The name has become more generic over time to refer to symmetric service at a variety of rates over a single loop. Once encoded, an inverse process called demodulation may recover the original signal. Modulation is performed to adapt the signal to a different frequency range than that of the original signal. The modem is necessary because the phone network transmits audio, not data bits. The modem is for compatibility with existing equipment.
Attenuation is signal loss due to the diminishing availability of signal energy, or signal power.
As a analog or digital signal traverses across a medium, it fades. High attenuation may lead to the inability to recover the signal on the far end. Signal repeaters may be used on the transmission path to periodically boost the signal strength. Baseband transmission is extremely limited to attenuation. Broadband much less so.