Optical Networks: A Practical Perspective - Part 55. 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. | 510 Control and Management Figure Forward and backward defect indicator signals and their use in a network. Alarm Management In a network a single failure event may cause multiple alarms to be generated all over the network and incorrect actions to be taken in response to the failed condition. Consider in particular a simple example. When a link fails all lightpaths on that link fail. This could be detected at the nodes at the end of the failed link which would then issue alarms for each individual lightpath as well as report an entire link failure. In addition all the nodes through which these lightpaths traverse could detect the failure of these lightpaths and issue alarms. For example in a network with 32 lightpaths on a given link each traversing through two intermediate nodes the failure of a single link could trigger a total of 129 alarms 1 for the link failure and 4 for each lightpath at each of the nodes associated with the lightpath . It is clearly the management system s job to report the single root-cause alarm in this case namely the failure of the link and suppress the remaining 128 alarms. Alarm suppression is accomplished by using a set of special signals called the forward defect indicator FDI and the backward defect indicator BDI . Figure shows the operation of the FDI and BDI signals. When a link fails the node downstream of the failed link detects it and generates a defect condition. For instance a defect condition could be generated because of a high bit error rate on the incoming signal or an outright loss of light on the incoming signal. If the defect persists for a certain time period typically a few seconds the node generates an alarm. Immediately upon detecting a defect the node inserts an FDI signal downstream to the next node. The FDI signal propagates rapidly and nodes further downstream receive the FDI and suppress their alarms. The FDI signal is also sometimes referred to as the alarm indication signal AIS . A node .
