dge router BAS = Broadband access server (see also access chapter) Figure 8.14 Example of a convergence that has started from the datacom side (LAN, IP) and reached the multimedia stage (MM server), and that has incorporated voice (telephony server, PBX, access node) in between. Both voice and multimedia require servers at the control layer 8.5 INTRODUCTION TO PACKET DESIGN So far a heterogeneous situation with a number of transfer modes in a number of devel- opment tracks has been described. The heterogeneous situation demands gateways for the user plane and the control plane. However, the situation is converging towards a more unified packet mode. Packet design becomes a crucial task for all actors. In a wide sense the task also embraces the content of the payload, apart from all signalling and encapsulating protocols. In fact, the design and content of the packets travelling in the network tells us a lot about end-user needs and the telecom business. See Figure 8.15 that for example could initiate a discussion on VoATM versus VoIP. Thus, a particular service might be differently implemented by different actors, resulting in different packets. SMS has SO far been carried by circuit mode, but this will change to packet mode (Figure 8.16). From Figure 8.15 the conclusion is clearly that the basic packets are far from enough, when for example using stratification and tunnelling. We suspect that quite a number of inputs are involved in the almost instantaneous packet generation. This is confirmed when checking the infrastructure support in IP best effort networks (Figure 8.17). It is also confirmed when looking at the need for information at the start of a packet communication after attaching, let us say a mobile terminal in GPRS to a packet network. The information referred to is called PDP (packet data protocol) context. It might be a logical association between a mobile station and a public data network running across a GPRS network. The context may define aspects such as routing, QoS, security, billing, type of