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I think all of us have experienced congestion management in our lives.

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You get to the airport and you want to check in, there's a queue or a line, so you have to wait before

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your bags can be processed.

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You get to security at the airport and you have to wait in a queue or a line you're driving on the highway

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or motorway and lanes have been closed.

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It results in congestion, which results in a queuing mechanism being used to filter the traffic.

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So queuing mechanisms manage congestion.

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Different queuing mechanisms can be used in the US.

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As an example, you have TSA PreCheck, so that helps you get through security quicker.

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If you've got a first class airplane ticket, you're going to be processed quicker or better at check

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in than if you've got an economy class ticket.

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An ambulance will take priority on a motorway or a highway when there's congestion versus a standard

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car.

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So different queuing mechanisms can be used to determine how traffic is processed.

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With round robin queuing mechanisms, all traffic is treated the same way.

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That means that everyone is treated the same way but results in delay for real time traffic.

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You could also implement a strict queuing mechanism where high priority traffic is always processed

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first.

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The advantage of that is voice packets will be transmitted first.

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But if there's a lot of voice traffic or high priority traffic, it could result in starvation of less

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important traffic.

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If you're constantly sending a lot of voice and video, your FTP traffic may never be transmitted because

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you are always transmitting video and voice first in a strict priority queuing mechanism.

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So with congestion management, we have queuing or buffering which determines the ordering of packets

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in the output buffers.

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In other words, it determines how traffic leaves a router interface or leaves a switch interface.

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You do have ingress queues, but for now we'll just concentrate on egress queuing mechanisms.

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Queuing is only required when there's congestion.

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If you're driving on the highway or motorway late at night and there are six lanes and there are only

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a few cars, there's no need to have a queuing mechanism.

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So queuing is used when there's congestion, hence the term congestion management when queues fill up,

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packets are reordered so that high priority packets are sent out of a router interface first rather

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than lower priority packets.

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So again, when this congestion, you may have high priority packets and low priority packets arriving.

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A queuing mechanism is used to determine which packets should be processed first and then they are scheduled

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for transmission out of the interface of a router.

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So when packets arrive they are put into classes and will be processed on how important they are or

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based on the queuing algorithm used.

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More important packets will be transmitted first before less important traffic as an example.

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So they'll be scheduled for sending out of the interface of the router before less important packets.

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So queuing is the logic of ordering packets in output buffers.

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Scheduling is a process of deciding which packets should be sent out next.

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Scheduling occurs regardless of whether there's congestion on the link.

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Queuing is only activated when congestion occurs, when queues fill up, packets are reordered so that

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high priority packets can be sent out before lower priority packets.