Notes/UNB/Year 4/Semester 2/CS3873/2024-01-22.md
2024-01-22 12:38:16 -04:00

2.8 KiB

Lecture Topic: Packet Switching Performance

Packet Switching

Congestion

A relevant example is air plane ticket overbooking. If an air plane has a capacity of 100 seats, and the probability of of a passenger showing up to their flight is 80%, then you can overbook ticket sales due to the probability of passengers not showing up

  • If 110 tickets are sold, the probability of more than 100 passengers is 0.0058%
  • If 115 tickets are sold, the probability goes up to 1.94%
  • If 120 tickets are sold, the probability is 15.17%
  • If 130 tickets are sold, the probability is 78.12%

Performance

Throughput: Rate (bits/time) at which bits are transferred between sender/receiver

  • Instantaneous: Receiving rate at any instant of time
  • Average: Receiving rate over a longer period of time

How fast a node (host or router) is transmitting depends on

  1. How fast the sender is sending
  2. How fast the link is transmitting

End-to-end throughput is constrained by rate of bottleneck link (the link of the minimum rate on an end-to-end path). The weakest link in the chain (of nodes) determines the throughput of the entire link.

Delay and Loss

Packets queue in a router buffer (Store and Forward)

  • They are delayed while waiting in the buffer for it's turn
  • Slowed down while the queue keeps growing (congestion)
  • Dropped (lost) if no free space in a full buffer

There is four sources of nodal delay:

  1. Node processing: Decoding the incoming electronic signal and accounting for distortion (e.g. wireless signal distortion), and verifying the correctness of the packet, and determining the output link. Usually very small (10^{-6} secs)
  2. Queuing: Time waiting at the output link for transmission. Amount depends on the congestion of the network.
  3. Transmission: L/R, L = Packet length, R = Link bandwidth
  4. Propagation: m/s m = Physical distance of link (e.g. 100m wire), s = propagation speed of link (e.g. speed of electricity)

The entire delay is the sum of all of these figures

Measuring queuing delay

Traffic intensity is a measure of congestion.

 \frac{L \times a}{R} 

a: Average packet arrive rate (packets/s) L: Packet length/size (bits/packet) R: Link bandwidth/rate (bps)

If this figure is 0, the delay on average is very small If this figure is 1, the delay is large If this figure is > 1, then more work arriving than serviced (severe congestion)

Note: There is a field called traffic engineering, and an important rule for this field is to not let the traffic intensity exceed 1.

Example: Delay

Consider only transmission delay and propagation delay. S sends 1 packet of length L to D over a single link of rate R and distance m. s is the speed of the link

L = 1 kb R = 100 kb/s m = 100 km s = 2\times10^8 m/s

d_{prop} = m/s = 10^5/(2\times 10^8) = 5 \times 10^{-4}