You can find SAP Basis introduction documents on here
Category: SAP Basis
SAP Basis Fundamentals
SAP R/3 is 3 tier architecture. You can see below picture. SAP Basis architecture presentation is here
About of Database Systems and Information Technologies
You can find SAP Basis introduction documents on here
SAP R/3 is 3 tier architecture. You can see below picture. SAP Basis architecture presentation is here