ARE CONTROL PLANE BENEFITS APPLICABLE TO SUBMARINE NETWORKS?

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1 ARE CONTROL PLANE BENEFITS APPLICABLE TO SUBMARINE NETWORKS? Damien Martinet, Julien Meuric, Yann Loussouarn (Orange Labs) Orange Labs, 2 avenue Pierre Marzin, Lannion, France Abstract: Over the past few years, major operators have started to operate their Time Division Multiplexing (TDM) core networks by using a control plane in order to (1) simplify their management and (2) save protection resources. In the short term, network topology auto discovery, end to end provisioning simplicity and restoration mechanism benefits could similarly facilitate the integration and operations within a submarine cable system operated by a consortium through a single control layer. 1. INTRODUCTION Traditionally, transmission network management is performed through a dedicated software suite. Figure 1: Management System Suite The Network Management System (NMS) provides end to end services provisioning and network supervision. The NMS is connected to (or it includes) the Equipment Management System (EMS), which allows all operations linked to one Network Element (NE) such as interfaces set up, cross-connection management Finally, the Local Craft Terminal (LCT) is used on field in order to connect directly when in front of the NE. The Data Communication Network (DCN) provides the link for the management information exchange between the NEs and the EMSs. The management of submarine systems and supported services is assigned to the Network Administrator (NA). The Network Operating Center (NOC) is in charge of system operations and maintenance. The different NEs involved in submarine cable networks are the Submarine Interconnection Equipment (SIE) for SONET/SDH operator backhauling interconnection, the Submarine /Terrestrial Line Terminating Equipment (SLTE/TLTE) dedicated to the challenged line side. The DCN is usually supported by dedicated SONET/SDH or OTN overhead bytes and protected by a different route to keep equipment management in case of cable failure (e.g. another submarine cable system). 2. CONTROL PLANE FUNCTION AND PRINCIPLE A control plane (CP) is used as a complementary tool to the network management system. It consists on moving an active management part into the NEs. Historically, CP comes from the packet layer operations. A CP provides autonomous mechanisms between NEs via protocols exchanges over the Signalling Communications Network (SCN). Copyright 2010 SubOptic Page 1 of 5

2 Figure 2: Logical Representation of CP Communication Each NE can learn network topology and resource, compute routes according to received requests and create, remove, modify network connections. By moving some part of the active components into NEs (i.e. closer to traffic), it provides also lower management constraints at NMS level and more operational flexibility. MULTI-VENDOR INTERWORKING Operators networks are almost based on different vendors solutions. NE internetworking is a clue in order to simplify their operations. Once all NEs work with a standardized CP (as GMPLS), it is easy to provision routes on the same level (SONET/SDH) over such network instead of provisioning everything manually from each dedicated suppliers NMSs. MULTI-LAYER INTERWORKING Several vendors do implement a control plane not only on SONET/SDH Optical Cross-Connects (OXCs) but also on Dense Wavelength Division Multiplexing (DWDM) nodes. In the short term, this could facilitate the integration and operations within an operator network. GMPLS Generalized MultiProtocol Label Switching (GMPLS) is a standardized control protocol suite defined by a collection of IETF RFCs. Service Protocol Topology OSPF or IS-IS Link Management LMP Signaling RSVP-TE Table 1: GMPLS Protocols GMPLS enables: (1) Network topology auto-discovery: link management and topology distribution; (2) Connection provisioning (with single or multiple routes): ingress node routing (per administrative domain) and end to end signaling (possibly across multiple layers and/or domains); (3) Failure recovery: fault localization, connection protection or rerouting and multiple failures resilience. The standardized GMPLS protocol suite allows to tackle internetworking issues where proprietary CP protocols do not. Figure 3: Multi -Layer Interworking In order to create a new service, the border OXC can request some resources (figure 3) to a lower layer (WDM) in order to reach another OXC. Then connections (wavelengths and STS-3s/VC-4s) can be established one after the other across both layers. TOWARD OPTICAL CONTROL PLANE Already deployed CPs in the transmission world are mostly related to electrical layers such as SONET/SDH or OTN cross connection. The needs for automatic optical mesh networks drive the introduction of a new optical layer and this layer have to be controlled. Standardized photonic CP mechanisms are not fully mature yet, mostly due to optical line impairments constraints. Those latest Copyright 2010 SubOptic Page 2 of 5

3 evolutions show that a CP is a key to provide the operators with more simplicity and automation in networks management. Then examining CP possible benefits for submarine cable network makes sense. 3. SUBMARINE NETWORK APPLICATION Over the past few years, CPs have been deployed over core meshed terrestrial networks. A CP is even native in some SIE implementations. Benefits to simplify the network management are clearly demonstrated. All the same CP uses seem to be feared in transmission networks: it gives the false feeling that the operator loses control on its network management in terms of security, provisioning, routing, layer interactions... SUBMARINE NETWORK BACKHAUL Some operators already use a CP in order to manage their sparse submarine network backhauling interconnections. Many OXCs are interconnected around the world by using many submarine cable systems on multiple routes. The operators take the opportunity to reroute traffic over the world by using submarine network systems as trunks. Figure 4: Operator Worldwide Interconnection It introduces more flexibility, automation and resilience in case of full cable failure where restorable subsea capacity is not strong enough (single point of failure). Anyway, operators can plan that kind of scheme, independently of the strategy use for the submarine cable network system itself. Nevertheless that kind of scheme may introduce latency and round trip delay issues. And of course, route diversity over multiple cables has a huge cost for operators. SUBMARINE NETWORK TOPOLOGY AUTO-DISCOVERY In some cases, the submarine cable networks are very long and the topology could be quite complex. Different suppliers could also be involved in different segments over the same cable system. Figure 5: Network Auto-Discovery A CP allows each NE to discover the network topology (link capacity and involved NE) once it is connected by one or more interfaces to its neighbors. Then, the topology information can be used by the management system. Even if most cable systems do not evolve in terms of numbers of NEs and SLTE Optical Channels (OCh) between two major upgrades, network auto-discovery provides a real time network topology and link availability in case of interfaces, boards, and NEs or links failure. Regarding the SONET/SDH layer, each SIE know the network in term of resources availability and routing constraints. Copyright 2010 SubOptic Page 3 of 5

