where message creation begins. end-user protocols such as ftp, http, &  smtp work on this layer  * measured in messages
this layer is where message creation-and, therefore packet creation begins. db access is on this level. end-user protocols such as ftp, smtp, telnet, and ras work at this layer.
end user of software that is used to accomplish a variety of tasks. 1. general purpose applications. 2. specialized applications.  3. mobile apps.