Quick general QOS question

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Sep 10th, 2008
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We are implimenting video services across our IP network. The lead engineer was discussing QOS. However, we have a DS3 that is only utilized 20%

There are some issues with call quality.

I was under the impression that Qos only comes into play when bandwidth becomes an issue?..i.e. if we started to hit 80% bandwidth utilization

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Jon Marshall Wed, 09/10/2008 - 08:42
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Generally speaking QOS is indeed for prioritizing one particular type of traffic over another when there is not enough bandwidth for both so you are right in what you say.

But some applications, voice being the most obvious one, are also very sensitive to any end to end delay in the delivery of packets. These packets could still be adversly affected even when there is enough bandwidth because each packet takes a certain amount of time to be transmitted onto the wire. If the voice packet gets "stuck" behind other packets then it could exceed the end to end delay. So even in a campus LAN when deploying vocie it is often necessary to prioritize the traffic even though there is enough overall bandwidth.


Giuseppe Larosa Wed, 09/10/2008 - 09:31
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Hello Richard,

some traffic types like VoIP are sensitive to variation on delay so the QoS by providing priority helps to have a limited jitter (delay variation).

Codecs have elasticity buffers to accomodate for some level of delay variation but if jitter is high they don't work well and voice quality drops.

For example we have been able to use VoIP phones on a satellite connection VSAT - satellite- VSAT by using LLQ.

At first we thought the absolute delay was too much but it worked.

the voip phones were 14 over a 4Mbps link.

Hope to help


Joseph W. Doherty Wed, 09/10/2008 - 10:00
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Often bandwidth utilization is misunderstood.

What does "only utilized 20%" really mean? Often, what it means is the circuit is at least using 100% of the circuit, but only 20% of the time. As Jon describes, some application traffic doesn't work well when "stuck behind", or delayed by, other traffic, which might very well happen 20% of the time with 20% utilization.

Further 20% utilization could also be the result of traffic that wants 300% bandwidth 10% of the time. In such situations, not only is some traffic delayed, but often some traffic is dropped. Also again, some application traffic is very sensitive to being dropped.

QoS comprises methods to manage traffic beyond just "best effort". It might be needed on a link which only shows 1% utilization or might not be needed on a link with 100% utilization. It's all a question of whether applications sending traffic obtain the performance they need to work well.

The fact that you say "There are some issues with call quality. " is a good indication you might need to look at supporting QoS.


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