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Now let's look at some of the congestion management queuing mechanisms.

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There are many queuing mechanisms.

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Some of these are older and are inefficient for modern, rich media networks.

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In other words, they were good in the past but are not good for voice over IP and video running across

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a data network.

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So let's start with a FIFO or first in first out queue.

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This consists of a single queue with packets that are sent in the exact order that they arrived.

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We've probably all experienced this queuing mechanism in the real world.

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In this example, we have people wanting to pay for their purchases.

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There's only one cashier that they can pay at people of service to in first come, first serve order.

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In other words, first in, first out.

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This lady arrived first, so she served first.

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This gentleman arrived second.

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So he served a second and so forth and so on.

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This is the front of the queue.

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This is the back of the queue.

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And people are served in that order.

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In the same way in a FIFO queuing algorithm on a router, packets are served in the order that they

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arrived.

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This is the front of the queue.

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This is the back of the queue.

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New packets are in queued to the back packets at the front of the queue or D queued and forwarded for

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transmission.

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The problem with this queuing mechanism is voice packets can be delayed by large data packets.

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Everyone is served in the same way, which may work well in some cases in the real world.

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But that wouldn't work.

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As an example, if there was an emergency and an ambulance as an example, needed to go to the front

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of the queue.

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And a truck carrying cement or dry goods, as an example, should wait for the ambulance.

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You don't want a slow moving truck or lorry in front of a ambulance.

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You want an ambulance to go to the front of the queue.

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In the same way you want a voice packets to be able to go to the front of the queue.

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So FIFO is not good for voice and video.

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Another older queuing mechanism is a priority queue.

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This consists of four queues that are served in a strict priority order.

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In this queuing algorithm, we had four queues a high medium, normal and low queue.

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By enforcing a strict priority, the lower priority queues are served only when the higher priority

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queues are empty.

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So when traffic arrives, if it's classified as important, it's put into the high priority queue.

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Classification could be done on protocol or source interface or some other criteria.

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Traffic in the high priority queue is always serviced first only when the high and medium queues are

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empty.

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Is the normal queue processed?

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The low priority queue is only processed when the high, medium and normal queues are empty.

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So this is the problem.

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The low priority queue could starve if there is constant traffic in the high, medium or normal queues,

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so low priority queues could be starved by higher priority queues.

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It was an older mechanism which is fine in the past but doesn't serve well for modern networks.

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Again, in a priority queue we have four queues high, medium, normal and low.

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High priority queues are always serviced before low priority queues.

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The problem here is it could result in starvation of lower priority queues.

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A third queuing algorithm is custom queuing.

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This consists of up to six queues serviced in a round robin fashion.

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In order to prevent starvation, it provides traffic guarantees.

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The problem with this method, however, is that it doesn't provide strict priority for real time traffic.

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So as an example, we've got incoming packets, they are classified into various queues.

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There can be up to 16 of them can be of different sizes.

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So you could provide more bandwidth to some queues compared to other queues.

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The problem with custom queuing, however, is that if you have important voice traffic arriving, it

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will only be serviced in its round or in its turn.

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So as an example, your voice is processed now and then.

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It's the turn of the second queue and a new voice packet arrives that a new voice packet is not going

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to be processed until Q3, Q4, Q5 and all the way up to queue 16 is serviced and then it comes round

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back to voice custom queuing uses a round robin queuing scheduler.

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So once voice is processed, it becomes the turn of the second queue.

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The scheduler doesn't come back to the first cue or the voice cue until it's processed all the other

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cues.

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So the problem with this method is it introduces delay.

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There's no priority for voice.

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So voice traffic often gets delayed, which introduces delay and jitter and affects voice quality.

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So neither FIFO priority queuing or custom queuing are ideal for a modern network.

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A fourth algorithm is weighted for queuing.

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This is an algorithm that divides the internet bandwidth by the number of flows, thus trying to ensure

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proper distribution of the bandwidth for all applications.

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It provides generally a good service for real time traffic, but there are no bandwidth guarantees for

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particular flows and some flows can actually starve other flows.