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The vbr3 codec, also known as Apple’s audiotoolbox, is an encoder that provides a series of AAC profiles tailored for low bit rates. Using the -profile:a option of libfdk_aac with a sample rate as low as 64 kb/s gives you CBR and VBBR, while -profile:a 2 gives you HE-AAC (version 1 and version 2) which are better suited to lower bit rates.

rt-vbr is a class of service that defines the bandwidth requirements for connections with real time (rt) data transfer characteristics such as compressed voice video conferencing, but with bandwidth needs that vary depending on the dynamics of the media being transmitted at any given moment. rt-vbr is characterized by a high PCR and SCR (sustained cell rate), and a higher MBS.

VBR is supported on BXM and IGX switches with the BXM, ASI, or UXM card. The BPX Command Line Interface (CLI) and Cisco WAN Manager accept the same connection policing and bandwidth parameters for both rt-vbr and nrt-vbr service.

When adding a rt-vbr or nrt-vbr connection to a port, the addcon command displays the same extended parameter prompts as previous releases. The displayed parameters include PCR, %util, FBTC flag, SCR, MBS, and policing type. The qbin values displayed at the ports and trunks used by rt-vbr and nrt-vbr connections can be configured independently of each other by specifying the yes ” response to the extended parameter prompt in the addcon command. Alternatively, the connection policing and bandwidth settings can be set at a global level by specifying the sctr ” value of the addcon command.

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