to-point WDM systems have been installed by several operators to fulfil growing capacity needs in the existing optical fibre network. These systems have usually been installed in optical cables where the traffic demand is very high and the number of available fibres is small. Because the investments in these systems are reasonably large it should be possible to utilise them when planning new optical network structures. These existing systems may be used in a meshed network, for construction of ring structures and for interconnection of network domains. 5.4.1.1 Meshed network In a meshed restoration or used for 1+1 nodes and an page 34 (48) optical network protection can be implemented by using optical dedicated protection methods. Existing point-to-point systems can be protection when fast protection switching is needed between certain alternative optical cable route is available between these nodes. The © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 limiting factor might be the capacity of the systems, because the WDM systems installed in the first stage usually carry only 4 or 8 bi-directional optical channels. In this case the existing point-to-point systems could be upgraded to the required capacity if it is technically possible and cost effective. Another possibility is to provide protection only for a certain part of the wavelengths according to the capacity available in the point-to-point system. 5.4.1.2 Ring structures Existing point-to-point WDM systems may be utilised in optical network architectures also when creating new ring structures. This is illustrated in Figure 12. Point-to-point WDM systems from node A to nodes B and C can be used when constructing a ring A-B-C. For this purpose the optical terminal multiplexers in nodes A, B and C have to be replaced by add-drop multiplexers. This means that from the original systems only the optical line amplifiers are used. If the capacity of the systems is also upgraded, the same line amplifiers cannot necessarily be used. When OMS-SPRing protection is going to be used an extra pair of fibres is needed between the nodes of the ring. B B b) a) A C C A OTM OADM OA Figure 24. Existing point-to-point WDM systems (a), used for constructing a ring structure (b) As a conclusion, it seems that constructing ring structures by using existing point-topoint systems is not a very cost effective solution. It can also include some technical problems depending on the technology used in the existing systems. 5.4.1.3 Interconnection of domains Existing point-to-point systems may also be used for the interconnection of domains in the architectures defined in Deliverable 1 of P709 (two-level OMS-SPRing, OMSSPRing - Optical Mesh, Optical Mesh - OMS-SPRing). This is useful only when the interconnecting link is so long that the normal interfaces of the OADMs or OXCs are not sufficient. Normally this is not the case because the domains to be interconnected are usually situated geographically close to each other. One reason for using point-topoint systems for the interconnection of domains could be the requirement for dual node interconnection. In this case even a longer route may be used for the secondary interconnection - at least as a temporary solution - if it is the only possible alternative. A limiting factor is then the maximum number of consecutive optical amplifier sections between regeneration points. © 1999 EURESCOM Participants in Project P709 page 35 (48) Volume 3: Annex B – Protection in the optical layer 5.4.2 Deliverable 2 Existing SDH systems If protection at the optical layer cannot be implemented it is possible to use existing SDH systems and equipment for protection in some parts of the network. This can be realised using different ring structures and dedicated protection or SDH restoration in a meshed network. SDH systems can also be used for the interconnection optical domains. One alternative is to use SDH restoration as a secondary protection when dedicated optical protection is used. This gives an opportunity to provide protection against two simultaneous failures in certain parts of the network. Optical network protection based merely on the use of SDH connections can only be an interim solution. If the target of planning is a fully optical network the protection should not be based on the use of the client layer. However, protection in the SDH layer can be used in addition to optical protection to improve the availability of the network. In this case the interworking of the two protection methods must be controlled in such a way that the possibility of conflicts is minimised. 