tions, Administration, and Maintenance (OA&M) ANSI-41 allows for control of the MSC-to-MSC trunks used for intersystem handoffs. 4 The trunks can be removed and reinstated, and they can be configured for loop-back testing. This capability only affects intersystem handoff, not roaming and not ordinary call completion. An MSC-to-MSC link has four states: Active Locally blocked Remotely blocked Locally and remotely blocked The trunk is locally blocked if it is removed from service at the local end, and it is re- motely blocked if it is removed from service at the other end. Loop-back testing has one end of a normal two-way trunk looped back at one end, as shown in Fig. 15.3, so that the signal received is immediately transmitted back the other way. Once the loop-back is in place, the trunk can be tested by sending signals along the trunk and making sure that the same signal comes back the other way. 15.5 Over-the-Air Service Provisioning (OTASP) OTASP allows a subscriber to get wireless telephone service without having to visit a retail establishment. When OTASP is working properly, it makes service activation easy, it avoids programming errors from manual data entry, and it reduces the costs as- sociated with operating retail wireless phone-center stores. Using only the radio link, the base station transmits encrypted data to the user ter- minal in order to program its number-assignment module (NAM). Typically, the poten- tial subscriber who already has a mobile telephone speaks to a highly trained service representative to obtain service. Financial information is provided by the subscriber and confirmed by the sales representative, and then OTASP is used to activate service for the user terminal. Over-the-air parameter administration (OTAPA) makes it possible to adjust pre- ferred roaming lists (PRLs), to update intelligent roaming databases (IRDBs), and to change the mobile identification number due to area-code changes. OTAPA works whenever the user terminal is powered on and requires no input from the subscribe