NEC UNIVERGE SV9100 Manual page 8

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cause "choppy" speech as the WAN link will suffer from congestion. In this case, a lower bandwidth
CODEC (such as G.729) may be more appropriate.
2.6 Bandwidth?
Available bandwidth has a major influence on voice quality in VoIP networks. Bandwidth is usually
expressed in the number of bits per second (bps) that can be transmitted over a network link. The
amount of bandwidth is usually limited by the service provider or the physical cables that are used for
transmission.
Examples of bandwidth:
Ethernet LAN 100Mbps
ISDN (one B-Channel) 64Kbps
ADSL 512Kbps/8Mbps (see asymmetric bandwidth section)
Bandwidth is typically shared between VoIP and other data applications. It is important to take into
account the amount of bandwidth required for voice and data applications. The amount of bandwidth
required for VoIP calls depends on several factors, including:
· Number of simultaneous calls
· CODEC used
· Frame Size
· Data Networking Protocol used
The more frames we encapsulate into each packet, the less bandwidth is required. This is because
each packet transmitted has the same header size - therefore if lots of very small packets are sent
we are also using bandwidth for lots of header information. If we add several frames to the packet, we
have fewer packets transmitted and therefore have less header information sent.
If we add lots of voice frames to each packet we know that we are using less bandwidth, but this
does have disadvantages. If we have a large packet size, and a particular voice packet is lost, this
will have a greater impact on the speech quality. If we are using small quantity of voice frames per
packet the effect of losing a packet is reduced.
As a general rule: The more frames we have per packet, the less bandwidth is used, but the quality
is lower.
Example one:
CODEC: G.729
Frame Size: 10ms
Voice Frames per Packet: 2
Voice Sample Size: 20ms (frame size x Voice Frames) Bandwidth Required: 24Kbps