Scope of application This Recommendation presents the possible elementary functions for the maintenance of the subscriber installation. The functions are to be considered as optional@ except when needed to meet specific network interface requirements found in Recommendations I.430 and I.431. These functions can be controlled by the local side (e.g. from the subscriber premises) and by a remote side [i.e. from an MSP (maintenance service provider)@ as described in Recommendation I.601]. It is the responsibility of the subscriber installation to ensure that only authorized MSPs are given access to the following functions.