Cisco VGD-1T3, Руководство по настройке программного обеспечения

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Configuring Voice over IP
Enabling QoS Features for VoIP
103
Cisco VGD 1T3 Voice Gateway Software Configuration Guide
Enabling QoS Features for VoIP
The following text briefly describes some of the most important QoS features that you can enable, and
cites references that you need to make informed decisions about the use and optimization of those
features. Features discussed include the following:
•
Congestion Management
– Weighted Fair Queuing
–
Low-Latency Queuing
– IP RTP Priority and Frame Relay IP RTP Priority
–
Resource Reservation
• Fragmentation and Interleaving
•
Traffic Shaping for Frame Relay
• Other Bandwidth-Reduction Features
–
Voice Encoding
– RTP Packet-Header Compression
–
Serialization Delay
– Voice Activity Detection
–
Jitter Buffering
References in the “Additional Resources” section on page 108 provide more information.
Tip
Should you have problems with QoS, try adding the following commands to your configuration:
• At the top-level configuration level:
io-cache enable
voice-fastpath enable
•
Under the Gigabit Ethernet interface:
ip route-cache
Congestion Management
Weighted Fair Queuing
You need to avoid congestion on backbone gateways serving high-traffic, high-speed networks. A
weighted-fair-queuing methodology called WRED (weighted random early detection) queues traffic
according to priority values that you set (you set voice traffic to critical, for example), sets different
packet-drop thresholds for each queue, and drops packets in lower-priority queues as necessary so that
higher-priority queues can be adequately served. This ensures that low-bandwidth conversations get
through, even in the presence of other high-bandwidth applications.