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Lecture #12
INT20CN Computer Networks
Tutorial #12
- What does CSMA/CD mean? CSMA/CD is sometimes referred to as the
"polite dinner table" algorithm? Can you think of a reason for
this?
- Ethernet/802.3 is a shared medium,
multi-access network technology.
- What do these terms mean?
- Does this pose any potential security risks? Explain. What about
a network based on a switching hub?
- What is interesting about the Ethernet/802.3 "MAC address"?
Discuss.
- In the lecture, it was stated that an Ethernet/802.3 collision
occurs when two stations start to transmit at the same time.
Discuss the meaning of the term at the same time
as used in this context.
- The 10baseT configuration has captured the Ethernet market from
thin wire. Discuss reasons why network planners and managers
might prefer this technology.
- Switching Hubs build a table which maps
destination MAC addresses to port numbers. How do they build this
table?
- An IP packet is "encapsulated" into an Ethernet frame for delivery
in the network -- this is IP "local delivery". How will this
encapsulation differ in the case where the packet is sent to a
router for "Internet Delivery"?
- In slide 6 of today's lecture, it is stated that the key difference
between Ethernet and IEEE 802.3 LANs is the meaning of the 16 bit
"type" field (used as a "length" field in 802.3). In many LANS,
Ethernet and 802.3 frames co-exist perfectly happily. How can a
receiving station know whether to treat the field as a
length or as a type?
- Assume[1] a one megabyte file must be
transferred across a network. Ignoring delays caused by waiting
for access and other overhead (ie, counting only the data
transferred), how long would it take to send the file across an
Ethernet? Across a Fast Ethernet?
- Research question: It's not required knowledge for this subject,
but you might care to investigate the Internet Address
Resolution Protocol (ARP), which provides a mapping
between the IP address of a host and its MAC address -- the
question is: How does a host discover the MAC-level
(Ethernet) address of another host on the same network, if all it
knows is its IP address? This mapping is obviously needed to
enable "local delivery" of a datagram.
- Engineering research question[2]: A
shared medium (non-switched) Ethernet is generally regarded as
heavily loaded (approaching overloaded, in fact) if the network
utilisation goes over (approximately) 20%. This, on the face of it,
seems a low value. What do you think is going on?
- Opinion question: ATM is probably the dominant
high-speed networking technology at present. What do you think is
the particular attraction of ATM networks over other high-speed
technologies, on the basis of the material presented in the
lecture?
- Serious Research question #1:In the lecture, it was mentioned that
Gigabit Ethernet is compatible with 10/100Mbps
Ethernet. What does this mean? It's OK to guess...
- Serious Research question #2: What is the historical origin of the
difference between the frame formats of Ethernet and IEEE 802.3?
[1] From
Comer,
Computer Networks and Internets 2/e P91 (paraphrased).
[2] You are not expected to understand
this.
These tutorial exercises accompany
Lecture #12.
See Prac #12 for the practical exercises
accompanying this tutorial.
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Copyright © 2001 by
Philip Scott,
La Trobe University.