he same service, but with different capabilities. The implemented new networks will probably lend many features from the described customer optimal architecture, but many actors want to retain a view of being special, and will continue with partly vertical features. A differentiation is sometimes necessary, especially in the complex control layer, in order to cope with different existing signalling systems that are connected to the new backbone. See Figure 3.17. Differentiation in the service layer arises when one operator deliberately tries to differentiate itself from the others. There are, indeed, also some prob- lems with open networks regarding confidential or company-sensitive information about the user (location, personal etc.), which may impact the transition. The first breakthrough for a layered view within telecentric networks was the intro- duction of SS7, the connectionless data network for signalling within PSTN, ISDN and PLMN. The separation of signalling and user traffic was also a step to split the network in a more functionally oriented way. The real 'vertical' networks of today are also layered to a considerable extent, in par- ticular at the transmission level. This makes the migration process cheaper and simpler. Let us take IP networks as an example, looking from the application down the stack. The power of TCP/IP applications is that the application generally can assume a simplis- tic view of the network (a basic packet-carrying network). This is because the complexity of the network is hidden from the application: the error-free end-to-end transmission chan- nel offered by TCP and the fast, 'best-effort' packet-sending interface offered by UDP are there for the application to use. The complexity of addressing, routing, re-transmissions, etc. is implemented in the TCP/IP protocol stack. The migration process implies a convergence process, since the target network is quite similar for the various vertical networks. When shifting from vertical to horizontal, we Understanding Changing Telecommu