Nortel VoIP Gateway, Configuration Manual

Page: 191 / 220

Implementing QoS in IP networks 191
Norstar VoIP Gateway Configuration Guide
If the intranet capacity is tight, and the IP telephony traffic is important, the installer or
administrator must consider making intranet measurements under load. Apply load using traffic
generator tools. The amount of load must match the IP telephony offered traffic estimated in the
VoIP Gateway Bandwidth requirements.
Decision: does the intranet meet IP telephony QoS needs?
The end of the measurement and analysis is a good indicator of whether the corporate intranet can
deliver acceptable voice and fax services. The Expected QoS level column in the table under
“Measurement procedure” on page 190 indicates to the installer or administrator the QoS level for
each site pair with the data.
To provide voice and fax services over the intranet, keep the network within a Good or Excellent
QoS level at the Mean+
σ
operating area. Fax services must not travel on routes that have Fair or
Poor QoS levels.
If QoS levels of some or all routes fall short of being Good, evaluate options and costs for
upgrading the intranet. The evaluation often requires a link upgrade, a topology change, or
implementation of QoS in the network.
To maintain costs, you can accept a Fair QoS level for the time for a selected route. A calculated
trade-off in quality requires the installer or administrator to monitor the QoS level, reset needs
with the end users, and respond to user feedback.
Implementing QoS in IP networks
This section describes information about implementing QoS in IP networks:
• “Traffic mix” on page 192
• “TCP traffic behavior” on page 192
Corporate intranets are developed to support data services. Accordingly, normal intranets are
designed to support a set of QoS objectives dictated by these data services.
When an intranet takes on a real-time service, users of that service set additional QoS objectives in
the intranet. Some of the targets can be less controlled, compared with the targets set by current
services, while other targets are more controlled. For intranets not exposed to real-time services in
the past, but which now need to deliver IP telephony traffic, QoS objectives for delay can set an
additional design restriction on the intranet.
One method of determining requirements is to subject all intranet traffic to additional QoS
restrictions, and design the network to the strictest QoS objectives. An exact plan for the design
improves the quality of data services, although most applications cannot identify a reduction of,
say, 50 ms in delay. Improvement of the network results in a network that is correctly planned for
voice, but over planned for data services.
Another plan is to consider using QoS in the intranet. This provides a more cost-effective solution
to engineering the intranet for non-homogenous traffic types.