Too much delay == problem.
Too much loss == problem.

Queue overflow == loss.

Q: Why don’t we just have a larger buffer?
A: Computation costs. If we need to perform operations on the buffer (searching, sorting), it will take longer.
Larger queue == Larger average delay

Packets queue in router buffers.
This occurs when packet arrival rates to link (temporarily) exceeed output link capacity

Types of Packet Delay

d_proc - Processing Delay

  • Time it takes for the router/switch to process the packet

  • Checking for bit errors

  • Determine output link

  • Typically < msec

AFFECTED BY PACKET SIZE

d_queue - Queueing Delay

  • Time it takes for the packet to leave the queue and be transmitted
  • Depends on the congestion of the router

d_trans - Transmission Delay

Time it takes to transmit the entire packet

Data is transmitted in a serial fashion (one at a time)

  • L - packet length (bits)
  • R - link bandwidth (bits per second)
  • d_trans = L/R

AFFECTED BY PACKET SIZE

d_prop - Propagation Delay

Time taken for a single bit to reach the destination.

  • d - length of physical link
  • s - propagation speed
  • d_prop = d/s

Queueing Behaviour

Traffic Intensity = aL/R
a - packet arrival rate (packets/sec)
L - packet length (bits)
R - link bandwidth (bits/sec)

  • Traffic intensity ~= 0 - Small queueing delay
  • Traffic intensity == 1 (Burst) - L(N-1)/2R
  • Traffic intensity == 1 (Continuous) - 0 (!!!?)
  • Traffic intensity > 1 - Average queueing delay of infinity.

Throughput

Throughput is the rate at which bits are transferred between sender/receiver.