5.5 Conclusions 5.5.1 Protection of individual channels The basic optical architectures implementing different protection methods may exhibit weak points, leading to vulnerability. On the other hand, there may be very important interconnections between sub-domains of a network, the availability of which has to be improved. Coming from these two reasons, the possibilities of protecting individual channels are investigated. In this discussion an individual channel is an electrical client signal. The target of this examination is to find out whether it makes sense to have individual channel protection in addition to the existing network protection or not. The results are shown in Table II, where various basic optical architectures and hierarchical networks are considered. In CS-Rings, the protection is performed entirely on the SDH MS layer. The weak point of a CS-Ring can be the SDH node where some signals are transited on the electrical path layer. However, this weak point can be eliminated by either: using 1+1 path protection on the HOP or LOP layer, or using the extra flexibility features of CS-Rings, which makes it possible to rearrange the logical order of nodes in a way that transiting through the electrical layer is no longer necessary. Anyway, individual channel protection is always possible in the client layer (SDH path protection or MSP) regardless of the optical architecture used. But it does not make sense in every case. When the network has weak points it can, obviously, be considered. But when the architecture is robust enough it does not seem to be necessary. page 36 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 Weak point eliminate weak point protect against double failure (note 1) individual prot. possibility Worth individual channel protection? CSR yes 1+1 SDH path prot. and/or changing the logical node order no 1+1 SDH path prot. proposed OMSPR no n.a. no 1+1 SDH path prot. or MSP not proposed CSR-CSR normal routing yes 1+1 SDH path prot. and/or changing the logical node order only on the same route 1+1 SDH path prot. proposed OMSPR-OMSPR normal routing no n.a. only on the same route 1+1 SDH path prot. or MSP not proposed note 1: Naturally, no ring can fully survive two span or node failures, independently of the ring type. However, in hierachical networks, when the failures happen on different layers, imunity agains such double failure can be achieved. Table II - Conclusions on protection of individual channels 5.5.2 Protection priority classes Network survivability performance is of utmost importance as aggregate traffic rates increase both for the network operator and for the user. Such an increase of traffic will occupy network resources initially planned to assure network protection. The definition of protection priority classes is possible, and suits both the operator and the user. The first one will be able to release some previously allocate resources for protection, will not degrade its reliability image for the user, and will get a revenue which is proportional to the degree of offered service. The users will be able to select the type of protection suited to its own needs and only be charged accordingly to that. A migration to a higher level of protection its always possible if the its protection requirements change. A possible definition based on percentage of user traffic protected and recovery delay is presented in section 5.2.1, and a proposition of simple implementation with existing protection schemes done. More elaborated schemes based on n:m protection, two level complex architectures and mixed protection restoration techniques seem very promising for offering diversified grades of survivability, however their planing maybe very complex and there are no available dedicated tools for this purpose. 5.5.3 Protection against simultaneous failures With the use of optical drop and continue functionality additional wavelengths are not needed for the interconnection of optical network domains. By this way protection against several, but not all, multiple failure cases can be achieved. Additional wavelengths may still be needed to further improve the survivability of the network. Nevertheless, the network cannot be protected against all simultaneous failure cases. In the case where drop and continue functionality is not available additional wavelengths are not necessarily needed for the interconnection of optical network domains. By this way protection against all single point failures and only some multiple failures can be achieved. The survivability of the network against multiple failures can be improved, to the level that the use of drop and continue functionality provides without additional wavelengths, by doubling the number of wavelengths used to interconnect the two nodes located in different optical network domains. This can provide protection against several, but not all, multiple failure cases. Additional wavelengths may still be needed to further improve the survivability of the network. Nevertheless, the network cannot be protected against all simultaneous failure cases. © 1999 EURESCOM Participants in Project P709 page 37 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 Full protection against all multiple failure cases is hard to achieve with reasonable use of resources. Therefore, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. 5.5.4 Existing infrastructures in optical network protection The best way to utilize existing point-to-point WDM systems is to implement 1+1 optical protection between certain nodes when fast protection switching is needed. It can also be used in addition to other optical protection methods in some parts of the network. The small capacity of the existing point-to-point systems may limit the level of protection. The technology used in the existing WDM systems may crucially limit their usage in ring structures. If optical terminal multiplexers cannot be easily upgraded to OADMs the construction of ring structures is not cost-effective. Interconnection of network domains using point-to-point systems is useful only when the distances between the domains are very long. Existing SDH systems and equipment can be used for protection in the optical network as an interim solution when fully optical protection methods are not available. It is also possible to use SDH layer for secondary protection in order to be able to protect certain connections against two simultaneous failures. page 38 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 6 Identification of constraints on network upgrading This chapter investigates the upgrading possibilities of Coloured Section Rings and hierarchical network consisting of CSRs. First the upgrading possibilities of CSRs are discussed. Based on these results, a more complex hierarchical network is examined. 