Nortel Networks Nortel Business Services Gateway BSGX4e - Manual (Page 26)

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Configuration process 3 Wizard pages
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QoS Groups
The Voice QoS group protects the data packets traveling to and from your VoIP phones. The
Control QoS group protects PPP and ARP signals that are needed to maintain an Internet
connection.
The default value for the Control group is 64,000 bps. Do not change this value.
The default value for the Voice group is 500 000 bps, which protects between
(approximately) 4 and 13 simultaneous calls, based on each VoIP call requiring between
approximately 39 000 bps to 133 000 bps of bandwidth.
The variation in the size of calls is due to different encapsulation types, and because VoIP
phones negotiate which codec to use. A codec is a program that determines the level of
compression of the VoIP traffic stream. VoIP phones decide which codec to use with each
call, and different codecs have different compression rates.
You may want to recalculate the Voice group if your quoted or actual upstream bandwidth is
much different than the default 800,000 bps.
‰
If the upstream rate is actually lower than the default value, you probably do not have to
change the Voice group bandwidth. Its 500,000 value works unless your upstream actual
rate drops below 626,000. It is not common to have your bandwidth drop that low.
‰
If the upstream rate is actually higher than the default value and you want to protect
more simultaneous calls, you can perform a simple calculation to determine the
maximum value you can enter into the Voice Bandwidth field:
— Calculate 90 percent of the upstream QoS bandwidth.
— Subtract 64 000 (for the Control group).
The result is the maximum value for the Voice Bandwidth field.
Here is an example calculation:
The maximum value you can enter into the Voice bandwidth field is 836000. At this
bandwidth:
The highest compression codec sustains [836,000 ÷ 39,200] 21 calls.
The lowest compression codec sustains [836,000 ÷ 132,800] 6 calls.
Typical VoIP traffic is a combination of calls using different compression rates, although the
high-compression codecs are more common. You need to consider the number of
simultaneous calls you want to support compared to your available bandwidth, and enter an
appropriate value up to the maximum that you just calculated.
Entering the maximum value into the
Voice Bandwidth field might not restrict bandwidth for
other functions. QoS actually reserves only the bandwidth needed for the current number of
calls and allows the remaining bandwidth to be available for other, non-protected functions
such as email and Web browsing. So you can be generous in reserving bandwidth without
Your service provider informs you that your upstream rate is
about 1 Mbps (1,000,000 bps).
a. 1,000,000 × 0.9 = 900,000
b. 1,080,000 – 64,000 = 836,000