Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

A comparison of the Internet Low Bit Rate Codec (iLBC) with legacy ITU-T

codecs is shown in Figure 18.2 [5]. The iLBC codec is described in [6] and [7].

High-performance Internet codecs have also been developed as open source

software, such as the SPEEX codec that uses audio sampling rates of 8 kHz, 16

kHz, and 32 kHz corresponding to audio bandwidths of 4 kHz, 8 kHz, and 16

kHz. The SPEEX codec [8] and [9] features are:

■■ Free software/open-source software

■■ Integration of wideband and narrowband in the same bitstream

■■ Wide range of bit rates available

■■ Dynamic bit-rate switching and variable bit rate (VBR)

■■ Voice activity detection (VAD, integrated with VBR)

■■ Variable complexity

Unfortunately, many VoIP users form their impressions of VoIP quality by

the limitations of existing IP-PSTN and IP-PBX gateways that have legacy 3.1

kHz audio codecs.

Figure 18.2 Performance of the Internet Low Bit Rate Codec (iLBC)

The tests were performed by Dynstat, Inc., an independant test laboratory.

Score system range: 1= bad, 2 = poor, 3 = fair, 4 = good, 5 = excellent

Courtesy of Global IP Sound

ilbcfreeware.org

0

1.5

2

2.5

3

3.5

4

510

Packet Loss (%)

http://www.ilbcfreeware.org

RFC 3951 : Internet Low Bit Rate Codec (iLBC)

RFC 3935 : RTP Payload for iLBC

MOS

iLBC

G.729A

15 G.723.1

306 Chapter 18