during a limited time period, or at a lower price. However, such solutions might need financial perseverance and are not considered is this chapter. The opposite solution, at least at bearer level, is to over-dimension the network, which would reduce ROCE. Regarding the QoS business model, let us return to the holiday sales at the beginning of Chapter 5. It was argued that the travel operator does not sell holidays by promoting the features of the plane. He sells the holiday. However, it would certainly be possible to have different travel classes for charter trips, similar to the differentiation in hotel standards. Service QoS Bearer QoS e2e QoS = For example: Adaptations (ALL parts) Computational resources Media mixing resources Enablers: CC/PP RTP, RTSP, Diffserv (examples) (WAP) ATM AAL 802.1p Figure 7.2 Perceiving QoS at service terminating points by sets of QoS enablers Quality of Service By paying a different overall price to the travel operator including flying standard class, the airline company gets paid for the provision of different classes of transport. 7.2.2 HISTORY QoS has always been an important part of telecommunication. Various measures have been used. The probability of congestion for traditional voice trunk calls is still indicated in the number of congested calls per hundred call attempts. Analogue circuits were particularly expensive and investments were expressed in number of circuits. Other measured circuit characteristics related to service quality in telephony networks are echo, cross talk, signal- to-noise ratio and lately bit error rate (BER). The characteristics were directly associated to the selected circuit, as well as the bandwidth, which was fixed and standard for all calls. A fairly common current perception of QoS is wider than, but not too far from, what has traditionally been called 'The transmission plan'. This plan caters for end-to-end perceived quality of voice in circuit-mode networks, originally in analogue networks and later in digital networks. The broad introduction