Slotted Aloha

Summary

This lecture introduces Slotted Aloha, a protocol enhancing the efficiency of Pure Aloha used in wireless LANs. Unlike Pure Aloha, where frames can be sent at any time, Slotted Aloha divides time into slots, allowing frame transmission only at the start of each slot. This reduces collision probability and doubles the maximum efficiency from 18.4% to 36%. The lecture compares both Aloha types, highlighting Slotted Aloha's advantages in collision reduction and throughput improvement.

Takeaways

• 📚 Aloha is a random access protocol designed for wireless LANs and shared mediums.
• 🔄 There are two types of Aloha: Pure Aloha and Slotted Aloha, with Slotted Aloha introduced to improve efficiency.
• 🕒 In Pure Aloha, stations can transmit at any time, leading to random and variable collision durations.
• 🔄 Slotted Aloha divides time into discrete intervals called slots, allowing transmission only at the start of each slot.
• 🚦 The vulnerable time in Slotted Aloha is reduced to the frame transmission time, cutting the collision window in half.
• 📈 The throughput of Slotted Aloha is G * e^(-G), where G is the number of stations transmitting, improving upon Pure Aloha's throughput of G * e^(-2G).
• 🔝 The maximum throughput for Slotted Aloha is 0.368 when G equals 1, doubling the maximum efficiency of Pure Aloha at 0.184.
• 🛑 Stations in Slotted Aloha must wait for the next time slot if they miss the current one, reducing collision probability.
• 📉 Collisions in Slotted Aloha are predictable and occur only at the beginning of time slots, unlike in Pure Aloha where they can occur at any time.
• 🆚 Slotted Aloha offers a more structured and synchronized approach compared to the simplicity of Pure Aloha, with a significant reduction in collisions and increased efficiency.

Q & A

What is Slotted Aloha?

Slotted Aloha is a random access protocol designed to improve the efficiency of Pure Aloha by dividing the channel time into discrete time intervals called slots. Data transmission is only allowed at the beginning of these time slots, which helps to reduce the probability of collisions.

How does Slotted Aloha reduce the probability of collisions?

In Slotted Aloha, since data transmission is only allowed at the beginning of each time slot, the vulnerable time for collisions is reduced to the frame transmission time, which is half of what it is in Pure Aloha, thus reducing the number of collisions.

What is the throughput of Pure Aloha?

The throughput of Pure Aloha is approximately 0.184 (or 18.4%) when the number of stations G is equal to 1.

What is the maximum throughput of Slotted Aloha?

The maximum throughput of Slotted Aloha is 0.368 (or 36.8%) when the number of stations G is equal to 1.

How does the vulnerable time differ between Pure Aloha and Slotted Aloha?

In Pure Aloha, the vulnerable time is twice the frame transmission time since frames can start at any time. In Slotted Aloha, it is only the frame transmission time because frames can only start at the beginning of a time slot.

What is the main advantage of Slotted Aloha over Pure Aloha?

The main advantage of Slotted Aloha is that it reduces the number of collisions by half and doubles the efficiency of Pure Aloha.

How does the time management differ between Pure Aloha and Slotted Aloha?

In Pure Aloha, time is continuous and not globally synchronized, whereas in Slotted Aloha, time is discrete and globally synchronized into time slots.

What happens if a station misses its allowed time slot in Slotted Aloha?

If a station misses its allowed time slot in Slotted Aloha, it must wait until the beginning of the next time slot to transmit data.

What is the probability of successful transmission of data packets in Slotted Aloha?

The probability of successful transmission of data packets in Slotted Aloha is given by the formula G * e^(-G) where G is the number of stations attempting to transmit at the same time.

How does the collision duration differ between Pure Aloha and Slotted Aloha?

In Pure Aloha, collision duration can vary because frames can start at any time. In Slotted Aloha, the collision duration is uniform and equals the duration of a time slot.

What is the main advantage of Pure Aloha?

The main advantage of Pure Aloha is its simplicity in implementation, as it does not require the synchronization of time slots like Slotted Aloha.