Let Tx be the length of time to accumulate x units of a resource. In queueing, the resource could be service. The authors derive a sufficient condition for the process (Tx,x≥0) to have stationary increments where Tx is an additive functional of a Markov process. This condition is satisfied in symmetric queues and generalized semi-Markov schemes with insensitive components. As a corollary, the authors show that the conditional expected response time in a symmetric queue is linear in the service requirement. A similar result holds for the conditional average residence time of an insensitive component in a GSMS.
