5.1. Introduction

The fundamental use of optical networks is to provide communication bandwidth from one geographic location to another. For instance, consider provisioning a link between two geographically separated IP routers. Such a link could be realized by establishing a SONET path layer connection between the routers through an optical network. This is shown in Figure 5-1, where the IP routers, R1 and R2, are designated as clients of the optical network. Figure 5-2 illustrates the details of the interconnections between O1 and O2 within the optical network, based on the discussion in the preceding chapters. Specifically, this figure shows OEO optical switching elements (called OXCs, see Chapter 1) and line systems that comprise the network. The establishment of a SONET path layer connection between the routers first requires the determination of the connection route within the optical network. Then, a series of SONET line layer connection segments must be established between the switches (LTEs) en route, and the switches must be cross connected properly so that an end-to-end connection is realized. This, in essence, describes connection provisioning in an optical network.

A provisioned connection may be subject to automatic protection and/or restoration (referred to as “recovery”). As described in Chapter 4, the recovery mechanism may be applied at each hop (e.g., linear APS) and/or end-to-end (e.g., path-level restoration). When end-to-end restoration is used, a physically diverse alternate route within the optical network must be determined, and the connection must be activated along the alternate path after a failure occurs along the primary path.

Both provisioning and recovery capabilities require control intelligence in the optical network. Such control intelligence may be centralized, say in a management system, with a well-defined communication interface between the management system and the network elements. On the other hand, the control intelligence may be distributed among the network elements. Or, a combination of the above methods may be used. Thus, an optical network consists of a transport plane, a control plane, and a management plane. The transport plane refers to the logic and hardware required for the physical transfer of data in the network. The control plane refers to the infrastructure and distributed intelligence that controls the establishment and maintenance of connections in the network. This intelligence is typically realized in the form of various communication protocols. Such protocols can be broadly classified into signaling, routing, and discovery protocols. The details of such protocols are covered in later chapters. The management plane refers to the systems, interfaces, and protocols used to manage the network and its services. In the rest of this book, the focus is mainly on the control plane. This chapter describes the high level architecture of the modern optical network control plane, which has evolved from IP, ATM, and MPLS control architectures. This evolution is described in this chapter. The following chapters contain detailed descriptions of the optical network control mechanisms. An overall roadmap is provided in this chapter to the material that follows.