ad of keeping all the IP links connected all the time (including spares for failure restoration), it is only necessary to keep the endpoint tails (a combination of a router port and a transponder) and some spare optical regenerators that are ready for rapid connection in the event of a failure or traffic surge. At any given instant only those tails are connected that are needed to support the traffic flow at that instant. As the traffic condition and network condition changes, the tails can be dynamically reconfigured (including optical regenerators, if needed) to match the new needs. This approach heavily relies on real-time traffic measurements that were mentioned before. First, end-to-end (edge-to-edge or core-to-core) and individual link measurements as well as quick failure detection at the packet IP and optical layers provide the trigger for capacity addition or deletion. Second, we also need a real-time view of available tails and regenerators to identify the optimal choice of link addition or deletion. Use of the above approach allows the average network utilization (under no-failure condition) to be increased from about 50% to about 70%. This approach does require some preconfigured spare endpoint tails and spare optical regenerators. However, overall savings compared to the traditional approach can be substantial. 16.2.5 USE OF AVAILABLE SPARE CAPACITY TO OFFER BANDWIDTH CALENDARING SERVICE Even after achieving a highly efficient network using techniques described above, there will still be some spare capacity available in the network. In Figure 16.4, we consider the intercity IP links of a typical ISP network. The horizontal axis shows the individual IP links arranged from largest to smallest capacities and the vertical axis shows the capacity and the maximum traffic at those links using a normalized scale. Each link has two directions with symmetric capacity but typically asym- metric traffic loads. For this reason, we show the traffic separately in the dominant direction and the other direction. Due 