 network element, only Media stream components Framing Framing Connectivity Connectivity Media gateway or (label) edge router Figure 11.1 Position of the edge node/media gateway and the principle for a conversion stack in the lower part Understanding Changing Telecommunications - Building a Successful Telecom Business, by A. Olsson C 2003 Ericsson och Student litteratur. ISBN: 0-470-86851-1 Understanding Changing Telecommunications - Building a Successful Telecom Business is necessary in order to understand the new network. The subject of edge nodes/media gateways was introduced in Chapter 3. This chapter will be complemented by a media gateway dimensioning to be found in Appendix 4. 11.1.2 TERMINOLOGY The term media gateway (MGW) has nothing to do with for example transmission media. It stands for a conversion of the media streams of voice, data, video or multimedia information. However, this is not necessarily what the MGW does. A true conversion of for example voice would imply a recoding, let us say from AMR to PCM (pulse code modulation) code. It is true that a media gateway contains coders, but the professional network designer tries to avoid recoding for QoS reasons (time) and for security reasons (the packet must be unpacked). So, the information or media stream can often be retained intact, whereas the transfer mode might change. Transfer mode can be seen as the way of packing information. An example is repacking PCM voice from TDM into ATM AAL1 cells. A more correct term is media framing. See the lower part of Figure 11.1. For a repacking of circuit mode voice to packetized voice the term circuit emulation is often used. An 'all-access' emulation to ATM is shown in Figure 11.2. Another common term is softswitch. This is a generic term for any open application pro- gram interface (API) software used to bridge a public-switched telephone network (PSTN) and a voice over Internet path (VoIP) by separating the call control functions of a phone call from the media gateway. The separation is usually performed 