IEC 62056-42:2002 specifies the physical layer services and protocols within the Companion Specification for Energy Metering (COSEM) three-layer connection oriented profile for asynchronous data communication. In annex A, an example of how this physical layer can be used for data exchange through the Public Switched Telephone Network (PSTN) using intelligent Hayes modems is given. Annex B gives an explanation of the role of data models and protocols in electricity meter data exchange. This bilingual version (2013-05) corresponds to the monolingual English version, published in 2002-02.