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Configuring Voice over IP
Additional Resources
108
Cisco VGD 1T3 Voice Gateway Software Configuration Guide
Tip
For more information and configuration options, see the VoIP over PPP Links with Quality of Service
(LLQ / IP RTP Priority, LFI, cRTP) document, available online at
http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html.
Jitter Buffering
Jitter occurs when there is a discrepancy between when a voice packet is expected to arrive and when it
actually arrives, causing discontinuity in the voice stream. Cisco devices handle jitter by buffering
received data and playing it back smoothly.
Default jitter-buffer settings are sufficient in most networks under normal situations. If you experience
choppy voice signals or poor voice quality, increase the size of the buffer. If you experience significant
overall network delay, decrease the size. If your network is noisy and you use jitter-prone applications
such as unified messaging server or interactive voice response, choose fixed mode and a relatively high
nominal value. Note that the trade-off for increasing jitter-buffer size is a corresponding increase in
delay.
Cisco jitter buffers are normally sized dynamically, and adaptive mode plus default buffer size should
suffice, but you can adjust mode and size as needed.
Additional Resources
In configuring VoIP and setting QoS parameters for your network, you will have to wrestle with a large
number of decisions and parameters. This chapter provides a brief overview on this very complex
subject. The following sources provide more information:
•
Cisco documents on IP telephony solutions:
http://www.cisco.com/univercd/cc/td/doc/product/voice/ip_tele/index.htm
•
Cisco feature modules:
http://www.cisco.com/univercd/cc/td/doc/product/software/
,
under listings for your Cisco IOS release
•
Cisco IOS documents:
–
Cisco IOS Quality of Service Solutions Configuration Guide
–
Cisco IOS Multiservice Applications Command Reference
Note
Start your search at
http://www.cisco.com/univercd/cc/td/doc/product/software/
and then go to your Cisco IOS release.
•
Commercially available books:
–
Davidson, J. Peters, J. and Gracely, B. Voice Over IP Fundamentals, Indianapolis, Indiana:
Cisco Press; 2000
–
Caputo, R. Cisco Packetized Voice & Data Integration, New York, New York: McGraw-Hill;
2000
•
VoIP references for Cisco devices:
–
Voice-over-IP Quick Start Guide, available online at
http://www.cisco.com/univercd/cc/td/doc/product/access/acs_mod/1700/voipqsg.htm.