510 CONTROL AND MANAGEMENT Figure 9.4 Forward and backward defect indicator signals and their use in a network. 9.5.4 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 9.4 shows the operation of the FDI and BDI signals. When a link fails, the node down- stream 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 detecting a defect also sends a BDI signal upstream to the previous node, to notify that node of the failure. If this previous node didn't send out an FDI, it then knows that the link to the next node downstream has failed. Note further that separate FDI and BDI signals are needed for different sublay- ers within the optical layer, for example, to distinguish between link failures and failures of individual lightpaths, or to distinguish between the failure of a section of the link between amplifier locations and that of the entire link. The exact types 9.5 Performance and Fault Management 511 Figure 9.5 Using hierarchical defect indicator signals in a network. Defect indicators are used at the OTS, OMS, and the various OCh sublayers. and behavior of defect indicators for the optical layer are being standardized cur- rently (ITU G.709). Figure 9.5 illustrates one possible use of these different indicator signals in a network. Suppose there is a link cut between OLT A and amplifier B as shown. Amplifier B detects the cut. It immediately inserts an OMS-FDI signal downstream indicating that all channels in the multiplexed group have failed and also an OTS-BDI signal upstream to OLT A. The OMS-FDI is transmitted as part of the overhead associated with the OMS layer, and the OTS-BDI is transmitted as part of the overhead associated with the OTS layer. Note that an OMS-FDI is transmitted downstream and not an OTS-FDI. This is because the defect information needs to be propagated all the way downstream to the network element where the OMS layer is terminated, which, in this case, is OADM D. Amplifier C downstream receives the OMS-FDI and passes it on. OADM D, which is the next node downstream, receives the OMS-FDI and determines that all the lightpaths on the incoming link have failed. Some of these lightpaths are dropped locally and others are passed through. For each lightpath passed through, the OADM generates OCh-TS-FDIs and sends them downstream. The OCh-TS-FDIs are trans- mitted as part of the OCh-TS overhead. At the end of the all-optical subnet, at OLT E, the wavelengths are demultiplexed and terminated in transponders/regenerators. Therefore the OCh-TS layer is terminated here. OLT E receives the OCh-TS-FDIs. It then generates OCh-P-FDI indicators for each failed lightpath and sends that down- stream to the ultimate destination of each lightpath as part of the OCh-P overhead. Finally, the only node that issues an alarm is node B. Another major reason for using the defect indicator signals is that defects are used to trigger protection switching. For example, nodes adjacent to a failure detect 512 CONTROL AND MANAGEMENT the failure and may trigger a protection-switching event to reroute traffic around the failure. At the same time, nodes further downstream and upstream of the failure may think that other links have failed and decide to reroute traffic as well. A node receiving an FDI knows whether it should or shouldn't initiate protection switching. For example, if the protection-switching method requires the nodes immediately adjacent to the failure to reroute traffic, other nodes receiving the FDI signal will not invoke protection switching. On the other hand, if protection switching is done by the nodes at the end of a lightpath, then a node receiving an FDI initiates protection switching if it is the end point of the associated lightpath. 9.5.5 Data Communication Network (DCN) and Signaling The element management system (EMS) communicates with the different network elements through the DCN. This DCN is usually a standard TCP/IP or OSI network (see Chapter 6). If the DCN is sufficiently well connected (2-connected, to be more precise), then the DCN can stay up even if there is a failure in the network. The DCN can be transported in several ways: 1. Through a separate out-of-band network outside the optical layer. Carriers can make use of their existing TCP/IP or OSI networks for this purpose. If such a network is not available, dedicated leased lines could be used for this purpose. This option is viable for network elements that are located in big central offices where such connectivity is easily available, but not viable for network elements such as optical amplifiers that are located in remote huts in the field. 2. Through the OSC on a separate wavelength (see Section 9.5.7). This option is available for WDM line equipment that processes the optical transmission section and multiplex section layers, where the optical supervisory channel is made available. For example, optical amplifiers are managed using this approach. However, this option is not available to equipment that only looks at the optical channel layer, such as optical crossconnects. 