This section of IEC 870-6 considers the standards related to layers 1-4 of the OSI reference model. Its objectives are as follows: – to describe the role and the functions carried out by each layer; – to list the relevant ISO basic standards; – to give general considerations and recommendations for the use of the standards in the application context of part 6. The use of these standards in their application to end systems (ES) and intermediate systems (IS) is described. An ES is one which contains all seven layers of the OSI reference model as well as an application process. An IS is a communication relay between sub-networks. ESs and ISs are considered in IEC 870-1-4 and IEC 870-6-1. The description of the functions carried out by each layer is intended to be fairly complete but makes no claim to being exhaustive. For more details, the relevant sections of ISO 7498 should be consulted as well as the listed ISO basic standards. Second, the ISO standards listed are the fundamental ones related to each layer; again, the list does not claim to be exhaustive. Finally, the general considerations and recommendations are intended to present the basic issues involved in the application of the standards and to lay down the fundamental manner in which they are to be used. The purpose is to codify the ground rules for making the specific comprehensive and detailed choices necessary for the definition of the functional profiles which are the subject of part 6-5 and subsequent parts. This section is organized according to the OSI layers. Because there is a strong interdependence in the choice of protocols in the three lowest layers, these layers are grouped into clauses of this section. Within these clauses, the organization is according to the type of transmission network. The treatment of each layer contains the following elements: – introduction: briefly describing the layer's function and role in the overall communication process; – reference documents; – services; • list of services and QOS parameters included in the standards; • guidelines and recommendations concerning the choice and use of these services; – protocols; • list of protocol classes, subsets, etc. included in the relevant standards; • guidelines and recommendations concerning the choice and use of these protocols.