NEC SL1100, Сетевое руководство

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NEC SL1100 Issue 6.0
Networking Manual 6 - 11
The common scenario for remote IP deployment is:

Implementation of an IP Phone with a public IP address talking with an
NEC SL1100 behind NAT. An example would be a telecommuter.
Implementation of an IP Phone behind a NAT, which connects to the
Internet, terminates in a NEC SL1100 behind a different NAT.
When selecting VPN equipment it is important to consider Quality of
Service. Generally, VPN hardware is connected to Internet connections
which are unreliable and out of the control of the customer. However, it is
possible to set prioritization on some VPN units for voice traffic. This does
not solve the unreliability of the Internet, but helps to ensure that the data
traffic to and from the LAN do not impair the quality of the voice traffic.
(Refer to
Section 2 QoS on page 6-1 ).

NEC strongly recommends that any VPN hardware used for VoIP has the
facility to prioritize voice traffic.
S
ECTION
5
CODEC
AND
B
ANDWIDTH
This section describes CODEC and bandwidth and their application with the NEC
SL1100 system.
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 SL1100 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
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.