3. Through the rate-preserving or digital wrapper inband optical channel layer overhead techniques to be described in Section 9.5.7. This option is useful for equipment that only looks at the optical channel layer and does not process the multiplex and transmission section layers, such as optical crossconnects. Also, it is available only at locations where the lightpath is processed in the electrical domain, that is, at regenerator or transponder locations. Table 9.1 summarizes the applicability of different DCN options available for each type of network element. We assume that OADMs are part of the line system that 9.5 Performance and Fault Management 513 Table 9.1 Different ways of realizing the DCN for different network elements. The OADM is assumed to have transponders for channels that are dropped and added, but not for channels that are passed through. Network Element Out-of-Band OSC Rate-Preserving Overhead or Digital Wrapper OLT with transponders Yes Yes Yes OADM Yes Yes Yes (for dropped channels) Amplifier No Yes No OXC with regenerators Yes No Yes All-optical OXC (no regenerators) Yes No No includes OLTs and amplifiers. Access to the optical supervisory channel is typically restricted to elements within a line system due to the proprietary nature of the OSC. In addition to the DCN, in many cases, a fast signaling network is needed between network elements. This allows the network elements to exchange critical informa- tion between them in real time. For instance, the FDI and BDI signals need to be propagated quickly to the nodes along a lightpath. Other such signals include infor- mation needed to implement fast protection switching in the network, the topic of Chapter 10. Just as with the DCN, the signaling network can be implemented using dedicated out-of-band connections, the optical supervisory channel, or through one of the overhead techniques. 9.5.6 Policing One function of the management system is to monitor the wavelength and power levels of signals being input to the network to ensure that they meet the requirements imposed by the network. As we discussed above, the acceptable power levels will depend on the signal types and bit rates. The types and bit rates are specified by the user, and the network can then set thresholds for the parameters as appropriate for each signal type and monitor them accordingly. This includes threshold values for the parameters at which alarms must be set off. The thresholds depend on the data rate, wavelength, and specific location along the path of the lightpath, and degradations may be measured relative to their original values. Another more important function is to monitor the actual service being utilized by the user. For example, the service provider may choose to provide two services, say, an ESCON service and an OC-3 service, by leasing a transparent lightpath to the user. The two services may be tariffed differently. With a purely transparent network, it is difficult to prevent a user who opts for the ESCON service from sending OC-3 514 CONTROL AND MANAGEMENT Figure 9.6 Different types of optical layer overhead techniques. The OSC is used hop by hop. The pilot tone is inserted by a transmitter and can be monitored at elements in an all-optical subnet until it is terminated at a receiver. The digital wrapper or rate-preserving overhead is used end to end across multiple subnets through intermediate regenerators. traffic. What this implies is that services based on leasing wavelengths will likely be tariffed based on a specified maximum bit rate, with the user being allowed to send any signal up to the specified maximum bit rate. 9.5.7 Optical Layer Overhead Supporting the optical path trace, defect indicators, and BER measurement requires the use of some sort of overhead in the optical layer. We have alluded indirectly to some of these overheads earlier, for example, the use of the SONET/SDH overhead to measure the BER and the use of the optical supervisory channel to carry some of the defect indicator signals. In this section, we describe four different methods for carrying the optical layer overhead. These methods are illustrated in Figure 9.6 and compared in Table 9.2. The pilot tone approach and the optical supervisory channel are useful to carry overhead information within an all-optical subnetwork. At the boundaries of each subnetwork, the signal is regenerated (3R) by converting into the electrical domain and back. The rate-preserving overhead and the digital wrapper can be used to carry overhead information across an entire optical network through multiple all-optical subnetworks. 9.5 Performance and Fault Management 515 Table 9.2 Applications of different optical layer overhead techniques. The different techniques apply to different sublayers within the optical layer namely, the optical transmission section (OTS), optical multiplex section (OMS), or optical channel-section (OCh-S) or optical channel (OCh) layers. The trace and defect indicator (DI) signals are defined at multiple sublayers. All-Optical Subnet End-to-End Application OSC Pilot Tone Rate-Preserving Digital Wrapper Trace OTS OCh-TS OCh-P OCh-P OCh-S OCh-S Dis Performance monitoring Client signal compatibility OTS None OMS OCh-P OCh-P OCh-TS None Optical power BER BER Any Any SONET/SDH Any Pilot Tone or Subcarrier Modulated Overhead Here, the overhead is realized by modulating the optical carrier (wavelength) of a lightpath with an additional subcarrier signal, as described in Section 4.2. This signal is also sometimes called a pilot tone. As long as the modulation depth of this signal is kept small compared to the data, typically between 5-10%, and the subcarrier frequency is chosen carefully, the data is relatively unaffected as a result. The pilot