Special Report: Next Generation Internet Applications
Data services growth, the cost of new cable deployment, and the need to better exploit the latent capacity of fiber optic cable motivate the introduction of DWDM equipment within the network. DWDM makes it possible to combine several transmissions onto the same fiber strand. For example, the DWDM in Figure 24 combines several high-speed SONET, ATM, and IP transmissions onto the same fiber. The combination makes much greater use of the fiber capacity and avoids costly construction of new fiber routes. Now, consider a high capacity communication network configured as shown in Figure 25. The ATM switches and IP routers are using the same high speed, OC-192 OEO electronics as the SONET multiplexer. There is no economy gained by passing the OC192 ATM and IP transmissions through the OC-192 SONET multiplexer. The SONET multiplexer is superfluous. Instead, it is more efficient to connect the ATM and IP equipment directly to the DWDM as shown in Figure 26. Figure 26 illustrates the current transitional trend in which DWDMs are displacing SONET multiplexers from the high capacity core network. The DWDM assumes the SONET multiplexing function. Note that the IP routers retain the SONET OC-x signaling interface as a standard for signaling and encapsulating packets across the DWDM channel. Likewise, the ATM switches retain the SONET OC-X signaling interface as a standard for signaling and encapsulating cells across the DWDM channel. Thus, the SONET multiplexers are displaced by the DWDM, but SONET multiplexer functions appear within the DWDM, ATM, and IP equipment. (C) IP-over-ATM-over DWDM The dizzying pace of today's service cycles and associated technology transitions is difficult to comprehend and even more difficult to convey to the marketplace. The photonic marketplace has a vernacular for describing the many configurations of the networks as they transition from one network configuration to another. The three SONET-based network configurations previously shown in Figure 23 are frequently referred to as: ATM-over-SONET (Figure 23 A), IP-over-SONET (Figure 23 B), and IP-over-ATM-over-SONET (Figure 23 C). They rely on SONET multiplexers within the network core to consolidate traffic onto fiber optic strands.
Networks resembling these are rapidly replacing SONET-multiplexed networks. These DWDMbased configurations are often referred to as: ATM-over-DWDM (Figure 26 A), IP-over-DWDM (Figure 26 B), and IP-over- ATM-over-DWDM (Figure 26 C). Note that all three configurations retain SONET signaling as the standard for the OEO interface between the electronic packet switching and DWDM equipment. Technically speaking, SONET continues to play a role in the network as a standard for signaling across an optical channel. However, SONET no longer exists in the network in the form of a separate piece of equipment. Instead, DWDMs assume the multiplexing role previously provided by SONET multiplexers. Industrial convention uses the terminology ATM-over-SONET, IP-over-SONET, and IP-over-ATM- over-SONET to denote the presence of discrete SONET equipment within the multiplexing layer of the network. Convention uses the terminology ATM-over-DWDM, IP-over-DWDM, and IP-over-ATM- over-DWDM to denote networks where DWDM equipment takes the place of discrete SONET equipment, even if SONET signaling is used at the OEO interface within the IP and ATM equipment. This report adopts this convention and will use the conventional terminology throughout the remainder of the document when referring to networks comprised of combinations of electronic and optical equipment.
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