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Practically impossible to regulate inbound bandwidth to insure quality, even when severely capping various traffic type's allowable bandwidth. You need to be able to manage the congestion point's egress. So, for example, when you note problem occurs when someone starts a larger download, you would need to manage the ISP side of the link.
Assuming you can manage the egress bottleneck, most L3 switches QoS features often make it difficult do such congestion management really well. For 20 Mbps, would recommend a small router with base IOS feature set to manage the congestion. Again, though, the ISP side could be an issue.
