Traffic Shaping Calculator
Compute token-bucket shaper parameters — committed burst, excess burst and interval — from CIR and Tc.
Inputs
Committed Burst Bc
500,000bits
Committed Burst Bc
62,500bytes
Excess Burst Be
250,000bits
Shaping Intervals per Second
100
Time to Send a Full Burst at Line Rate
0.750ms
Step by step
Values used
Committed information rate (CIR) = 50 Mbps; Shaping interval Tc = 10 ms; Excess burst as share of Bc = 50 %; Physical line rate = 1,000 Mbps
Traffic Shaping
Bc = CIR × Tc, so Tc = Bc ÷ CIR; the shaper releases Bc bits every Tc and 1000 ÷ Tc intervals fit in a second.
Committed Burst Bc
= 500,000 bits
Committed Burst Bc
= 62,500 bytes
Excess Burst Be
= 250,000 bits
Shaping Intervals per Second
= 100
Time to Send a Full Burst at Line Rate
= 0.750 ms
How it works
A shaper is a bucket refilled with Bc bits every Tc milliseconds; traffic is metered out at line rate until the bucket empties, then waits for the next interval. Small Tc values give smoother output and lower jitter, at the cost of more scheduler work. Tc chosen too large produces bursts long enough to overflow carrier buffers and drop voice, while Tc too small wastes CPU and can under-run the committed rate.
Formula
Traffic Shaping
Bc = CIR × Tc, so Tc = Bc ÷ CIR; the shaper releases Bc bits every Tc and 1000 ÷ Tc intervals fit in a second.
- CIR
- Long-term average rate the shaper enforces
- Tc
- Token replenishment interval, typically 4–10 ms for voice
- Be
- Extra bucket depth that lets unused credit be spent later
Frequently Asked Questions
How is Traffic Shaping calculated?
Bc = CIR × Tc, so Tc = Bc ÷ CIR; the shaper releases Bc bits every Tc and 1000 ÷ Tc intervals fit in a second. A shaper is a bucket refilled with Bc bits every Tc milliseconds; traffic is metered out at line rate until the bucket empties, then waits for the next interval. Small Tc values give smoother output and lower jitter, at the cost of more scheduler work.
Why does Traffic Shaping matter?
Tc chosen too large produces bursts long enough to overflow carrier buffers and drop voice, while Tc too small wastes CPU and can under-run the committed rate.
What values do I need to enter?
This calculator takes 4 inputs: Committed information rate (CIR), Shaping interval Tc, Excess burst as share of Bc, Physical line rate. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.