ng System 7 (SS7) before actually switching the trunks. This is discussed in Chap. 14. Every step of this process has alternative outcomes. The callING party may not com- plete a full telephone number, the line may be busy, or the callED party may not pick up the receiver. There are alternatives internal to the telephone network, but they hap- pen far less often. There can be equipment failures, or all the circuits can be busy. The addition of custom calling features, call waiting and three-way calling, adds a number of other possibilities to the sequence of events during a call. However, it is im- portant to remember, in all the complexity and sophistication of telephone network de- sign, that the primary mission of the telephone network has been to complete voice calls from one person to another. 12.2 Quality of Service Telephone service can be evaluated in several ways, but there are three basic areas where a call can go wrong. It can fail to complete, it can fail while stable, or it can sound 12.2.1 Ineffective attempts Users make calls, and some of them do not get through. The simplest case is when the callED party does not answer or the line is busy. These events, however, are not fail- ures of the telephone system. If the call does not go through due to a failure of the tele- phone system, it is called an ineffective attempt on the part of the telephone system. And when the telephone system fails to complete a call because it has no more re- sources to devote to calls, then we say the call is blocked. Blocked calls are discussed in some detail in Chap. 23. A wireless telephone system may have the resources to complete a call, but the radio link is too poor to set up the call. The radio link can fail because its call setup signal- ing fails or because the voice quality is below the minimum standard. Code division multiple access (CDMA) technology designers face a tradeoff: The system can deny ser- vice to a caller when the system is getting close to its maximum level, or it can serve the caller at the risk 