6.1 Upgrading Coloured Section Rings If the capacity of the CSR is fully utilised, and new links are needed between certain ring nodes, we can insert new nodes within the existing stations. This possibility is investigated in chapter 0. In principle, the structure of the ring can be modified either by inserting totally new nodes at different stations, or turning the ring into a mesh, or changing the CSR into a real optical ring (OMS-SP Ring). These possibilities are considered in chapter 0. Another upgrading issue can be the modification of the existing protection mechanism discussed in chapter 0. The upgrading possibilities mentioned above are summarised in Table III. In this document the results of EURESCOM Project P615 are implicitly used concerning the CSR features. Wherever these results are used a clear reference is given to the relevant P615 document. Capacity extension inserting new node within station Structure upgrade inserting new nodes Change in protection 1+1 to 1:1 (secondary traffic) change to mesh change to OMS-SP-Ring Table III - CSR upgrading possibilities 6.1.1 Capacity extension The capacity of an existing CSR can be increased by inserting a new node within the station. The solution is depicted in Figure 25 and Figure 26. In the presented solution, only one new connection is shown (two additional nodes), therefore instead of ADM, terminal multiplexers (TM) are used. However, the TMs can be changed to ADMs later on, by inserting another wavelength. If optical amplifiers are used, the maximum number of nodes is limited to 9 (see P615 Deliverable 1, Volume 2, Chapter 3.4.2: physical limitations of CSR). Therefore, by the upgrade we cannot exceed this number. There is no interruption in the traffic during the insertion of new OADMs thanks to the self-healing feature of CSR. Since the maximum number of nodes in CSRs is 9 (see P615 Deliverable 1, Volume 2, Chapter 3.4.2), the ring upgrade must be made accordingly. In optically amplified CSRs, the maximum number of wavelengths in CSR is not a limitation factor, therefore it is not necessary to be considered in the upgrade. Two additional OADMs per wavelengths are needed. The power budget has to have enough safety margin to work properly with these additional insertion losses. © 1999 EURESCOM Participants in Project P709 page 39 (48) Volume 3: Annex B – Protection in the optical layer Station A TM Deliverable 2 Station B ADM ADM CS-Ring new TM ADM ADM new equipment Station D Station C Figure 25. Inserting new nodes into CSR as capacity extension W W W TM ADM P OADM OADM OA OADM OADM new equipment P P OA OADM OADM new Figure 26. Inserting a node into CSR 6.1.2 Change in structure 6.1.2.1 Inserting new nodes An example can be seen in Figure 27. In the figure, 3 new nodes are put into an existing CSR. The number of necessary additional wavelength is equal to the number of new nodes. There is no interruption in the traffic, provided the nodes are inserted one after the other. As the maximum number of nodes in optically amplified system is 9 (see P615 Deliverable 1, Volume 2, Chapter 3.4.2), we should consider that in the upgrade. page 40 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 If any of the new nodes has to have an optical connection to any of the existing nodes, it requires to change the ADM to a DXC at the existing node as well. This should be avoided as it means traffic interruption. Station E Station A Station B ADM ADM ADM Station F A D M CS-Ring ADM ADM Station G E, F, G Station D new stations ADM Station C new s new equipment Figure 27. Three new nodes of an existing CSR 6.1.2.2 Changing CSR to mesh or OMS-SP Ring Changing CSR to mesh or OMS-SP Ring is not possible since the optical elements of the CSR are entirely different, and their working principles are absolutely different. 6.1.3 Changing the protection The CSR utilises 1+1 protection handled by the add-drop multiplexers on the SDH client layer. In this case, the entirely protection capacity is reserved, and unused in normal operation. Changing the 1+1 protection to 1:1 scheme can make sense, as we can use the reserved capacity for low priority traffic. This traffic having secondary importance will immediately be lost when a network failure occurs, and the working traffic is switched to the protection routes. (See Figure 28.) With this solution, additional traffic can be routed by the same ring, and therefore in terms of capacity extension, it is relating to the capacity extension of the ring discussed in chapter 6.1.1. © 1999 EURESCOM Participants in Project P709 page 41 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 Changing 1+1 to 1:1 protection requires that the ADM equipment shall be able to handle the 1:1 MSP scheme that is quite common in SDH. This modification in the ADMs can be made