18 August 1998
Figure 3-9. Central or Island Station LAN using a Star Topology.
a. A dial-up modem with auto answer security callback and manual originate capabilities will be
used for remote interface between the central or island station and a remote location, such as the UMCS
supplier's diagnostics facility. The modem allows the supplier's personnel to perform system diagnostic
checks and programming from their facilities. However, the security callback feature terminates the
connection after auto answer, and then automatically dials a previously established and programmed
number, preventing dial-in access to the system from unauthorized locations. The modem's manual
originate capabilities allow on-site maintenance personnel to communicate with the supplier's home office
to transmit data as required to resolve field problems. The manual originate capabilities are also used by
utilities/UMCS operations personnel to establish communication between a central station and an island
station in the event that the normal network communication circuits fail.
b. Network modems provide communication between geographically separated LANs or between a
remote processor, such as an operator workstation, and a LAN. In some systems, this function is
provided by LAN devices referred to as remote bridges.