Depending the Layer 2 technology that you use , the header can change :
- Ethernet Frame => 18 bytes
- Frame Relay/FRF.12 => 6 bytes
- MLP => 6 bytes 13 bytes
- ATM => 5 bytes
- MLP over Frame Relay => 14 bytes
For Layer 3 , you can use the following values
- IP Header = 20 bytes
- UDP Header = 8 bytes
- RTP Header = 12 bytes
So it gives you the following amount
- IP+TCP+UDP Header => 40 bytes
With CRTP enabled, you can then benefit of a huge saving of compression of this payload.
Indeed , from 40 bytes , you can decrease this payload from 2 up 4 bytes.
Let’s look this with an IP Call in G729 , here is the bandwitdh used without CRTP enabled:
8000 * ( 40 + 20) / 20 = 24000 bps = 24 Kbps
Now , let’s enable CTRP to see the saving :
8000 * ( 2 + 20) / 20 = 8800 bps = 8,8 Kbps
You are performing a saving of more or less 70 %
So you can see that on low links it is a benefit to enable CRTP in order to save your bandwidth.