This part of IEC 62055 specifies a physical layer protocol of the STS for transferring units of credit and other management information between a client (typically a HHU) and a server (an STS-compliant electricity payment meter), typically over a direct local connection. It is complementary to the application layer protocol specified in IEC 62055-41 and should be used in conjunction with that standard. This standard is not applicable to payment metering systems employing monetary-based tokens, complex tariffs and currency-mode accounting functions. It is only intended to support the STS functionality as defined in IEC 62055-41 and it does not support the additional functionality required for extended use that includes monetary-based tokens and complex meter functions such as tariffs, real time clocks and currency-mode accounting. If such extended use were required in the future, then it would need new work on this part of IEC 62055 as well as on IEC 62055-41. It is intended for use across a range of payment meters developed by different manufacturers and to ensure compatibility between these products and other client devices. It specifies a client/server communications protocol that: • transfers STS-compliant tokens from a client device to a payment meter server; • reads the result from the payment meter after transfer and execution of the token; • transfers management data from the client device to the payment meter server; • reads management data from the payment meter server and transfers same to the client device. NOTE Although developed for payment systems for electricity, this standard can also be applied to other utility services, such as water and gas.