Client server architecture can be categorised into 2-tier, 3-tier and n-tier (multiple tier). Client/server models started being adopted since late 1980’s. At the time, client/server models were mainly in 2-tier in form of file server. The function of the server was mainly to transfer files from the shared storage to desktop clients connecting to the server. The configuration can be shown in the figure below. 
This approach was mainly applied with X-base style programmes such as dbase, foxpro and clipper etc.. With the development of multi-user X-base technology in 90’s and the emergence of the GUI in desktop computers, the file server in the client/server model was gradually replaced with database server. The main communication between the clients and server were mainly SQL. Since 1992, various tools including Microsoft Visual Basic, Borland Delphi and Sybase’s PowerBuilders were developed to be adopted in 2-tier client/server models. The 2-tier client/server configuration at the time can be shown below:- 
The performance of the 2-tier architecture was satisfactory for connecting clients less than 100. The performance of client/server with 2-tier architecture in large organisation with number of clients exceeding 100 becomes unacceptable. There were two major limitations with 2-tier architecture at the time. They were the “live connections” between the clients and servers and the lacking of flexibility and compatibility with different database management systems from proprietary vendors such as Oracle, Sybase and Infromix etc. The “keeping alive” connection situation between the clients and servers limits the number of connecting clients in the models. On the other hand, any change in database management system would require all the procedure codes at the server to be re-generated manually. The above limitations resulted in the introduction of an additional tier, the third tier (Schussel, 1995).
Reference
Schussel, G. (1995). Client/Server:Past, present and future. Retrieved March 9, 2008 from http://www.dciexpo.com/geos/dbsejava.htm
