The HFC (Hybrid Fiber Coax) is used for cable Internet access. HFC uses both coaxial cables and fiber. In downstream channel of HFC, every packet travels from head end to every home. In upstream channel of HFC, every packet sent by a home travels to head end.
Since the cable Internet uses a shared medium to transmit the packets, the HFC transmission rate is shared among the users.
Collision is occurred when multiple sources sent the packets simultaneously into the shared medium. For example, in upstream channel of HFC, multiple homes sent packets to head end simultaneously. Thus, collisions are possible in upstream channel.
But in downstream, every packet is sent by only single source called head end. Thus, collisions are not possible in downstream.