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As the title states!

  • What is QoS
  • When is it good to use it?
  • When is it not good to use it?
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5 Answers 5

up vote 6 down vote accepted

QoS is Quality of Service. It's meant to flag traffic that should receive special attention. VOIP, for instance, should be low latency and put at the front of the queue. The problem with that is that anyone could mark their traffic just to be treated nicely, even if were a bulk download that had no special requirements. So across the internet almost nobody respects this flag and therefore it's irrelevant.

Within a network that you control you could implement this in your routers, and perhaps switches, to give priority to traffic that needs it.

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FYI, SLA is a service level agreement if you don't know what it is. That is an agreement between you and another party that you're guaranteeing a certain level of service. –  SQLChicken May 27 '09 at 15:01
    
SQLChicken: true enough if your SLA covers end-to-end, which can very well be the case in some circumstances. Not all, though: if my VOIP phone connects to your VOIP phone and transits 7 networks in the wild, where is QoS and our (separate provider) SLAs now? –  dwc May 27 '09 at 16:30
    
Accepting for the mention of which devices! –  squillman May 27 '09 at 16:45
    
Is there no benefit to be expected from the QoS features of IPv6? –  Nick Chammas Jan 3 '12 at 19:08

QoS is quality of service. Think of it as an SLA between bandwidth and a certain application or port. For instance you have a VoIP phone on your desk that has a network jack built-in that you plug your computer into. Normally that data connection is just for that, data, but in this case your bandwidth is shared between data and voice data. Applying QoS on that port tells the port "OK, I will guarantee the voice data 40% of this bandwidth at all times no matter how much data is coming through". What does this mean for you? If you're downloading a 2 GB file and a call comes through the QoS guarantees that your call won't sound like its broken up and coming from a mysterious doctor in Myst. Yes, your download rates might take a hit for a bit but your voice data is what you're trying to preserve and make sure comes through. QoS is also usually applied to Video Conferencing lines so that the video doesn't stutter because its not getting the sufficient bandwidth needed to keep everything in sync.

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+1 for bringing up SLAs –  dwc May 27 '09 at 16:31
    
+1 for mention of streaming applications other than VoIP! –  squillman May 27 '09 at 16:46
    
So where is the QoS applied in the case of a VOIP handset with ethernet in and ethernet out to a PC? Presumably at the switch? Does the switch determine between voice (VOIP/SIP) and data traffic? –  TomC Aug 28 at 7:16

To add to the other comments

QoS has a few different capabilities/features - one of them being CoS. Class of Service allows you to simply prioritize specific traffic by adding the class of service bits to the frame header (per IEEE802.1p). This is commonly used in VoIP for marking these kind of traffic to a high class number like 3, 4, 5. This is the simplest way of configuring QoS on your network, other implementations include traffic shaping, intelligent congestion rules, and scheduling which are considered more advanced, and needing greater QoS configuration knowledge to implement successfuly.

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+1 for CoS talk –  squillman May 27 '09 at 16:47

QoS is Quality of Service and is used to give priority to some packets over others. For example, if you use Voice over IP, you may want to configure your network to give priority to those packets to ensure they are routed with a higher priority than other traffic. You could also use it to de-prioritize file sharing traffic, assuming it isn't blocked entirely.

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In my home business network, I use my Linksys router to enable priority based QoS. This is protocol and port based rules. My configuration:

  1. VoIP has high priority
  2. Bittorrent downloading and my FTP download requests have medium priority
  3. Bittorrent seeding and connections from people downloading files from my FTP have low priority
  4. Everything else has normal priority which is between medium and low.

You can also define rate control based QoS. This is normally source or destination IP based rules. Sometimes you can also limit it to a particular protocol/port on that IP. This configuration is useful if you have a particular machine or service on a machine that should take priority over other machines or services in your network.

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On your home business network, you give higher priority to BT than your actual work? Do you work in entertainment? –  jedberg May 27 '09 at 16:34
1  
Heh. Only BT and FTP downloading. Basically, if I start downloading a file from BT or FTP on my work network, it usually means that I need it for work and I don't mind it taking priority over web surfing and e-mail and such that I'm doing while waiting for the file to finish. –  deinspanjer May 27 '09 at 19:18

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