2024-01-22 12:38:16

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UNB/Year 4/Semester 2/CS3873/2024-01-22.md

@@ -0,0 +1,56 @@
+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
+
+$d_{prop} = m/s = 10^5/(2\times 10^8) = 5 \times 10^{-4}$

UNB/Year 4/Semester 2/CS3873/CS2024-01-22.md

@@ -1,8 +0,0 @@
-Lecture Topic:
-# 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%