Queueing and packet drops in a router

Several sources share a single output link. Increase the traffic intensity and watch the queue build up, the buffer fill and the router start dropping packets.

0.70
Go to:
8
Arrivals:
Packet size:
Speed:

Buffer occupancy

0 / 8
avg: 0.00

Queueing delay

0.00
average, in transmission times (T = L/R)

Link utilisation

0 %
time spent transmitting

Dropped packets

0.0 %
0 of 0 arrived

Delivered

0
throughput: 0.00 pkt/T
Buffer occupancy over time (last 60 T). Red marks show instants when at least one packet was dropped.
Drop probability vs ρ.
Mean delay (queue + transmission) vs ρ.

The curve is the theoretical M/M/1/N reference (Poisson arrivals, exponential sizes, N = buffer + 1 packet in transmission). The filled dot is the theoretical value for the current ρ; the hollow one is what the simulation has measured since the last parameter change (it needs a few hundred arrivals to settle). Try “Regular” and “Bursty”: same ρ, different behaviour.

Space pause · R reset · ←/→ adjust ρ (with the slider focused). Time unit: T = L/R, the time to transmit one packet.

About this visualization

Simulation of a router with four sources, a finite buffer and one output link. The student varies the traffic intensity ρ = λ·L/R (0.05 to 1.6), the buffer size (1 to 16 packets), the arrival pattern (Poisson, regular or bursty) and the packet size (fixed or exponential). Queue occupancy, queueing delay, link utilization, drops and throughput are measured and compared with the theoretical M/M/1/N curve, which is exact when packet sizes are exponential. It shows that queues grow with ρ, that packets are dropped even when ρ < 1, and that with ρ > 1 dropping becomes structural.

Concepts: traffic intensity, queueing delay, finite buffer, packet drop, link utilization, packet switching, M/M/1/N queue, bursty traffic.

Formulas:

Network Architecture I, Session 1.3 · Enrique de la Hoz, University of Alcalá · License CC BY-SA 4.0 · Source and catalogue on GitHub · DOI 10.5281/zenodo.23285664