vice is 1 in 100 calls lost during the busy hour. Other elements to be listed under QoS are: Can connectivity be achieved? Delay before receiving dial tone (dial tone delay). Post dial(ing) delay (time from the completion of dialing the last digit of a number to the first ring-back13 of the called telephone). This is the primary measure of signaling quality. Availability of service tones [e.g., busy tone, telephone out of order, time out, and all trunks busy (ATB)]. Correctness of billing. Reasonable cost of service to the customer. 13 Ring-back is a call-progress signal telling the calling subscriber that a ringing signal is being applied to the called subscriber's telephone. 1.5 STANDARDIZATION IN TELECOMMUNICATIONS Responsiveness to servicing requests. Responsiveness and courtesy of operators. Time to installation of a new telephone, and, by some, the additional services offered by the telephone company. One way or another, each item, depending on the service quality goal. will have an impact on the design of a telecommunication system. 1.5 STANDARDIZATION IN TELECOMMUNICATIONS Standardization is vital in telecommunications. A rough analogy is that it allows world- wide communication because we all "speak a standard language." As the reader progresses through this book, he/she will find that this is not strictly true. However, a good-faith attempt is made in nearly every case. There are international, regional, and national standardization agencies. There are at least two international agencies that impact telecommunications. The most encompassing is the ITU (International Telecommunication Union) based in Geneva, Switzerland, which has produced literally over 2000 standards. Another is the International Standardization Organization (ISO) that has issued a number of important data communication standards. Unlike other standardization entities, the ITU is a treaty organization with more treaty signatories than the United Nations. Its General Secretariat produces the Radio Regula- tions. This document set is the