Other Bandwidth-Reduction Features - Cisco AS5350 - Universal Access Server Configuration Manual

Enabling QoS Features for VoIP

Other Bandwidth-Reduction Features

Voice Encoding
The Cisco AS5350 and Cisco AS5400 gateways offer the following codec (coders/decoder)
methodologies for encoding (digitizing and, optionally, compressing) voice:
•
•
•
Choosing a coding methodology is a matter of balancing trade-offs among several factors, principal
among them those listed for various methodologies in
Table 6-8
Methodology
G.711 PCM
G.729a CS-ACELP
G.723.1 MP-MLQ
1. High bit rate is optimal for voice quality, because the original voice signal is better represented; low bit rate is optimal for
2. Perceived quality is measured in standardized mean-opinion-score (MOS) studies.
Tandem switching (also called dual encodings or dual compressions) can cause additional problems.
Note
Digital calls routed to a tandem (toll) office are converted there to analog form for processing, and then
reconverted to digital form for further transmission. Converting and reconverting in this way more than
about twice distorts signals irreparably. If your calls are subject to significant toll-office processing,
choose PCM if you have sufficient bandwidth. It is also recommended that you employ a Cisco IOS
Multimedia Conference Manager (H.323 gatekeeper) or management application such as Cisco Voice
Manager to help manage these types of processes.
Other factors that might enter into your decision, or that you can use to tweak performance, include the
likelihood of multiple tandem encodings and how you handle packet fragmentation.
For more information and configuration options, see Voice over IP Quality of Service for Low-Speed
Tip
PPP Links, available online at
http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html
Cisco AS5350 and Cisco AS5400 Universal Gateway Software Configuration Guide
6-20
G.711/PCM (pulse-code modulation): Digitizes, does not compress
G.729a/CS-ACELP (conjucate structure - algebraic code excited linear prediction): Digitizes and
compresses
G.723.1/MP-MLQ (multipulse multilevel quantization), 6.3 or 5.3 kbps: Digitizes and compresses
Trade-Offs Among Codec Methodologies
Bit Rate (kbps)
64 (very high)
8 (low)
6.3/5.3 (low)
network performance, because packets are less apt to be delayed or dropped.
Table 6-8.
Processing
Frame Size (ms) Required (mips)
1
(low is optimal) (low is optimal)
0.125 (low) 0.34 (low)
10 (med) 10 (med)
30 (high) 16 (med-high)
Chapter 6 Configuring Voice over IP
Perceived Quality
(1=bad, 5=excellent)
2
(high is optimal)
4.1 (high)
3.7 (med)
3.9 (med)
OL-3418-02 B0