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When do I need to use QoS for VoIP bandwidth support?

Is your call quality suffering? Network bandwidth may be growing, but demands on it are, too. Here's what you need to know to use QoS to help solve VoIP call issues.

Meeting VoIP bandwidth requirements is more important than ever. Fortunately, today's networks have more bandwidth than ever.

In the LAN, 100 Mbps -- or even Gigabit Ethernet -- typically connects endpoints. Wireless networks based on the 802.11n or newer 802.11ac standards support hundreds of megabits of bandwidth. Backbone networks are commonly 10 Gb, and even WAN speeds are exploding, with Gigabit services increasingly available.

Given all this bandwidth, network engineers may wonder if they still need quality of service (QoS) for VoIP bandwidth support. The answer is a resounding yes. Why? Because applications that use large amounts of bandwidth, such as video conferencing and streaming, are increasing almost as fast as bandwidth capacity expands.

While calls actually require little VoIP bandwidth -- typically about 30 Kbps for wideband compressed codecs -- applications like data transfers and video can quickly consume available bandwidth, leading to VoIP quality issues.

Limitations of QoS for VoIP

The biggest culprit in voice quality issues is jitter -- the variation in delay across the network. Spikes in available VoIP bandwidth or congestion at oversubscription points in the network cause jitter problems. One example is when Gigabit Ethernet ports connect to a 10 Gb uplink.

QoS can prioritize VoIP traffic to ensure VoIP is not affected by congestion. It can also protect VoIP traffic from network utilization spikes, such as software distributions, companywide video broadcasts and other high-bandwidth applications. Finally, QoS applied to wireless access points can ensure VoIP bandwidth needs gets priority when the Wi-Fi network is congested.

In the WAN, QoS is still necessary, as well, both to avoid jitter-related issues and minimize latency by prioritizing the handling of VoIP packets at WAN routers. WAN networks are especially in need of QoS for voice traffic, as they are often oversubscribed, and they are carrying an ever-increasing amount of bandwidth both due to the increasing use of video, as well as software-as-a-service applications.

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