Module 10: Application Layer Multicast and Link Layer Services

Question 1

Application Layer Multicast (ALM)

Answer 1

Question 2

What are the advantages of ALM?

Answer 2

No need for changes to IP or routers

No need for ISP cooperation

End hosts can prevent other hosts from sending

Easy to implement reliability: use hop-by-hop retransmissions

Question 3

What are the disadvantages of ALM?

Answer 3

lack of knowledge about network:
-does not have direct access to unicast routing
-does not know about network topology

stretch: ratio of latency in the overlay to the latency in the underlying network

Question 4

How is the disadvantage of Stress in ALM solved?

Answer 4

stress: number of duplicate packets sent over by the same physical link

solution: periodically probe overlay neighbors to pick best neighbor that meets application-level utility function (e.g. latency, available bandwidth, both)

Question 5

DHT multicast tree formation

Answer 5

group is associated with key

root of group is node that owns key

any node that wants to joins ends message to root, leaving forwarding state along path

Question 6

In a multicast tree, if the root wants to broadcast to n leaves of a subtree, it has to send n messages to the root of that subtree.

Answer 6

false

Question 7

ISO OSI Reference Model for Layers

Answer 7

Question 8

Which of the following layers use end-to-end protocols?

Answer 8

Transport (TCP) and Application

Question 9

Adaptors communicating

Answer 9

Question 10

Link Layer Servives

Answer 10

Encoding
Framing
Error detection
Flow control

Question 11

Encoding

Answer 11

Question 12

problem with simple approach to encoding

Answer 12

Question 13

simple approach to address the long strings of 0s and 1s

Answer 13

Question 14

encoding alternatives (NRZI and Manchester)

Answer 14

Question 15

encoding alternatives (4B/5B)

Answer 15

Question 16

Framing

Answer 16

Question 17

More Framing

Answer 17

Question 18

What is a potential downside with the counter-based approach?

Answer 18

count field gets corrupted

Question 19

Error Detection

Answer 19

Question 20

Techniques for error detection

Answer 20

parity checking
checksum
cyclic redundancy check

Question 21

Parity Check

Answer 21

Question 22

link-layer broadcast media

Answer 22

Question 23

Methods to handle contention in end-to-end layers is similar to that in the link layer.

Answer 23

True

Question 24

Multiple Access Protocol

Answer 24

Single shared broadcast channel
- avoid having multiple nodes speaking at once
- otherwise, collisions

MAP
- distributed algo for sharing the channel
- algo determines which node can transmit

Question 25

classes of techniques

Answer 25

channel partitioning

taking turns

random access

Question 26

Which of the following is false about TDMA?

Answer 26

packet transmission can span multiple slots

true stuff:
Each machine can only occupy one time slot at a time
Transmission channel gets partitioned into fixed length slots

Question 27

channel partitioning: FDMA

Answer 27

Question 28

Taking Turns MAC protocols

Answer 28

used in bluetooth

Question 29

Which of the following MAC protocols utilize the technique of “taking turns”?

Answer 29

Token Passing

Question 30

key ideas of random access

Answer 30

Question 31

Which of the following is incorrect about random access concepts?

Answer 31

Carrier sense means that the receiver should check if anyone else is already sending data to it before accepting data sent from a source

correct stuff:
Collision detection means that when a node discovers that other nodes are sending data after its data is sent out, it stops sending data and retries at a later time
Randomness refers to nodes backing off and restarting after a random amount of time

Question 32

aloha transmissions strategy

Answer 32

Question 33

variation: slotted aloha

Answer 33

Question 34

slotted aloha example

Answer 34

Question 35

efficiency of slotted aloha

Answer 35

Question 36

ethernet uses CSMA/CD

Answer 36

Question 37

Explain CSMA/CD Random Access: exponential backoff

Answer 37

Question 38

ethernet frame structure

Answer 38

Question 39

Minimum Packet Size

Answer 39

Question 40

Limitations on Ethernet Length

Answer 40