Special Report: Next Generation Internet Applications
4.1.2 Protection and Restoration Protection and restoration switching are essential in DWDM networks. The prospect of failures in high- speed DWDM networks creates too much risk. DWDM capacity cannot be effectively managed, protected, and restored using IP, SONET, or ATM switching. Therefore, a routing scheme that uses protocols like MPLS or MP'S must provide the intelligence needed within the optical network to perform routing, wavelength assignment, fault detection, protection and restoration switching. Current switching methods within DWDM networks are slow when compared to SONET or ATM.
Therefore, routing and wavelength assignment within DWDM networks must occur much less frequently than in an electronic ATM or IP network. The most likely scenario will be to use static light path establishment with dynamic light path optimization. Static light path establishment employs offline routing schemes to optimally construct the optical network. Dynamic optimization uses automated methods to adjust the static light path configurations in response to changing traffic demands. DWDM protection could take two forms. Pre-provisioned protection transmits signals over a primary and a backup path.
The receiving device chooses between the paths based upon the relative signal quality of each path. Preemptive protection sends high priority traffic along a primary path and low priority traffic along a predetermined protection path. High priority traffic preempts the low priority traffic and seizes the protection path if the primary path fails.
Until high-speed photonic switching devices are available, ATM switches or IP routers will define the primary light path and restoration light paths. The OXCs will establish primary paths and restoration paths by interacting with the routers. Signaling between routers and OXCs will create light paths, delete light paths, modify light paths, and inquire on path status. The control plane requires mechanisms for neighbor discovery, link status updates, route computation, and path establishment. Restoration mechanisms may perform local restoration of links between adjacent OXCs or end-to-end restoration along alternate paths. Restoration paths may be 1+1 reserved capacity (similar to SONET ring protection methods) or shared protection where primary light paths share disjoint protection paths as a means of conserving network capacity and avoiding unnecessary traffic congestion.
4.1.3 Submarine Networks Submarine cable networks are an important but little discussed portion of the global communications infrastructure. Yet they provide all-important paths for connecting continents and supporting international commerce. Submarine networks provide services at the top of the global photonic infrastructure hierarchy as depicted in Figure 29.
Submarine cables are built and operated by consortiums of communications carriers from across the globe. A typical undersea cable consortium may involve 30 or more carriers from as many countries in the planning, construction, and management of a single cable system. A carrier consortium will jointly invest from several hundreds of millions to over $1 billion dollars in a single cable spanning the Atlantic or Pacific Ocean.
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