[2] The physical layer synchronizes a small packet of tones to the zero-crossing of the power line's voltage.
In multiphase power lines, a separate signal might be sent on each phase to speed up the transmission.
The standard's low speed is caused by the limited number of bits per power line cycle.
The speed is also limited by noise, and the local jitter of the AC line's zero crossing.
The high reliability comes from its reliable timing system (i.e. zero crossing), high signal to noise ratio (frequencies are chosen to avoid common power line noise), lack of intermodulation distortion, and adaptive signal detection.
(It is common for dirty insulators to arc at the highest point of the voltage, and thus generate a wide-band burst of noise.)
The silence and the preamble allow the receiver's signal processing to measure the channel's noise ratios.
The higher link-layer strongly resembles HDLC, except with a novel feature that allows selected stations to retransmit messages.
S-FSK (spread frequency-shift keying) is a modulation scheme that combines some of the advantages of classical spread-spectrum modulation (immunity against narrow-band interference) with some of the advantages of classical frequency-shift keying (FSK is low complexity).