Separating the Control and Switching Planes

Let's take a different approach to looking at the characteristics of multiservice switching networks. This section goes one step deeper into explaining these virtual networks by delving into the switch architecture. How does a switch need to be designed in order to build these networks?

We can start by studying a provider that offers different services, such as voice and ATM, over separate networks. Analyzing the telephony switches and ATM switches that make up these infrastructures, we find that typically they are dedicated pieces of equipment in which the control plane in those switches is indivisible from the switching plane. Moreover, in the software architecture of those switches, usually the control logic is aware of the different cards in the switch and the specifics of the interfaces and even drivers. This section investigates the advantages that a modular approach like the one outlined in Figure 1-4 brings.

Figure 1-4. Modular Architecture of a Multiservice Switch

The multiservice switch shown in Figure 1-4 has different modules or components. Their functions are outlined in Table 1-1.

Table 1-1. Multiservice Switch Modules and Their Functions

Module	Function
Controlled switch	This is the module responsible for traffic switching.
Controller(s)	This is the module responsible for managing the controlled switch. It acts like the "brain" of the controlled switch.
SCI	Switch Control Interface. The interface that manages the communication between the controller and the controlled switch.

The following parallels can be made:

• The controller is analogous to the system's "brain."

• The controlled switch is comparable to the "muscles" of the system performing the switching.

• The SCI connecting the preceding two items is equivalent to the "nervous system," bringing information from the switch to the controller and commands from the controller to the switch.

This analogy is shown in Figure 1-5.

Figure 1-5. Functions of Each Constitutive Module in a Multiservice Switch

The controller operates independently of the controlled switch. Different controllers operate completely independently of each other. Resource partitioning (discussed later) enables the multiservice functionality of the controlled switch.

This architecture provides separation of routing and forwarding, meaning routing running in the controller and forwarding performed by the controlled switch. This gives you the ability to design the best routing software controlling the best switch, which could be practically impossible otherwise. This also presents the framework for implementing technologies that rely on that separation of routing and forwarding, such as MPLS.