pective of whether it is run over circuit or packet-switched (ATM and/or IP) networks. Conversational (voice, videophone) Streaming Interactive Background Transactional Messaging (SMS, MMS etc) Figure 5.7 Services classes used in this book Services - IP telephony, i.e. voice services defined as part of multimedia standards, run over IP and provided separately or as a component of a multimedia service. Conversational Mode Requirements Conversational mode is used when two or more persons are involved in a conversation demanding short response times and a certain and stable quality. Some main examples are voice conversation (e.g. telephony), audio conferencing and video conferencing. The main requirements are: Voice quality - low, medium or high ('hi-fi') depending on the application and usage cost. The technical correspondence is the frequency range of the transferred voice. Traditionally 300-4000 Hz has been a requirement among network operators, specified by the ITU. Picture quality - low, medium or high depending on the application and usage cost. Quality can be described primarily by the following parameters: - black/white including different grey scale granularities or colour with different fidelity levels, for example normal colour (8-bit representa- tion), high colour (16 bit), true colour (32 bit) - resolution represented by pixels/square inch. Normal alternatives are 640x480 (lowest quality), 800x600, 1024x768, 1280x1024 and 1600x1200 (high- est quality). Delay - minimal/low. The delay consists primarily of network propagation delay (which can never be zero), also by buffering, format conversions (e.g. A/D and D/A conversions) and any other type of handling of the information stream. The ITU has in recommendation G.114 specified the following values for one-way delay for voice (telephony): - 0-150 ms - acceptable for most user applications 150-400 ms - acceptable provided that the operators/administrations are aware of the transmission time impact on the transmission quality of the user application - 400+ 