 corridors, and entrances. Some surveillance applications allow guards to press a button to unlock a door, and thereby remotely regulate admission to a building. Amtrak incorporates a unique video surveillance application to control and manage movable bridges in Atlantic City, New Jersey from a control center in Philadelphia, Pennsylvania. Four cameras, two on each side of a moveable bridge, relay pictures of the bridge to the control center. When a boat approaches the bridge, the control center performs a visual check for trains before remotely opening the bridge only long enough to let the boat pass. Before the use of surveillance technology, a bridge keeper was on location at all times. The installation pays for itself, and provides better visibility-the cameras provide four views of the bridge rather than the former bridge attendant's single view. Amtrak has since expanded the use of surveillance to Connecticut. A Houston, Texas company offers continuous surveillance services to its customers without the high cost of security guards. Its system remains dormant until someone passes through a photoelectric or infrared beam or until someone manually presses an alert button. Then, the system notifies an operator at the company's control center. The operator may use the audio function of the system to advise the intruder to retreat, or may notify local police. The system allows operators to identify false alarms. Moreover, since the system captures intruders on videotape, it thoroughly serves the interests of justice. Federal, state, and local law enforcement agencies used videoconferencing codecs to conduct security surveillance of both internal and external grounds during the 1996 Republican National Convention in San Diego. Stationed at the San Diego City Emergency Operations Center, the Joint Incident Command Post, and the San Diego Convention Center, law enforcement agencies simultaneously observed strategic areas, and surveyed the convention site and adjacent areas. Employing interactive, encryption-based co