When a voice gateway receive a call , you can divide your call in an inbound and outbound leg.
Let’s see how the inbound priority matching .
- incoming-called-number (called number “DNIS” information used)
- answer-address (calling number “ANI” information used)
- destination-pattern (calling number “ANI” information used)
- voice-port (that has dial port configured)
During WAN failure , we can observe that H323 will not preserve absolutely the call when the failover occurs as there is no more any signal from the CUCM.
An unofficial workaround is to use the command h225 timeout to suppress the keepalive between the Call Manager and the gateway.
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no h225 timeout keepalive
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Hi ,
RTP ports are used on top of UDP to carry on the voice payload/traffic.
The common range of ports are 16384 and 32767 .
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.
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
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