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Here’s the thing you need to ask yourself whenever you build a network to support Voice over IP, do I need to implement traffic prioritization? Most people when they hear that VoIP is going to share the same kit as data quickly assume that Quality of Service is an absolute must. Also in the same vain when huge volumes of network traffic is causing data loss, choppy sounding speech we all think that switching on traffic prioritization will solve all your problems. Quality of service is not the man for all seasons as one might expect.

Quality of service is the better treatment of something to the detriment of another. Here is how I like to imagine QoS; I imagine walking into my local bank on a busy and as always having to choose which queue to join. Do I go for the shortest queue? Sod laws states it’ll be the slowest or do I take my chances with the longer queue?

Would it not be fairer if the queues were populated by different types of customers, for instance we may decide that queue 1 is reserved for the richest of clients, queue 2 is set aside for well off ones and queue 3 for your everyday punter. I would then stroll into the bank safe in the knowledge that I will always join the rich queue. So in I go to the bank only to be turned away and pointed towards the 3rd queue. What happened there?

At the entrance of the bank we have a member of staff called a classifier who’s job it is to determine (by the banks standards) if you merit going into the 1st, 2nd or 3rd queue. Within a network device a classifier is a process that is configured by the network administrator. The classifiers role is to look within the body of the frame(L_2) or packet (L_3) and find whatever field it has been configured to look for and from that field it can then place the traffic into the correct queue. In my network I will always want to place real time voice traffic in the priority queue.

Therefore they will stop you and ask how much money you have; the answer will result in you being placed into the appropriate queue, oh by the way, once you are in the queue there is not queue jumping, the order in any queue is first in and first out.

So everyone is now placed into the queue, now what?, the 1st queue seems to be moving along nicely but your queue 3 seems to be going nowhere, the reason is that no longer is there an individual teller per queue but only on teller to serve all three queues.

You notice that they are paying more attention to the 1st queue, well that seems unfair, well that’s why Quality of Service is also referred to as managed unfairness, someone is getting better treatment to your detriment.

So who decides who is going to get served next and from which queue? That job falls to a role called the scheduler who’s responsibility is to make sure that each queue is service for the appropriate amount of time, that time is again determined by the bank. The bank might decide that the 1st queue will serve at a ratio of 10:5:2. Not looking good for me in the 3rd queue.

Getting back to the original thought when do we need Quality of Service. If I walk into the bank early in the morning when there are no customers then there is no need to queue, I can bypass the empty queues and go directly to the teller. In a network scenario if you have little data and voice traffic on your network queues will never be used as the interfaces can transmit data at such high rates that there is little chance of and serialization delay (Amount of time taken to put the data onto the wire).

On the other hand, if news break out that the bank is going bust there will be a run on the banks, the place will be packed out with people trying to get at their money, the 1st queue will overflow with all the rich people trying to get their money along with the well off and the normal customers in queue 3. At this point the classifier at the door is going to have to start turning people away. In a network scenario if you have excessive traffic on you network, you will have excessive traffic of all kinds that will fill you interface queues, therefore all traffic types will suffer.

The only thing the bank can do is order the teller to work faster. In a network scenario you need to get faster interfaces. Quality of service can only do so much but better to have it than have a complete free for all.