MAC Protocols for PLC

MAC Layer

        The task of a MAC protocol is to manage the channel allocation / re-allocation between a number of PLC subscribers and transmission of different kinds of services.  The MAC protocol that need to support a PLC system has to provide features for realization of different teleservices that can be grouped as follows:

                                1.  Connection oriented service, like telephony and other CBR (constant Bite Rate) services

                                2.  Connectionless services without QoS guarantees (e.g. Internet)

                                3.  Specific PLC services

                                4.  Data transmission with QoS guarantees (like VBR - Variable Bit Rate - services)

        PLC networks must support the classical telephone service, internet, more sophisticated services with higher QoS requirements like home automation, energy management, security...  These should all be included into the MAC layer.

MAC Protocols

        A MAC protocol specifies a resource sharing strategy - access of multiple users to the network transmission capacity - applied to a multiple access scheme.

        To achieve the goals we have set above, we will not be able to use token passing or polling.  These two strategies do not maintain the QoS when additional network stations are added on.  Sending rights become longer making both protocols not suitable for the time-critical services.  So, we will use reservation MAC protocol.

        Because of similarity in signaling organization between PLC and wireless networks, caused by usage of similar transmission schemes and a similar disturbance scenario we can implement the same MAC protocols.  We will analyze ALOHA and Polling with dedicated reservation.

ALOHA:  Access time with ALOHA based protocols are significantly less than with polling when there are seldom transmission requests and when there is a small probability of collisions on the signaling channel.

Polling:  When collision probability is increased, with increasing network load or number of subscribers in the PLC network, polling protocol has a clear advantage.

        Both of these protocols can be improved.  The ALOHA based protocol can be improved with a small probability of collision in the signaling channel.  The disadvantages of the polling protocol can be improved by inserting a contention component into the protocol making them more robust against disturbances and decreasing the access time.

Hybrid Access Protocols

        A hybrid access method contains both dedicated and random protocol components.  So, a random component can be added to a polling based protocol to make it more robust and to decrease the access times.

        The idea is to have a free dedicated channel that is randomly accessed only by transmission demands, so that the transmitted data segments cannot collide and a station can try to make a transmission request over free data channels without waiting for a dedicated slot. 

Conclusions

        Reservation MAC protocols are the best solution of PLC systems, since they can carry hybrid traffic with variable data rates ensuring a high network utilization.