 the Terms of Use as given at the website. Quality of Service (QoS) Quality of Service (QoS) We can think of latency and jitter as statistical moments, average and variability. There is some mean delay in a data connection, its latency, and some standard devia- tion or variance, its jitter component. This is a simplification because the distribution of packet times is often a far cry from a bell-shaped statistically normal distribution. However, it gives us a visualization of the issues in maintaining QoS under time constraints. The third impairment in a data system is loss of information. A packet can be lost be- cause it simply does not get there, or it can be lost because it gets there with errors, as we discuss in the next section. In either case, a data connection loses some fraction of its packets. Since a subscriber pays good money to have his or her data arrive safely, it sounds negligent to admit that we lose some of them. However, it is not as terrible as it sounds because voice and video services are adequate with losses or errors as high as 1 percent of the bits, and end-to-end data subscribers have their own software applica- tions to ensure that their data losses are detected and either repaired or resent. The three impairments, latency, jitter, and loss, interact with each other in con- tributing to service impairment. Consider a voice telephone call where a total delay of over 200 ms from speaker to listener starts to become annoying. A wireless voice-coded data stream goes from one user terminal through a radio link to a base station, through a backhaul pipe to a mobile switching center (MSC), from the MSC through the public switched telephone network (PSTN) to a telephone, a landline telephone in this exam- ple. The voice data stream might tolerate a bit error rate (BER) of 1 percent. This means that 99 percent of the packets have to be both correct and timely. If 0.5 percent of the bits have errors and 0.5 percent of the packets take too long, then we have our 1 percent BER. If we design a pack