Section A — 1 Mark Questions
1. What does MAC stand for? [1 Mark]
Answer: Medium Access Control.
2. In which layer of the OSI model do MAC protocols operate? [1 Mark]
Answer: Data Link Layer.
3. What is the main purpose of MAC protocols? [1 Mark]
Answer: To control access to a shared communication medium among multiple stations.
4. In pure ALOHA, when does a station transmit a frame? [1 Mark]
Answer: A station transmits as soon as it has a frame ready to send.
5. What is the vulnerability period of pure ALOHA? [1 Mark]
Answer: 2 × frame transmission time.
6. In slotted ALOHA, time is divided into what? [1 Mark]
Answer: Discrete time slots.
7. What is the vulnerability period of slotted ALOHA? [1 Mark]
Answer: 1 × frame transmission time.
8. What does CSMA stand for? [1 Mark]
Answer: Carrier Sense Multiple Access.
9. In 1-persistent CSMA, what does a station do if the channel is busy? [1 Mark]
Answer: It continues to sense the channel continuously until it becomes idle.
10. In non-persistent CSMA, what does a station do if the channel is busy? [1 Mark]
Answer: It waits for a random period of time before sensing again.
11. What is the main drawback of 1-persistent CSMA? [1 Mark]
Answer: High probability of collisions when multiple stations wait for the channel to become idle.
12. What does CSMA/CD stand for? [1 Mark]
Answer: Carrier Sense Multiple Access with Collision Detection.
13. When is a collision detected in CSMA/CD? [1 Mark]
Answer: During the transmission phase of the frame.
14. What happens to a frame in CSMA/CD when a collision is detected? [1 Mark]
Answer: The transmission is aborted, and a jam signal is sent.
15. What does CSMA/CA stand for? [1 Mark]
Answer: Carrier Sense Multiple Access with Collision Avoidance.
16. Why is CSMA/CA used instead of CSMA/CD in wireless networks? [1 Mark]
Answer: Because wireless networks often cannot detect collisions while transmitting.
17. Which mechanism is often used in CSMA/CA to acknowledge successful receipt? [1 Mark]
Answer: Positive Acknowledgement (ACK).
18. What is the role of the IFS (Inter-Frame Space) in CSMA/CA? [1 Mark]
Answer: To allow for priority access and separation between frames.
19. Does CSMA/CD eliminate all collisions? [1 Mark]
Answer: No, it only detects them and stops transmission to save bandwidth.
20. What is the purpose of binary exponential backoff? [1 Mark]
Answer: To determine the wait time before retransmission after a collision.
Section B — 2/3 Mark Questions
1. Differentiate between pure ALOHA and slotted ALOHA. [3 Marks]
Answer: In pure ALOHA, stations transmit at any time, leading to a vulnerability period of 2T. In slotted ALOHA, transmissions are restricted to predefined time slots, reducing the vulnerability period to T. Consequently, slotted ALOHA achieves double the maximum throughput of pure ALOHA.
2. Briefly explain the 1-persistent CSMA protocol. [2 Marks]
Answer: In 1-persistent CSMA, a station senses the channel. If it is idle, it transmits immediately with probability 1. If it is busy, the station continuously monitors the channel until it becomes idle, then transmits.
3. Why is CSMA/CA necessary in wireless local area networks? [2 Marks]
Answer: CSMA/CA is used because wireless stations cannot reliably detect collisions while transmitting (the transmitted signal masks the weaker incoming signal). It avoids collisions by using techniques like inter-frame spacing and reservation signals.
4. Describe the role of a jam signal in CSMA/CD. [2 Marks]
Answer: When a station detects a collision in CSMA/CD, it aborts the current transmission and sends a jam signal. The purpose of this signal is to notify all other stations that a collision has occurred so they discard the corrupted data.
5. Explain the non-persistent CSMA strategy. [3 Marks]
Answer: If a station finds the channel idle, it transmits. If the channel is busy, it waits for a random period of time before re-sensing the channel. This reduces the probability of collisions compared to 1-persistent, but may introduce delays if the channel is idle for long periods.
6. Compare p-persistent CSMA with 1-persistent CSMA. [3 Marks]
Answer: 1-persistent is a special case of p-persistent where p=1. In p-persistent, if the channel is idle, the station transmits with probability p, or waits for the next slot with probability 1-p. This allows multiple stations to share the channel more effectively when idle, reducing contention compared to 1-persistent.
7. Define the concept of the vulnerability period in MAC protocols. [2 Marks]
Answer: The vulnerability period is the time interval during which a new transmission from one station will collide with another station's transmission, causing a collision.
8. What is the purpose of binary exponential backoff in collision resolution? [3 Marks]
Answer: After a collision, the stations involved wait for a random interval before attempting retransmission. The range of this random interval doubles with each consecutive collision. This helps avoid repeated collisions by spreading out the retransmission attempts.
