NEC UNIVERGE SL2100, Networking Manual

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5.1 CODECs
CODEC (COder/DECoder) uses the technology of encoding and decoding a signal. For VoIP, this
specifically refers to the algorithm used to convert analog speech to digital data for transmission on an
IP network.
The NEC SL2100 system supports three different CODECs:
• G.711
This is the ITU-T recommendation for coding of speech at 64kbps using PCM (pulse code
modulation). This CODEC is often described as uncompressed as it uses the same sampling rate as
Time-Division Multiplexing (TDM). G.711 has a MOS
**1
score of 4.2 but uses a large bandwidth for
transmission. This CODEC is not commonly used due to the bandwidth required, although it can be
acceptable in LAN environment (i.e., IP Phones connected over a 100Mbps LAN).
• G.722
G.722 is an ITU standard CODEC that provides 7kHz wideband audio at data rates from 48 to
64kbps. This is useful in a fixed network Voice Over IP applications, where the required bandwidth
is typically not prohibitive, and offers a significant improvement in speech quality over older
narrowband CODECs such as G.711, without an excessive increase in implementation complexity.
• G.726
G.726 is an ITU-T ADPCM speech CODEC standard covering voice transmission at rates of 16, 24,
32, and 40kbit/s. It was introduced to supersede both G.721, which covered ADPCM at 32kbit/s,
and G.723, which described ADPCM for 24 and 40kbit/s. G.726 also introduced a new 16kbit/s rate.
The four bit rates associated with G.726 are often referred to by the bit size of a sample as 2-bits, 3-
bits, 4-bits, and 5-bits respectively.
• G.729A
This ITU-T recommendation describes the algorithm for coding of speech signals at 8kbps using
CS-ACELP (conjugate-structure algebraic code-excited linear prediction). This CODEC samples the
analog signal at 8000Hz and uses a frame size of 10ms. This CODEC has a MOS score of 4.0.
G.729 is the most commonly used CODEC for NEC SL2100 VoIP installations. This is due to the
fact that it offers high compression (and therefore low bandwidth) while maintaining good speech
quality.
Packet Size:
Each CODEC has a set frame length. This is the time that the frame encapsulates. For G.729 and G.
711 the frame length is 10ms. It is possible to configure the packet size in the NEC SL2100
programming. To do this, we tell the NEC SL2100 how many frames to encapsulate into each packet
for transmission.
For example, the G.729 has a frame length of 10ms - the packet size is set to 3 (in Program 84-11-01).
This gives a 10ms x 3 = 30ms packet.
5.2 Bandwidth
The bandwidth required for VoIP calls depends on several factors, including:
• Number of simultaneous calls
• CODEC used
• Frame Size
• Data Networking Protocol used
The more frames encapsulated into each packet, the less bandwidth is required. This is because each
packet transmitted has the same header size. Therefore, if numerous very small packets are sent then
**1. The Mean Opinion Score (MOS) provides a numerical measure of the quality of human speech at the
destination end of the circuit. The scheme uses subjective tests (opinionated scores) that are mathematically
averaged to obtain a quantitative indicator of the system performance.
SL2100 ISSUE 1.0
6-8 Network Design Considerations