Since the advent of computer data networking, large and small business corporations, the banking industry, government, and military organizations, and today even the personal home computer user have all raised the concern for network security. Extensive research and development efforts have been exerted to develop sophisticated firewall and intrusion detection technologies. Most of these technologies are designed to monitor the gateways of local networks to the public Internet. Unfortunately, they rarely scrutinize the activity on the network itself.

Because most networks are not being monitored inside their gateways, they are vulnerable to intrusion from dial-up modem access to any device residing on the network. A Telephone Intrusion Detection System was designed and developed to defend against this vulnerability. The system was developed using a commercially available DSP board and T1 interface card for monitoring Primary Rate ISDN trunk lines. Primary Rate ISDN circuits provide the ideal monitoring point because they transport all phone calls between a central office switch and a facility's private branch exchange. The associated application integrated software that extracted detail call information such as calling number, called number, call direction, call plan and call blocking pertaining to each call and software that discriminated call type as either voice, fax or modem. With this capability, the system demonstrated its ability to perform as a firewall to block unauthorized call types such as attempts to place modem calls on voice-only telephone lines. When an unauthorized call is determined, the application sends a call termination command to the private branch exchange via a serial port connection.

The objective of this system is to eliminate the backdoor entry to a local network by only allowing remote dial-up connectivity to authorized connections such as Remote Access Server that would monitor the connection and protect the network.