Chapter 42 VoIP overview 365Networking Configuration GuideLANA Local Area Network (LAN) is a communications network that serves users within a confinedgeographical area. For BCM, a LAN is any IP network connected on the BCM system. Often, theLAN can include a router that forms a connection to the Internet. A BCM can have up to two LANconnections.Key VoIP conceptsThe following explains a few commonly used VoIP terms.QoSQoS (Quality of Service) is technology that determines the maximum acceptable amount oflatency, and balances that with the quality of the VoIP connection. BCM and network routers useQoS to ensure that real time critical IP packets, such as voice packets, are given higher routing andhandling priority than other types of data packets.Silence suppressionSilence suppression technology identifies the periods of silence in a conversation, and stopssending IP speech packets during those periods. Telco studies show that in a typical telephoneconversation, only about 36% to 40% of a full-duplex conversation is active. When one party inthe conversation is quiet for more than a few hundredths of a second, voice packet transmission issuppressed until the party starts talking again. This is half-duplex. There are important periods ofsilence during speaker pauses between words and phrases. By applying silence suppression,average bandwidth use is reduced by the same amount. This reduction in average bandwidthrequirements develops over a 20-to-30-second period as the conversation switches from onedirection to another. Refer to “Silence suppression” on page 529.CodecsThe algorithm used to compress and decompress voice over IP networks and VoIP trunks isembedded in a software entity called a codec (COde-DECcode).Refer to “Codec rates” on page 549 for a listing of the supported codes and their transmissionrates.• The G.711 Codec samples the voice stream at a rate of 64 kbps (kilo bits per second), and isthe Codec to use for maximum voice quality. Choose the G.711 Codec with the compandinglaw (alaw or ulaw) that matches your system requirements.• The G.729 Codec samples the voice stream at 8 kbps. The voice quality is slightly lower usinga G.729 but it reduces network traffic by approximately 80%.• The G.723 Codec should be used only with third party devices that do not support G.729 orG.711.
