such as positioning services. 5.4.1 CONVERSATIONAL CLASS 1. Video telephony - Video telephony is a real-time service, which is the most demand- ing type. Due to the nature of video compression, the BER requirement for video is even more stringent than that of speech. It is indeed very important to distin- guish between conversational real-time services and other services, because of the development towards multimedia paired with the original basic mismatch between the conversational services and the packet-based technologies. 2. Voice telephony - The integrity of the existing telephony service is a key concern for service providers. The following distinction is useful, recognizing that classical telephony and IP telephony are really two different services: - Classical telephony, i.e. the existing PSTN telephony service with its characteristics and service features, irrespective 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 gr