Optical Networks - _9_1 Basic Concepts_106

Optical Networks - _9_1 Basic Concepts_106 - 9.1 Basic...

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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 3, such as the IP layer. There are several reasons why this is the case. For instance, each layer can protect against certain types of failures but probably not protect against all types of failures effectively. In this chapter, we will focus primarily on layer 1 restoration, but will also briefly discuss the protection techniques applicable to layers 2 and 3. The rest of this chapter is organized as follows. We start by outlining the basic concepts behind protection schemes. Many of the protection techniques used in today’s telecommunication networks were developed for use in SONET and SDH networks, and we will explore these techniques in detail. We will also look at how protection is implemented in some of today’s protocols in the client layer and in particular Ethernet, IP, MPLS, and Resilient Packet Rings. Next we will look at protection functions in the optical layer in detail, and then discuss how protection functions in the different layers of the network can work together. 9.1 Basic Concepts A great variety of protection schemes are used in today’s networks, and the notions of working paths and protect paths are fundamental to understanding them. Working paths carry traffic under normal operation; protect paths provide an alternate path
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514 Network Survivability to carry the traffic in case of failures. Working and protection paths are usually
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