Subscriber Module Priority defines the QoS mode of the SM with values of Normal, High, and Low.
Normal allows the QoS Classification Rules, Broadcast Priority, and Multicast Priority to be followed for this SM. The SM gives priority to the packets as defined in the rules which could be Low, High, or VoIP. Normal priority will allow data to be added to the appropriate High, Low, and VoIP queues based on the QoS rules. This is the default setting. If no rule is defined for a packet, then the packet priority will be Low.
High forces all non-VoIP traffic to have high priority and thus be at an advantage to other SMs that do not use High. The SM places all data other than VoIP in the High queue. It will be given higher priority than SMs configured with Low and Normal when there is contention for bandwidth under the AP.
Low forces all non-VoIP traffic to have low priority and thus be at a disadvantage to other SMs that pass data at High. The SM will place all data that is not VoIP in Low priority queue. It will be given lower priority than SMs configured with Normal and High when there is contention for bandwidth under the same AP.
For a detailed explanation of ePMP's QoS mechanisms, see: http://community.cambiumnetworks.com/t5/ePMP-Networking/Quality-of-Service-Mechanisms-Explained/td-p/52921