NEC Univerge SV9100, Manual

Page: 315 / 766

Issue 2.0
SV9100 Networking Manual 5-3
users. If packet drops become epidemic, the quality of all transmissions
degrades. Packet loss rate must be less than five percent for minimum quality
and less than one percent for toll quality.
2.2 Voice Quality Improvements
This section describes various techniques that can be used to improve the voice
quality.

Increase available bandwidth:
This can sometimes be the most basic solution and the easiest of the solutions.
If running a System IP Phone using G.711 with a 30ms fill time over Ethernet, for
only one call, 90Kbps of bandwidth is needed. If that same user only has a 64K
line, they do not have a decent IP voice call. The user can increase the
available bandwidth to slightly exceed the 90Kbps requirements and their voice
quality dramatically increases. This solution might not be viable if no more
bandwidth is available.

Use a different CODEC:
The CODEC contains possible compression algorithms to be used on the voice.
Lets take the example above again. The user only wants one voice line over a
64Kbps data connection. They also want to maintain their current fill time of
30ms. Change to a G.729. For one line, only 34Kbps is required for a call. This
fits well within the 64Kbps of available bandwidth.

Increase the number of frames per packet:
To continue with the example above, the user has moved to a G.729 CODEC.
But now, the user wishes to add two more System IP Phones. Their current
64Kbps line can handle one call, because it is only 34Kbps. Two more System
IP Phones would increase the total to 102Kbps so obviously there is not
sufficient bandwidth.
The user can now increase the fill time to 50ms. This reduces the bandwidth per
call to 19.8Kbps (3x 19.8 = 59.4Kbps). The savings in bandwidth comes from
the fact that with a longer fill time, fewer packets are needed to send the voice.
With fewer packets, less header information needs to be attached and
transmitted.

Change Layer 2 Protocols:
Ethernet is most commonly used for IP packets. Unfortunately, Ethernet has a
fairly large overhead of 34 bytes. So every IP voice packet going over Ethernet
has a 34-byte Ethernet header attached to it. As the number of packets add up,
this header data can become significant. Frame Relay has a 7-byte header and
Point-to-Point Protocol (PPP) has a 6-byte header. With this decrease in header
length at layer 2, some significant savings in bandwidth use can be achieved.
The down side to this is that most networks may not have these services
available, where Ethernet is very widely used. This is usually outside the control
of the installer and therefore NEC strongly advises users to do more research
on other layer 2 protocols before trying to implement them in their voice
network.

Implement Quality of Service (QOS):
Now, assume a derivative of the above example. The user needs only one voice
line over their 64Kbps connection. They are using G.729 with a 30ms fill time.
This requires 34Kbps of their available bandwidth. Also assume that this line is