The main reason Generalised Processor Sharing is useful as a model, is to make clear the idea of a round. In work conserving schedules, the time spent by a given packet in a given flow in the router depends on the number of other packets from other flows, ahead of it, waiting to be transmitted to the next hop. The round advances but it isn't a measure of elapsed real time, as it depends on the number of flows currently in the round, which varies as sources start (and stop) sending, so flows join (or leave) the system.
Of course, if all flows were constant packet size and constant packet rate, and we only admitted as many flows as would fit in the capacity along the path, exactly, then the round would map to a wall clock (real time) - but flows are often variable in number, in packet rate and in packet size.
What the model lets us do is
a) reason about how fair and accurate a real scheduler is
b) work out admission control (can we accept a flow in the call setup/open loop flow control paradigm) or not - i..e for variable flows as above, described using some simple parameters like a linear bounded arrival process with a peak and burst size , that want some sort of minimum capacity guarantee (even if just roughly) - will they fit (without excessively disrupting the existing / established set of flows of packets?
c) what latency we might get for that flow, due to queuing and scheduling inaccuracies (op top of the basic latency due to simple propagation delay over the whole end-to-end path).
In the next section, we'll look at the exceptional setup & requirements in data center networks, where we may be able to make simplifying assumptions about the traffic and topology, and hence employ a simpler approach to schedulers.
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