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 jitter 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.
Try:
http://www.cisco.com/warp/public/788/voice-qos/jitter_packet_voice.html
http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html
