Of Rules That Govern Data Communication.  A Protocol Defines

ALOHAnet Ravi.H.B

Protocol  Set

of rules that govern data communication.  A protocol defines what is communicated, how it is communicated and when it is communicated.

MAC Protocols  Medium

Access Control.  Many nodes share a common channel.  Goals    

Fair among users High efficiency Low delay Fault tolerant

ALOHAnet  Norman

Abramson – University of Hawaii in

1970.  Used for satellite communication in the pacific.  Basic “transmit at will” protocols.  ALOHAnet uses shared medium for transmission.

ALOHA Protocol  OSI

layer 2 protocol  Used for LAN networks  Broadcast topology  Contention type

ALOHA  Two  

rules

If you have data to send, send the data. If the message collides with another transmission, try re-transmitting later.

 Successful

receipt of the frame at the satellite means no collision or overlap.  ALOHA depends on the ability of a node to detect or learn that a collision has occurred.

ALOHA 





Any terminal is allowed to transmit without considering whether channel is idle or busy If packet is received correctly, the listener transmits an ACK. If no ACK is received, re-transmission of data

ALOHA - Types  Pure

ALOHA  Slotted ALOHA

Pure ALOHA  Any

node with a packet of data to transmit can transmit onto the shared channel.  Collision will occur in 2 scenarios 



Another node is already transmitting when this node starts. Another node starts to transmit while this node is transmitting

Slotted ALOHA  Enhancement

of Pure ALOHA.  A node can send only at the beginning of a new timeslot.  Thus, collisions are reduced.

Throughput of Pure ALOHA  In

Pure ALOHA, the vulnerable period for a node’s transmission starts T seconds before it initiates its frame transmission and ends T seconds after it completes its frame.  Max. Throughput is 18.4%  It occurs at G=0.5

Throughput of Pure ALOHA 0.2 0.18 0.16

Throughput (S)

0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0

1 2 3 AverageNum beroffram esperunittim e(G)

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Throughput of Slotted ALOHA  In

Slotted ALOHA, the vulnerable period of a node’s transmission is only the T seconds prior to the start of its frame.  Max. Throughput is 36.8%  It occurs at G=1

Throughput of Slotted ALOHA 0.4 0.35

Throughput (S)

0.3 0.25 0.2 0.15 0.1 0.05 0 0

1 2 3 AverageNum beroffram esperunittim e(G)

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Thank You