Data access arrangement

The term data access arrangement (DAA) has the following meanings: Data access arrangements are an integral part of all modems built for the public telephone network.

While DAA now describes an integral component of a device that connects to the telephone network, during the 60s and 70s it described a separate device mandated by the Bell System, connected between the telephone line and non-Bell equipment, typically a modem.

Following the Carterfone decision, which required Bell to allow customers to attach any non-harmful equipment to their network, Bell mandated that subscribers use PCAs/DAAs - purchased exclusively from Western Electric - to ensure the network was protected.

[1][2] At the time, some subscribers believed that the DAA was a scheme by AT&T to penalize and discourage use of non-Bell modems and recover lost profits from hardware sales,[3] and the FCC began investigations into the legality of the practice.

A manual DAA required a call to be initiated (or answered) as normal, at which point it could then be connected to the third-party device, while an automatic DAA allowed an attached device to be connected without human intervention, important for receiving modem use.