This European Telecommunication Standard (EN) specifies the stage 3 description of the Supplementary Service Discreet Listening (SS-DL) for the Terrestrial Trunked Radio (TETRA). The DL supplementary service enables an authorized user to listen to one or more communications between TETRA subscribers Mobile Station (MS) without any indication to any user that the communication is being monitored. As options@ the authorized user is able to intrude into the existing call and is able to forcefully clear the monitored call without call owner consent (in the case of a group call). Man-Machine Interface (MMI) and charging principles are outside the scope of the present document. The supplementary service stage 3 description is preceded by the stage 1 and the stage 2 description of the service@ according to the method described in ITU-T Recommendation I.130 [i.2]. The stage 1 description specifies the service from the user's point of view. The stage 2 description identifies the functional capabilities of SS-DL and the information flows needed to support the supplementary service as specified in its stage 1 description. And the present stage 3 description specifies the protocols at the air interface and at the various Inter-System Interfaces (ISI) to support SS-DL. NOTE: According to ITU-T Recommendation I.130 [i.2]@ the stage 3 description of any telecommunication service addresses the network implementation aspects. Consequently it comprises two steps: the specifications of all protocols at the various reference points involved in any of the service procedures (notably the service operation) are the first step of the stage 3 description@ and the specifications of the functions of the corresponding network entities are its second step. The latter have not been provided since they can be derived from the specification of the functional entity actions in the stage 2 description. The present document is applicable to Voice plus Data individual call or group call; more specifically to the following entities: ? the MS of the monitoring user during an individual call or a group call; ? the monitoring user Switching and Management Infrastructure (SwMI) in an individual call or a group call; ? the group controlling SwMI and the participating SwMI for a group call; ? the monitored user present SwMI for an individual call; ? and@ optionally@ the home SwMI of the monitoring/authorized MS@ for managing the supplementary service DL.