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Optical Networks - _1_4 The Optical Layer_13

Optical Networks - _1_4 The Optical Layer_13 - 1.4 The...

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1.4 The Optical Layer 15 Optical networks based on the architecture described above are already being deployed. OLTs have been widely deployed for point-to-point applications. OADMs are now used in long-haul and metro networks. OXCs are beginning to be deployed first in long-haul networks because of the higher capacities in those networks. 1.4 The Optical Layer Before delving into the details of the optical layer, we first introduce the notion of a layered network architecture. Networks are complicated entities with a variety of different functions being performed by different components of the network, with equipment from different vendors all interworking together. In order to simplify our view of the network, it is desirable to break up the functions of the network into different layers, as shown in Figure 1.5. This type of layered model was proposed by the International Standards Organization (ISO) in the early 1980s. Imagine the layers as being vertically stacked up. Each layer performs a certain set of functions and provides a certain set of services to the next higher layer. In turn, each layer expects the layer below it to deliver a certain set of services to it. The service interface Layer 1 Layer 1 Layer i Layer i Layer 1 i+ Layer 1 i+ ... ... NE NE NE NE Service access point Connection Figure 1.5 Layered hierarchy of a network showing the layers at each network element (NE).
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16 Introduction to Optical Networks between two adjacent layers is called a service access point (SAP), and there can be multiple SAPs between layers corresponding to different types of services offered. In most cases, the network provides connections to the user. A connection is established between a source and a destination node. Setting up, taking down, and managing the state of a connection is the job of a separate network control and management entity (not shown in Figure 1.5), which may control each individual layer in the network. There are also examples where the network provides con- nectionless services to the user. These services are suitable for transmitting short messages across a network, without having to pay the overhead of setting up and taking down a connection for this purpose. We will confine the following discussion to the connection-oriented model. Within a network element, data belonging to a connection flows between the layers. Each layer multiplexes a number of higher-layer connections and may add more overhead to data coming from the higher layer. Each intermediate network element along the path of a connection embodies a set of layers starting from the lowest layer up to a certain layer in the hierarchy. It is important to define the functions of each layer and the interfaces between layers. This is essential because it allows vendors to manufacture a variety of hard- ware and software products performing the functions of some, but not all, of the layers, and provide the appropriate interfaces to communicate with other products performing the functions of other layers.
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