802.3

Question 1

In order for a node to be able to detect if another node transmitted during its transmission what should be true

Answer 1

Time_tx >= 2Time_propagation

Question 2

What is the formula for ethernet efficiency

Answer 2

1 / 1 + 5(Time_prop/Time_tx)

Question 3

True/False - In CSMA/CD, a node is prohibited from sending back to back frames

Answer 3

True

Question 4

For ethernet what is the amount of time a node waits during the backoff period?

Answer 4

K x 512 bit times

Question 5

In ethernet what is the binary exponential backoff algorithm

Answer 5

choose random k from 0 to K = (2^n) - 1
n = number of collisions so far

Question 6

In ethernet what is the maximum value of n (number of collisions) in the exponential backoff algorithm

Answer 6

10

Question 7

Describe the CSMA/CD TX Loop

Answer 7

• Sender listens to the medium
• When sender senses medium is idle it starts tx
• If sender tx entire frame w/o detecting another tx - success
• If sender detects another tx while tx’ing, it aborts and send a jamming signal
• After aborting the sender enters back-off period
• After back-off period sender returns to listen to the medium

Question 8

Describe the CSMA/CD RX Loop

Question 9

True/False - A Layer2 switch is transparent to end nodes

Answer 9

True - End nodes do not know that they are sending through a switch

Question 10

Define Collision Domain

Answer 10

The set of nodes competing for shared resources

Question 11

What are the characteristics of a L2 Switch

Answer 11

• Transparent
• Plug and Play
• Self Learning
• Store and Forward
• Recognizes MAC addresses

Question 12

For a Hub with R Mbps / Port, What is the avg throughput / node and the max throughput for a node

Answer 12

Avg - R / # Nodes, Max - R

Question 13

For a Switch with R Mbps / Port, What is the avg throughput / node and the aggregate throughput for the switch

Answer 13

Avg - R, Aggregate - R x # Nodes

Question 14

True/False - In a switch setup with no dedicated TX/RX wire, eacy physical port of a switch is a collision domain

Answer 14

True

Question 15

Define Broadcast Domain

Answer 15

The set of nodes that rx a local broadcast

Question 16

True/False - Switches isolate broadcast domains

Answer 16

False - The entire switched network is a single broadcast domain

Question 17

Describe the 3 Switch Modes of Operation

Answer 17

Forwarding - If src & dst mac are on a diff phy port and the switch knows which phy port the dst is on.
Filtering (Drop) - src & dst mac share same phys port
Flooding - dst mac is not in the table, frame goes to every port except the one it came on

Question 18

True/False - for a switch to learn about a node the node has to send a frame

Answer 18

true

Question 19

Define Time-to-Live in the context of the switch forwarding table

Answer 19

The time for which an entry stays in the switch forwarding table before being removed