3Com CoreBuilder 7000, Operation Manual, Page: 118 / 264

118 C
HAPTER
5: E
STABLISHING
ATM C
HANNELS
Sustained cell rate (SCR) — average cell rate over time, taking into
account burst traffic
The GCAC algorithm consists of the following steps:
1 Read the peak cell rate (PCR) and the sustained cell rate (SCR) from the
traffic contract of the call.
2 Add up SCRs of all existing connections at the switch to give the
aggregate requested cell rate of existing connections (ARR).
3 Calculate the cell rate margin (CRM), a measure of the difference
between the PCR and the SCR summed over all existing connections,
multiplied by the variance factor (VF) a a relative measure of the cell rate
margin normalized by the variance of the aggregate rate.
The actual allocated capacity (AAC) is the sum of ARR and CRM.
The available cell rate (ACR) is the difference between the maximum
usable cell rate through the switch (MaxCR) and the AAC.
4 Calculate the additional capacity (DeltaCR) needed to carry the new
connection from its CRM and SCR
5 Compare DeltaCR with ACR. If DeltaCR < ACR accept the connection,
otherwise reject it.
Figure 40 shows the cell rate utilization for the entire CoreBuilder 7000
switch for the VBR and CBR Service Classes.
Aggregate Sustained Rate
Actual Allocated Capacity
Maximum Usable Cell Rate
Cell Rate Margin
Available Cell Rate