This object is used to specify the type of test to run for the ISDN-PRI D channel or all channels. There are no results stored locally for a loopback test. Loopback of the whole ISDN-PRI physical user port is supported where all the D and B channels are looped back simultaneously, along with individual loopbackof the D channel. AAL2 Loopback is also supported on the D channel. In order to run these tests, the CO-IWF must first set the value of this object appropriate to the test that needs to be run. The CO-IWF should then set the value of ifAdminStatus in the corresponding ifEntry to 'testing'. The CO-IWF should verify that the test is in progress by checking that the value of ifOperStatus in the corresponding ifEntry is equal to 'testing'. Once testing is complete the CO-IWF must set the ifAdminStatus to a value other than 'testing'. Note that the LES device must not allow a CO-IWF to manipulate the ifAdminStatus of physical user ports that do not belong to its managed CP-IWF.