without influencing the existing traffic. normal secondary normal traffic traffic traffic W W ADM P P OADM OA OADM OA OADM OADM Figure 28. Using 1:1 MSP in the ADM of a CSR node 6.2 Upgrading CSR-CSR Hierarchical Networks In this section only upgrading is considered relating to transit demands, i.e. demands crossing the upper level CSR are taken into account. The other upgradings are identical to the separate CSR upgrading issues discussed in section 0. If the capacity of the network is fully utilised, and new links are needed between nodes located on different rings, we can either insert new wavelengths or insert new nodes within the existing stations, as discussed in section 0. These two possibilities applied especially to the CSR-CSR network are investigated in the following chapters. The structure can be modified by inserting totally new nodes at different stations. Such modifications such as changing the CSR-CSR hierarchical network to OMSSPR - OMS-SPR or other structure is not considered, as it is not possible (see section 0). The modification of the existing protection mechanism discussed in section 0 is also relevant to CSR-CSR networks. These upgrading possibilities are summarised in Table IV. Capacity extension inserting new node within station Structure upgrade inserting new nodes Change in protection 1+1 to 1:1 (secondary traffic) Table IV - CSR-CSR upgrading possibilities 6.2.1 Inserting new nodes within a station The capacity of an existing CSR-CSR network can be increased also by inserting new nodes within the stations. The principle of the solution is shown in Figure 29. The changes within the stations are identical with of Figure 26. The wavelength allocation can be made separately for each ring. page 42 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer The limitations in number of wavelengths and number of nodes are the same as in separate CSRs, discussed in chapter 6.1.1.2, as the rings are optically separated. There is no interruption in the traffic during the insertion of new OADMs thanks to the self-healing feature of CSR. Two additional OADMs per wavelengths are needed. The power budget has to have enough safety margin to work properly with these additional insertion losses. If inserting new nodes step by step, there is no traffic interruption. Because of inserting two nodes on the lower level as the originating and end point of the traffic, four new nodes insertion is required on the higher ring. © 1999 EURESCOM Participants in Project P709 page 43 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 Station A ADM ADM Lower level CS-Ring new s DXC DXC new equipment I1 I2 Upper level CS-Ring I3 I4 DXC DXC Lower level CS-Ring ADM ADM Station B Figure 29. Inserting new nodes within stations into CSR-CSR network 6.2.2 Inserting new nodes Inserting entirely new nodes is the same as within the station, therefore the same are valid as described in chapter 6.2.2. 6.2.3 Inserting new rings on the lower level A new lower level ring can be integrated into the network, using the existing upper level ring for transiting. On the upper level, new hub nodes can be inserted or just simply new nodes at the existing hub stations. page 44 (48) © 1999 EURESCOM Participants in Project P709 Volume 3: Annex B – Protection in the optical layer Deliverable 2 These all can be made on the same principles described in previous chapters. 6.2.4 Changing protection Each CSR of the network utilises 1+1 protection handled by the add-drop multiplexers or DXCs. In this case, the entirely protection capacity is reserved, and unused in normal operation. Changing the 1+1 protection to 1:1 scheme can make sense, as we can use the reserved capacity for low priority traffic. This traffic having secondary importance will immediately be lost when a network failure occurs, and the working traffic is switched to the protection routes. (See Figure 28.) With this solution, additional traffic can be routed via the network. Changing 1+1 to 1:1 protection requires that the ADM and DXC equipment shall be able to handle the 1:1 MSP scheme. This modification can be made without influencing the existing traffic. 6.3 Conclusions This section addresses the upgrading possibilities of Coloured Section Rings and hierarchical networks consisting of CS-Rings. In the CS-Ring hierarchical network the rings are optically separated. Therefore, the same rules for network upgrading are valid in both cases: Inserting new nodes into a CS-Ring hierarchical network, either in a new station or within a station, can be made without any problem, as far as some rules (such as the maximum number of nodes or power budget) are kept in mind. It makes sense to use 1:1 protection in the SDH ADMs and DXCs, to transport additional secondary traffic, if the required additional tributary ports are available in the SDH equipment. The upgrading issues of CS-Ring - CS-Ring networks are summarised in the table below. degree of "intervention" traffic interruption limitation benefit Inserting new node low no max. 9 nodes in total (note 1) new capacity Changing protection: 1+1 to 1:1 low no new capacity for secondary traffic note 1: according to Deliverable 1 of P615, the maximum number of nodes in a CS-Ring using OAs is 9, due to power budget limitations Table V - Upgrading CSR-CSR network © 1999 EURESCOM Participants in Project P709 page 45 (48) Volume 3: Annex B – Protection in the optical