Orignially developed for the military, anyone remember when 1024 PCM was considered missle code? That's what it was! This explanation is intended for voice communication, but the theory is the same, just a different media to convert, and please remember that PCM is a digital way of transmitting code instead of a nice signwave or analog PPM.
TDM/PCM Interface: The TDM/PCM interface carries the fixed bandwidth media signal. It consists of 32 data streams (128 timeslots/stream = 8Mbps/stream), an 8KHz Frame Sync Pulse, and an 8.192MHz clock. The data streams are segregated into sequential groups of four: Send In (SI), Receive Out (RO), Receive In (RI), and Send Out (SO). Each logical channel uses a timeslot from each of the four streams (SI, RI, RO, SO). Thus, the 4,096 timeslots on the TDM/PCM interface allow a maximum of 1,024 logical channels.
Hope you can see from the above that the time slots are many, convert that into individual position requests to be decoded to command the servos. Then Futaba has a short blurb on its PCM without getting into details. http://www.futaba-rc.com/faq/faq-pcm1024.html

Hope this mush helps.. Got to go detox now!