s are call waiting, call transfer, and conferencing. These supplementary services are undeniably useful and many users take advantage of them. However, gaining access to them is pretty complicated. Because the interface used for this purpose is usually the keypad of a telephone set, the user has to learn different codes for different services. If the service is not accessed very often, remembering codes becomes difficult. A non-atypical example of service invocation is typ- ing "*22# telephone number #" in order to turn on call forwarding, compli- cated enough that it will probably require the user to look up procedures before nearly every forward. We will see that the introduction of better user interfaces not only makes for more frequent usage, but also makes protocols evolve more rapidly, since the users can use newly introduced features in a more friendly way. ISUP supplementary services are implemented in a distributed manner. Users gain access to new services when the local exchange to which they are connected is upgraded to support new features. Therefore, service avail- ability depends on the local exchange and its business plan. Moreover, not all users of the network have access to the same services since all the local exchanges of the network cannot be upgraded at the same time and with the same version of software and the same hardware. Even users accessing the same service might expect different performance behavior, depending on which switch they are requesting the service from. Intelligent Network (IN) Services These limitations of ISUP with respect to service provision prompted implementation of the Intelligent Net- work (IN) services, which utilizes Intelligent Network Application Protocol (INAP). Although ISUP services were distributed, IN services are clustered Signalling in the Circuit-Switched Network in a centrally located node. This node, called Service Control Point (SCP), can be accessed by any user in the network, SO providers can deliver new services simply by upgrading the node-a big ad