## 2 Comparing the Telco Approach to Internet Approach  
### The Telco Approach, the Evolution of the PSTN  
When we take a quick look at the evolution of the telecommunication networks over the past hundred years, we can see it is a prodigious evolution. At present the telecommunication networks face another step in their more or less dynamic development.  
Let us explain the main principles of the PSTN architecture. The end terminal is of a limited intelligence, as most of the activities are ensured by the network components. Originally call-control signaling serves for setting up the circuit (previously done by the telephone exchange). This process precedes the communication itself. Several types of interfaces and PSTN signaling are defined: DSS1 (Digital Subscriber Signaling System No. 1) for the User to Network Interface (UNI), and ISUP/SS7 for the Network to Network Interface (NNI). Each digital telephony exchange has software implemented and is coupled with hardware that basically ensures switching on a centralized basis. The telecommunication network has a hierarchical structure.  
The next stage in the telecom networks development was the development of ISDN technologies, the aim of which was the integration of various types of services within one network, as it follows from the denomination \"Integrated Service Digital Network\". ISDN can be considered to be one of the first attempts to create an integrated network for many digital services. A very important step in the development of telecommunication networks was the introduction of Intelligent Networks (IN), which allows the use of the existing infrastructure for providing new services implemented within the framework of IN platform (with service logic) on top of existing PSTN/ISDN without the need to implement these services practically into all exchanges of the network. Number portability and carrier (pre-) selection are examples of successful services made possible by IN.  
A specific case, coming also from the telecommunication world, was a design of broadband ISDN, at present known under the name Asynchronous Transfer Mode (ATM), which was a second significant attempt to create a universal network based on packet transmission. The question as to whether one should attempt to create another universal Next Generation Network (NGN) based on the IP protocol and internet technologies, is justified.  
The main reason for using the IP protocol is its universality and it conforms with our goal to use a similar infrastructure for both telco and internet services. This has happened probably due to the expansion of the internet and data networks, as well as the development of new technologies that enable voice transmission via packet networks (mainly on an IP basis). The internet itself as a global network has become widespread and allows new forms of communication and services (e-mail, ftp, www, _etc._), but basically is not suitable for real time voice transmission. This is especially due to the fact that in the Transmission Control Protocol (TCP)/IP and User Datagram Protocol (UDP)/IP protocol combinations intelligence is concentrated in terminal equipment (PCs, servers), rather than in the network, so cannot easily guarantee quality of service.