Principles of Communications Week 6 11-13/11/2025

 We've revisited flow and congestion control - one way of visualising the progress of an adaptive flow&congestion control protocol is the time sequence diagram:-

but note this is a massive over-simplification as really what you see here is an ideal with only one source and (apparently) only one bottleneck queue. In reality, in FIFO queues, traffic from multiple sources mixes and interferences (causing high variance in delay, hence round trip time, and very unpredictable loss. If we had Round Robin (by flow) queues, things might be a bit better, but how much? That is what we look at under Scheduling, and with the Generalised Processor Sharing model, can see how close to some ideal of "isolation" between flows, we can get. 

With FIFO queues, RTTs and rates (as estiamted from data or ack packet inter-arrival times are going to be varying fairly chaotically, as the ensemble of flows at any bottleneck will not be coordinated in any special way as they all have different RTTs and perhaps just different performance senders, maybe different packet sizes, possibly different inter-packet timing at transmit time, etc etc

A combination of open loop (admission control) and closed loop (feedback) would allow sources to simply operate at a flat rate (the requested rate) or adapt above it as capacity is made free...

The trade off in flow control and scheduling is all about overall system complexity.