Another form of QoS is Variable Bit Rate (VBR). It may be used in cases where an adapter would like two megabits per second, but can live with an average of only one megabit per second. It could negotiate for 2 megabits for a certain number of seconds in each minute, as a peak, with no cells lost. The switch guarantees an average, over time, of one megabit per second, with two megabits provided when available. It can also guarantee an average peak bandwidth per period, in addition to the flat average. This is less expensive that CBR, because although the adapter requested a peak of two megabits per second, the carrier has to provide an average of only one. If other traffic needs to be put on the backplane of the switch and is running out of bandwidth, it can use the second megabit, because it only guaranteed an average of one. It might even go as low as 512K per second, as long as it can keep the guaranteed average.
VBR can be used efficiently for voice traffic, taking advantage of silence suppression. During pauses in a normal conversation, other data can be sent on the vacant bandwidth. For example, a video conferencing application, with the audio over a speaker, could use VBR with silence suppression. Because VBR gives an average bandwidth, software and algorithms can be sent when the peak bandwidth is available. This works when the average is adequate to transfer the voice traffic in compressed, silence-suppressed form. When less flexibility exists, CBR may be necessary, but it usually isn't required for video on demand, video conferencing, and such. Those can typically be done with VBR.