Information Technology Reference
In-Depth Information
Bandwidth
Vo I P c a l l s n e e d t o m e e t b a n d w i d t h a n d d e l a y p a r a m e t e r s . T h e a m o u n t o f b a n d w i d t h r e -
quired depends on the codec used, the Layer 2 protocol, and whether voice-activity detec-
tion (VAD) is enabled. For the purpose of call control, you can use the following
bandwidth requirements (minimum values) for VoIP design:
G.729 calls use 26 kbps.
■
G.711 calls use 80 kbps.
■
When you're designing for VoIP networks, the total bandwidth for voice, data, and video
should not exceed 75 percent sustained of the provisioned link capacity during peak
times. Best practice is to provision/plan for no more than one-third of any links for the pri-
ority queue/real time traffic. Use the following formula to provision interface speeds:
Link capacity = [required bandwidth for voice] + [required bandwidth for video] + [re-
quired bandwidth for data]
The remaining bandwidth is used by routing, multicast, and management protocols.
VA D
As we listen and pause between sentences, typical voice conversations can contain up to
60 percent silence in each direction. In circuit-switched telephone networks, all voice calls
use fixed-bandwidth 64-kbps links regardless of how much of the conversation is speech
and how much is silence. In multiservice networks, all conversation and silence is packe-
tized. Using VAD, you can suppress packets of silence. Silence suppression at the source
IP telephone or VoIP gateway increases the number of calls or data volumes that can be
carried over the links, more effectively utilizing network bandwidth. Bandwidth savings
are at least 35 percent in conservative estimates. VAD is enabled by default for all VoIP
calls. In real-world practice, is it suggested that VAD be avoided because it creates quality
issues and breaks applications such as fax and modem transmissions.
Ta ble 1 4 -1 2 shows how much bandwidth is required based on different parameters. Notice
that for G.729 bandwidth is reduced from 26.4 kbps to 17.2 kbps with VAD and to 7.3
kbps with VAD and cRTP enabled.
Ta b l e 1 4 -1 2
Vo I P B a n d w i d t h R e q u i r e m e n t s w i t h c R T P a n d VA D
Te c h n i q u e
Codec Bit
Rate (kbps)
Payload
Size
(Bytes)
Bandwidth
Multilink PPP
(MLP) or FRF.12
(kbps)
Bandwidth with
VAD MLP or
FRF.12 (kbps)
Bandwidth with
cRTP and VAD
MLP or FRF.12
(kbps)
G.711 (64)
240
76
50
43
G.711 (64)
160
(default)
83
54
44
G.726 (32)
120
44
34
29
22
G.726 (32)
80
(default)
50
35
33
23
