Information Technology Reference
In-Depth Information
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.726 (24)
80
38
27
25
17
G.726 (24)
60
(default)
42
27
27
18
G.728 (16)
80
25
18
17
12
G.728 (16)
40
(default)
35
19
23
13
G.729 (8)
40
17.2
9.6
11.2
6.3
G.729 (8)
20
(default)
26.4
11.2
17.2
7. 3
G.723.1 (6.3)
48
12.3
7. 4
8.0
4.8
G.723.1 (6.3)
24
(default)
18.4
8.4
12.0
5.5
G.723.1 (5.3)
40
11.4
6.4
7. 4
4.1
G.723.1 (5.3)
20
(default)
17.5
7. 4
11.4
4.8
Calculating Voice Bandwidth
The CCDA test expects the designer to be able to calculate some basic voice bandwidth
estimates. Use the following assumptions when calculating voice bandwidth:
IP/UDP/RTP header uses 40 bytes.
■
cRTP reduces the IP/UDP/RTP header to 2 or 4 bytes.
■
The WAN Layer 2 header adds 6 bytes on a point-to-point circuit.
■
Vo i c e p a c k e t s i z e = ( L a y e r 2 h e a d e r ) + ( I P/ U D P/ RT P h e a d e r ) + ( v o i c e p a y l o a d ) .
■
Vo i c e p a c k e t s p e r s e c o n d ( p p s ) = c o d e c b i t r a t e / v o i c e p a y l o a d s i z e .
■
Vo i c e b a n d w i d t h ( b p s ) = ( v o i c e p a c k e t s i z e ) * ( p p s ) .
■
As an example, calculate the WAN bandwidth used at a site that will have 10 concurrent
G.729 calls with cRTP and a default voice payload of 20 bytes.
From this description, we obtain the following:
G.729 codec is used: 8 kbps codec bit rate.
■
cRTP = 2-byte IP/UDP/RTP header.
■
