The present document defines the Terrestrial Trunked Radio system (TETRA) supporting Voice plus Data (V+D). It specifies the air interface@ the inter-working between TETRA systems and to other systems via gateways@ the terminal equipment interface on the mobile station@ the connection of line stations to the infrastructure@ the security aspects in TETRA networks@ the management services offered to the operator@ the performance objectives@ and the supplementary services that come in addition to the basic and teleservices. The present part describes the security mechanisms in TETRA V+D. It provides mechanisms for confidentiality of control signalling and user speech and data at the air interface@ authentication and key management mechanisms for the air interface and for the Inter-System Interface (ISI). Clause 4 describes the authentication and key management mechanisms for the TETRA air interface. The following two authentication services have been specified for the air-interface in ETSI ETR 086-3 [i.3]@ based on a threat analysis: ? authentication of an MS by the TETRA infrastructure; ? authentication of the TETRA infrastructure by an MS. Clause 5 describes the mechanisms and protocol for enable and disable of both the mobile station equipment and the mobile station user's subscription. Air interface encryption may be provided as an option in TETRA. Where employed@ clause 6 describes the confidentiality mechanisms using encryption on the air interface@ for circuit mode speech@ circuit mode data@ packet data and control information. Clause 6 describes both encryption mechanisms and mobility procedures. It also details the protocol concerning control of encryption at the air interface. The present document does not address the detail handling of protocol errors or any protocol mechanisms when TETRA is operating in a degraded mode. These issues are implementation specific and therefore fall outside the scope of the TETRA standardization effort. The detail description of the Authentication Centre is outside the scope of the present document.