Posts Tagged ‘oversubscription’


Note: The following post is an excerpt from the full QoS section of IEWB-RS VOL1 version 5.

Peak shaping may look confusing at first sight; however, its function becomes clear once you think of oversubscription. As we discussed before, oversubscription means selling customers more bandwidth than a network can supply, hoping that not all connections would use their maximum sending rate at the same time. With oversubscription, traffic contract usually specifies three parameters: PIR, CIR and Tc – peak rate, committed rate and averaging time interval for rate measurements. The SP allows customers to send traffic at rates up to PIR, but only guarantees CIR rate in case of network congestion. Inside the network SP uses any of the max-min scheduling procedures to implement bandwidth sharing in such manner that oversubscribed traffic has lower preference than conforming traffic. Additionally, the SP generally assumes that customers respond to notifications of traffic congestion in the network (either explicit, such as FECN/BECN/TCP ECN or implicit such as packet drops in TCP) by slowing down sending rate.

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