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Selecting the Proper Voice Compression
The best voice quality is achieved by using the ADPCM 32 compression. The maximum quality comes at
the expense of the highest utilized bandwidth. The largest voice compression is achieved by using
G.729A. If you are using this algorithm, you will save on bandwidth and still achieve voice quality that is
regarded as near toll. If absolute conversation quality is your focus and bandwidth is no object, you
probably want to select ADPCM 32. If bandwidth is a priority you will employ G.729A.
If you are using different compression algorithms for each individual user (port), use the following formula
to establish your aggregate data bandwidth needs.
A
B
x 16 +
x 32 +
A: number of G.729A
B: number of ADPCM 24
C: number of ADPCM 32
D: Total Bandwidth
Divide this bandwidth by either 64 or 56, in order to establish the correct number of DS0 channels to be
used.
Example: If your DS0s on your CSU/DSUs are set up for 56Kbps, use 56 and if they are set up for
64Kbps DS0s, use 64.
From the website, http://www.MCK.com/, you can use a Bandwidth Calculator. This Calculator
automatically determines the bandwidth needed for your applications when you enter values for the
following:
•
Number of telephones
•
Name and version of the PBX/KSU.
With the help of the Bandwidth Calculator, you can quickly create tables like the one below.
Table 14:
Computed EXTender IP Bandwidth
Compression Type (w/phone signaling)
# Phones
G.729a
@13k
4
52k
8
104k
12
156k
16
208k
24
312k
Note: These values were calculated using an overhead of 40, and 2 packets per window. By default, the
PBXgateway II is set to 2 packets per window.
PBXgateway II System Administrator's Guide
C
D
x 40 =
ADPCM24
ADPCM32
@31k
@40k
124k
160k
248k
320k
372k
480k
496k
640k
744k
960k
G.711
@77k
308k
616k
924k
1232k
1848k
172
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