ontent assurance and dimension reception billing value/attitude (peer) Roles dimension QoS, Se SM = Service Application logic, Application management ROCE etc. NM = Network Stable economic management TCP/IP FP = Fundamental technical plans Backbone Invest- ments Figure 3.38 XML used as middleware for B2B communication. B1-B3 = Business processes 3.11.3 LAYERING DRAWBACKS When trying to satisfy everyone's needs the result can be that nobody is happy. In optimized vertical networks with a minimum of overhead and a firm control over the whole stack, the quality tends to be better. Another problem is that with open horizontal layers there will be no differentiation among the actors. Therefore there is also a resistance, Networks and Technologies especially in the service layer. Considerable layering also initiates management challenges. See for example Chapter 12, which presents a layer interconnect protocol called GMPLS. 3.12 THE DISTRIBUTED NETWORK DIMENSION The distributed dimension of the network, for example according to Figure 3.41, must be designed for a multi-service offering with end-to-end security and QoS. Further the deregulated multi-operator environment requires agility and flexibility and a number of interconnection points. Interworking between nodes is provided by means of control sig- nalling or management/built-in control. 3.12.1 ENABLER EXAMPLES: SIP, SDP The session initiation protocol (SIP) is an application layer protocol. It is also a signalling protocol for setting up sessions between clients over a network, i.e. the Internet. These sessions do not necessarily have to be Internet telephony sessions. SIP could just as well be used for setting up gaming sessions or for distance learning where a lecture is streamed out to the participants. SIP can be transported over ATM, frame relay and X.25 as alternatives to TCP/IP. The service description protocol (SDP) communicates several session descriptors: Session name and purpose Media type (e.g. audio, video etc.) Media transport protocol (RTP/UDP/IP, 