CAR Explanation

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

• 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)

To work out what the Bc value should be on a 512k link, you need to decide what your Tc should be. A big deciding factor is the most used/important application. For data applications doing large file transfers, a larger Tc is generally recommended. For voice you want the smallest possible Tc, to avoid voice packets having to wait a large amount of milliseconds for the next interval before being sent.

Firstly lets calculate using a Tc of 125ms on a 512k pvc. Using the formula above:

1. Bc = CIR x (TC/1000)
2. Bc = 512000 x (125/1000)
3. Bc = 64000

If we need to use a TC of 10ms on the same pvc:

1. Bc = CIR x (TC/1000)
2. Bc = 512000 x (10/1000)
3. Bc = 5120

If we need to use a TC of 8ms on the same pvc:

1. Bc = CIR x (TC/1000)
2. Bc = 512000 x (8/1000)
3. Bc = 4096

To get the Bc value, I use the following formula and work out the result much quicker.

• Bc = CIR/Intervals per second

To get the intervals per second, 1000ms/Tc.

Example a Tc of 125 ms  (1000ms/125ms) = 8 intervals

• 512000/8 = 64000 Bc                     'Tc of 125ms = 8 intervals per second'

Using the rest of the above examples :

• 512000/100 = 5120 Bc                    'Tc of 10ms = 100 intervals per second'
• 512000/125 = 4096 Bc                    'Tc of 8ms = 125 intervals per second'