Assignment5: Link and Physical Layers
CS390 Data Networking
LINK LAYER: ETHERNET
One key feature of the link layer is collision detection and avoidance. Back in the old
days of the Ethernet, hosts on the same Ethernet network had to detect and avoid
frame collisions. Although this isn’t a problem for modern Ethernet networks, the
principal is the same for many types of shared media networks like wireless.
Consider this old-fashioned Ethernet network:
1
2
3
Assuming no one gets regular messages from node to endnote three…
1) Why is there a possibility of collision?
2) How does Ethernet handle Collisions?
3) Does the Ethernet network in the lab have collisions (Why or why not)?
Create a simulation of the following network using NetSim installed on the lab
computers ( simulation -> new -> lan -> Ethernet -> traditional):
And him
1
2
Right click on Node2, set:
- the transmission type to point to point
- the traffic type to data
- the destination to node 1
Run the simulation.
1) Supply a screen snapshot of the network statistics from the simulation.
2) Why are there no collisions?
3) The inter-arrival time is the average time between frame transmissions.
Using this information and some simple math can you explain why the
simulation generation rate is approximately 0.588 Mbps?
4) Since no one is competing for the network media why is the utilization of the
network media not 100%?
5) Revisit the simulation. What can you do to improve the utilization?
6) Supply a screen snapshot of the utilization report and the delay report.
7) As you improve the utilization, the delay time for frames increases
dramatically especially when utilization nears 100%. Can you explain this
phenomenon?
PHYSICAL LAYER
1) Give the sequence of bits for the following signal. Assume that Manchester
encoding is being used (thanks to Srinivasan Seshan @ Carnegie Mellon for this
diagram):
+0.85
V
0
-0.85
0.1µs
2) What parity (even/odd) is the resulting bit sequence?
3) Consider a 4 bit generator G=1 0 0 1, what is the value or R when given the
following data bits:
a. 10101010
b. 10010001
c. 10100011
d. 01010101
4) An application developer comes to you with a complaint. She says that her
application sends a lot of repetitive character data over the network from a
source to a destination machine. At the application layer, her messages average
around 1K per message. However, when she looks at the message at the link
layer, (via WireShark) she sees on average 2K per message. Can you explain
this?
5) Assuming a 1Gbps network hub similar to the NetSim configuration from the
LINK LAYER problems above, how long (in seconds) will a host wait before
transmitting a response after receiving a datagram from the network layer?