There are some technical differences between PPM and PWM but on all recievers, including PCM receivers, the signal sent from the receiver to the servo is PWM, i.e. a positive going pulse that varies from 1 to 2 milliseconds in width depending on input command occuring once every frame of transmitted signal, typically every 15 to 25 milliseconds. In general the frame rate of PPM is faster than PCM, i.e. the servo is updated more often using PPM than it is using PCM although the difference is to small to worry about.