HP Survivable Branch Communication zl Module powered by Microsoft Lync Planning and Design Guide 2011-02

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Design Considerations
Planning Audio Quality
In both the LAN and the WAN, the VoIP traffic competes with data traffic. Real-
time traffic such as VoIP traffic requires special handling due to its special
needs. VoIP traffic is low latency; it cannot tolerate delays. In addition, voice
traffic must have low jitter: that is, the delay for each packet transmitted
should be similar so that the receiver does not hear a difference in
conversation.
The traffic’s codec affects its sensitivity to jitter and lag. RTAudio, which Lync
clients typically use for local and remote Lync calls, is less sensitive to jitter
and lag than G.711, which Lync clients use for PSTN calls when Media Bypass
is enabled. (Without Media Bypass, Lync clients might use RTAudio to transmit
the PSTN calls; the Mediation Server then translates them to G.711.)
Because Media Bypass is recommended, and the branch’s calls will probably
include a mix of RTAudio and G.711-encoded calls, you should configure the
infrastructure to prioritize all VoIP traffic. This prioritization ensures that
traffic is transmitted in a timely manner. In fact, to ensure high quality, even
when all calls use RTAudio, the network infrastructure should prioritize the
traffic—in particular, on WAN links.
Ensuring QoS in the LAN
A LAN provides enough bandwidth that you might be less concerned about
QoS for VoIP traffic. However, even when, on average, the LAN provides
enough bandwidth for timely VoIP service, bursts of traffic (particularly large
data transfer packets) could cause jitter. Therefore, you should always imple-
ment QoS throughout the entire LAN infrastructure.
Layer 2 devices use 802.1p, a protocol that inserts a class of service (CoS)
mark in the 802.1Q VLAN tag, to provide QoS. The 802.1p protocol classifies
Ethernet traffic into eight CoS categories, each of which is assigned to a queue.
Switches forward traffic in queues associated with higher 802.1p values
preferentially. With 802.1p, administrators typically assign VoIP traffic to class
5 or 6.
Depending on your switches’ capabilities and configuration, they might sup-
port two, three, four, or eight queues. Most HP switches support eight by
default, but on some switches you can change the number of supported
queues. Leave the default setting when you want to define traffic more
granularly. However, fewer queues translate to larger buffers for each queue,
which can improve overall QoS. Therefore, you should generally enable just
the number of queues that you need for types of traffic that require different
handling.