Random Access Protocols: ALOHA (Pure vs Slotted)

Random access protocols allow multiple stations to transmit over a shared communication medium without requiring scheduled turns. In such systems, transmissions may overlap, creating collisions; a receiver typically cannot successfully decode any frame if two or more frames overlap beyond the capture model.

Two canonical random access schemes are Algorithm ALOHA variants:

• keyword Pure ALOHA: stations transmit whenever they have data; frames can start at arbitrary times.
• keyword Slotted ALOHA: time is divided into equal slots; a station transmits only at slot boundaries.

The core idea behind performance is the probability that a station transmits during a time interval where no other station transmits, so the frame is collision-free. In ALOHA, successful throughput is maximized at specific offered load values, yielding well-known peak throughputs.

Key term map for this section:

• random access protocol
• collision
• throughput
• offered load
• slot

Footnotes

1. Computer Networking: A Top-Down Approach - Discusses MAC concepts, random access, and collision-based models. ↩

2. ALOHA protocol (IEEE/intro materials compilation) - Defines pure vs slotted ALOHA and their synchronization/slot assumptions. ↩

Random Access Protocol: What it means in practice

A random access protocol is a medium access control (MAC) mechanism designed for shared media. Each station transmits independently when it has data, rather than following a centralized schedule. Because stations do not coordinate, overlapping transmissions are possible.

ALOHA specifically assumes:

• Frames can collide when transmissions overlap in time.
• Collided frames are not recoverable at the receiver and must be retransmitted (via some higher-layer or MAC retransmission policy, often with random backoff in real systems).

In this model, the probability of a successful transmission depends on:

1. How many stations are active,
2. The rate at which they attempt transmissions,
3. Whether they are synchronized to slot boundaries (slotted) or not (pure).

Why overlap window matters:

• With pure ALOHA, any frame that begins within one frame-time of another frame’s start can overlap and cause collision.
• With slotted ALOHA, collisions only occur if another transmission happens in the same slot (or, equivalently in the model, within the vulnerable period aligned to slots).

Footnotes

1. ALOHA protocol (IEEE/intro materials compilation) - Defines pure vs slotted ALOHA and their synchronization/slot assumptions. ↩ ↩²

Differentiate: Pure ALOHA vs Slotted ALOHA

1) Timing / synchronization

• Pure ALOHA: stations transmit at arbitrary times (frame starts can occur anytime).
• Slotted ALOHA: time is divided into equal slots; stations transmit only at slot boundaries (requires synchronization).

2) Vulnerable period and collision likelihood

• Pure ALOHA has a larger vulnerable period because any overlap in continuous time can cause collision.
• Slotted ALOHA reduces the vulnerable period because transmissions start only at slot edges, cutting the number of interfering start times that lead to partial overlap.

In the standard analysis (Poisson model of attempts):

• Pure ALOHA throughput: $S_{pure}(G) = G e^{-2G}$
• Slotted ALOHA throughput: $S_{slotted}(G) = G e^{-G}$

3) Maximum throughput (peak efficiency)

From maximizing the respective $S(G)$ functions:

• Pure ALOHA peak at $G=\\tfrac{1}{2}$: $S_{pure,max} = \\frac{1}{2e} \\approx 0.184$
• Slotted ALOHA peak at $G=1$: $S_{slotted,max} = \\frac{1}{e} \\approx 0.368$

4) Practical implications

• Slotted ALOHA typically achieves higher throughput due to reduced collision probability.
• However, it requires time synchronization, which may add complexity and overhead in real deployments.

Footnotes

1. ALOHA protocol (IEEE/intro materials compilation) - Defines pure vs slotted ALOHA and their synchronization/slot assumptions. ↩ ↩²

2. [ALOHA throughput derivations and classic peak values] (https://en.wikipedia.org/wiki/ALOHA_(media_access_control)) - Provides the standard throughput expressions $S_{pure}=G e^{-2G}$ and $S_{slotted}=G e^{-G}$ and their maxima $1/(2e)$ and $1/e$. ↩ ↩²

Summary: Key differentiators (exam-ready)

Footnotes

1. ALOHA protocol (IEEE/intro materials compilation) - Defines pure vs slotted ALOHA and their synchronization/slot assumptions. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. [ALOHA throughput derivations and classic peak values] (https://en.wikipedia.org/wiki/ALOHA_(media_access_control)) - Provides the standard throughput expressions $S_{pure}=G e^{-2G}$ and $S_{slotted}=G e^{-G}$ and their maxima $1/(2e)$ and $1/e$. ↩ ↩² ↩³ ↩⁴