Firstly, when there is enough bandwidth for all the traffic QoS (Quality of Service) doesn’t apply. QoS  only ‘kicks in’ when your WAN connection reaches 80% of its capacity.

Secondly, the percentages are relative. As a simple example, say that we have a 512/128 ADSL connection, and at one instant in time we have a VoIP call (64Kbps), BitTorrent using 100Kbps, you’re sending an email with some holiday photos attached (30Kbps), and there are no VPN tunnels active.  These total 244Kbps, and so won’t all fit through our 128Kbps uplink at the same time, so QoS becomes active. Let’s say that the router’s QoS rules are:

Rule Class Name Reserved_bandwidth Ratio
implied built-in FXS ports have first priority
1 VPN 50%
2 HTTP, SMTP, POP3 30%
4 all other traffic 20%

Results are:

  1. The VoIP call uses all the bandwidth it needs, leaving 64Kbps for other data.
  2. Since no VPNs are active, the VPN rule is not applied.
  3. The 30% and 20% are scaled to the remaining bandwidth resulting in up to 38Kbps (64Kbps*60%) for the email.
  4. Since the email only needs 30Kbps, the balance of 34Kbps (64 – 30) is available for BitTorrent.

Note that QoS is an advanced topic, using complex formulae with effects over time, and actual results will not be as straight-forward as the above example.