Although I am taking the R&S Lab Exam my vast Experience is mainly i VOIP, I work for ~7 years with VoIP Company's and in the last 5 Years in the same Group Z-Group I have worked in the past years with more then 500 Carriers and with that I believe that I have the privileged to talk about it a little bit. As the Sys Admin in my company I have been loyal to Cisco Class 4 Equipment (mostly AS5300 AS5350 and AS5350XM I have also worked with MG200 AS5400 AS5800 and the small 26xx and 36xx), QoS is one of the major topics in the VoIP env as there is a growing number of Bandwidth Consuming Applications.in my company I am considering the QoS planing and deploying one of the most important part of deploying a successful and friendly VoIP Env. QoS is a way to make your network bandwidth utilization optimized without investing more bandwidth or separate env for voice and data. When you say that your network is full you should consider taking a survey see what is passing trough your network, one of the most usful tools I am using today is NBAR (Network Based Application Recognition)nbar is using PDLM (Protocol description Language Module) or in other words pre defined scripts that enabling the Cisco to recognize Applications and Protocols passing trough the Machine One of the first steps to deploy QoS is to recognize and define your traffic and some use also the term color your traffic. when you have colored traffic it is very easy to play with it, like we use tagging (mostly in OSPF) we can tag this is RTP that is P2P and so on...and after you have Classified your traffic you can say for example: if I want to give RTP priority over everything else I want to Drop P2P if they use more then 20KbpsI want to random detect (Work only with TCP called also WRED) and drop low classified packets randomly so I will not pass the threshold of allowed bandwidth. QoS allow us to set Queuing and Priorities to traffic patterns and that way: 1) Assuring important traffic over low important traffic 2) Allowing Administrator identify problems during and also before the happen 3) in some way also adding some security policy as you can defign that all P2P or IM should be Dropped and not allowed Most business's consider such applications as work disturbing Few Things regarding NBAR as it is grate it also has one major disadvantage, it is CPU consuming task and if you have already busy router you will have a problem, note that hardware issues are also one of the problems in achieving QoS. you must know your equipment and its capabilities. For now I will conclude this section of QoS and soon I will add part 2 to this with more notes mainly on Voice today and different types of implementations Scenarios.