Hello Adrian
One of the fundamental components of achieving traffic shaping is the Tc or time interval used. You see, a physical interface is able to send data at either its maximum speed or a speed of 0Kbps. In the lesson, we’re looking at a serial interface with a physical bandwidth of 128Kbps, so at any given point in time, the interface can send data at 128Kbps or at 0Kbps and not any values in between.
To achieve intermediate speeds, such as say 64kbps, traffic shaping will cause the interface to transmit at 128Kbps for 50% of the time, and 0Kbps for the other 50% of the time, thus giving an average over time of 64Kbps. To achieve a speed of 32Kbps, we would transmit at 128Kbps for 25% of the time, and at 0Kbps for 75% of the time.
To implement this practically, we need to define a time interval that will be used to define when to transmit and when not to transmit. The time interval must be sufficiently small in order to provide the “illusion” of a continuous flow of data. This time interval is called Tc or Time Constant or Commit Time.
Tc is calculated using the formula Tc=Bc/CIR. In the lesson:
- Bc is 8000 bits
- CIR is 64000bps
So Tc = 8000/64000 = 0.125 seconds.
This results in 8 intervals per second.
The Tc can vary depending on the Bc and the CIR.
I hope this has been helpful!
Laz