LWC Training Corp.

Voice over IP Online Course

Lesson 4 - Quality of Service

Review: Providing QoS

This module looked at the issue of Quality of Service in so far as Voice over IP is concerned.

Section A: QoS factors
This section looks at the major factors which affect QoS, particularly bandwidth, delay, jitter, lost packets and echo. The relationship between delay and compression is examined.

Section B: Bandwidth - Share or restrict
Bandwidth is a major factor in QoS simply because if affects the other factors. Two approaches can be taken to a lack of bandwidth. The first approach simply restricts connections when bandwidth runs out. This approach is used by IntServ and also ATM and Frame Relay. The other approach is to let all traffic on but give priority to traffic that is delay sensitive. This is the approach taken by DiffServ.

Section C: Providing QoS
This section identifies some of the techniques used to provide QoS. Over provisioning is a simple answer to bandwidth woes but is not practical for expensive WAN links. Rationing bandwidth is the answer here. Different techniques are used to tackle delay, jitter and lost packets.

Section D: Packet Level Techniques for Providing QoS
This section looks at the changes made to packets by various QoS techniques. 802.1Q/p adds an extra field to the packet as does MPLS. Precedence and DiffServ make use of the ToS field already present in the IP header.

Exercise 4-1: Quality of Service

1. Compressing voice data may deteriorate the quality of the signal by introducing
a) Jitter
b) Echo
c) Noise
d) Delay

2. Which of the following techniques restricts access to the data channel when there is not enough bandwidth?
b) ToS
c) DiffServ
d) 802.1p

3. IP Precedence and DiffServ are mutually exclusive because
a) Both use the same queues in the router switch
b) IP Precedence is a proprietary protocol of Cisco whereas DiffServ is controlled by the IETF
c) They both use the same ToS field in the IP header
d) Routers are configured for DiffServ whereas switches are configured for IP Precedence

4. Which one of the following does not make use of information in the voice packet or change the voice packet structure in order to implement QoS?
a) 802.1p
d) DiffServ

5. If a single packet in a voice conversation were lost, how would the user notice the effect?
a) He wouldn’t notice
b) There would be a hiss on the line
c) There would be a gap in the conversation
d) The person on the other end would repeat what he said

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