demon dialing
The use of a computer to dial a number repeatedly (usually to a crowded modem pool) in an attempt to gain access immediately after another user has hung up.
demon dialing: automated redial for modem access
Demon dialing is an automated technique where a computer program repeatedly dials a telephone number at high frequency, typically targeting a shared modem pool or access line. The goal is to catch a connection the instant another user disconnects, seizing the available slot before human operators or competing automated systems can. This was particularly prevalent in the 1980s and 1990s when dial-up internet access was scarce and modem pools were heavily congested during peak hours.
The practice works because modems in a pool answer calls sequentially. When one drops a connection, there is a brief window, sometimes only milliseconds, before that line becomes unavailable again or another caller connects. A demon dialer eliminates human reaction time by issuing redial commands in rapid succession, sometimes dozens per second depending on the modem and telephone line speed. The program typically logs connection attempts, records which numbers answer, and monitors for successful handshakes with the target system.
Technical constraints and side effects
Demon dialing's effectiveness depends on several factors. The speed of the modem itself limits redial frequency, as does the telephone network's ability to process repeated connection attempts. Most public access providers quickly blacklisted numbers showing signs of automated dialing, and telephone companies began charging premium rates for lines receiving high volumes of incoming calls. The technique also created significant load on switching equipment and tied up trunk lines unnecessarily.
The term "demon" derives from Unix system administration, where background processes running without user interaction were colloquially called demons. The implied relentlessness and autonomy of a daemon process mapped neatly onto the behavior of an automated dialer working tirelessly in the background. By the mid-1990s, as internet service providers expanded capacity and broadband became available, demon dialing became obsolete; modern systems use queuing and callback features instead.
Some service providers responded by implementing rate limiting, call throttling, and account lockouts triggered by excessive redial attempts. These countermeasures are now standard in telecommunications infrastructure and have applications beyond modem pools, including protection against auto-dialing attacks on voice systems.