The key advantages of adopting point-to-point are pay-as-you-grow capability, provisioning flexibility, digitized signal transparency, and the ability to evolve seamlessly into a full-fledged mesh network. This enables network planners to tackle today’s key challenge—fiber exhaust—with simple point-to-point solutions, while at the same time putting in place the building blocks of a sophisticated future optical network. They may still, of course, support their legacy rings with metropolitan DWDM as required, or even create ring-on-ring and ring-to-mesh topologies.

Unlike some topologies, the point-to-point strategy requires little upfront planning; random growth is quite acceptable. Even more importantly, network operators can pay as they grow, adding channels and equipment as they add customers. Furthermore, point-to-point enables service providers to receive revenue from an initial low-cost investment as soon as the first customer is activated. This is in stark contrast to the majority of network solutions that essentially require the entire network to be planned, built, and paid for before the first potential customer is even contacted. As a result, point-to-point is an ideal solution for competitive local exchange carriers (CLECs) seeking to enter new markets while minimizing start-up expenditures. Also, by utilizing the digitized signal transparency, exiting traffic can be handled on one wavelength while allowing the network to expand one channel at a time and allowing customers to be signed up without regard to traffic types (IP, ATM, video, SONET/synchronous digital digital hierarchy (SDH), enterprise systems connection (ESCON), ISC, Gigabit Ethernet, etc).

By adding an OXC to point-to-point trunks, the entire nature of the system evolves from a simple growth requirement to a sophisticated, all-optical network (AON) that has DWDM as its transport technology. The increased sophistication provided by optical routing dramatically increases capabilities. Some of this routing capability may initially be in the metropolitan DWDM in the form of dynamic optical add/drop (OAD) modules (see Figure 3). Even though capabilities are extensive, however, the core network is extremely simple to plan and expand.

Figure 3. Adding an OXC

Existing DWDM network management systems already have the ability to provision point-to-point circuits, one channel at a time. Furthermore, it is possible to plan wavelengths on the fly and use wavelengths independently on each hop. For example, hop one might contain four channels while hop two might contain eight channels (see Figure 4). Eventually, with an OXC installed between each hop, system-level wavelength planning becomes unnecessary, and more flexible utilization of previously installed DWDM becomes available. There is absolutely no need to reserve a wavelength through the entire network just to have the ability to provide a channel to a specific site sometime in the future, as is now required in a TDM or OAD multiplexer (OADM)–based system.

Figure 4. On-the-Fly Expansion per Hop

As for reliability, all circuit protection issues can be handled through features of the metropolitan DWDM and the OXC, including wavelength translation between hops. If regeneration is used, a point-to-point circuit that leverages a cross-connect can cover vast distances. For example, a firm in Southern California could build an end-user circuit using cross-connected city metro systems through many communities throughout the state without having to install a specific long-haul route, as long as there is connectivity between the various metro areas along the way. The additional advantage is that the system’s many points of presence make it a blended metropolitan and long-distance system so that delivering service to customers along the routes is very convenient. Even alternate routing choices of more than two paths can be built for each channel independently, without resorting to costly ring-layer, protect-all-channels designs.

A mesh topology is also an ideal compliment to the high demand for data networking and packet-based transport. While telecommunications providers have typically built networks based on rings as a result of their self-healing and rerouting properties, data backbones have always been mesh designs. This is also partly a result of the complex interconnection of IP routers. Certainly, a mesh topology shared by both IP and transport networks provides an elegant solution. Moreover, a mesh with optical routing can also provide self-healing capabilities without the complexity associated with ring software. Alternate routing through the mesh either by the router, switch, or optical network becomes an apparent solution.