effects of failures or taking capacity temporarily out of service for maintenance. A single incident like a fiber cut could result in the failure of multiple network elements. Prior to the use of SDN, IP service planners would decide between which pairs of routers link capacity would be needed, which would then be used as an input to the capacity planning of the layer 1 network. Turning up the new capacity could often take months. With SDN, links between routers can be turned up dynamically in response to network failures or changing traffic condi- tions. As a result, interrouter links can be run at significantly higher utilizations but spare capac- ity must be planned in the layer 1 network to accommodate interrouter links being turned up on demand. 16.3.2 HIGHLIGHTS OF BENEFITS FROM SDN Some of the key benefits of SDN in the capacity planning process are described below: Pre-SDN Need to build end-to-end IP links that are statically routed over optical links. Long lead-time (several months) for ordering IP/optical equipment. Decisions about where capacity will be augmented are made several months before the capacity will actually be utilized. Forecast accuracy is very important. If the forecast is off, then capacity may be in the wrong place even after several months. This is particularly true since the entire end- to-end IP link is static. Post-SDN Only need to build endpoints (tails consisting of router port plus colorless/direction- less transponder) and spare regenerators at strategic places and make the connec- tion dynamically between any two available tails (including regenerators if needed) depending on the traffic and network failures. Faster provisioning with SDN cuts down equipment ordering lead-time from several months to a few weeks. Decisions about where capacity will be augmented are made only several weeks before the capacity will actually be utilized. Building the Network of the Future Uncertainty about forecast accuracy is less important since capacity planning operates on a time scale of a few