layer 7 Deliverable 2 Conclusions This document has studied protection techniques for optical networks. Focus has been on two-level network architectures. The architectures that have been studied use dual node interconnection because in large capacity core networks disjoint alternative routing is necessary. It has been found that the introduction of optical drop and continue functionality and optical switching functionality in the interconnection nodes is an effective means of improving the survivability of interconnected optical network domains. In the case of interconnected OMS-SPRings this functionality provides protection against the failure of one or both interconnecting nodes (each on different rings, but on the same interconnection), or the connection between the two interconnecting nodes. The architecture can also protect against a single failure in each of the rings provided that these failures are not terminating node failures, or these failures do not combine to affect both interconnections between the rings. The problem of wavelength conflicts, or misconnections, under protected state in OMS-SPRings has been addressed. In this case, wavelengths are analogous to the time slots in SDH. As such, misconnections will occur at the optical channel level under conditions similar to the SDH case: node or multiple span failures without lowpriority traffic in the protection wavelengths, and single span failures with lowpriority traffic in the protection wavelengths. The proposed solution for the misconnections is based on wavelength squelching, using the overhead channel associated to the optical channels to support an Optical Channel AIS signal. This solution would maintain the independence of the optical layers from the client layers, as the client signals would not be directly manipulated. An issue left for further study is the proper way of linking the AIS alarm at the Optical Channel Layer to the management system of the client signal (typically SDH), in order that the client would be able to reject the misconnected signals. The CS-Ring architecture presents a weak point when traffic electrically transits through a node, as this traffic is unprotected against failures in that node. This weakness can be avoided either by re-ordering the nodes logically, in order to avoid transits through the electrical layer, or by the implementing 1+1 path protection for this traffic. Network survivability is of utmost importance as aggregate traffic rates increase. A possible protection priority class definition based on percentage of user traffic protected and recovery delay is presented in this document. Also a proposition of simple implementation with existing protection schemes is done. More elaborated schemes based on n:m protection, two level complex architectures and mixed protection restoration techniques seem very promising for offering diversified grades of survivability, however their planing maybe very complex and there are no available dedicated tools for this purpose. Full protection against all multiple failures is hard to achieve in two level optical network architectures without wasting wavelengths. Most efficient protection against multiple failures can be achieved by using optical drop and continue functionality in the interconnecting nodes. However, protection at client layer is sometimes required if there is a need to fully protect the client layer traffic against multiple failures in the optical network. page 46 (48) © 1999 EURESCOM Participants in Project P709 Deliverable 2 Volume 3: Annex B – Protection in the optical layer The best way to utilise existing point-to-point WDM systems is to implement 1+1 optical protection between certain nodes when fast protection switching is needed. The small capacity of the existing point-to-point systems may limit the level of protection. The technology used in the existing WDM systems may crucially limit their usage in ring structures. If optical terminal multiplexers cannot be easily upgraded to OADMs the construction of ring structures is not cost-effective. Interconnection of network domains using point-to-point systems is useful only when the distances between the domains are very long. © 1999 EURESCOM Participants in Project P709 page 47 (48) Volume 3: Annex B – Protection in the optical layer Deliverable 2 References [1] ITU- T Recommendation G.842 – Interworking of SDH network protection architectures (04/97) [2] ETSI TS 101 010 V1.1.1 (1997-11) Transmission and Multiplexing (TM); Synchronous Digital Hierarchy (SDH); Network protection schemes; Interworking: rings and other schemes [3] EURESCOM P615 PIR 2.2, document BT22-03a.doc – Choice of network study cases and specifications of comparison scenarios [4] EURESCOM P615 PIR 2.2, document BT22-05a.doc – Traffic uniformity, planning complexity and traffic interruption aspects of the OMS-SPRING architecture with wavelength reuse [5] EURESCOM P615 PIR 2.3, document TE23-04c.doc – Dimensioning results of ring architectures [6] ITU-T Recommendation G.841 – Types and Characteristics of SDH Network Protection Architectures (07/95) [7] EURESCOM P615 Deliverable 1 – Understanding Optical Network Architectures, Vol. 2 [8] ETSI TS 101 009 V1.1.1 (1997-11) Transmission and Multiplexing (TM); Synchronous Digital Hierarchy (SDH); Network protection schemes; Types and characteristics page 48 (48) © 1999 EURESCOM Participants in Project P709