If I (say) allocate 80% to VPN, does this leave only 20% of my bandwidth for other data types?
Posted by Gabriel Yu on 15 April 2008 01:44 AM
Firstly, while there is enough bandwidth for all the traffic, there is no problem and QoS is not applied. QoS (Quality of Service) 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 is active.  Lets say that the router's QoS rules are:

Rule Class Name
Reserved_bandwidth Ratio 
impliedbuilt-in FXS portshave first priority
4all other traffic20%
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.
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