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Optical Networks - _7_4 Optical Crossconnects_89

Optical Networks - _7_4 Optical Crossconnects_89 - 452 WDM...

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452 WDM Network Elements We can modify the two example architectures in Figure 7.9 by replacing the power splitters with 1 × N WSSs. For these designs as well as the designs in Figure 7.8, using WSSs rather than optical splitters or couplers has the advantage of improving power loss. A disadvantage is that WSSs are more expensive. Also note that the ROADMs may still require optical amplifiers to be placed between components to compensate for power losses. So what would an ideal OADM look like? Such an OADM (1) would be capable of being configured to drop a certain maximum number of channels, (2) would allow the user to select what specific channels are dropped/added and what are passed through under remote software control, including the transponders, without affecting the operation of existing channels, (3) would not require the user to plan ahead as to what channels may need to be dropped at a particular node, and (4) would maintain a low fixed loss regardless of how many channels are dropped/added versus passed through. 7.4 Optical Crossconnects OADMs are useful network elements to handle simple network topologies, such as the linear topology shown in Figure 7.4 or ring topologies, and a relatively modest number of wavelengths. An additional network element is required to handle more complex mesh topologies and large numbers of wavelengths, particularly at hub locations handling a large amount of traffic. This element is the optical crossconnect (OXC). We will see that though the term optical is used, an OXC could internally use either a pure optical or an electrical switch fabric. An OXC is also the key network element enabling reconfigurable optical networks, where lightpaths can be set up and taken down as needed, without having to be statically provisioned. Consider a large carrier central office hub. This might be an office in a large city for local service providers or a large node in a long-haul service provider’s network. Such an office might terminate several fiber links, each carrying a large number of wavelengths. A number of these wavelengths might not need to be terminated in that location but rather passed through to another node. The OXC shown in Figure 7.10 performs this function. OXCs work alongside SONET/SDH network elements as well as IP routers, and WDM terminals and add/drop multiplexers as shown in Figure 7.10. Typically, some OXC ports are connected to WDM equipment and other OXC ports to terminating devices such as SONET/SDH ADMs, IP routers, or ATM switches. Thus, the OXC provides cost-effective passthrough for express traffic not terminating at the hub as well as collects traffic from attached equipment into the network. Some people think of an OXC as a crossconnect switch together with the surrounding OLTs. However, our definition of OXC does not include the surrounding
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7.4 Optical Crossconnects 453 IP ATM SONET SDH OXC OLT Figure 7.10 Using an OXC in the network. The OXC sits between the client equipment of the optical layer and the optical layer OLTs.
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