Abstract: A method and apparatus for providing troubleshooting capability in a four wire telephone line employing a V.32-type or other unbalanced modem in a local communication link at one or both ends of the telephone line for a data network. Each modem has its own transmitter and receiver. An activatable but normally inhibited analog loopback is installed externally of the modem within the local link with which the modem is associated, with no loopback connection between the transmitter and the receiver within the modem itself. The external loopback is activated while the modem is induced to transmit a call connection signal, so that by virtue of the external loopback the signal is fed back to the receiver of the same modem to permit testing of the operativeness of the modem and the related portion of the link. The external loopback may be activated from terminal equipment on the local link with which the modem is associated, or may be activated from a point of the network remote from the local link.

Abstract: Security of communications in a packet-switched data communications system is enhanced by introducing terminal and host security devices into the system in communicative relationship with a terminal and a host processor, respectively. In response to a user-initiated data entry at the terminal, the terminal security device generates an initial data packet indicative of user authorization or not, but which is unsuited for processing by the addressed processor, ahead of additional data packets containing user-entered message data to be processed by the addressed processor. The host security device intercepts and processes the initial data packet and, if user authorization is indicated therein, replaces it with an artificial data packet solely to render the additional packets amenable to processing by the addressed processor and thereby to establish a communications session between user terminal and processor-associated database to which access was requested.