ACARS via Satellite
I work for the subcontractor manufacturer of the major players and we make Inmarsat Classic Aero H+ SATCOM equipment. Here is how it works.
The SATCOM locks onto the P channel signal (600/1200/10500 bps - its dependent on what the network broadcasts and the antenna type) coming from the ground network. This provides the timing reference.
When the SATCOM wishes to log onto the network, it sends a R channel burst (600/1200/10500 bps again dependent as above) to the network using random slotted aloha (and repeats if it does not get a response signal unit back).
When it wants to send an ACARS message, it uses this R channel burst to request resources from the network. The network allocates a timeplan to the terminal, and it then sends slotted T (600/1200/10500 bps) channel bursts on a network controlled frequency to the ground. The size of the bursts are 19 bytes each, so many will be needed to send an average ACARS message.
If the pilot wishes to talk to the ground, then the R Channel mechanism is used to ask for a call setup, then the network assignes a pair of C channels (8400bps), one in each direction, and another tranceiver in the box is turned on to pass voice. 1 or 2 C channels can be supported per plane. The MCDU is hooked up to the SATCOM unit (SDU) and a menu is presented to the pilot from the SDU to allow them to select who to call - the menu is aircraft specific.
Hope this helps.