An Asynchronous Communications Server (ACS) is a dedicated personal computer (PC) or expansion card that allows other network nodes to access serial ports or modems
. An ACS server allows users to dial into a long-distance telephone network
from local area networks. They can also use modems, fax cards, serial ports, or other asynchronous network components across networks. Asynchronous Communications Servers are also called dial-in/dial-out servers or modem servers.