Getting Started
Ensuring Voice Quality
Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide 35
1
Bandwidth Requirements
Depending on how you have your IP phones configured, each call requires 55 to
110 kbps of the bandwidth in each direction. For example, using G.729 with an
average business-grade broadband Internet connection supporting 1.5 Mbps
downstream and 384 kbps upstream, a total of seven (7) simultaneous
conversations can be reliably supported with adequate bandwidth available for
file downloads.
We recommend using Cisco SPA IP phones with QoS-capable networking
equipment that can prioritize the VoIP traffic. A QoS-enabled device prioritizes the
packets going upstream to the ISP.
The table approximates the bandwidth budget for each side of the conversation
(in each direction) using different codecs and the number of calls the network
might support. The table is based on the following assumptions:
• Bandwidth calculated with no silence suppression, as the use of silence
suppression can reduce the average bandwidth budget by 30 percent or
more.
• 20 millisecond of payload per RTP packet
For more information about bandwidth calculation, refer to the following web sites:
http://www.erlang.com/calculator/lipb/
http://www.packetizer.com/voip/diagnostics/bandcalc.html
Codec Estimated
Bandwidth
2 Calls 4 Calls 6 Calls 8 Calls
G.711 110 kbps 220 kbps 440 kbps 660 kbps 880 kbps
G.722 110 kbps 220 kbps 440 kbps 660 kbps 880 kbps
G.726-40 87 kbps 174 kbps 348 kbps 522 kbps 696 kbps
G.726-32 79 kbps 158 kbps 316 kbps 474 kbps 632 kbps
G.726-24 71 kbps 142 kbps 284 kbps 426 kbps 568 kbps
G.726-16 63 kbps 126 kbps 252 kbps 378 kbps 504 kbps
G.729 55 kbps 110 kbps 220 kbps 330 kbps 440 kbps
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