Question on quality of service

Answered Question
Jun 25th, 2009

Hi Gents,

I have taken this excerpt from the QOS SRND for Campus Design: "Queueing algorithms are activated only when a device is experiencing congestion and are deactivated when the congestion clears" my question is - What if you have a Gig interface that is not overutilized but has n x 1500 byte packets waiting to be processed before your voice packet? So Im guessing the voice packet will have to wait and incur the delay right and only if the interface is 100% (bandwidth) utilized then the queues come into play and place voice packets into a priority queue, or am I understanding this wrong? M

I have this problem too.
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Correct Answer by francisco_1 about 7 years 5 months ago

let say without QoS enable on our network, the default is FIFO, first in, first out and the switch is not priotizing your voice from data.

With Qos, depending on what you want to do, you have different queuing method or congestion management you can use. With priority queuing for example you can always prioritize voice traffic from data. Some of the QoS mechanism will not work until congestion happens and the bandwidth you reserve is used by other queues until congestion kicks in

see this for different QoS mechansim http://www.cisco.com/en/US/tech/tk543/tk544/tsd_technology_support_protocol_home.html

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Correct Answer
francisco_1 Thu, 06/25/2009 - 02:06

let say without QoS enable on our network, the default is FIFO, first in, first out and the switch is not priotizing your voice from data.

With Qos, depending on what you want to do, you have different queuing method or congestion management you can use. With priority queuing for example you can always prioritize voice traffic from data. Some of the QoS mechanism will not work until congestion happens and the bandwidth you reserve is used by other queues until congestion kicks in

see this for different QoS mechansim http://www.cisco.com/en/US/tech/tk543/tk544/tsd_technology_support_protocol_home.html

Joseph W. Doherty Thu, 06/25/2009 - 03:25

Congestion isn't 100% utilization, it's when a packet is queued (and delayed). You can have (transient) congestion at any utilization level, 100% and under, excluding 0%. You might not have congestion even at 100%.

QoS often concerns itself with congestion management, or queue management. Generally, packet transmission re-ordering. In your example where there are n x 1500 byte packets that had arrived before voice packets, the voice packets would often be dequeued before those packets.

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