Questions: Multiple Access Protocols: ALOHA, CSMA, and Taking Turns

Question 1

Under the standard model with many active nodes, what is the maximum fraction of time slots that carry a successful frame in slotted ALOHA?

• A. About 18%
• B. About 37%
• C. About 50%
• D. 100%

Answer: (B)

The maximum efficiency of slotted ALOHA is 1/e, about 0.37. Unslotted (pure) ALOHA reaches only 1/(2e), about 0.18, because a frame can collide with frames that began during a window twice as long.

Question 2

Classic Ethernet's binary exponential backoff has a node, after its n-th collision on a frame, choose K at random from {0, 1, ..., 2^n - 1} and wait K x 512 bit times. After the third collision, what is the range of K?

• A. {0, 1, 2}
• B. {0, 1, 2, 3}
• C. {0, 1, ..., 7}
• D. {0, 1, ..., 15}

Answer: (C)

With n = 3, 2^3 - 1 = 7, so K is chosen from 0 to 7. Doubling the range after each collision spreads retries out more widely as the channel gets busier. Ethernet caps n at 10 for this choice.

Question 3

IEEE 802.11 Wi-Fi uses CSMA/CD, aborting a transmission as soon as it detects a collision.

• T. True
• F. False

Answer: False

802.11 uses CSMA/CA (collision avoidance). A wireless sender cannot reliably detect a collision while transmitting, and some collisions are caused by hidden terminals it cannot hear. So it waits random backoff times before sending and relies on acknowledgments to learn whether a frame got through.

Question 4

Ten nodes share a channel using TDMA, each owning one fixed time slot out of every ten. Why does this perform poorly when only one node has data to send?

Model answer: The active node may use only its own slot, so it gets just one tenth of the channel's capacity while the other nine slots go unused. Channel partitioning wastes capacity when traffic is bursty or few nodes are active.

Random access protocols avoid that waste because a lone active node can use the full channel. They lose efficiency instead when many nodes are active and collisions become frequent.