System Administration Guide: Advanced Administration

Modem Description

Modems can be set up in three basic configurations:

A modem connected to your home computer might be set up to provide dial-out service. With dial-out service, you can access other computers from your own home. However, nobody outside can gain access to your machine.

Dial-in service is just the opposite. Dial-in service allows people to access a system from remote sites. However, it does not permit calls to the outside world.

Bidirectional access, as the name implies, provides both dial-in and dial-out capabilities.