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.
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
An Erlang is defined as 1 hour of conversation.
Erlangs = Hours_of_phone_use_in_a_month/22 * 0.15 *1.10
The higher is the codec complexity , the fewer number of calls that the DSP can process
MGCP is Master/Slave protocol and MGCP endpoints are controlled by the Control Agent ( Most of the case , it will be the CUCM)
MGCP uses the UDP/2427 for its communication between the CUCM and the GW. It is also a text-based master/slave protocol used by a media gateway controller (MGC) and Call Agent to control media gateways. However , TCP/2428 is used to exchange keepalives between endpoints and Call Agent, as well for MGCP PRI/BRI backhaul.
MGCP MGCs and MGs can be described as follows:
- MGC/Call Agent: this element possesses call control intelligence and controls MGs (example : Call Manager)
- MG: This device provides translation between data packet and audio signals reveived over VoIP networks and other networks such as the PSTN ( example: IOS gateway)
As defined for SIP , here are the transport layers used by Skinny/SCCP Protocol:
- TCP Connection with CUCM
- UDP Connection between the endpoints
SCCP is a Cisco proprietary protocol master/slave protocol that Call Manager and other call agents can use to communicate with devices and endpoints as Cisco IP Phones.
SCCP uses port TCP/2000 and TCP/2443 for SCCPS