Introduction To Packetized Voice - Nortel Remote Gateway 9110 Reference Manual

Standard 1.6

Introduction to packetized voice

The Public Switched Telephone Network (PSTN) has evolved into a stable, high
quality communications medium. When you take a telephone off-hook, dial tone
is immediate. Connection time is quite fast and clarity of the audio is excellent.
With the development of packetized communications such as Voice over
Internet Protocol (VoIP), the same high quality experience can be realized -
when engineered properly.
To correctly engineer a VoIP system, you must use subjective testing results
performed by numerous companies and independent labs to understand the
factors that affect the quality of the VoIP. The main factors are categorized as
follows:
audio levels
The PSTN has been built around clear rules of Loss Level Planning – the
level of transmitted and received audio. The audio levels are such that the
conversation is comfortable and highly intelligible. You can converse
without having to strain to hear the caller.
extra audio (echo, clicks, pops)
Extra audio can be created in many ways. The most common extra audio is
echo, or a reflection of your own speech. In traditional PSTN telephony,
echo is present, but the Loss Level Planning reduces the amplitude, and the
small amount of echo delay makes it unperceivable. With VoIP systems, the
inherent audio processing and network delays can make the echo
perceivable, if the echo is not cancelled/managed properly.
Clicks and pops can be heard when packet loss is occurring on the network.
The severity of the click or pop is determined by the size and duration of
the packet loss experienced, as well as the audio compression and
decompression scheme (Codec) chosen. A Codec compresses audio,
reducing the amount of bandwidth required to have a conversation.
— G.711 (64 Kbps data rate - equivalent to a wired telephone [no
compression])
— G.726 (32 Kbps data rate - reduces the required bandwidth by one half
in trade for a slightly lower audio fidelity)
— G.729A (8 Kbps data rate - high level of compression in trade for lower
audio fidelity)
Remote Gateway 9100 Series Network Engineering Guidelines
June 2005
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