# Working out Bc values quickly

First lets review some basic shaping definitions.

CIR (Committed Information Rate)

• Dictates the output rate one aims to average per second on the circuit/interface.
• Book formula : CIR = Bc / (Tc/1000)

Tc (Time-Interval)

• It is the time in milliseconds into which a second is divided for transmission intervals.
• The Tc can’t be adjusted directly, but it can be changed by setting the Bc to a specific value..
• The maximum value of Tc is 125ms (8 intervals per second) and the minimum value is 10ms (100 intervals per second).
• Actually 8ms (125 intervals per second) on distributed platforms. On distributed platforms, the Tc must be defined in 4-ms increments. The nearest multiple of 4 ms within the 10-ms target is 8 ms.
• Book formula :  Tc = (Bc / CIR) x 1000

Bc (Committed Burst Rate)

• Bc is the number of committed bits allowed to be sent per interval (Tc) to conform with the target-rate (CIR) per second.
• If Bc worth of bits are sent every interval in a second, the output rate is the CIR.
• Book formula : Bc = CIR x (Tc/1000)