Header

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.

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VPN Overhead

Please , don’t forget also that VPN mechanisms add a lot of overhead to your voice packets and it depends also of the technology that you deploy (L2TP for Layer 2 or IPSEC for Layer 3).

So as rule , we can add 30 to 60 bytes of overhead per voip packet when you transmit the packet over the VPN paths.

QOS – MQC

Here is as short recap/representation of the MQC ( Modular QoS CLI)

Step #1 : Classify Traffic => CLASS-MAP

Step #2 : Assisgn Policies to the traffic classes => POLICY-MAP

Step #3 : Apply the policy to an interface => SERVICE-POLICY

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