to the subscriber compared to approaches such as fiber-to-the-curb or fiber-to-the-home. (Chapter 12 provides a comprehensive discussion of fiber-to-the-x and PONs.) MSPPs MSPPs are specialized optical access systems that enable carriers to get a quick start in offering a full range of services. Typically, MSPPs reside at the carrier's POP or at the customer's site. They incorporate WDM, allowing service providers and customers to take advantage of the fact that a fiber can carry much more than the The Optical Core: Overlay Versus Peer-to-Peer Networking Models single wavelength that SDH/SONET fibers today carry. With MSPPs, carriers can also offer customers different grades of service, as well as telephony and other multi- media offerings. Today's MSPPs are based largely on proprietary technology, SO they vary from provider to provider, but they all share some basic characteristics, including the following: Fiber access-Optical interfaces can use SDH/SONET, Gigabit Ethernet (GigE), or 10GigE, and they can include an automatic restoration capability. Digital multiplexing-MSPPs can use TDM, a form of packet access, or a combination of both. Support for WDM-Many MSPPs incorporate CWDM, SO a single fiber pair can support multiple customers. Coarse wavelength division multi- plexers were developed for metro and campus environments. They use a less powerful laser source, reducing their capabilities but also reducing their costs and making it easier to justify using them for the type of short- haul applications metro and campus environments represent. Because the laser sources are less powerful with CWDM, the channels need to be spaced further apart (at 100GHz or 200GHz intervals), and one result of this is that fewer channels can be derived. Generally, CWDM is limited to no more than 64 channels. But because of the lower cost of the light sources, CWDM devices are much less costly to build and can be more justified in their deployment in the metro network. The Optical Core: Overlay Versus Peer-to-Peer Networking Model