"I've learned a lot in theory, but saw the effects in reality as well. QoS is importand."
I too consider QoS important. Unfortunately, most don't consider using it except when there's need to support real-time traffic, like VoIP and/or video conferencing.
Now mounting my soapbox. . .
Three stories I like to tell (probably have noted one or more, over the years on these forums), which show benefit of QoS, are centered around a fairly good sized German international firm I worked at a couple of decades ago.
The German firm split its network management into 3 regions, Europe/Africa, Americas, and Asian/Pacific. Although German networking management had over site for the whole international network, they allowed the two other regions much freedom in how corporate networking goals were obtained. (Later, when the firm did start to use real-time apps like VoIP and video-conferencing, then QoS was "needed", but of course [laugh] they [German HQ] weren't much interested in our several years experience using QoS.)
When I came on-board, in the America's HQ, I suggested QoS might mitigate the constant complaints, from the other offices, how poorly the networked performed.
America's network management allowed me to implement QoS between our regional HQ and all the other regional offices (varied between 80 to 100).
Interestingly, the WAN lead engineer, thought QoS was just useless voodoo until the morning he came in and noted his phone was lit like a "christmas tree" with most remote offices complaining what's wrong with the network?
He discovered one of our HQ WAN routers crashed, the prior night, rebooted itself, but without its QoS configuration. He reapplied the missing QoS, and the phone calls stopped. (BTW, this was pre any real-time traffic being on the network.)
About a year later, we were hosting the twice a year principle network engineers' meeting, where all 3 regions engineers met. (NB: the meetings rotated through the regions.) One big complaint, from the other regions was "no matter how much bandwidth, we appear to add [to the WAN], users keep complaining how poorly the network performs". I noted, since we've implemented QoS, we no longer get those complaints and now very seldom upgrade WAN bandwidths (BTW, at that time, possibly saving us about $100,000 a month). Other regions' engineers response, don't see any need for QoS, as we've don't have any real-time traffic.
Lastly, we had a major regional office in South America, whose dual (E3) WAN links ran at 100% all day long during business hours. Those links carried all forms of traffic, from VoIP (by that time we had moved to it), to video-conferencing, to email, to Citrix like apps, to "critical" web apps, to SAP apps, to FTP and Windows file copy traffic, to database replications, to client backups, to other sundry apps. All worked as expected, without complaints. Why? QoS!
So, again, I too consider QoS important.
