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T1/E1 Digital Voice Port Adapter Installation and Configuration
OL-3592-02
Chapter 4 Configuring the PA-VXA, PA-VXB, and PA-VXC
Configuring Voice over Frame Relay
For example, to enable VAD for VoIP dial peer 108, enter the following:
dial-peer voice 108 voip
destination-p1408528
vad
session target ipv4:10.0.0.8
Configuring Voice over Frame Relay
You need to take certain factors into consideration when configuring VoIP for it to run 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 will be 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 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 MTU size—Voice packets are generally small. By lowering the maximum transmission unit
(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.
–
Use RSVP or IP precedence to prioritize voice traffic.
–
Use compressed RTP to minimize voice packet header size.
•
Traffic shaping—Use adaptive traffic shaping to throttle back the output rate based on the backward
explicit congestion notification (BECN). If the feedback from the switch is ignored, packets (both
data and voice) might be discarded. Because the Frame Relay switch does not distinguish between
voice and data packets, voice packets could be discarded, which would result in a deterioration of
voice quality.
–
Use RSVP, compressed RTP, reduced MTU size, and adaptive traffic shaping based on BECN
to hold the data rate to the CIR.
–
Use generic traffic shaping to obtain a low interpacket wait time. For example, set Bc to 4000
to obtain an interpacket wait of 125 ms.
