lity is obtained by imple- menting end-to-end services in which a network failure won't lead to a loss of state information essential for the service. As long as the end systems do not crash, the service can still be provided. Access Tightly Tied to Service Provision We have seen how PSTN signalling has evolved throughout the years, and why its evolution has been closely related to that of the user plane. When new multiplexing techniques or transmission mechanisms are created for the user plane, signalling pro- tocols have to evolve to exploit the new mechanisms. Traditionally, new sig- nalling protocols were developed in order to carry new information about the user plane. Signalling protocols specify the types of user plane that can be used in conjunction with them in detail. For instance, international ISUP defines three types of user plane: 64-kbps unrestricted, speech, and 3.1-kHz audio. This is because, in the PSTN, access to the network and service provi- sion is coordinated. Therefore, if a user accesses the PSTN, the services he or she is able to access will be those provided by the protocol used for access. If DSS-1 is used, for instance, the caller will be provided with a voice channel. He or she cannot then access the PSTN and request another kind of service. In the next chapter, we'll see that on the Internet environment, access and service provision are completely separated. A user can (and often does) connect to the Internet through a particular Internet Service Provider (ISP) and get e-mail services through an Internet portal. Access provider and ser- vice provider in this situation are different. As one might expect, the Internet promotes a different approach to design signalling protocols from the PSTN. We will see that SIP does not restrict the number of types of sessions that can be established. A caller can use SIP to establish a VoIP session, but he can also use SIP in order to establish, say, a gaming session. SIP could even be leveraged to establish a traditional telephone call. Thus, Inte