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Information transfer is an essential factor in automation systems. For reasons of practicability, the development of industrial communication networks focused on horizontal aspects, i.e., the provision of suitable automation networks on the various levels. Conversely, vertical integration has become a topic of interest only in the recent past and mostly concerns the interconnection of field-level networks and IP-based office-type networks. Such interconnection can reasonably be achieved via gateways. This survey paper reviews concepts and defines a set of criteria that are relevant for the design of such gateways. Furthermore, it compares the applicability of common Internet communication standards like SNMP, LDAP, SQL, native web technologies, OPC XML-DA, and OPC UA as higher-level access protocols to field-level data, thus providing a basis for vertical integration on an application level. For comparison, a dedicated proprietary protocol is discussed as well.