Introduction to the STS

This document provides a brief introduction to the Standard Transfer Specification.
WHAT IS STS ? STS is a secure message system for carrying information between a point-of-sale and a meter, and is currently finding wide application in electricity metering and payment systems. STS is not limited to this application (see the section: Future enhancements), but because of the wide interest in this area, this document describes the electricity application.

STS is a South African industry specification described in *NRS 009-6, section 6 to 9, and part 7 which are being considered for publication as publicly available standards through IEC TC 13 working group 15 †

The STS specification does not specify any physical characteristics (such as shape or size) of a prepayment meter.  Please note that only the following three specification documents are applicable for STS certification:  NRS009-6-6:2002; NRS009-6-7:2002 and NRS009-6-9:1997  - any other documents linked from the STS website are for general background information (or as implementation examples) only.  The STS is currently in the process of being adopted by IEC as IEC 62055 parts -41 and -51 and will be published under these numbers in due course.  However, these IEC documents have NOT YET been officially released and consequently the NRS documents, listed above, still constitute the official STS specification at this time. STS Articles of Association


The sketch above illustrates a typical system. The example illustrates a consumer making a payment at the vending station and a token being generated to pass the information concerning this payment on to the meter. To date, disposable magnetic card tokens, as well as number based tokens for keypad entry at the meter, are using the STS standard to transfer the information.

STS is centred around the information transferred to the meter - this includes the manner in which the vending station encodes the token with the information, and the way in which the meter interprets the information.

WHY USE STS? Critical to the successful operation of any payment system are the issues of security and inter-operability of equipment supplied by different manufacturers. STS fully addresses these key issues:


Security issues are of prime importance to the utility supplier and the consumer. The use of the STS standard prevents:

  • Fraudulent generation of tokens from hit and miss attempts at entering the correct number;
  • Fraudulent generation of tokens from a stolen vending station;
  • Fraudulent generation of tokens from legitimate vending stations outside of the utility's area;
  • Fraudulent use of tokens which have already been used;
  • Tampering of legitimate tokens e.g. to change the value
STS provides the facility of generating (e.g. credit transfer) tokens which can only be used by the intended meter, and furthermore in the case of credit tokens, can only be used once in that meter.

In order to achieve the above security, the standard defines the following:

  • the use of advanced encryption techniques, which are at all times hidden from the consumer -the system is easy to use.
  • the use of very secure key management procedures, including the manner in which keys are generated and transported.
  • Required functionality at both the vending station and the meter.
Inter-operability Users have long struggled with various systems performing the same or similar functions but which are not compatible with one another, and with components of these systems which are not inter-operable with those from different manufacturers or suppliers.

STS is an open system specification which defines transfer tokens which may be generated at a vending system from any one of a number of suppliers, and used in STS compliant meters from any one of a number of suppliers. The functionality at the vending station and the response of meters to certain transfer numbers is also specified in order to achieve inter-operability. This allows utilities to mix and match equipment from suppliers of their choice.

The Association ensures inter-operability between system components from different manufacturers, by:

  • Ensuring that manufacturing members make encryption keys available to other manufacturers at the request of the utility using the equipment;
  • Accrediting and maintaining a list of equipment test laboratories which ensure correct STS functionality of equipment;
  • Ensuring consistent manufacturer identity codes and meter serial numbers
WHERE IS STS TODAY? Where did STS start? The STS Standard Transfer Specification was initiated by Eskom (the major South African electricity utility) in 1993 through their need to develop compatibility between meters and vending systems from different suppliers. Also paramount was the need to ensure sufficient system security to prevent fraud. The STS venture has been a considerable success, and has enabled different manufacturers vending systems to provide compatible tokens/credit transfer to any STS compliant meter (from a number of manufacturers). How widely is it used? A large number of manufacturers are producing vending stations, meters, encryption key management equipment and complete systems. Almost 2 million certified STS compliant meters are in use in South Africa today, as well as almost 1000 vending stations. Internationally, the system is in use in many countries and on all continents.

What is the STS Association? During 1997 the STS Association was formed to take over the STS technology, maintain the necessary infrastructure, promote the technology internationally, and further develop the standard to meet emerging international demands for additional functionality.

It is a non profit association of members, championed by the founder members: Conlog (Pty)Ltd., Energy Measurements (Pty)Ltd., Eskom, and Schlumberger Measurement and Systems (Pty)Ltd.

The main motivating factors for establishing the Association were as follows:

  • Transfer the responsibility for the maintenance and enhancement of STS to the manufacturers;
  • Provide the manufacturers with a stake in STS and thereby ensure that they support its use outside the context of Eskom;
  • The Association would be better placed to provide STS support services to parties outside Eskom.
Objectives include:
  • STS standardisation and enhancement;
  • Technical guides;
  • Accreditation testing;
  • Maintaining an approved accreditors register
  • Maintaining an approved list of STS-compliant equipment;
  • Key management and key security and certification.
Membership: All parties that are interested in the technology and who agree to participate in furthering the aims and objectives of the Association are encouraged to Join the Association. Interested parties could include utilities (electricity, water, gas and other), vending equipment or meter manufacturers, standardisation entities, consultants, academics and others involved in this type of market/technology.

All categories of members are able to influence/participate in the administration and future development of the technology.

Categories of membership The Association has 2 categories of members (apart from the founding members):

  • Ordinary members - these are members, typically manufacturers, requiring access to the confidential tables and manufacturing information. These members are required to enter into a special agreement which covers the safeguarding of the information confidentiality.
  • Associate members - these are members such as user utilities, academics and interested parties who do not require access to the confidential tables and manufacturing information. They pay lower Association fees than Ordinary members.
Ownership of the technology

The STS Association owns the STS technology and licences manufacturers to use it.

Security The STS Association carries out the necessary functions to assure the security of STS systems. These include the following:

  • Obtaining the agreement of manufacturers on rules regarding the handling of confidential information and procedures
  • Approving equipment and processes for generating and transferring encryption keys
Future Development

Technical working groups will further enhance STS to meet the emerging needs of users.

FUTURE ENHANCEMENTS Additional Functionality Development of additional functionality is envisaged e.g. the transfer of tariff information and currency credits rather than Kwh transfer; the use of 2 -way communication; etc.

New Applications In addition to the electricity utility application discussed above, STS can easily be expanded to include other utilities such as gas and water.STS transfer numbers, rather than being hand carried, may be transmitted using various communication standards such as DLMS ‡

* NRS - The National Rationalisation of Standards body is responsible for co-ordinating user requirements on a national scale prior to these being tabled as national specifications. † IEC - The International Electrotechnical Commission is an international body which generates standards relating to electrical systems - It's Technical committee TC13 is responsible for electricity metering, and working group WG15 for payment systems. ‡ DLMS - 'Device Language Message Specification' is a protocol allowing meters and meter system components from different manufacturers to communicate with one another.