CBR,VBR and ABR?
Well this came from ATM (Asimetrical Transfer Mode) This transfer mode like MPLS allowed for QOS.
in CBR the ATM swich reservs a constant amount of data for the trafic,
VBR - variable bit rate there is no constant bit rate availoble to a switch, if the trafic is low a user can get more bit rate, and if the trafic is high less.
ABR - averidge bit rate not availoble
Well in ABR the QOS offered to the end user is an averidge usualy between a maxiun and a minimun bit rate this is simular as VBR but here the QOS is deffined better.
As for video, well in the begining of mpeg1 it's use to be defined for use in digital systems like atm, ant thet's where it's name comes from.
CBR - definative file size variable qualati
VBR - definative qualaty variable file size
ABR - something in between as the file size and the qualaty are defined with in reasnoble borders.