TheJitter is generally caused by congestion in the IP network. The congestion can occur either at the router interfaces or in a provider or carrier network if the circuit has not been provisioned correct information.
The easiest and best place to start looking for jitter is at the router interfaces since you have direct control over this portion of the circuit. How you track down the source of the jitter depends greatly on the encapsulation and type of link where the jitter happens. Typically, ATM circuits do not experience jitter when configured correctly due to the constant cell rate involved. This gives a very consistent latency. If gitter is seen in an ATM environment, examination of the ATM configuration is necessary. When ATM works correctly (no dropped cells), you can expect jitter to be a non-issue. In Point-to-Point Protocol (PPP) encapsulation, jitter is almost always due to serialization delay. This can easily be managed with Link Fragmentation and Interleaving on the PPP link. The nature of PPP means that PPP endpoints talk directly to each other, without a network of switches between them. This is so that the network administrator has control over all interfaces involved.(Check here)
Jitter in a Frame Relay Environment
Three parameters need to be addressed to find the jitter in a Frame Relay environment:
For sample configurations and information related to configuring this, refer to VoIP over Frame Relay with Quality of Service.
You need to ensure that you are shaping the traffic that leaves the router to the actual Committed Information Rate (CIR) that the carrier provides. Verify this by looking at the Frame Relay statistics and check with the carrier. The first place to look is at the Frame Relay statistics. Use the show frame-relay pvc xx command , where xx is the Data-link connection identifier (DLCI) number. You should receive output similar to this: