
Efficient Networking 161
IP Telephony Configuration Guide
Each site supports four VoIP ports. Assume the codex is G.729 Annex B, 20 ms payload.
Assuming full-duplex links, peak bandwidths per call are between 24.8 kbit/s and 27.6 kbit/s peak
transmission or approximately 28 kbit/s. This is shown in the following figure, taken from the
table under “Bandwidth requirements on full duplex links” on page 156.
Figure 57 Network loading bandwidth
Route R1-R2 needs to support four VoIP Calls. R4-R5 needs to support eight VoIP calls. The
incremental peak bandwidth for VoIP traffic is therefore:
R1-R2 peak VoIP Load = 4(28 kbit/s) = 112kbit/s
R4-R5 peak VoIP Load = 8(28kbit/s) = 224kbit/s
With Business Communications Manager VoIP gateway bandwidth requirements and
Traceroute measurements, the R4-R5 link is expected to support the Santa Clara/Richardson,
Santa Clara/Tokyo and the Ottawa/Tokyo traffic flows. The other IP telephony traffic flows do not
route over R4-R5. A peak of eight calls can be made over R4-R5 for the four IP telephony ports
per site. R4-R5 needs to support the incremental bandwidth of 8 x 12 = 96 kbit/s.
To complete this exercise, the traffic flow from every site pair needs to be summed to calculate the
load on each route and loaded to the link.
Enough link capacity
The following table sorts the computations so that for each link, the available link capacity is
compared against the additional IP telephony load. For example, on link R4-R5, there is capacity
(568 kbit/s) to allow for the additional 96 kbit/s of IP telephony traffic.
Codec Type
Payload
Size
PPP B/W FR B/W
No SP
Silence
Suppression
No SP
Silence
Suppression
ms peak
(kbit/s)
peak
(kbit/s)
Avg
(kbit/s)
peak
(kbit/s)
peak
(kbit/s)
Avg
(kbit/s)
G.729
(8 kb/s) 30 19.2 9.6 21.1 10.6
Table 41 Link capacity example
Link Utilization (%)
Available
capacity
kbit/s
Incremental IP telephony load
Enough
capacity?
End
Points
Capacity
kbit/s Threshold Used Site pair
Traffic
kbit/s
R1-R2 1536 85 75 154 Santa Clara/Ottawa
Santa Clara/Tokyo
15.5 Yes
R1-R3 1536
R2-R3 1536
R2-R4 1536
R4-R5 1536 85 48 568 Santa Clara/Richardson
Ottawa/Tokyo
Santa Clara/Tokyo
24 Yes
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