A smart modem can dial, hang up and answer incoming calls automatically. Can…

Concept: a modem is a peripheral that carries out commands; it does not originate them. A smart (Hayes-compatible) modem exposes a command interface — the AT command set standardised as ITU-T V.250 — on its serial line, and whichever host program owns that interface decides when the device goes off-hook, dials, answers a ring or hangs up. Call control therefore lives in the program running on the computer, while the modem contributes the signalling and the line hardware.

Application: the communication software on the computer opens the serial port, puts the modem into command mode and writes AT strings to it:

1. ATDT 24680 — take the line off-hook and dial the number using tone dialling.

2. ATS0=1 to arm auto-answer, or ATA to answer a ring that is already in progress.

3. +++ followed by ATH — escape from data mode back to command mode and hang the line up.

The modem firmware parses each string, performs the line operation and replies with a result code such as OK, RING or CONNECT 9600. Every one of those instructions is composed and timed by the program running on the computer, so the appropriate instructions for dialling, answering and hanging up are provided by the communication software.

Cross-check — what each of the other candidates governs:

• Error detection protocols (MNP 4, ITU-T V.42): how blocks of user data are checksummed, acknowledged and retransmitted across a connection that is already up.

• Link access procedure (LAPM inside V.42, LAPB on X.25): data-link-layer framing that numbers, sequences and acknowledges frames on an established link.

• Telecommunications: the engineering field and the carrier plant — subscriber lines, exchanges and switching networks — across which the signals travel.

Each of those three acts on a call that already exists or names the surrounding infrastructure, so the source of the dial, answer and hang-up instructions is the communication software on the host computer.