The easiest way would be to create a physical representation of the fieldbus nodes and to provide this structure on the gateway. This approach is impressively simple and easy to implement but has the following shortcomings:
A common fieldbus abstraction is built around the value, which is the very basic element of fieldbus nodes. Echelon for example introduces a concept of so-called ``data points'' that basically represent pieces of information. These data points reside in a fieldbus node and contain the value that can be read and written with some accompanying properties, such as a timestamp, the data type or the status of the value as depicted in figure 28.
Common interface specifications for Internet/fieldbus gateways, as described in chapter 3, use very similar abstraction techniques.
Internet/fieldbus gateways may also aggregate data points to a structure that is presented to client applications. This way, data point elements in the gateway, such as the value, are only place holders63, for their counterparts in the fieldbus node. Such a data structure in an Internet/fieldbus gateway has to be configured, which means that fieldbus data is assigned to certain data points that are aggregated to some data structure in the gateway. The following basic data structures may be used:
The addressing scheme of data points in this structure is simple. However, every data point must have a unique address, which demands an address space that is big enough when high numbers of data points are assigned to an Internet/fieldbus gateway.
Hierarchical trees may be used to represent physical, geographical or logical structures. The addressing scheme itself describes the location of a data point in the hierarchical structure. The address itself consists of the child's address and all parent addresses, which are separated by delimiters65.
Fieldbus systems may have multiple viewpoints66, therefore Internet/fieldbus gateways may assign one data point to multiple data structures. This way, multiple data point trees may coexist in one gateway and offer clients different perspectives of fieldbus systems.