4 END TO END SERVICE PROVISIONING ON SIE An interesting issue tackled by a control plane is the end to end service provisioning. Traditionally, it is performed manually from the NMS and requires that the operator perform independent routings and provisioning for each layer/domain on customers demands. With a CP, as NEs have knowledge of the network topology, it is easy to compute a route for the services, depending on resource availability, link metrics (omnibus spans for example may cost more than express ones) and by protocol exchange to establish cross connections along the route. Service routing is computed by the ingress node. It processes requests and initiate path setup, then intermediate nodes process the signaling messages along the path till the egress (cross-connection creation, VOA adjustment ). Figure 6: End to End Service Provisioning Depending on resource availability and link metrics, SIE#1 receive the request (figure 5) and computes the relevant route in order to carry a new SDH VC-4-4C service. For very simple architectures such as point-to-point submarine cables, the benefits of such mechanism are not obvious. For more complex submarine networks (lot of landing stations, lot of segments, multi vendor environment, route diversity on different fiber pairs ), the activation of a control plane on SIEs is a strong improvement in order to simplify their management. Network management automation is efficient when operator get few different NMS s, with a lot of network activity in terms of service provisioning/tear down requests. Regarding the end to end service provisioning on SLTEs, it is not a usual case to add extra wavelengths in order to raise the capacity between two upgrades. RESTORATION RELEVANCY IN LOW MESHED NETWORK A main reason that accelerates CP introduction in terrestrial core networks is saving resource allocated to recovery, which is possible thanks to the restoration option added by a CP. The more meshed a restoration-enabled network is, the more robust the network is (larger routing alternatives): this commonly heard statement is true but restrictive. Indeed submarine cable systems are not much meshed, enabling CP can prevents multiple failures better than legacy protection mechanisms. Keep in mind that Earth is a single point of failure [1], whatever the recovery mechanism; they can not prevent a cable failure. Protections at SIE level and SLTE level are designed before cable setup. Legacy protection on submarine cable networks are mostly MSP 1+1 on client side and MSP 1:N on line SONET/SDH side. Then, sometimes SNCP is used on SIEs and optical SNCP provisioned over SLTEs. Those schemes protect SIE board or interface failure. Transoceanic MS-SPRing allows a better resiliency but this protection scheme is more costly and seems to wear off. Legacy protections provide services, link and Optical Channel protection when restoration mechanism provides a full network protection. The restoration mechanism based on GMPLS mechanism allows to share efficiently recovery resource while legacy SONET/SDH and optical protection Copyright 2010 SubOptic Page 4 of 5

5 schemes implement 1:N or 1+1 with fixed protection resources (group of protection). What is more operational needs sometimes require extra capacity while the submarine cable system is completely full: in that case, legacy protection may be partly disabled to reallocate some of the former protection capacity to new traffic; this ends up with some unprotected traffic whereas enabling restoration would keep a minimum level of recovery. MSP 1:N Restoration Routing Operator NE or Operator Bandwidth Not Optimized Optimized Granularity STM-64 VC-4 Sharing Mx1:n Static (M:N) n Dynamic Multiple failure No Yes Service recovery 50 ms seconds Table 2: Legacy and CP Recovery Comparison Restoration also permits a finer granularity (STS-3/VC-4) instead of entire STM-N or Optical Channel. Obviously legacy SONET/SDH protection schemes can still be used even when a control plane is activated. As a drawback of dynamic activation, restoration recovery time is longer (seconds) than legacy SONET/SDH ones (50 ms). Information exchange is performed through the SCN, which is very sensitive. Figure 7: Legacy Protection Legacy MSP 1:N protection (figure 6) between SIE#1 and SIE#3 does not protect any express fiber fault, whereas restoration allows to reroute services via SIE#2. If the submarine cable network is own by a single operator, the submarine resources may be shown as a link such as terrestrial route. Protection/restoration schemes could be left to client responsibility over different submarine cable systems. Anyway if restoration does not recover a cable failure as fast as route diversity over multiple submarine cable systems would, it can be a good intermediate solution to improve network resiliency at lower cost. No extra hardware and equipment, no extra civil work or fiber pairs are needed, most of the time only software implementation or activation. 4. CONCLUSION Regarding very simple topologies, control plane use does not provide many additional features. However, when submarine cable networks become more complex in terms of landing station number (and associated SIEs), segment number (mono or multi vendors), available routes (express, omnibus, local ring), a control plane could in short term facilitate integration and operations within a submarine system operated by a consortium through a single control layer. In longer term, it could ease integration between such system with other piece of equipment, such as SLTEs, and operator backhauling networks. As there is no limitation due to CP activation and as it is possible to operate legacy domains without any change regarding the current way to manage the network, it could be interesting to consider a CP in order to simplify management and even improve resiliency of submarine cable networks. 5. REFERENCES [1] D. Cooperson, "undersea network: Traffic growth and resiliency in the spotlight", Ovum White Paper, May [2] E. Mannie et al., RFC 3945: GMPLS Architecture [3] J.P. Lang et al., RFC 4272: RSVP-TE Extensions in Support of End-to-End GMPLS Recovery Copyright 2010 SubOptic Page 5 of 5