Modems

CEMP-E

TI 811-12

18 August 1998

Computer

or

Peripheral

Hub

Computer

or

Peripheral

Figure 3-9. Central or Island Station LAN using a Star Topology.

12. MODEMS.

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.

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