tone itself may be amplitude or frequency modulated at a low rate, say, a few kilobits per second, to carry additional overhead information. At intermediate locations, a small fraction of the optical power can be tapped off and the pilot tones extracted without receiving and retransmitting the entire signal. Note that the pilot tones on each wavelength can be extracted from the composite WDM signal carrying all the wavelengths without requiring each wavelength to be demultiplexed. The pilot tone frequency needs to be chosen carefully. First, it should have min- imal overlap with the data bandwidth. For instance, a lightpath carrying SONET data at 2.5 Gb/s has relatively little spectral content below 2 MHz, and a pilot tone in the 1-2 MHz range can be added with minimal impact to the data. The pilot tone frequency also needs to lie above the gain modulation cutoff of the erbium-doped op- tical amplifiers, which is typically around 100 kHz (see Section 3.4.3). Tones below this frequency will cause the amplifier gain to vary with the pilot tone amplitude, causing this modulation to be imposed on other channels as undesirable "ghost" 516 CONTROL AND MANAGEMENT Figure 9.7 The optical supervisory channel, which is terminated at each amplifier lo- cation. tones or crosstalk. The pilot tone frequency can also be chosen to lie above the data band, in this example, say, above 2.5 GHz, but it is relatively more expensive to process signals at higher frequencies than at lower frequencies. The advantages of the pilot tone approach are that it is relatively inexpensive and that it allows monitoring of the overhead in transparent networks without requiring knowledge of the actual protocol or bit rate of the signal. The disadvantages are that it cannot be used to monitor the BER, and the pilot tone can be modified only at the transmitter or at a regenerator and not at the intermediate nodes. Thus it can be used for the OCh-TS trace function inside a transparent subnetwork between regenerator points, but cannot be used to insert FDI and BDI signals at intermediate nodes without a regenerator. The trace function can be accomplished using pilot tones in several possible ways. For example, each lightpath could have a unique pilot tone frequency, which by itself serves as the trace. Alternatively, we could have a unique pilot tone frequency for each wavelength, and the pilot tone can be modulated with a digital signal containing a unique lightpath identifier. Optical Supervisory Channel In systems with line amplifiers, a separate OSC is used to convey information asso- ciated with monitoring the state of the amplifiers along the link, particularly if these amplifiers are in remote locations where other direct access is not possible. The OSC is also used to control the line amplifiers, for example, turning them on or turning them off for test purposes. It can also be used to carry the DCN, as well as some of the overhead information. The OSC is carried on a wavelength different from the wavelengths used for carrying traffic. It is separated from the other wavelengths at each amplifier stage and received, processed, and retransmitted, as shown in Figure 9.7. The choice of the exact wavelength for the OSC involves a number of trade-offs. Figure 9.8 shows the usage of various wavelength bands in the network for carrying 9.5 Performance and Fault Management 517 Figure 9.8 Usage of wavelengths in the network. Traffic is carried on the O (original), S (short), C (conventional), or L (long) wavelength bands. Raman pumps, if used, are located about 80-100 nm below the signal. traffic, for pumping the erbium or Raman amplifiers, and for the OSC. The OSC could be located within the same band as the traffic-bearing channels, or in a separate band located away from the traffic-bearing channels. In the latter situation, it is easier to filter out and reinsert the OSC at each amplifier location. However, we need to locate the OSC away from the Raman pumps if they are used in the system. Perhaps the only advantage of locating the OSC in the same band as the traffic-bearing channels is a slight reduction in amplifier noise. For instance, if a two-stage amplifier design is used, the in-band OSC can be filtered out after the first stage along with the amplifier noise that is present at this wavelength. For WDM systems operating in the C-band, the popular choices for the OSC wavelength include 1310 nm, 1480 nm, 1510 nm, or 1620 nm. Using the 1310 nm band for the OSC precludes the use of this band for carrying traffic. The 1480 nm wavelength was considered only because of the easy availability of lasers at that wavelength it happens to be one of the wavelengths used to pump an erbium-doped fiber amplifier (EDFA). For the same reason, however, there can be some undesirable interactions between the OSC laser and the EDFA pump, so this is not a popular choice. After going through some of these trade-offs, the ITU has adopted the 1510 nm wavelength as the preferred choice. This wavelength is outside the EDFA passband, does not coincide with an EDFA pump wavelength, and lies outside the C- and L-bands. Note, however, that this wavelength falls in the S-band and may also overlap with Raman pumps for the L-band. 518 CONTROL AND MANAGEMENT Yet another choice used by some vendors is the 1620 nm wavelength, on the outer edge of the L-band. This choice avoids most of the