rnet protocols are implemented in a much more modular way than PSTN protocols. Chapter 1 Conclusions We have seen that the PSTN implements the intelligence of the system in the network rather than in the terminals. In addition, signalling related to call establishment is tightly tied to management of the user plane. This overloads protocols and makes them less general for use in another type of network. Taking the previous statements into consideration, we infer that the par- adigm behind SS7 does not suit IP networks, which are designed for maxi- mum flexibility. SS7 is an excellent performer in circuit-switched networks but cannot take advantage of the very flexibility that IP networks provide. Therefore, signalling protocols to be used in IP environments need to be designed keeping the IP paradigm in mind. It will be seen that SIP suits it very well. CHAPTER Packet Switching, IP, and the IETF Chapter 2 In this chapter, we will learn how the Internet is built, how packet switch- ing works, and how Internet protocols are designed. This information is vital context for understanding the motivation behind the way the Session Initiation Protocol (SIP) works. In addition, a familiarity with how the Internet Engineering Task Force (IETF) works emphasizes how mature SIP and its different extensions really are. It is important for basic IP literacy to know the process that every Internet protocol follows before becoming an Internet standard. Packet Switching We saw in Chapter 1, "Signalling Protocols in the Circuit-Switched Net- work," that circuit switching was originally developed for analog transmis- sions. Circuit switching provides a dedicated path from source to destination that is suitable for transmitting continuous analog signals such as voice. Frequency Division Multiplexing (FDM) was the most widespread multiplexing technique in circuit-switched analog networks. The appearance of digital technology enabled circuit switching to evolve. Information is transmitted between systems in binary format using O's and 