If Ci and Ti are the Worst Case Execution Time and the minimum interarrival time of task , using a CBS with parameters (Qs=Ci,Ts=Ti) a bandwidth is reserved, hence it is guaranteed that the assigned deadline will not be postponed (hard guarantee), otherwise only a stochastic guarantee can be performed.
Using a CBS to implement Constant Bandwidth resource allocation, the following lemma can be proved (see [1]):
In [5,6] Deng and Liu use a similar mechanism to schedule multiple applications in an open environment, realizing the resource allocations with a Total Bandwidth Server (TBS) [13,14] or a similar aperiodic server (the Constant Utilization Server). The problem with these servers is that they cannot be used when tasks' WCETs are not known, so an error in the WCET estimation would compromise the correct behavior of the system.
The Resource Reservation abstraction [12] does not introduce the
concept of reserved bandwidth, but it is similar to CBS
in that it realizes a CBA.
Like CBS, it allows to perform hard guarantee, but, being based on
fixed priority (DM), it performs worse in overload situations (see Figure
1). In facts, when a task executes for Ci time units in its
period Ti and does not complete(the reservation
is depleted), the rest of the task is scheduled
in background.
Using dynamic priority, CBS schedules the "depleted tasks" by postponing
deadlines.