Error Analysis

Step 1: Function of a modem.
A modem is a device that performs modulation and demodulation of signals.
Step 2: Modulation and demodulation roles.
During transmission, the modem acts as a modulator by converting digital signals into analog signals. During reception, it acts as a demodulator by converting received analog signals back into digital form.
Step 3: Conclusion.
Hence, the modem acts as a modulator in transmitting mode and as a demodulator in receiving mode.