9. How does CSMA/CA attempt to avoid collisions? [2 Marks]
Answer: CSMA/CA attempts to avoid collisions by requiring stations to wait for a specific duration (Inter-Frame Space) before transmitting, even if the channel is sensed as idle, and by using acknowledgements.
10. Explain the concept of carrier sensing. [2 Marks]
Answer: Carrier sensing is the process where a station listens to the transmission medium to determine if another station is currently transmitting. If the medium is silent, it is considered idle; otherwise, it is busy.
Section C — 5 Mark Questions
1. Explain the working mechanism of CSMA/CD and the steps taken upon detecting a collision. [5 Marks]
Answer:
Working Principle
CSMA/CD is a contention-based protocol where stations listen before talking. If the medium is idle, the station starts transmitting. If it is busy, the station waits. While transmitting, the station continues to monitor the channel for signal interference.
Collision Handling Steps
- Detection: If a station detects a signal strength higher than its own transmitted signal, it identifies a collision.
- Abort: The station stops the data frame transmission immediately.
- Jam Signal: It transmits a short jam signal to ensure all other stations recognize the collision.
- Backoff: The station waits for a random time interval determined by the binary exponential backoff algorithm.
- Retransmission: After the waiting period, the station restarts the process of sensing the channel and attempting transmission again.
2. Describe the differences between pure ALOHA and slotted ALOHA including their throughput characteristics. [5 Marks]
Answer:
Pure ALOHA
In pure ALOHA, a station sends a frame whenever it has data. There is no synchronization. Because transmissions can overlap at any point, the vulnerability period is twice the frame time (2T). The probability of a successful transmission is low, resulting in a maximum throughput of 1/2e ≈ 18.4%.
Slotted ALOHA
Slotted ALOHA introduces time slots. Stations must wait for the beginning of the next slot to transmit. This synchronization forces transmissions to either miss each other completely or collide within the same slot. This reduces the vulnerability period to 1T, effectively doubling the efficiency. The maximum throughput is 1/e ≈ 36.8%.
Comparison Table
| Feature | Pure ALOHA | Slotted ALOHA |
|---|---|---|
| Synchronization | None | Time-slotted |
| Vulnerability Period | 2T | T |
| Max Throughput | 18.4% | 36.8% |
3. Discuss the three persistence methods of CSMA (1-persistent, Non-persistent, and P-persistent). [5 Marks]
Answer:
1-Persistent
The station senses the channel and transmits immediately if it is idle. If busy, it waits continuously until the channel is free. It is greedy but leads to many collisions if multiple stations are waiting.
Non-Persistent
The station senses the channel. If idle, it transmits. If busy, it does not wait continuously; instead, it waits for a random amount of time and senses again. This reduces collisions but increases channel latency during idle periods.
P-Persistent
This is used in slotted channels. If the channel is idle, the station transmits with probability 'p' or delays transmission until the next slot with probability '1-p'. This balances the trade-off between collisions and throughput effectively by spreading transmission probability.
4. Explain the CSMA/CA protocol architecture and its use of acknowledgments. [5 Marks]
Answer:
Introduction
CSMA/CA is used in wireless networks where collision detection is physically infeasible. It focuses on 'avoiding' collisions before they occur.
The CA Mechanism
- Carrier Sensing: The station senses the medium.
- Inter-Frame Space (IFS): Even if the channel is idle, the station must wait for a specific duration called the IFS. This allows higher-priority frames to be sent first.
- Contention Window: If the channel remains idle, the station picks a random backoff time. This timer only counts down while the channel is idle.
Acknowledgements
Since the sender cannot detect a collision, it may not know if the receiver successfully got the frame. Therefore, the receiver must send a Positive Acknowledgement (ACK). If the sender does not receive an ACK within a specified time, it assumes a collision occurred and retransmits using an exponential backoff.
5. Elaborate on the factors affecting the performance of random access MAC protocols. [5 Marks]
Answer:
1. Number of Stations
As the number of active stations increases, the contention for the shared medium grows. This leads to an exponential increase in the number of collisions, drastically reducing total system throughput.
2. Channel Load
When the offered load exceeds the channel capacity, random access protocols enter a state of saturation. In ALOHA-based systems, this results in a rapid collapse of throughput due to excessive retransmissions.
3. Propagation Delay
In CSMA/CD, the time it takes for a signal to reach the other end of the medium (propagation delay) is critical. If the propagation delay is long relative to the frame transmission time, a station might finish sending before a collision is detected, rendering the detection mechanism ineffective.
4. Protocol Overhead
Mechanisms like jamming signals, backoff timers, and IFS contribute to control overhead. While these are necessary for collision management, excessive overhead in low-traffic scenarios can reduce overall channel utilization.
Conclusion
Optimal performance requires a balance between collision management and transmission latency. As networks scale, random access protocols often give way to scheduled or reservation-based MAC protocols to maintain stability.