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As a result, carriers are now paying a lot of attention to minimizing life cycle costs, as opposed to worrying just about up-front equipment costs. We start with a brief introduction to network management concepts in general and how they apply to managing optical networks. We follow this with a discussion of optical layer services and how the different aspects of the optical network are managed. 9.1 Network Management Functions Classically, network management consists of several functions, all of which are im- portant to the operation of the network: 1. Performance management deals with monitoring and managing the various parameters that measure the performance of the network. Performance man- agement is an essential function that enables a service provider to provide quality-of-service guarantees to their clients and to ensure that clients comply 495 496 CONTROL AND MANAGEMENT with the requirements imposed by the service provider. It is also needed to pro- vide input to other network management functions, in particular, fault manage- ment, when anomalous conditions are detected in the network. This function is discussed further in Section 9.5. 2. Fault management is the function responsible for detecting failures when they happen and isolating the failed component. The network also needs to restore traffic that may be disrupted due to the failure, but this is usually considered a separate function and is the subject of Chapter 10. We will study fault manage- ment in Section 9.5. 3. Configuration management deals with the set of functions associated with manag- ing orderly changes in a network. The basic function of managing the equipment in the network belongs to this category. This includes tracking the equipment in the network and managing the addition/removal of equipment, including any rerouting of traffic this may involve and the management of software versions on the equipment. Another aspect of configuration management is connection management, which deals with setting up, taking down, and keeping track of connections in a network. This function can be performed by a centralized management system. Alternatively, it can also be performed by a distributed network con- trol entity. Distributed network control becomes necessary when connection setup/take-down events occur very frequently or when the network is very large and complex. Finally, the network needs to convert external client signals entering the op- tical layer into appropriate signals inside the optical layer. This function is adap- tation management. We will study this and the other configuration management functions in Section 9.6. 0 Security management includes administrative functions such as authentication of users and setting attributes such as read and write permissions on a per-user basis. From a security perspective, the network is usually partitioned into do- mains, both horizontally and vertically. Vertical partitioning implies that some users may be allowed to access only certain network elements and not other network elements. For example, a local craftsperson may be allowed to access only the network elements he is responsible for and not other network elements. Horizontal partitioning implies that some users may be allowed to access some parameters associated with all the network elements across the network. For ex- ample, a user leasing a lightpath may be provided access to all the performance parameters associated with that lightpath across all the nodes that the lightpath traverses. 9.1 Network Management Functions 497 9.1.1 Security also involves protecting data belonging to network users from being tapped or corrupted by unauthorized entities. This part of the problem needs to be handled by encrypting the data before transmission and providing the decrypting capability to legitimate users. 5. Accounting management is the function responsible for billing and for developing lifetime histories of the network components. This function doesn't appear to be much different for optical networks, compared to other networks, and we will not be discussing this topic further. For optical networks, an additional consideration is safety management, which is needed to ensure that optical radiation conforms to limits imposed for ensuring eye safety. This subject is treated in Section 9.7. Management Framework Most functions of network management are implemented in a centralized manner by a hierarchy of management systems. However, this method of implementation is rather slow, and it can take several hundreds of milliseconds to seconds to communi- cate between the management system and the different parts of the network because of the large software path overheads usually involved in this process. Decentralized methods are usually much faster than centralized methods, even in small networks with only a few nodes. Therefore, certain management functions that require rapid action may have to be decentralized, such as responding to failures and setting up and taking down connections if these must be done rapidly. For example, a SONET ring can restore failures within 60 ms, and this is possible only because this process is completely decentralized. For this reason, restoration is viewed as more of an au- tonomous control function rather than an integrated part of network management. Another reason for decentralizing some of the functions arises when the network becomes very large. In this case, it becomes difficult for a single central manager to manage the entire network. Further, networks could include multiple domains administered by different managers. The managers of each domain will need to communicate with managers of other domains to perform certain functions in a coordinated manner. Figure 9.1 provides an overview of how network management functions are im- plemented on a typical network. Management is performed in a hierarchical manner, involving multiple management systems in many cases. The individual components to be managed are called network elements. Network elements include optical line terminals (OLTs), optical add/drop multiplexers (OADMs), optical amplifiers, and optical crossconnects (OXCs). Each element is managed by its element management system (EMS). The element itself has a built-in agent, which communicates with 498 CONTROL AND MANAGEMENT Figure 9.1 Overview of network management in a typical optical network, showing the network elements (OLTs, OADMs, OXCs, amplifiers), the management systems, and the associated interfaces. its EMS. The agent is implemented in software, usually in a microprocessor in the network element. The EMS is usually connected to one or more of the network elements and communicates with the other network elements in the network using a data commu- nication network (DCN). In addition to the DCN, a fast signaling channel is also required between network elements to exchange real-time control information to manage protection switching and other functions. The DCN and signaling channel can be realized in many different ways, as will be discussed in Section 9.5.5. One example is the optical supervisory channel (OSC), shown in Figure 9.1, a separate wavelength dedicated to performing control and management functions, particularly for line systems with optical amplifiers. Multiple EMSs may be used to manage the overall network. Typically each EMS manages a single vendor's network elements. For example, a carrier using WDM line systems from vendor A and crossconnects from vendor B will likely use two EMSs, one for managing the line systems and the other for managing the crossconnects, as shown in Figure 9.1. The EMS itself typically has a view of one network element at a time and may not have a comprehensive view of the entire network, and also of other types of network 9.1 Network Management Functions 499 elements that it cannot manage. Therefore the EMSs in turn communicate with a net- work management system (NMS) or an operations support system (OSS) through a management network. The NMS has a networkwide view and is capable of managing different types of network elements from possibly different vendors. In some cases, it is possible to have a multitiered hierarchy of management systems. Multiple OSSs may be used to perform different functions. For example, the regional Bell operating companies (RBOCs) in the United StatesmVerizon, Southwestern Bell, Bellsouth, and U.S. West (now part of Qwest) use a set of OSSs from Telcordia Technologies: network monitoring and analysis (NMA) for fault management, trunk inventory and record keeping system (TIRKS) for inventorying the equipment in the network, and transport element management system (TEMS) for provisioning circuits. These systems date back a few decades, and introducing new network elements into these networks is often gated by the time taken to modify these systems to support the new elements. In addition to the EMSs, a simplified local management system is usually provided to enable craftspeople and other service personnel to configure and manage individual network elements. This system is usually made available on a laptop or on a simple text-based terminal that can be plugged into individual elements to configure and provision them. 9.1.2 Information Model The information to be managed for each network element is represented in the form of an information model (IM). The information model is typically an object-oriented representation that specifies the attributes of the system and the external behavior of the network element with respect to how it is managed. It is implemented in software inside the network element as well as in the element and network man- agement systems used to manage the network element, usually in an object-oriented programming language. An object provides an abstract way to model the parts of a system. It has certain attributes and functions associated with it. The functions describe the behavior of the object or describe operations that can be performed on the object. For example, the simplest function is to create a new object of a particular type. There may be many types, or classes, of objects representing different parts of a system. An important concept in object-oriented modeling is inheritance. One object class can be inherited from another parent object class if it has all the attributes and behaviors of the parent class but adds additional attributes and behaviors. To provide a concrete example in our context, an OLT typically consists of one or more racks of equipment. Each rack consists of multiple shelves and multiple types of shelves. Each shelf has several slots into which line cards can be plugged. Many different types of line cards exist, such . Chlamtac, A. Ganz, and G. Karmi. Lightpath communications: An approach to high-bandwidth optical WAN's. 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