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Chapter 9 Network Survivability > 9.1 Basic Concepts - Pg. 513

9.1 Basic Concepts 513 additional 10 ms time allocated to detect or discover the failure.) This restoration time requirement came from the fact that some equipment in the network drops voice calls if the connection is disrupted for a period significantly longer than 60 ms. Over time, operators have become accustomed to achieving restoration on these time scales. However, in a world dominated by data, rather than voice traffic, the 60 ms number may not be a rigid requirement, and operators may be willing to tolerate somewhat larger restoration times, particularly if they see other benefits as a result, such as higher bandwidth efficiency, which in turn would lead to lower operating costs. On the other hand, the restoration time requirements could get more stringent as data rates in the network increase. A downtime of 1 second at 10 Gb/s corresponds to losing over a gigabyte of data. Most IP networks today provide services on a best- effort basis and do not guarantee availability. That is, they try to route traffic in the network as best as they can, but packets can have random delays through the network and can be dropped if there is congestion. Survivability can be addressed within many layers in the network. Protection can be performed at the physical layer, or layer 1, which includes the SONET/SDH, Optical Transport Network (OTN), and the optical layers. Protection can also be performed at the link layer, or layer 2, which includes MPLS, Ethernet, and Resilient Packet Ring. Finally, protection can also be performed at the network layer, or layer