Scope of application This Recommendation covers the maintenance of that part of the ISDN subscriber primary rate access which is controlled by the network. The Recommendation follows the maintenance principles as defined in Recommendation M.20 and applies to the primary rate access connected to the local exchange. The scope of this Recommendation is to describe the minimum functions required to maintain the subscriber primary rate access. They are applicable to any primary rate access. The principle of controlled maintenance (as defined in Recommendation M.20) is applied for maintaining the subscriber primary rate access. Controlled maintenance is a method of sustaining a desired technical performance by the systematic application of supervision@ testing and performance sampling in order to minimize preventive maintenance and to reduce corrective maintenance.