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@fibresystemsmag | www.fibre-systems.com


FEATURE SOFTWARE-DEFINED NETWORKING


network are working in harmony with maximum efficiency. Te benefits from an SDN-controlled multilayer approach fall into three categories: optimisation, restoration, and provisioning.


Multilayer optimisation Multilayer optimisation continuously reorganises Layer 0-1-2-3 network elements to handle both existing and incremental new service requirements in the most efficient manner, delaying the need to add new resources for new connectivity requests. Let’s look at a simple example. Assume that router A connects to router B which connects to router C, and that traffic has been rising on these links, straining their capabilities. Te traditional approach would be to increase the link interface speeds, and in addition perhaps add processing power and buffer memory to intermediary router B. All expensive propositions. With a holistic multilayer approach, we


quickly see that in fact a significant amount of the traffic is in fact packets being routed from A to C, which are just transiting B. Further, we discover that by reshuffling resources we can use the optical network to engineer a new connection directly from A to C for this traffic, bypassing B. Tis uses relatively less-expensive optical resources instead of router resources. By multiplying this simple example a thousand-fold to reflect the much more


Multilayer provisioning enables new services and revenue streams, often based on variations of bandwidth on demand


extensive and complex service demands of the real world, it is easy to see how network-wide resources can be optimised. Another role of multilayer optimisation is


policy alignment. All networks are governed by various high-level policies such as fibre fill load balancing, shortest routes, maximum number of node hops, transit delays, and shared risk groups that ensure that primary and backup paths do not share any common fibre throughout their lengths. In the current situation, it is difficult to track when these


Figure 2: The benefits of multilayer restoration


driſt out of spec, and even once this is discovered fixes tend to be isolated patches. By using an SDN-based multilayer approach that is continuously scanning the overall network situation, it is much easier to catch problems early on and implement improvements to keep all network layers humming together as a fine-tuned machine.


Multilayer restoration An outstanding attribute that end-users expect from telecommunications services and the networks that support them is very high availability. At the highest level, we refer to five nines or 99.999 per cent availability, which is a downtime of only five minutes per year. Achieving high availability, however, comes at a very high cost. In packet networks the usual standard is


that every core network router must be connected to at least three other routers, so as to be able to withstand at least two separate link failures. In other words, if a link fails, the router soſtware will initiate routing of that port’s traffic in another direction. Similarly in optical transport networks most links use some form of 1:N protection, where N is oſten 1, so that resources are sitting completely idle simply waiting to be activated in the event of a failure. Clearly this adds a huge expense compared to a world where no failures occur or where long service outages were tolerated. Tis is further exacerbated by the current


approach where protection and restoration schemes are localised to the packet and transport networks. Tis provides an opportunity for multilayer restoration


approaches to dramatically reduce capital costs. In fact, various quantitative studies of multilayer optimisation and restoration indicate typical capex savings of more than 30 per cent from the status quo, with the bulk oſten coming from the restoration piece. Multilayer restoration implements savings


through two basic approaches. Te first is outright elimination of excess ports on routers, by reducing for example the minimum number of ports on a core router from three to two ports. By homing these router ports onto an OTN switch or a reconfigurable optical add-drop multiplexer, in the event of a link failure, the SDN controller can immediately direct packet traffic through an alternate optical network path instead of relying on a pure router layer solution. Te second approach relies on migrating


Layer 0 wavelength-switched optical network (WSON) and Layer 1 automatic switched optical network (ASON) restoration architectures from their current independent distributed control to integrated centralised SDN control. Tis provides a big picture view that is much better equipped to recognise and implement restoration schemes, both within the transport network, and working with the packet and other services layers.


Multilayer provisioning Te cloud is changing users’ expectations from services. Tey want a portal interface through which they can order services and have them turned up, turned down, or modified, in real time. SDN-based multilayer provisioning is a key for enabling this experience in the telecom


Issue 15 • Spring 2017 FIBRE SYSTEMS 27


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