Furthermore, the scheme is non-preemptive that is, as long as low priority DBs can block
optical paths, no complete isolation is achieved. Yet, starvation of low priority classes is
possible if the offered traffic load of high priority bursts is high and not controlled.
??? Active.dropping-based.QoS.scheme: In this scheme, a burst dropper (hardware) is
implemented in front of every core node (Chen, Hamdi, & Tsang, 2001). Dependent
on a dropping policy, some BCPs and their corresponding DBs are dropped before
reaching the reservation unit. Therefore, the admission to the outgoing wavelengths
is controlled, enabling the core nodes to locally control the offered load of certain
service classes to maintain network resources for other service classes. Active Dropping-
based QoS scheme intervene before congestion occurs, as the selective dropping
of DBs is initiated according to the data traffic profile to guarantee that the higher
priority classes have higher chances to make successful reservations. However, this
scheme suffers from a major disadvantage which is the absence of feedback from the
core nodes to the edge nodes, and thus traffic volume of different classes cannot be
controlled. Furthermore, isolation between different traffic classes is not guaranteed.
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