Voice Coding and Compression Techniques


Recall that voice digitized with PCM is sent at 64 kbps. You can reduce the bandwidth required by voice in many ways, as detailed in the following sections.

Voice Compression

Voice compression reduces the size of the voice payload while still maintaining the quality at an acceptable level. As a result of technology advances, a variety of International Telecommunications Union (ITU) standards exist for voice coding/decoding and compression; these standards are known as codecs. Some of these standards and the bandwidth they use for one voice channel are summarized in Table 7-1. The quality of the resulting voice is represented by the average of the results of a wide variety of listeners' opinions, on a scale of 1 (bad) to 5 (excellent), known as the mean opinion score (MOS), which is also shown in Table 7-1. Notice the variety of bandwidth and MOSs; both should be considered when choosing a codec.

Table 7-1. Voice Coding and Compression Standards[5]

ITU Standard Codec

Bandwidth (kbps)

MOS

G.711

64

4.1

G.723

6.3/5.3

3.9/3.65

G.726

16/24/32

3.85

G.728

16

3.61

G.729

8

3.92


Key Point

In general, G.729 is the recommended voice compression technique over most WAN networks because of its combination of low bandwidth and high MOS.


Note

Using a single codec in WANs is recommended; this can also simplify the bandwidth-requirement calculations.


A DSP is a hardware component within a voice-enabled router that digitizes, compresses, and packetizes voice. Some of the codecs require more processing resources than others, so the number of voice calls that can be supported by one DSP depends on the codec implemented. The codecs are divided into two categories, known as medium complexity and high complexity, for this purpose. A single DSP can process up to four voice calls for medium-complexity codecs or up to two calls for high-complexity codecs. The two categories are summarized in Table 7-2.

Table 7-2. Medium-and High-Complexity Codecs[6]

Medium-Complexity Codecs

High-Complexity Codecs

G.711

G.723

G.726

G.728

G.729a

G.729

G.729ab

G.729b


Note

G.729a (also known as G.729 Annex-A) is a medium-complexity variant of G.729 with slightly lower voice quality. G.729b has more features than G.729. G.729ab is a medium-complexity variant of G.729b with slightly lower voice quality.


Voice Activity Detection

Another way to reduce the bandwidth required by voice calls is to use voice activity detection (VAD). On average, about 35 percent of a call is in fact silence; when VoIP is used, this silence is packetized along with the conversation. VAD suppresses the silence, so instead of sending IP packets of silence, only IP packets of conversation are sent. The network bandwidth is therefore being used more efficiently and effectively.

Note

In some cases, Cisco recommends disabling VAD, for example, when faxes are to be sent through the network. VAD can also degrade the perceived quality of the call, because when VAD is enabled, the silence is replaced by comfort noise played to the listener (by the device at the listener's end of the network). If this is causing problems, VAD can be disabled.


Compressed Real-Time Transport Protocol

Recall that voice conversation packets are sent using RTP, which runs on top of UDP. Thus, VoIP packets consist of two parts: the voice and the headers (IP/UDP/RTP). The voice samples can be compressed, and therefore their size can vary depending on the codec usedfor example, a typical G.729 call has 20 bytes of voice samples. The headers, though, are always 40 bytes, which can be a significant amount of overhead.

RTP header compression, called cRTP, compresses this 40-byte header to 24 bytes. The cRTP compression can be configured on an interface and is recommended on low-bandwidth WANs (up to 2 megabits per second [Mbps]). The cRTP compression is not recommended on higher-speed interfaces, though, because it is CPU intensive; the use of CPU resources outweighs the benefits on these faster links.

Note

The cRTP compression is configured on an interface and therefore must be configured on both ends of a WAN if it is used.





Campus Network Design Fundamentals
Campus Network Design Fundamentals
ISBN: 1587052229
EAN: 2147483647
Year: 2005
Pages: 156

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