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Cisco
multi-layer networking These characteristics of the optical layer are a necessary but insufficient to achieving a true low cost and ultra-efficient solution. What good is a high degree of network agility, if at the end it relies on slow, complex, and manually- intensive processes to implement a change in the network? The only way to achieve an agile network is to involve the layer that drives the need for optical agility – namely the service layer (which is typically an IP network). How multi-layer collaboration saves cost Network Manager Distributed Control
Central control (PCE/SDN)
The IP layermust closely interact with the optical network to optimize how optical resources and IP resources are used. Using such interaction, the network can quickly move optical capacity to where it is needed by the IP layer, insteadof today’s approach, of over-provisioning static IP links to address different possible changes in traffic patterns over a static optical layer. Consider the example in Figure 1 to best understand the difference between today’s approach to network planning and the desired future approach. The figure depicts on the left two traffic scenarios. Each scenario implies different capacity needs from one router to three other routers (in Gbps units). These scenarios could result from different failures in the network, from unexpected traffic growth, to changes in peering arrangements. Eitherway,thenetworkdesignmust accommodate both scenarios without having to redeploy gear. Figure 1(a) show how this is done using today’s static IP network over a static optical network. The planner must provision each link to an adjacent router to account for the maximum capacity needed for bothscenarios. Inthisexample,this means 2x100G wavelengths to the first and second router and 3x100G wavelengths to the third router. A design with an agile network in mind is shown in Figure 1(b). Here the planner must consider the total number of wavelengths needed to accommodate both scenarios – 5x100G wavelengths
Core router
Optical layer
Figure 3 - Agile multi-layer architecture
in this case, but how they are distributed amongst the neighbours is not important, since they can be redistributed quickly by the network. In summary, the planning process is changing from provisioning each link to the maximum needed capacity to provisioning each node to the maximum total capacity. The application that best captures the value of this new approach is multi-layer restoration. In this case, the IP layer relies on the optical layer to restore failed links in the event of an optical layer failure, using the same router interfaces and transponders – as shown in Figure 2. This reuse of interfaces is the key reason for the significant savings achieved by the scheme – in the order of 40% of router interfaces and transponders on several real European core network models – not to mention the associated rack- space and power. Control architecture of a multi-layer network Restoration against optical failures is just the start. Many other applications have been
identified: from optical layer optimization, to optical bypass of routers, to disaster recovery. Some of these applications (including restoration) require fast reaction and a high degree of availability, pointing to the need for a distributed control plane between layers. Others require a high degree of sophistication in understanding how a proposed change in the network will impact the routing of traffic flows in the IP layer and the resulting impact of its service- level agreement (SLA) – pointing to the need for centralized control with a high degree of user interaction. We believe that the best architecture is a hybrid one, with both distributed and centralized control elements. In addition, a new type of network management is needed to provide the operator with sufficient information and control over such an automated network – as shown in Figure 3. Ori Gerstel Principal Engineer Converged Routing and Optical Group, Cisco
The Internet of Everything - which will turn billions of devices into active users of the internet - will have an unknown, yet dramatic impact on the network.
| Optical Connections 2013 | www.opticalconnectionsnews.com | 19
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