# 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)

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:

```Bc = CIR x (TC/1000)
Bc = 512000 x (125/1000)
Bc = 64000
```

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

```Bc = CIR x (TC/1000)
Bc = 512000 x (10/1000)
Bc = 5120
```

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

```Bc = CIR x (TC/1000)
Bc = 512000 x (8/1000)
Bc = 4096
```

.

It is not difficult, but this usually needs to be worked out on a piece of paper. If you lazy like me, you want to work it out in your head. I do this by simplifying the actual formula.

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'
```

## 10 thoughts on “Working out Bc values quickly”

1. Naveed says:

I love ur explanation but i hv a question:

As you said that min intervals are 8 and maximum are 100. What is the criteria to define how many intervals suit the given CIR. Thanks

1. Ruhann says:

A question is rarely around how many intervals required.
The intervals is a product of the TC. You will almost always be given 2 parts of the formula whether it be the CIR and TC, or BC and the CIR, etc.

hth :)

1. naveed says:

i do understand what u r saying. but i was asking from a service provider point of view where u have only CIR and u hv to decide the Tc & then Bc. So on what bases u decide how many intervals will suit a given CIR as a CIR can be a 100Mb or 128Kb?

2. Ruhann says:

From a SP perspective there is no one value that will suite all.
Some SPs leave the defaults values, ie. not specify the BC.
But for a customer it might be better to asses the traffic breakdown to determine the TC. If its is voice traffic primarily, the a small TC would be best. For a large data transfers, a larger TC would be best, else some value in the middle can be used.
Asses it on a case by case basis.
Hth

2. Florin B. says:

Hi mate,

What about Be computation? Any hints?

1. Steven C says:

Using this method, the Be woud be calculated:

Be = ( AIR (Physical Interface Rate) / Interval per second ) – Bc

3. Joe Heli says:

is CIR can be calculated ?! if i have a 10Mbps shared BW and 100 customers with speed 6/4 Mbps and the data transfer is 40GB/Month !! what is the CIR ?!

1. Ruhann says:

All depends on your allocated over-subscription levels if at all.
With no oversubscription, each client having a 64k circuit you should be ok.

4. Colby says:

Can you tell me what the numbers would be for a T3?

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