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294 Avaya Application Solutions IP Telephony Deployment Guide
Overview
QoS
In particular, Quality of Service (QoS) becomes more important in a WAN environment than in a
LAN. In many cases, transitioning from the LAN to the WAN reduces bandwidth by
approximately 99%. Because of this severe bandwidth crunch, strong queuing, buffering, and
packet loss management techniques have been developed. These are covered in more detail in
the Quality of Service guidelines
chapter.
Recommendations for QoS
For many customers, including small and medium, simplicity is more effective than complex
configurations when implementing QoS for voice, data, signaling and video. If traffic engineering
is done properly and sufficient bandwidth is available, especially for WAN links, voice and voice
signaling traffic can both be tagged as DSCP 46. This Class of Service (CoS) tagging will place
both types of packets into the same High Priority queue with a minimum of effort. The key is to
have enough bandwidth to prevent any packets from dropping.
For large enterprises and Multi-National companies, a stratified approach to CoS makes sense.
This allows maximum control for many data and voice services. For this environment, Avaya
recommends using DSCP 46 (Expedited Forwarding) for voice (bearer), but voice signaling and
especially IPSI signaling could have their own DSCP values and dedicated bandwidth. This
would prevent traffic, like voice bearer from contending with signaling. Although the
configuration may be more complex to manage and administer, the granularity will give the best
results and is recommended as a best practice.
At the routers, Avaya recommends using strict priority queuing for voice packets, and
weighted-fair queuing for data packets. Voice packets should always get priority over
non-network-control data packets. This type of queuing is called Class-Based Queuing (CBQ)
on Avaya data networking products, or Low-Latency Queuing (LLQ) on Cisco routers.
Codec selection and compression
Because of the limited bandwidth that is available on the WAN, using a compressed codec
allows much more efficient use of resources without a significant decrease in voice quality.
Avaya recommends that IP Telephony implementations across a WAN use the G.729 codec
with 20-ms packets. This configuration uses 24 Kbps (excluding Layer 2 overhead), 30% of the
bandwidth of the G.711 uncompressed codec (80 Kbps). For more information on bandwidth,
see IP
bandwidth and Call Admission Control on page 216.