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Configuring Voice over IP
Call-Admission Control
You can gracefully prevent calls from entering your Cisco VGD 1T3 voice gateway from the PSTN when
certain resources—such as CPU, memory, and interfaces—are not available to process those calls. Such
intervention is called call-admission control.
If your system experiences high CPU usage, large call volumes, or occasional large numbers of
simultaneous calls, you need to control two specific aspects of call-admission control: call spikes and
call thresholds. Doing so is especially important if you handle transactions involving debit cards, which
require AAA and similar types of support.
Configure call spikes to limit the number of incoming calls over a short period of time. Configure call
thresholds to define under which circumstances system resources should be enabled.
For more information and configuration options, including how to configure limits on call spikes and
Tip
call thresholds, refer to the following document:
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Fragmentation and Interleaving
Transmission of voice packets, usually small (60 to 240 bytes) in size, can be unduly delayed in networks
that also transmit large data packets. Fragmenting large data packets into smaller ones and interleaving
voice packets among the fragments reduces jitter and delay. Use fragmentation and interleaving in
conjunction with a congestion-management technique such as IP RTP Priority, RSVP, or both if you have
a low-bandwidth (< 1.5 Mbps) WAN circuit, but not if you have a high-bandwidth (> 1.5 Mbps) WAN
circuit. The recommended fragmentation and interleaving methodology is FRF.12 for Voice over Frame
Relay, Multilink PPP for VoIP-over-PPP leased lines.
For more information and configuration options, see the following:
Tip
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Traffic Shaping for Frame Relay
You must regulate traffic flow so that packets arrive at their destination only as fast as the destination
can handle them. You do so by buffering packets that are generated faster than a configured value, and
releasing them at that value. It is especially important that you enable traffic shaping in Frame Relay
networks, but not in conjunction with RSVP. Do not enable traffic shaping with PPP leased lines.
Call Admission Control for H.323 VoIP Gateways, available online at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122limit/122x/122xa/
122xa_2/ft_pfavb.htm
For FRF.12, Frame Relay Fragmentation for Voice, available online at
http://www.cisco.com/warp/public/788/vofr/fr_frag.html
For Multilink PPP, VoIP over PPP Links with Quality of Service (LLQ / IP RTP Priority, LFI, cRTP),
available online at
http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html
Cisco VGD 1T3 Voice Gateway Software Configuration Guide
Enabling QoS Features for VoIP
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