Specifications
Network Requirements and Preparation
9 – 12 ShoreTel, Inc.
Resultant Voice Quality
As stated earlier, the ShoreTel 6.1 system has been recognized for excellent voice
quality. This is a result of the excellent hardware and software design that minimizes
latency and dynamically adapts to the effects of jitter, packet loss, and echo introduced
by the network.
There are two subjective testing methods that are used to evaluate voice quality. A
method called Mean Opinion Score (MOS) is an open test in which a variety of listeners
judge the quality of a voice sample on a scale of 1 (low) to 5 (high). There is general
industry agreement on the theoretical maximum MOS value on a per codec basis that
can be achieved (see Table 9-7).
Both the MOS test method and an interactive test method were used by Miercom. The
interactive test focused on the conversational quality of the call. The results are shown
in Table 9-8. The ShoreTel MOS scores are higher than the industry-standard values.
This is likely a result of the subjective nature of the head-to-head test, which scores a
relative ranking rather than an absolute ranking.
WAN Technology Choices
Minimum Bandwidth Requirements
The minimum WAN bandwidth required to deploy a voice switch at a site depends on
the number of calls expected. With ADPCM, a single call consumes 52 Kbps, and if this
call becomes a conference call, another 52 Kbps is needed, yielding a total of 104 Kbps.
From a broadband perspective, the first available technology is 128 Kbps (ISDN),
which leaves only 24 Kbps for other IP traffic. For teleworking applications, where only
a single call is needed, 128 Kbps can be used. For other sites on the voice network, the
minimum bandwidth recommended is 384 Kbps.
Table 9-7 Theoretical MOS Scores
Codec Data Rate (Kbps) MOS
Linear 128 4.5
G.711 64 4.1
ADPCM 32 3.85
G.729a 8 3.85
Table 9-8 ShoreTel MOS and Interactive Test Results
Codec Data Rate (Kbps) MOS Interactive
Linear 128 Not tested Not tested
G.711 64 4.46–4.87 4.66
ADPCM 32 3.96–4.05 4.33
G.729a 8 Not tested Not tested