Optical Networks: A Practical Perspective - Part 61. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 570 Network Survivability Transponder a Transponder Figure Comparison of a 1 1 OMS and b 1 1 OCh protection schemes. Working Protect Transponder b 1 1 OMS Protection This is perhaps the simplest optical layer protection scheme and is shown in Figure a . Because of its simplicity it has been implemented by several vendors in their OLTs. The composite WDM signal is bridged onto two diverse paths using an optical splitter. At the other end an optical switch is used to select the better among the two signals based primarily on detecting the presence or absence of light signals. The split incurs an additional 3 dB loss and the switch also adds a small amount of loss 1 dB . An alternative implementation uses optical amplifiers on each of the fibers and a passive combiner to combine both directions at the receiver. At any time one amplifier is turned on and the other is turned off. This has the advantage of avoiding a single point of failure in the system the selector switch in other implementations but may be more expensive to implement. Optical Layer Protection Schemes 571 1 1 OMS Protection This scheme is similar to the SONET 1 1 scheme discussed in Section and the benefits are similar support for low-priority traffic and also the ability to have N working systems share a single protection system. Compared to the 1 1 scheme of Figure a a typical implementation uses a switch at the transmitter instead of a splitter resulting in a somewhat lower total loss in the path. Just as in the SONET equivalent an APS protocol is needed to provide coordination between the two ends of the link. OMS-DPRing The OMS-DPRing dedicated protection ring is similar to a SONET UPSR except that it operates at the OMS or optical line layer whereas the UPSR operates in the SONET path layer. It can also be thought of as an optical unidirectional line-switched ring ULSR . One possible implementation of an OMS-DPRing Bat98 is shown in Figure .
