With the implementation of ETCS Level 2, suppliers of ETCS on-board and Radio Block Centre (RBC) subsystems are facing different challenges. The UNISIG specifications describe interfaces, functions and quantitative safety targets for ETCS on-board sub-systems. However, requirements for the RBC only refer to technical interoperability and quantitative safety targets. Functional requirements for the RBC must thus mainly be derived from the customer specifications and from the RBC hazard analysis. Using the example of granting a first movement authority, this article demonstrates how requirements are derived by systematically taking into account transmission and computation delays at RBC interfaces and how implementation variants arise under consideration of different railway infrastructures.