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Guest Column
Voice over Packet Protocols
VoIP and VoATM (VoAAL1, VoAAL2)
(continued)
The
table below summarizes the status of the common VoP protocols
(VoAAL1, VoAAL2, VoIP) in regards to voice encoding and
transmission delays.
|
Protocols
Issues
and Challenges |
| |
VoAAL1
(same
applies to legacy VoTDM services) |
VoAAL2 |
VoIP |
Encoders
Waveform Support:
G.711 / G.726 |
Yes |
Yes |
Yes |
Encoders
Low Bit Rate Coder Support:
G.723/G.728/
G.729/etc. |
No |
Yes |
Yes |
Delays
Echo Impairment |
No
- Generally do not need echo cancellation |
Borderline
- May need echo cancellation; depends on configuration |
Yes
- Generally needs echo cancellation |
Delays
Interactive Speech Impairment*
*
There is no cure for this type of
impairment |
No |
No |
-
No with G.711/G.726
-
Borderline
with low bit rate coders G.723/8/9 in complex networks |
Table 1: VoP voice encoding and transmission delays
Encoders - On the
encoder front, VoAAL1 generally only supports waveform
encoders G.711 and G.726. VoAAL2 and VoIP support all commonly
used voice encoders: waveform encoders such as G.711/G.726, or
complex vocoders such as G.723, G.728, G.729, EFR (also
referred to as low bit rate coders).
Delays
- On the delay front, VoAAL1 is the only Voice over Packet
protocol that can generally avoid the need for universal echo
cancellation because it intrinsically generates small delays.
In the case of VoAAL2, the delays generated are most often
borderline. Depending on the system design or configuration,
the use of universal echo cancellation may or may not be
prescribed. In the case of VoIP, because of the typical
magnitude of delays generated, the use of universal echo
cancellation is usually prescribed. Furthermore, when complex
vocoders (G.723, G.728, G,729) are used with VoIP, overall
end-to-end network system design must be very carefully
controlled to avoid transmission delays beyond the 150-200
milliseconds threshold where interactive speech impairment
starts to become a quality impairment for which there is no
technical cure.
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