10.4. SWITCHED OPTICAL BACKBONE
With IP traffic continuing to grow at a healthy rate , scalability of IP backbones is one important problem, if not the most important, facing service providers today. Historically, IP backbones have consisted of core routers interconnected in a mesh topology over ATM or SONET/ SDH links. With the advent of WDM technology, service providers are now connecting core routers directly over point-to-point WDM links. This architecture, referred to as IP over WDM, is illustrated in Figure 10.15a . Figure 10.15 shows an IP traffic flow from point of presence (PoP) 1 to PoP 4 passing through PoP 2 as an intermediate Pop .
 Transit traffic at PoP 2 (for this IP flow) uses IP router ports. In IP over WDM, traditional transport functions such as switching, grooming, configuration, and restoration are eliminated from the SONET/SDH layer. These functions are moved to the IP layer and accomplished by protocols like MPLS .
Figure 10.15. Alternative architectures for interconnecting IP routers: (a) lP over WDM and (b) IP over OTN.
In an alternative approach, referred to as IP over OTN, routers are connected through a reconfigurable optical backbone, or OTN, consisting of SONET/SDH OXCs interconnected in a mesh topology using WDM links. The core optical backbone consisting of such OXCs takes over the functions of switching, grooming, and restoration ...