|
Guest Column
Voice over Packet Protocols
VoIP and VoATM (VoAAL1, VoAAL2)
(continued)
Bandwidth
Requirements
The table below summarizes
and compares the absolute bandwidth performance of the VoAAL1,
VoAAL2, and VoIP protocols. The performance of legacy TDM
networks, where 64 kbps bandwidth for voice is typically
reserved, is also included for reference.
|
Bandwidth
Efficiency
|
| |
G.711
kbps/Figure
of merit(1) |
G.726
(Compression = 2:1)
kbps/Figure
of merit(1)
|
Complex
Vocoders
(G.723
/ G.729)
(Compression
= 8:1)
kbps/Figure
of merit(1)
|
|
Legacy
TDM
(ex:
T1 circuits) |
64kbps
/ 1 |
32kbps
/ .5 |
Not
Applicable |
|
VoAAL1
(One
ch. per VC) |
72kbps
/ 1.12
(6
ms packet)
|
36kbps
/ .56
(12
ms packet)
|
Not
Applicable |
| VoAAL2 |
77kbps
/1.20
(5.4
ms packet. delay – 44
byte CPS packet)
|
38kbps
/.59
(11
ms packet. delay – 44
byte CPS packet)
|
12kbps
/ .19
(20
ms packet. delay – 20
byte CPS packet)
|
| VoIP |
80kbps
/ 1.25
(20
ms packet. delay - 160
byte payload IP packet) (2)
|
48kbps
/.75
(20
ms packet. delay - 80
byte payload IP packet) (2)
|
24kbps
/.37
(20
ms packet. delay - 20
byte payload IP packet) (2)
|
|
(1)
“1” is equivalent to 64kbs. The lower the number,
the better the relative efficiency.
(2)
Packet payload size is restricted to limit the
packetization delay to 20 milliseconds. Header is fixed
at 40 bytes. |
Table 2: VoP bandwidth performance
G.711/G.726 - Whenever G.711 or G.726 encoders are used, VoAAL1 theoretically offers the best possible performance. In the case of trunking applications, this is true only when all channels are active on a VoAAL1 circuit. In practice however, since normally all channels on a trunk are never fully active, VoAAL1 is actually less efficient than VoAAL2 and VoIP. In cases where silence suppression is used VoAAL1 is even less efficient, as silence suppression is a feature not supported by this protocol. As a result, VoAAL2 and VoIP are always (even theoretically) more efficient than VoAAL1. Note that typically VoAAL2 is slightly more efficient than VoIP because ATM packets have smaller headers relative to standard payloads than IP packets.
Complex Vocoders - VoAAL1 does not support complex vocoders, so it is not in the race here. As can be readily seen in Table 2, VoAAL2 provides better bandwidth performance than VoIP for otherwise equal performance (i.e. equal delays). This is because ATM packets feature smaller headers than IP packets and also because VoAAL2 supports trunking. In the end however, although the above comparisons are interesting, this performance disparity in favor of VoAAL2 will have very little bearing on which protocol/encoder pairs are retained for broad deployment. Other factors more related to the delays, echo impairment, and popularity of protocol preference will prevail.
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