SAP Gateway 
Purpose
The SAP Gateway carries out CPI-C services within the SAP world, which are based on TCP/IP. These services enable SAP Systems and external programs to communicate with one another.
CPI-C services can be used either in the ABAP program or for the external programs via the interfaces.
As RFC (remote function call) is based on CPI-C, all RFC connections also pass through the SAP Gateway.
Implementation Considerations
In the SAP System,
an SAP Gateway is started for each application server
(see 
Architecture of the
SAP Web AS).
For certain constellations, an SAP Gateway is used as a separately installed SAP instance or used outside an SAP System:
· Communication with an R/2 System (CUA interface to R/2)
· Starting external partner programs on remote systems which do not recognize any remote shells (Windows)
· Application-specific decoupling of communication (in certain cases)
You should install the SAP Gateway in the same way as you install all other SAP programs, that is in the executable files directory of the corresponding system tree.
Integration
The following topics are related to the gateway environment:
System Configuration
for Parallel RFCs
Features
The SAP Gateway documentation is divided into the following sections:
Architecture of the SAP Gateway
Monitoring and Error Handling of the SAP Gateway