 with the service requirements covered in this chapter You here what is said. You here who is talking. This requires a sufficient signal level without too much noise or echo. The quality is often measured by letting a test panel compare the sound between telephony from different sources and rating the perceived quality. The average rating is called the MOS (mean objective score) value. ITU-T Recommendation P.800 provides the following scale: 5 = excellent; 4 = good; 3 = fair; 2 = poor; and 1 = bad. The current target for VoIP is class 4, which is the same as ordinary circuit-mode voice systems. Another current target for VoIP is called carrier grade. It means extremely high reliability with a down time of about 5 minutes per year, known as five-nines reliability (99.999%). Understanding Changing Telecommunications - Building a Successful Telecom Business Delay and Jitter Delay is an important area for conversational real-time services like telephony and video telephony. There are two aspects regarding delay: Total delay. Delay variation or jitter. Voice is an isochronous service, which means that the decoded signal should have the same frequency and frequency stability as the sent signal. Deviations depend mainly on impact from active elements in the path, but also on rerouting. If the stream of information is uneven (also called 'jittery') or the values arrive in disorder, it becomes difficult to hear what is said and who is talking. What is called adaptation protocols in the enabler part are intended to cancel the deviations, and to restore the isochronous relation. Jitter is eliminated by means of delaying packets individually to restore the original intervals. The price to pay is an increased overall delay. The process is called 'dejitterization'. The total delay gets worse at certain occasions: Connections over a long distance (especially when one or more satellite link(s) is used - a satellite hop adds a delay of about 240 ms). Voice is coded using advanced compression algorithms to save bandwidth (as in a 