lity measures informing this trade-off have been standardised. One such measure is the perceptual evaluation of speech quality (PESQ) scale which compares different voice cod- ing schemes with a benchmark of a clean analogue reference [8]. Another method is the mean opinion score (MOS) which averages the perception of listener on the scale of 1 (bad) to 5 (excellent). Table 5.4 shows a typical MOS comparison for several voice coders. As expected the perceptual quality degrades as lower bit rates are used to represent voice. The new wideband AMR codecs also can have a much better quality of voice reproduction and are used in new wireless standards such as 4G voice over LTE (VoLTE) [9]. Lower call quality generally results in unsatisfactory communications and may result in early call termination, therefore reducing potential operator revenue. Indeed it has been shown that telephone calls tend to last longer if the voice quality is high, and shorter if the quality is low [11]. Adaptive rate coding allows a network operator to use the best quality source coding schemes during periods of low traffic to deliver the highest possible perceptual quality. When the network is congested, the coding rate can be reduced to accommodate more calls. Case Study 5.2: Skype Skype is primarily a VoIP application, launched in 2003 to enable peer-to-peer telephony. Initially designed for personal computer (PC)-to-PC communications, enhancements have been made to enable instant messaging, phone-PC telephony, mobile application, teleconferencing and video telephony. Furthermore, Skype has entered the WiFi hotspot market, and is estimated to provide Skype subscribers access to 2 million hotspots glob- ally. As of August 2013, the number of subscribers was more than 300 million [12]. PC-to-PC Skype telephony, or the technology of VoIP, has remained free and has been the main reason behind its subscriber growth. VoIP services existed long before Skype. The first VoIP services were offered in the mid-1990s, and ITU VoIP standard H.323 was pu