Nortel BCM50 Installation And Configuration Manual

Also see for BCM50: Configuration guide Installation and maintenance guide Keycode Installation Guide Integration guide Installation and configuration guide

Contents

22 Chapter 2 IP telephone overviewN0027269 01N0027269 01CodecsThe algorithm used to compress and decompress voice is embedded in a software entity called acodec (COde-DECode).Two popular codecs are G.711 and G.729. The G.711 Codec samples voice at 64 kilobits persecond (kbps), while G.729 samples at a far lower rate of 8 kbps. For actual bandwidthrequirements, refer to the BCM50 Networking Configuration Guide (N0027156); note that theactual kbps requirements are slightly higher than the label suggests.Voice quality is better when using a G.711 Codec, but more network bandwidth is used toexchange the voice frames between the telephones.If you experience poor voice quality and you suspect it is due to heavy network traffic, you canachieve better voice quality by configuring the IP telephone to use a G.729 Codec.The BCM50 supports these codecs:• G.729• G.729 with VAD (Voice Activity Detection)• G.711-uLaw• G.711-aLawJitter BufferVoice frames are transmitted at a fixed rate, because the time interval between frames is constant.If the frames arrive at the other end at the same rate, voice quality is perceived as good. In manycases, however, some frames can arrive slightly faster or slower than the other frames. This iscalled jitter, and degrades the perceived voice quality. To minimize this problem, configure the IPtelephone with a jitter buffer for arriving frames.This is how the jitter buffer works (assume a jitter buffer setting of five frames):• The IP telephone firmware places the first five arriving frames in the jitter buffer.• When frame six arrives, the IP telephone firmware places it in the buffer, and sends frame oneto the handset speaker.• When frame seven arrives, the IP telephone buffers it, and sends frame two to the handsetspeaker.By using a jitter buffer, the arriving packets are delayed slightly in order to ensure a constant rateof arriving frames at the handset speaker.Note: You can change the codec on a configured IP telephone only if it isconnected to the BCM50, or if Keep DN Alive is enabled for an offline telephone.Note: You can change the jitter buffer on a configured IP telephone only if it isconnected to the BCM50, or if Keep DN Alive is enabled for an offline telephone.