hn Wiley & Sons, Ltd 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 by means of the H.248 protocol, presented in Chapter 3. See Figure 11.3. Concluding this introduction, the 'all-access network architecture' with a common core interfacing a multitude of access types and other networks, is the basic reason for the use Voice or data Voice or data PSTN, ISDN, is packed is taken out PSTN, ISDN, PLMN, IP, FR, X.25 into AT