Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
Configure VAD for a VoIP Dial Peer
To disable the transmission of silence packets and enable VAD for a selected VoIP dial peer, use the
following global configuration commands:
The default for the vad command is enabled; normally, the default configuration for this command is
the most desirable. If you are operating on a high bandwidth network and voice quality is of the highest
importance, you should disable VAD. Using this value results in better voice quality, but it also requires
higher bandwidth requirements for voice.
For example, to enable VAD for VoIP dial peer 108, enter the following:
Router(config)# Dial-peer voice 108 voip
Router(config-dial-peer)# vad
Configure Frame Relay for VoIP
You need to take certain factors into consideration when configuring VoIP so that it runs smoothly over
Frame Relay. A public Frame Relay cloud provides no guarantees for QoS. For real-time traffic to be
transmitted in a timely manner, the data rate must not exceed the committed information rate (CIR), or
there is the possibility that packets are dropped. In addition, Frame Relay traffic shaping and RSVP are
mutually exclusive. This is particularly important to remember if multiple data link connection
identifiers (DLCIs) are carried on a single interface.
For Frame Relay links with slow output rates (less than or equal to 64 kbps), where data and voice are
being transmitted over the same permanent virtual circuit (PVC), we recommend the following
solutions:
• Separate DLCIs for voice and data—By providing a separate subinterface for voice and data, you
can use the appropriate QoS tool per line. For example, each DLCI would use 32 kbps of a 64-kbps
line.
– Apply adaptive traffic shaping to both DLCIs.
– Use RSVP or IP precedence to prioritize voice traffic.
– Use compressed RTP to minimize voice packet size.
– Use weighted fair queuing to manage voice traffic.
• Lower maximum transmission unit (MTU) size—Voice packets are generally small. By lowering
the MTU size (for example, to 300 bytes), large data packets can be broken up into smaller data
packets that can more easily be interwoven with voice packets.
Note Lowering the MTU size affects data throughput speed.
• CIR equal to line rate—Make sure that the data rate does not exceed the CIR. This is accomplished
through generic traffic shaping.
Command Task
Step 1
dial-peer voice number voip Enter the dial peer configuration mode to configure a VoIP dial
peer.
Step 2
vad Disable the transmission of silence packets .