MAC protocols for PLC networks must overcome difficulties stronger than those found in other systems such as the need to cope with several hops and with very noisy environments. Polling protocols are widely used in PLC based AMR (Automatic Meter Reading) systems but they present several drawbacks, as their inherent impossibility to automatically associate new meters to the system. Contention protocols over PLC do allow for automatic association of new meters but they suffer from low efficiency basically due to the multihop nature of PLC based AMR systems. A combination of both types of protocols seems a wise option. One possibility is to partition the time into two periods, one for polling and another one for contention, but it requires resource consuming network-wide synchronization in a system that must be especially inexpensive. Another possibility is to superimpose both types of communication in a proper manner. The choice has advantages and disadvantages and affects also the topology control function. A simulation model of the PLC system has been carried out based on OPNET Modeler. The paper presents this model and results that basically compare the two aforementioned alternatives of protocol combination. An analytical model developed by the authors in a previous work is used to complement the simulation, allowing for the assessment of the system behavior in a stable topology situation.