problems above, except that we have to be careful about separating this channel from a traffic-bearing channel toward the edge of the L-band. The OSC can be used to carry OTS traces and defect indicators, as well as OMS and OCh-TS defect indicators. Rate-Preserving Overhead The idea here is to make use of the existing SONET/SDH overhead that is used with most of the signals entering the optical layer. This overhead includes several bytes that are currently unused. Some of these bytes can be used by the optical layer. These bytes can also be used to add forward error correction (FEC), which improves the optical layer link budget. This technique can be used only at locations where the signal is available in electrical form, that is, at regenerator locations or at the edges of the network. Unlike the pilot tone method, it cannot be used inside a transparent optical subnetwork. The advantages of this method are the following: First, it can be used with the existing equipment in the network. For example, a new network element with this capability can communicate with other network elements of the same type through intermediate WDM and SONET equipment that is already present in the network. Second, it retains the existing hierarchy of bit rates in the SONET/SDH standards, without the need for creating a new hierarchy of rates that would be needed with the digital wrapper technique to be discussed next. This allows existing SONET/SDH chipsets, such as clock recovery circuits, receivers, modulators, and overhead processing chips, to be used without requiring the development of a new set of components to support the new rates. The disadvantages of this method are the following: First, the number of unused bytes available is limited and may not offer sufficient bandwidth to carry all the optical layer overhead and FEC. Second, while the SONET/SDH standards specify the set of unused bytes, several vendors have already made use of some of these bytes for their own proprietary reasons, which makes it difficult to determine which set of bytes are truly unused! Third, it does not work with signals that don't use SONET/SDH framing, such as Fibre Channel or Gigabit Ethernet (see Chapter 6). Digital Wrapper Overhead Here, a new set of overhead bytes is added to the signal as it enters the optical layer and removed when the signal is handed back to the client layer. This scheme offers essentially the same capabilities as the rate-preserving overhead discussed above. The 9.6 Configuration Management 519 digital wrapper defines a new set of overheads associated with the optical layer and can be used instead of the SONET/SDH overhead. It is being standardized in the ITU. The advantages of this method are the following: First, sufficient overhead bytes can be added so as to provide adequate FEC and support the DCN as well as to allow for future needs. Second, a new standard based on this technique would allow better interoperability among multiple vendors through regenerators. Third, the technique is not limited to SONET/SDH signals. The wrapper can be used to encapsulate a variety of different signals, such as Fibre Channel and Gigabit Ethernet. The main disadvantages of the digital wrapper approach are that it is not suitable for use with legacy equipment, and that it requires the development of a new set of components to support the new hierarchy of bit rates. However, new components have already been developed to support the wrapper, and it is now available on many WDM products. The digital wrapper is ideally suited to carrying OCh-section and path layer traces and defect indicators, as well as providing other overheads for management, such as those used by an automatic protection-switching (APS) protocol for signaling between network elements during failures. 9.6 Configuration Management We can break down configuration management functions into three parts: manag- ing the equipment in the network, managing the connections in the network, and managing the adaptation of client signals into the optical layer. 9.6.1 Equipment Management In general, the principles of managing optical networking equipment are no different from those of managing other high-speed networking equipment. We must be able to keep track of the actual equipment in the system (for example, number and location of optical line amplifiers) as well as the equipment in each network element and its capabilities. For example, in a terminal of a point-to-point WDM system, we may want to keep track of the maximum number of wavelengths and the number of wavelengths currently equipped, whether there are optical pre- and power amplifiers or not, and so forth. Among the considerations in designing network equipment is that we should be able to add to existing equipment in a modular fashion. For instance, we should be . A. The OMS-FDI is transmitted as part of the overhead associated with the OMS layer, and the OTS-BDI is transmitted as part of the overhead associated with the OTS layer. Note that an OMS-FDI. CONTROL AND MANAGEMENT Figure 9.4 Forward and backward defect indicator signals and their use in a network. 9.5.4 Alarm Management In a network, a single failure event may cause multiple alarms. modulated at a low rate, say, a few kilobits per second, to carry additional overhead information. At intermediate locations, a small fraction of the optical power can be tapped off and the