SONET and SDH-based optical transport networks have been deployed extensively. It is therefore important to understand the fundamentals of these technologies before delving into the details of the control plane mechanisms. After all, the optical network control plane is a relatively recent development. Its primary application in the near term will be in SONET/SDH networks. In this context, it is vital to know about the low-level control mechanisms that already exist in SONET and SDH and how they help in building advanced control plane capabilities. The next chapter continues with a description of another key topic relevant to the control plane, that is, protection and restoration mechanisms in SONET and SDH networks. Following this, the subject of modern optical control plane is dealt with in earnest.