Acronym for Constant Bit Rate. A compression scheme in which each unit of input material is always compressed to the same output size. For MPEG-2 video, for example, this means that the compressed data always has the same data rate (that is, bytes per second), even when the input material is very easy to encode. 

The term may also imply that there is a guaranteed upper limit on the latency that data incurs while in transit. lf any of the samples are lost or delayed during transmission, the reconstructed speech signal may not be intelligible as the human ear is very intolerant to speech gaps. lt is therefore necessary to ensure that the full 64 Kbps can always be carried and the transmission delay will be effectively constant.

CBR keeps bit rate more constant, while VBR makes bit rate differ more. You were receiving frame drops because VBR was using more bandwidth than your connection could handle.