A UE accessing the IM CN subsystem, and the IM CN subsystem itself, utilise the services provided by the DOCSIS cable access network to provide packet-mode communication between the UE and the IM CN subsystem.
From the perspective of the UE, the necessary IP-CAN bearer for signalling is transparently available to the UE.
The UE is not directly involved in requests for IP-CAN bearer(s) for media flow(s). The IM CN interacts with the PCRF in the DOCSIS IP-CAN to establish IP-CAN bearer(s) for media flow(s), on behalf of the UE.