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GPS, or Generalized Processor Sharing, is the best approximation to the Fair Fluid Model. In the fluid model all flows are infinitesimally divisible, each flow contributes a fixed percent of the total flow at any one time. GPS takes infinitesimally small amounts from all queues and serves it, so for any time interval all queues will have been served fairly according to their assigned weights. It is easy to see that GPS is an ideal approximation to the Fair Fluid Model.
In the following diagram the Green packets have been assigned 50% of total bandwidth. Both the Purple and Red flows are assigned 25% each of the total bandwidth. At any one point queue serves 50% of the Green flow, 25% of The Purple flow and 25% of the Red flow.
The Fair Fluid Model can’t be implemented because data networks transfer bits and only one bit can be served at a time. Just as the Fluid model is unattainable so to is pure GPS. Data networks use packets, which aren’t infinitesimally divisible. Routers are required to finish serving one packet before starting the next one. PGPS, or Packet Based Generalized Processor Share, has been developed to have the same properties as GPS, except that packets are served.