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The present document defines the stage 3 specifications of the Supplementary Service Pre-emptive Priority Call  
(SS-PPC) for the Terrestrial Trunked Radio (TETRA). 
SS-PPC enables a user to have preferential access to the network resources in a TETRA system in times of congestion 
including pre-emption of calls. SS-PPC is applicable for pre-emptive priorities including the emergency priority.  
SS-PPC includes the capability to pre-empt resources needed for higher priority calls and the capability to pre-empt 
users from ongoing calls in order to move them to higher priority calls. SS-PPC specifies the definition, activation, 
deactivation and interrogation for the usage of pre-emptive call priorities in the TETRA system. The Switching and 
Management Infrastructure (SwMI) applies the SS-PPC priorities when it allocates the resources for calls. The SS-PPC 
operations are defined for the SwMI and for the Mobile Station (MS). 
SS-PPC is defined to subscribers of one TETRA system, but the subscribers may be located in several TETRA systems 
and the information flows may be delivered over the Inter System Interface (ISI). SS-PPC is invoked for calls within 
one TETRA system or for calls that extend over ISI to several TETRA systems. 
Man-Machine Interface (MMI) and charging principles are outside the scope of the present document. 
Supplementary Service stage 3 specification is preceded by the stage 1 and the stage 2 specifications of the service. 
Stage 1 describes the functional capabilities from the user's point of view. Stage 2 defines the functional behaviour in 
terms of Functional Entities (FEs) and information flows. Stage 3 gives a precise description of the supplementary 
service from the implementation point of view. It defines the protocol for the service and the encoding rules for the 
information flows. It defines the processes for the FEs and their behaviour. The described protocols and behaviour 
apply to the SwMI, for the MS and may be applied over the ISI between TETRA systems. Aspects relating to all 
supplementary services are detailed in ETSI EN 300 392-9 [3].