The busy period of a permit buffer in a window-controlled virtual route is analysed using the method of sub-busy-periods partition. A non-linear recurrence equation for busy period p.d.f. (transformable into a continued fraction) is derived. Mean busy-period length is calculated; some similarities to the open M/G/l system are indicated. The limiting case of the busy-period equation, as both window size and virtual route length approach infinity, is investigated. It is shown that in the limit the permit buffer is equivalent to the M/M/l system from the busy and idle period points of view. The accuracy of the M/M/l model is inspected via simulation and numerical computations. The M/M/l model renders power optimization in a window-controlled virtual route easy, It has been shown that transmission power is optimized at a window size equal to the virtual route length, independently of an offered load. A simple operational rule for permit power optimization is also provided.