vance in service manage- ment. Services are implemented in the SCP with scripts-a set of rules and instructions that contain the particular service logic. Scripts come in two types: system and group. The system script performs number analysis in response to the number entered by a user, and then calls the proper group script to execute the actual service logic. Distributed throughout the network are a number of IN service triggers called Service Switching Points (SSPs). An SSP analyzes the number entered by the user and decides whether intelligent network services are needed. It then contacts the SCP, which in turn determines the actions required to provision the service invoked. The Paradigm Behind SS7 We've already touched on how signalling in the PSTN has evolved from its analog origins advanced. Now let's analyze the architectural principles behind the telephone network and how they differ from those used to design the Internet. This comparison will go a long way toward explaining why SIP is a true revolution, and not an evolution of existing signalling systems. Two main architectural differences exist between the PSTN and the Internet: Intelligence in the PSTN is concentrated in the network rather than the terminals. Access to the network and the services provided are tightly related. Intelligence Inside the Network SS7 makes a clear distinction between terminals and the network. It operates in the network-trunk sig- nalling-while other protocols like DSS-1 bridge the network and its ter- minals. SS7 proposes an architecture that enables the use of very simple terminals with limited responsibilities. On the Internet, the opposite holds true. Intelligence is pushed to end-user equipment, and the network is kept as simple as possible. We will see in a later chapter that this enables still faster and more flexible implementations of new services. Chapter 1 Following out the logic of the Internet paradigm, a signalling protocol should support intelligent end systems instead of building a network that makes decis