PDF user manual for CopperEdge 150

As a practical matter, however, a user of a real-world data linkcan almost always transmit and receive at a much faster ratethan the nominal baseline indicated by the CIR. As we know,most networked data communication is bursty in nature, thatis, it consists of bursts of packets (some of which may be verylarge), separated by relatively long idle periods. Thus, there isanother, typically larger, number that represents a burst of datawhich the network will accept with a reasonable probability ofreliable delivery. This is the CE150 CommittedBurst (bcMax).The third congestion-management setting specifies the amountof data which, though in excess of the Committed Burst, can stillbe handled by the network and the circuit under normal conditions.This amount is the ExcessBurst (BE). However, becausethis ExcessBurst does exceed the commitment, it necessarily isat greater risk of causing or encountering congestion on the link.Frames that fall within the BE range are automatically markedwith the discard-eligible (DE) bit, which means that if the networkbecomes congested, the frame will simply be dropped. Data withinthe commitment time interval that exceed the value of BE willbe discarded, regardless of whether the link is congested or not.The following illustration, adopted from the CCITT standard oncongestion management, shows the relationship between thethree variables within the commitment interval (T c ).3HUÃ&&,77Ã,Bcmax + beAccess (facility) RateData Volume(Bits)bebcmaxCommitted Information Rate1st Frame(Committed)2nd Frame(Committed)3rd Frame(Committed)Time4th Frame(DiscardEligible)5th Frame(DiscardEligible)6th Frame(Discarded)TCThroughput Management Variables&RSSHU(GJH,QVWDOODWLRQDQG2SHUDWLQJ*XLGH