VoIP Components

To transfer voice data on the same network with e-mail and Web traffic, a new and different
set of components is required. Some of these components are:
Codecs
TCP/IP and VoIP Protocols
IP telephony servers and PBXs
VoIP gateways and Routers
IP phones and softphones
Understanding Codecs
A codec (which stands for “compressor/decompressor” or “coder/decoder”) is the hardware
or software that samples analog sound and converts it to digital bits, which it outputs at
a predetermined data rate. The codec often performs compression as well, to save bandwidth.
There are dozens of available codecs, each with its own characteristics.
Codecs have odd-looking names that correspond to the name of the ITU standard that describes
their operation. For example, the codecs named G.711u and G.711a convert from
analog to digital and back with relatively high quality. As with most things digital, higher
quality implies more bits, so these two codecs use more bandwidth than lower-speed codecs.
Lower-speed codecs, such as G.726, G.729, and those in the G.723.1 family, consume less
network bandwidth. However, low-speed codecs impair the quality of the audio much more
than high-speed codecs, because they compress the digital transmission with lossy compression
– compression that loses some of the original data. Fewer bits are sent, so the receiving
side does its best to approximate what the original audio sounded like, but it’s not a highfidelity
recreation.
The table below describes some common VoIP codecs. The middle column in the table
shows the rate at which the codec generates its output. The “Packetization Delay” column
refers to the delay a codec introduces as it converts from analog to digital and back. We’ll
see in later chapters that this fixed amount of delay can affect the quality of the call as perceived
by the listeners.
Codec
Name
Nominal
Data Rate
Packetization
Delay
G.711u 64.0 kbps 1.0 ms
G.711a 64.0 kbps 1.0 ms
G.726-32 32.0 kbps 1.0 ms
G.729 8.0 kbps 25.0 ms
G.723.1
MPMLQ
6.3 kbps 67.5 ms
G.723.1
ACELP
5.3 kbps 67.5 ms

Codecs use sophisticated techniques for coding and compression. You’ll see names that
stretch the limits of your math background, like Multi-Pulse Maximum Likelihood
Quantization (MPMLQ) and conjugate structure Algebraic Code Excited Linear Predictive
(ACELP) compression. The names tell how the codecs do their job; consider these topics
beyond the scope of this book.
Packet loss concealment (PLC) is an additional feature available with the G.711u or G.711a
codecs. PLC techniques reduce or mask the effects of data loss during a telephone conversation.
PLC does not add delay or have bad side-effects, but it makes the G.711 codecs
more expensive to manufacture. Because of its cost, PLC is relatively rare today.