 vendors and standardization bodies. A truly horizontal network is built up by autonomous layers. Each layer has its own management, control (if any) and resources. Each layer is built on standards-based components and the interfaces between the layers are also standardized. A layered architecture allows each layer to evolve independently as technology evolves. Multiple service networks can share the same transport network. The horizontal network architecture can provide all types of communication and appli- cation services, only restricted by the capabilities of terminals and the used access. Applications will/should thus be independent of any core/radio technology/fixed access technology. In such a service-converged situation users from GSM, UMTS, PSTN/ISDN, CATV etc. will be able to use the 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 si