Configuring QoS for VoIP on a LAN

Configuring QoS for VoIP on a LAN
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While VoIP doesn’t require a lot of bandwidth, good voice quality requires a network environment that delivers low latency and jitter and keeps packet loss to a minimum. In practical terms, that means a quality of service (QoS) mechanism engineered to assure that competing traffic — think file transfer or backup functions — doesn’t demand so much network bandwidth that VoIP traffic is hamstrung and session voice quality suffers as a result.
VoIP QoS is particularly crucial at points where one network device is connected to another or to the internet. This requires two separate processes:
VoIP QoS is particularly crucial at points where one network device is connected to another or to the internet.
At the VoIP application level, IT staff has to configure the application to use the priority bits of the 802.1p Layer 2 header and — usually — the priority bits of the TCP Layer 3 header. This is how the application signals it wants a higher priority for its traffic.
Every transfer point — switch, router, access point (if wireless LAN clients are used) — has to be configured to match the priority queues in these devices to priorities set by the applications.
If this sounds complicated, it’s because it is complicated. Moreover, it never worked. For starters, Layer 2 QoS tags vanish when the VoIP packet hits a Layer 3 boundary. This disappearing act made them close to useless and explained why TCP priority tags were also required since they could cross Layer 3 boundaries. Still many applications did not set tags, and virtually all switches and routers required manual QoS configuration.
VoIP overseen more effectively
Today, VoIP over LAN can be managed much more effectively. First, QoS is automatic. Second, wired and wireless LANs are engineered with so much bandwidth that session traffic can happily coexist without even approaching saturation.
But, if some sort of traffic management is needed, it is applied automatically through real-time packet inspection. Most network devices — even low-end ones — are powerful enough and smart enough to diagnose the applications funneling through them. Once VoIP over LAN traffic is detected, the devices place that traffic in high-priority queues. In the event congestion occurs, the device allocates additional bandwidth to resolve potential quality issues.
To ensure good performance, check the configuration settings for each device across your VoIP over LAN deployment. Many will likely default to providing QoS. With others, you may need to enable QoS options, but those controls should be applied automatically from that point on. It is rare to find any device requiring manual queue configuration, though some may offer that option for more advanced needs.
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