A data set is an older telecommunications term for a modem or similar interface unit that translates digital signals from terminal equipment into audio tones for transmission over the public switched telephone network. These devices were essential for connecting early mainframes and teletypewriters before the advent of direct digital subscriber lines. They typically sat on a desk next to the phone and required manual dialing.
The technician is coming out to swap the data set on the leased line because the terminal keeps dropping the connection to the mainframe.
Related terms
Data terminal equipment
Any end-user device that acts as the source or destination of digital information on a network.
Acoustic coupler
An early modem interface that connected a computer to a telephone network by physically cradling a telephone handset.
POTS
Plain old telephone service is the traditional analog voice transmission network using copper wires.