Constant Bit Rate (CBR)
The main objective of the CBR service class is to support real time applications such as streaming voice or video. It is well known that voice quality in VoIP packet networks is very sensitive to packet latency and jitter. Thus, the CBR service class in the Aperto® system was created with the following set of features that help minimize latency and jitter to maintain excellent voice quality:
- CBR (and CIR) flows are subject to Admission Control and once a flow is admitted it is guaranteed its bandwidth for the duration that it is active.
- CBR flows are Buffered Separately from each other and from flows in the CIR and BE classes. In contrast, the majority of other products mix up voice and data in the same buffer, so that voice packets get queued up behind data.
- CBR service flows are given strictly Higher Priority versus CIR and BE service flows, which implies that the system serves CIR and BE packets only after it has finished transmitting all the outstanding CBR packets.
- In the upstream, the system uses an optimized mode for CBR, known as the Unsolicited Grants mechanism. This mechanism bypasses the normal request-grant mechanism for upstream traffic by having the BSU give automatic grants to a CBR flow at periodic intervals.