date, cellular-access networks have been optimized in a two-dimensional space Understanding Changing Telecommunications - Building a Successful Telecom Business VolPoW IP service flexibility WCDMA with ROHC Pure IP Ideal solution Spectrum efficiency WCDMA circuit switched voice Speech quality Figure 7.3 A main target is QoS, also spectrum efficiency and service flexibility whose axes are voice quality and spectrum efficiency. IP introduces a third dimension in terms of service flexibility, but adds protocol overhead, contradicting the goal of spectrum efficiency. See Figure 7.3. The major challenges ahead for VoIP are QoS and interworking with legacy networks and systems. In fact, very few issues regarding telecommunications, if any, have raised such strong and passionate polemic as QoS. Few telecom areas have been targeted by such intense research and frenetic development since the mid-1990s. 7.3 PERCEPTION OF QOS 7.3.1 SERVICE REQUIREMENTS The eye and ear are sensitive to delays in interactive conversation, but maybe even more sensitive to delay variations. They are less sensitive to bit errors, especially the ear, since one bit represents very little information, and the ear/eye/brain will fill in small missing information. For multimedia, lip synchronization (= limited skew) is important. The computer on the other hand isn't very sensitive to delay, but even one bit error can be devastating. See Figure 7.4. As already mentioned, an overall goal of this chapter is to support the implementation of a service plan as far as QoS is concerned. Considering the present power of the user, let us start with the user perception of service quality. See Figure 7.5. 7.3.2 TELEPHONY Perceived Quality Perceived quality end-to-end at application level is what matters for the user. What is it that determines whether we perceive the quality of a phone call as good or bad? Quality of Service Transparency = Bandwidth Delay Delay variations Listen Listen Bit error rate Figure 7.4 Different demands from different terminating points