ABSTRACT
The standard application models are fairly easy to congure, and they provide an adequate level of detail for modeling commonly used applications such as e-mail, FTP, and remote login. However, the standard applications only model a two-tier communication paradigm and do not allow for modications of the simulated application protocols. To address this issue, OPNET also provides facilities for modeling custom applications, which could represent nonstandard, multitier applications that follow a user-dened protocol. For example, the standard application model for e-mail only simulates a two-tier client-server message exchange, where the client periodically sends newly written messages to its e-mail server and also independently polls the server to retrieve e-mails destined for it. However, in real life, an e-mail application behaves slightly differently. The main steps in sending an e-mail message from one client to another include the following:
• Client A writes an e-mail message to client B. • Client A uploads this e-mail message to its e-mail sever. • The e-mail server for client A relays the message to the e-mail server for
client B. • Client B downloads the e-mail message sent by client A from its e-mail
server.
