An unusual question - to be honest we don't document the product in terms of the OSI layers so we can only give a very basic answer as we just don't have anything that can provide details of how Backup Exec uses the layers.
Most of Backup Exec sits at the Application Layer and makes requests down through the other layers as needed.
Here's where it gets a bit complicated though as some of our inter-process communication uses network protocols
For instance every session between the Backup Exec Job Engine and Backup Exec Remote Agent uses the NDMP Protocols and opens up requests using the NDMP ports (even when the activity in question is running against ONLY the Backup Exec Server. Which then means the communications goes down to at least that required for a localhost requests (I guess Network layer) and down to Physical if an actual remote system is involved (meaning the network card and cable is used)
Then you have to add in the device we are reading from to do a backup and where we are writing the backup to - which basically means for any one backup or restore operation you have requests all the way down to physical layer (either against the storage or the network) being a regular thing.
Sorry I can't really be more detailed than this.
