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ALOHABy: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 eG / 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) slotsSuccess (S), Collision (C), Empty (E) slots

Throughput of SLOTTED ALOHAThe throughput is the fraction of slots that contain a successful transmission = P(success) = Ge-G . G < 1 => too many idle slots.

G > 1 => too many collisions.

P(success) = GeG = 1/ e 0.36 .

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

Throughput comparison Stabilized pure aloha T = 0.184 = (1/ (2e)) Stabilized slotted aloha T = 0.368 = (1/e) 0.4S = throughput = goodput

(success rate)

0.3

Slotted Aloha0.2

0.1

Pure Aloha0.5 1.0 1.5 2.0

G = offered load = Np

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