Chapter 6 Network Design Considerations; Section 1 Introduction; Section 2 Qos - NEC Sl2100 Networking Manual

Network Design Considerations
S 1 INTRODUCTION
ECTION
This chapter explains some issues that should be considered when planning a NEC SL2100 VoIP
installation. This is a generalized explanation and therefore does not discuss vendor-specific issues
and solutions. Typically, different solutions are implemented by different manufacturers.
S 2 Q S
ECTION O
Quality of Service (QoS) is one of the most important factors for VoIP. This refers to the perceived
quality of speech and the methods used to provide good quality speech transmission. Several factors
that affect speech quality and several mechanisms can be used to ensure QoS.
This chapter also describes the problems that can occur and some possible solutions. Each network
equipment manufacturer (NEC, 3Com, Cisco, etc.) has slightly different methods of implementing QoS
and these are not discussed in this document. This chapter provides an overview to classify voice
traffic on the NEC SL2100 so that the network equipment can impose QoS.
2.1 QoS Definitions
This section lists common definitions used with QoS for VoIP.
Latency (Delay):
If at any point the usage on the network exceeds the available bandwidth, the user experiences delay,
also called latency. In more traditional uses of an IP data network, the applications can deal with this
latency. If a person is waiting for a web page to download, they can accept a certain amount of wait
time. This is not so for voice traffic. Voice is a real time application, which is sensitive to latency. If the
end-to-end voice latency becomes too long (150ms, for example), the call quality is usually considered
poor. It is also important to remember that packets can get lost. IP is a best effort networking protocol.
This means the network tries to get the information there, but there is no guarantee.
Delay is the time required for a signal to traverse the network. In a telephony context, end-to-end
delay is the time required for a signal generated at the talker's mouth to reach the listener's ear.
Therefore, end-to-end delay is the sum of all the delays at the different network devices and across
the network links through which voice traffic passes. Many factors contribute to end-to-end delay,
which are covered next.
The buffering, queuing, and switching or routing delay of IP routers primarily determines IP network
delay. Specifically, IP network delay is comprised of the following:
• Packet Capture Delay
Packet capture delay is the time required to receive the entire packet before processing and
forwarding it through the router. This delay is determined by the packet length and transmission
speed. Using short packets over high-speed networks can easily shorten the delay but potentially
decrease network efficiency.
Networking Manual
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