Aloha Ppt

VNS Institute of Technology Presentation on

ALOHA By:Anurag mishra

Introduction • Aloha means affection, love, peace, compassion and mercy. • state nickname of Hawaii, the "Aloha State". • In 1970s NORMAN ABRAMSON & his colleagues at Hawaii university .

Broadcast Channels • Also referred to as Multi-access channels/Random access channels. • MAC (Medium Access Control) determines the next arrival. • Examples of random access MAC protocols: • ALOHA,LAN. • CSMA, CSMA/CD. • Wireless radio.

PURE (unslotted) ALOHA • Pure Aloha: simpler, no synchronization • Packet needs transmission: • send without awaiting for beginning of slot • collision probability increases: • packet sent at t0 collide with other pkts sent in [t0-1, t0+1].

Throughput of PURE ALOHA • Probability is given by Pr[k] = Gk e–G / k! • P(success) = e-G x e-G = e-2G • Throughput (success rate) = Ge-2G • Max throughput at G = 1/2, Throughput = 1/2e ~ 0.18 • Advantages of unslotted aloha are simplicity and possibility of unequal length packets.

SLOTTED ALOHA • 1972 Robert founded slotted aloha. • Nodes are synchronized. All frames same size • If collision, node retransmits frame in each subsequent slot with prob. p until success.

Success (S), Collision (C), Empty (E) slots

 Success (S), Collision (C), Empty (E) slots Success (S), Collision (C), Empty (E) slots

Throughput of SLOTTED ALOHA The throughput is the fraction of slots that contain a successful transmission = P(success) = • Ge-G .

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G < 1 => too many idle slots.

G > 1 => too many collisions.

P(success) = Ge−G = 1/ e ≈ 0.36 .

Aloha is inherently unstable and needs algorithm to keep it stable .

Throughput comparison

(success rate)

S = throughput = “goodput”

• Stabilized pure aloha T = 0.184 = (1/ (2e)) • Stabilized slotted aloha T = 0.368 = (1/e) 0.4 0.3

Slotted Aloha 0.2

0.1

Pure Aloha 0.5

1.0

1.5

2.0

G = offered load = Np

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