This part of IEC 62055 specifies the application layer protocol of the STS for transferring units of credit and other management information from a point of sale (POS) system to an STScompliant payment meter in a one-way token carrier system. It is primarily intended for application with electricity payment meters without a tariff employing energy-based tokens, but may also have application with currency-based token systems and for services other than electricity. It specifies: • a POS to token carrier interface structured with an application layer protocol and a physical layer protocol using the OSI model as reference; • tokens for the application layer protocol to transfer the various messages from the POS to the payment meter; • security functions and processes in the application layer protocol such as the Standard Transfer Algorithm and the Data Encryption Algorithm, including the generation and distribution of the associated cryptographic keys; • security functions and processes in the application layer protocol at the payment meter such as decryption algorithms, token authentication, validation and cancellation; • specific requirements for the meter application process in response to tokens received; • a scheme for dealing with payment meter functionality in the meter application process and associated companion specifications; • generic requirements for an STS-compliant key management system; • guidelines for a key management system; • entities and identifiers used in an STS system; • code of practice for the management of TID roll-over key changes in association with the revised set of base dates; • code of practice and maintenance support services from the STS Association. It is intended for use by manufacturers of payment meters that have to accept tokens that comply with the STS and also by manufacturers of POS systems that have to produce STScompliant tokens and is to be read in conjunction with IEC 62055-5x series. STS-compliant products are required to comply with selective parts of this International Standard only, which is the subject of the purchase contract (see also Clause C.11). NOTE Although developed for payment systems for electricity, the standard also makes provision for tokens used in other utility services, such as water and gas.