tell@oscar.cs.unc.edu (Stephen Tell) (10/24/89)
At Duke University, where I was a student until recently, they are installing a similar system designed by fellow EE and friend. Briefly, here's how it works. A weatherproof phone is installed by the front door; its line runs through the custom hardware located down in the basement. These are speakerphones of some sort, with DTMF dial and large buttons for 'on' and 'off.' A delay in the on/off hook makes manual pulse- dialing impossible, which is important (see below). The phone line is restricted to local calls only. A visitor calls a resident's 7-digit phone number, and asks to be let in. The resident presses '#' and a solenoid unlocks the door. The box down in the basement designed by my friend works somthing like this. It has a DTMF decoder, and listens as the visitor dials. Stored in EPROM are the phone numbers of all of the residents of that dorm. If there's a match, it listens for the resident to dial '#'. When it hears a '#', it breaks the line on the speaker-phone side, and makes sure that it is still hearing the '#', so only the resident and not the visitor can trigger the lock. Pulse dialing is forbidden because someone could pulse dial any number in town, then dial a number in the dorm in DTMF, and convice the person who answered to press the '#' key. Storing the number list in EPROM is ok because phone numbers are fixed in rooms by the Duke Telephone system, and don't change from year to year. (Duke owns a 5ESS, and is the largest private phone company in North Carolina, I'm told. Somthing like 15,000 lines) If phone numbers ever did change, they could run the thing over to the EE department and burn a new EPROM. Steve Tell tell@cs.unc.edu CS Grad Student, UNC Chapel Hill. Former EE/CS student, Duke University, Durham, NC