PDF of this tutorialOptical Bidirectional Line Switched Rings
Optical-ring architectures utilize reconfigurable OADMs; the ring architecture is a familiar scheme to the telecommunications industry and now applied to the optical domain (see Figure 18). The optical ring uses the same principles as the fiber ring to provide protection against equipment and network failures (see Figure 19).
Figure 18. Optical Ring Architecture
Figure 19. Optical Rings
Network elements have intelligent software that senses a module failure or break in its fiber connection and automatically routes traffic in the opposite direction around the fiber ring. This architecture allows service providers to guarantee that customers' connection will not go out of service. However, the network elements now support multiple optical wavelengths as opposed to multiple DS–3 circuits. In the case of a fiber break, the optical network will automatically reroute up to 40 optical signals in less than 50 milliseconds.
Because optical rings are most cost effective over large networks, the switching time is critical. One technology planned for implementation is called network protection equipment (NPE), which significantly reduces the switching time required in large optical networks. Instead of routing traffic from network elements adjacent to a fiber cut, the OBLSR using NPE redirects the traffic from the node where it enters the ring. This redirection prevents the traffic from being backhauled across the network, which greatly improves overall switching time.
Optical Cross-Connect (OXC)
Efficient use of fiber facilities at the optical level obviously becomes critical as service providers begin to move wavelengths around the world. Routing and grooming are key areas that must be addressed. This is the function of the OXC, as shown in Figure 20.
Figure 20. OXC Block Diagram
Digital cross-connect systems are deployed en masse and provide the critical function of grooming traffic (DS–0, DS–1, and DS–3) to fill output ports on the system efficiently. Today, output ports can be at the DS–3, OC–3, or OC–12 level. For this reason, it is critical to ensure that those pipes are full of traffic when they exit the cross-connect system. In the optical domain, where 40 optical channels can be transported on a single fiber, a network element is needed that can accept various wavelengths on input ports and route them to appropriate output ports in the network. To accomplish this, the OXC needs three building blocks (See Figure 21):
- fiber switching—the ability to route all of the wavelengths on an incoming fiber to a different outgoing fiber
- wavelength switching—the ability to switch specific wavelengths from an incoming fiber to multiple outgoing fibers
- wavelength conversion—the ability to take incoming wavelengths and convert them (on the fly) to another optical frequency on the outgoing port; this is necessary to achieve strictly nonblocking architectures when using wavelength switching
Figure 21. Optical Cross-Connects
