EIS InfoBook Multi-[name] description, undated

NYC 9/11 Public Portal Document

the highest speed connectivity occurring over a local area network, thus improving performance. The public or private TCP/IP connection between the application server and the remote database server provides for the query and retrieval of data. Placing the application server on a LAN (meaning faster data exchange) means that the application can achieve a high level of processing (computer models, for example, of chemical reactions or weather conditions) or contain a subset of cashed data (for example geographic information system data) which it is not desirable to transfer over the network communications lines.

In the first case (the left hand part of the illustration) the presentation layer is very thin, located at a client site with the public or private TCP/IP network connection going from the client to the remote application server which is co-located (on a local area network) with the data server. In the second case we have just discussed, the presentation client and the application server are located on the same local area network, thus allowing for rapid exchange of large volumes of information, and are linked to the remote data server by the TCP/IP network. (A third model can exist in which a client is remote from the application server which is remote from the database server as well.) All of these implementations can be in use at the same time.

The third part of the multi-tier client/server architecture is illustrated on the far right of the illustration. In this case both a browser and application software can exist on a client workstation on a local area network. This will result in two type of database access taking place, both a three-tier and a two-tier client server. The three-tier uses the browser through the local (or remote) application server to the database server. At the same time, and in a different window, an application can be running with direct access to the data server (thus, a two-tier client/server).

Under what conditions might this be desirable?

Some applications are not suited to a three-tier approach. For example, a computer model that requires significant sensor input. Such a model would have sensor input coming in by RS232 ports into the computer that contains both the browser and the application. The application might very well be processing that sensor data. A second application might be a computer model used only by a handful of engineers that periodically employ the data from the sensors to create model output. If the model

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