Section 6 Quality Of Service (Qos) Implementation; Figure 6-3 Network Bottleneck Example - NEC Sl2100 Networking Manual

ISSUE 1.0 SL2100
bandwidth is also being used for a large amount of header information. If we add several frames to the
packet, less packets are transmitted and therefore have less header information sent.
If we add many voice frames to each packet, less bandwidth is being used. However, this does have
disadvantages. If there is a large packet size, and a particular voice packet is lost, this has a greater
impact on the speech quality. If a small quantity of voice frames per packet is being used, the effect of
losing a packet is reduced.
As a general rule: The more frames per packet, the less bandwidth is used, but the quality is also
lower.
Examples:
Example 1: CODEC: G.729 Frame Size: 10ms Voice Frames per Packet: 2 Voice Sample Size: 20ms
(frame size x Voice Frames) Bandwidth Required: 24Kbps
Example 2: CODEC: G.729 Frame Size: 80ms Voice Frames per Packet: 8 Voice Sample Size: 80ms
(frame size x Voice Frames) Bandwidth Required: 12Kbps
S 6 QUALITY OF SERVICE (Q S) IMPLEMENTATION
ECTION O
Section Voice Quality Improvements on page 6-2 discusses some of the problems associated with
voice quality. This section describes how QoS can be implemented on data networks to provide the
"best case" for VoIP traffic.
Not all network hardware supports QoS and each manufacturer has their own methods of
implementing QoS. The explanations below are as generic as possible. The installer/maintainer of the
data network should be familiar with the QoS characteristics of their equipment and should be able to
configure the equipment accordingly.
Quality of Service is commonly used to describe the actual implementation of prioritization on network
hardware. This prioritization (at Layer 2 and Layer 3 of the OSI model) is described in Figure 6-1 Layer
2 Diagram (802.1Q) on page 6-3.
6.1 Prioritization
When data is transmitted through a network, bottlenecks can occur causing the available bandwidth to
be reduced or the data to increase. This impacts the packet delivery.
Consider data communication between the two computers shown in the diagram Figure 6-1 Layer 2
Diagram (802.1Q) on page 6-3. The Hosts can transmit data at 100 Mbps. When a packet from Host
A, destined for Host B, reaches the router, the available bandwidth is reduced to 256Kbps and the
packet flow must be reduced. Figure 6-3 Network Bottleneck Example on page 6-10 shows a
diagram of this condition.
Networking Manual 6-9