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Threshold to turn on layer 2 QoS

Posted on 2009-07-15
Last Modified: 2012-05-07
Network is Ethernet 100BASE-TX with switch in the middle. 8 ports.

At what point should I start to worry about QoS. Right now I do not go above 15 Mbps as being the total offered node. i.e. Worst case 15 Mbps offered to one port and clients at say two ports  7.5 Mbps at each. 50% VoIP and 20% TCP and 30% UDP.

I think I do not need QoS yet, but is there a norm or some accepted industry practice say when you need to start worrying. For the UDP and VOIP I have no reliability built in.

Note: The switch is supposed to be 100BASE-T standard.
Question by:MikeDKJ
  • 2
LVL 17

Expert Comment

ID: 24864857
If you're using VoIP, QoS is mandatory, no matter what others may think or how much bandwidth your currently using. High burst traffic, even though it won't reflect on the overall throughput, will impair voice. Voice uses RTP streaming so if it get's interrupted for any reason, you'll get dead air, missing speach, or the converstation will just break up. QoS overcomes this by alloting the appropriate bandwidth and packet service needed so that VoIP takes precendenc over all other traffic. With voice being 50% of your bandwidth, it's just a matter of time before you start to have problems.

Author Comment

ID: 24864929
No VOIP is not 50% of the bandwidth. 100BASE-TX on paper should be able to do full-duplex 100Mbps. I am offering 7.5 Mbps of VOIP (say 20 msec samples). If the peak is 15 Mbps - then at 15% do I need QoS? I know that turning on is always good but why try to fix it if it is not broken?

LVL 17

Accepted Solution

mikecr earned 250 total points
ID: 24866272
At 40% of bandwidth, you're taxing ethernet. So if you're using 40% of a 100Mbit link, you're effectively running at 40 Mbit. If you reach half that point, or 20 Mbit, your going to have problems with VoIP. It may not be broken now but it will be as you progressively increase your network traffic. Other considerations are what type of queueing does the switch do. If it's FIFO, that's not good enough for VoIP. Traffic bursts are what kills you. Normal traffic RTP can get along with, it's the bursts such as downloads, uploads, or increased access to things such as Terminal Servers which utilize smaller packets which can cause problems. The more packets your switch must pass, the more chance your going to have problems with your VoIP.

With that said, if you're not overutilizing your current bandwidth and you see no future increase in traffic due to any more network/web apps, then all will be good. This is a determination on your part. I've worked with more VoIP implementations than you can imagine and I found out that even the lowest traffic networks can have problems. A good test is running streaming audio every now and then from your favorite news site such as CNN or play music across the internet using Winamp. If your quality is not there, then it's time to implement QoS but it only works for what's on your network. Once the packet leaves your network, such as crossing an ISP's SIP trunk, you have no control over it.

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