One of the challenges in designing distributed applications is the data distribution model, which consists of all participating systems and the messages to be exchanged by the system. Data distribution in R/3 to R/3 environment is easily achieved but there are several bottlenecks in R/3 to non-R/3 scenarios – for example data mapping - as non-R/3 systems seldom have as rich an organizational representation as R/3 has. The following figure illustrates the concept of data distribution using ALE.

Enterprise business objects of a SAP system are distributed with the help of BAPIs (Business Application Programming Interfaces). ALE acts as a layer that incorporates the inter-linking of business objects at the application level. The ALE layer provides the following services:

• Applications services: this layer provides ALE with an interface (for instance: BAPI) to R/3 to facilitate data exchange to or from external R/3 systems.
• Distribution services: the onus of filtering and converting messages exchanged between SAP and non-SAP systems is on the distribution layer of ALE. This service is the core service and acts as a sandwich layer between application and communication layers.
• Communications services: ALE supports synchronous as well asynchronous communication. Synchronous messaging is used for the direct reading of control data, while asynchronous messaging is used for transmitting or receiving application data.

The implementation of ALE is achieved with the help of external converters (ALE converters), which connect SAP applications to non-SAP applications. In simple words, external converters are generic-format conversion programs, which contain specific converter functions. The capabilities of the external converter will be discussed later in this article.