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Week 10: Multiple-Access Protocols, ARP

#Multiple-Access Protocols

#Multiple-Access Protocols 1

Why does CSMA not completely eliminate collisions?

  1. MAC addresses are not unique.
  2. CRC fails too often.
  3. Switches forward packets that collide with each other.
  4. Hosts that are far from each other have inconsistent views due to propagation delay.

#Multiple-Access Protocols 2

What is the purpose of exponential backoff?

  1. Reducing collisions under congestion.
  2. Accounting for the effects of propagation delay.
  3. Reducing CRC errors.
  4. Increasing the bandwidth of the link.

#Multiple-Access Protocols 3

Which of these is NOT a downside of token-passing?

  1. Delays can be high.
  2. Delays can be unbounded.
  3. There is a single point of failure.
  4. Token ring management adds overhead.

#Multiple-Access Protocols 4

After detecting a collision, what does an Ethernet NIC do?

  1. Continue transmitting the frame
  2. Send a jam signal and abort transmission
  3. Recalculate the CRC
  4. Change its MAC address

Answer with a number, like 1.

#Multiple-Access Protocols 5

Which of the following combinations results in the highest efficiency for Ethernet?

  1. High propagation delay, high transmission delay
  2. High propagation delay, low transmission delay
  3. Low propagation delay, high transmission delay
  4. Low propagation delay, low transmission delay

Answer with a number, like 1.

#Multiple-Access Protocols 6

An Ethernet NIC using exponential backoff has experienced 3 collisions. Which of the following is NOT a possible amount of slots it can wait before retransmission?

  1. 0
  2. 3
  3. 7
  4. 8

Answer with a number, like 1.

#Multiple-Access Protocols 7

What type of Wi-Fi scanning requires the host to send a request initially?

  1. Active scanning
  2. Passive scanning
  3. Collision scanning
  4. Backoff scanning

Answer with a number, like 1.

#Multiple-Access Protocols 8

Why does Wi-Fi use CSMA/CA instead of CSMA/CD?

  1. Wireless devices can easily detect collisions.
  2. Wireless devices cannot reliably detect collisions while transmitting.
  3. CSMA/CA is faster than CSMA/CD.
  4. CSMA/CD requires coordination between different hosts.

Answer with a number, like 1.

#Multiple-Access Protocols 9

Host A is a Wi-Fi device. It sends a RTS (request-to-send) and receives a corresponding a CTS (clear-to-send) reply granting host A permission to send data. Host A then sends its data, but doesn’t receive an ACK. What COULD NOT have been a cause for this?

  1. The data frame host A transmitted was corrupted or unable to reach the AP.
  2. A hidden Wi-Fi host that could hear neither the RTS nor the CTS transmission began transmitting at the same time, causing interference with the data frame.
  3. The ACK signal was sent by the AP, but it was lost or corrupted.
  4. A different Wi-Fi device, host B, simultaneously received a CTS signal from the same AP granting host B permission to send data. Host B decided to transmit a frame at the same time, causing a collision.

Answer with a number, like 1.

#ARP Simulation

#ARP Simulation 1

Consider the following network, where hosts A, B, and C are behind a switch. Initially, all ARP caches are empty. We are using a simplified version of MAC addresses where they are 2 bytes long instead of 6 bytes.

ARP simulation topology

Why can’t host A immediately send a packet to host B? Answer in 10 words or less.

#ARP Simulation 2

Host A sends an ARP request. What are the values of the following 4 fields?

  1. Sender IP address
  2. Sender MAC address
  3. Target IP address
  4. Target MAC address

Answer in a comma-separated list of addresses, like 1.1.1.1,AA-AA,2.2.2.2,BB-BB.

#ARP Simulation 3

Host B sends an ARP reply to A’s request. What are the values of the following 4 fields?

  1. Sender IP address
  2. Sender MAC address
  3. Target IP address
  4. Target MAC address

Answer in a comma-separated list of addresses, like 1.1.1.1,AA-AA,2.2.2.2,BB-BB.

#ARP Simulation 4

Are host A’s request and host B’s response unicast or broadcast?

  1. A’s request is broadcast. B’s response is broadcast.
  2. A’s request is broadcast. B’s response is unicast.
  3. A’s request is unicast. B’s response is broadcast.
  4. A’s request is unicast. B’s response is unicast.

Answer with a number, like 1.

#ARP Simulation 5

Which of the following devices now have an entry in their ARP cache?

  1. Host A
  2. Host B
  3. Host C
  4. The switch

Answer with a comma-separated list of numbers, like 1,2,3.