12.1. Introduction
As carriers build global and national networks, it becomes necessary to systematically partition these networks into control domains, that is, subnetworks in which a particular control paradigm or protocol is employed. Control domains are often referred to simply as domains (see Chapter 5).
From a network control perspective, the motivations for dividing a network into domains are:
To define administrative boundaries
To allow for scalability of routing and signaling
To isolate portions of the network for security or reliability, and
To accommodate technology differences between systems, for example, by partitioning a single carrier's network into separate single vendor subnetworks
Figure 12-1 illustrates the partitioning of a network into three domains. An interdomain interface is the signaling and routing interface between two network elements (or controllers, see Chapter 7) in different domains.[1] An intradomain interface is the signaling and routing interface between two network elements in the same domain. These interfaces are illustrated in Figure 12-1. An interdomain interface is likely to have different characteristics than an intradomain interface as the domain boundary exists for the purpose of hiding certain aspects of the domain from the outside world.
[1] Such nodes are also called border nodes (see Figure 12-1).
Figure 12-1. Partitioning a Network into Domains
The domain concept as used here is orthogonal to the concept of layering in transport networks. In the transport network, layers are technology specific and are used for multiplexing, performance monitoring, and fault management, with different layers providing different capabilities in each of these areas. A routing protocol for optical networks must include information particular to the technology layer in which it is being used.
In the rest of this chapter, we describe interdomain control requirements and routing architectural models. Presently, interdomain control is at early stages of standardization and experimentation [ITU-T02b]. Thus, the following descriptions are conceptual and high-level rather than being protocol oriented as in earlier chapters.