Hi,

Here are few definitions regarding Single Tier and Multi Tier Concept in ORACLE.

Oracle Single-Tier

The Single-Tier ODBC Drivers for Oracle are a single component installed on a data-consuming machine - depending on the architecture in which the driver is deployed, either a client such as a desktop, a workstation or an application-server machine. No components need be installed on the database server.
The Open Link Single-Tier Drivers for Oracle are a Type-B class of driver developed using the Oracle CLI.

The Oracle drivers for versions 7, 8 and 9 require SQL*Net, Net-8 or Net-9 installed. For Oracle 10g and above, we use the Oracle Instant Client, which obviates the need for the RDBMS-specific comms layer.

Oracle Multi-Tier

The Open Link Multi-Tier ODBC Driver for Oracle comprises two parts. On the server you install a request broker which handles incoming connections, and a database agent which makes the connection to the database itself.
On data-consuming machines (clients or application-servers), you install a generic client. This approach presents an extra layer of abstraction, such that by installing one component on each client, you can connect to several servers, or several different databases via the same broker.
Multi-tier is ideally suited to larger installations where the network administrator wishes to handle security rules and performance configurations centrally.

The Open Link Multi-Tier Drivers for Oracle are a Type-A class driver developed using the OCI call level interface.

For your reference i have attached the Images for Single-Tier and Multi-Tier.

Attached Images

	ORACLE Single-Tier Architecture Diagram.JPG (32.5 KB, 3 views)
	ORACLE Multi-Tier Architecture Diagram.JPG (42.0 KB, 4 views)

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-Murali..