This document discusses Pure Aloha, a random access protocol where multiple stations can transmit data simultaneously, potentially leading to collisions and garbled data. Under Pure Aloha, stations can transmit whenever they have data without coordination. If an acknowledgment is not received within an allotted time, the station will wait a random back-off time and resend the data. The throughput of Pure Aloha is maximized when frame lengths are uniform, and any overlap between frames results in both being destroyed and needing retransmission. The maximum throughput of Pure Aloha is 0.184 when half of stations have data to transmit.

1. SATELLITE PACKET SWITCHING:
PURE ALOHA

2. ALOHA
• Aloha is a random access protocol
• It was actually designed for WLAN but it is also applicable for shared medium
• In this multiple stations can transmit data at the same time and can hence lead to
collision and data being garbled
• Types :
Pure aloha
Slotted aloha

3. COLLISION

4. PURE ALOHA

5. PURE ALOHA
• Pure ALOHA allows stations to transmit whenever they have data to be sent
• When a station sends data it waits for an acknowledgement
• If acknowledgement doesn’t come within allotted time then station waits for
random amount of time called back-off time(tb) and resends the data

6. PURE ALOHA
• Since different station waits for different amount of time, the probability of the
further collision decreseas
• The throughput of a pure aloha is maximized when frames are of uniform length
• Whenever two frames try to occupy the channel at the same time there will be a
collision and both will be garbled
• If the first bit of anew frame overlaps with just the last bit of a frame which is
almost finished,both frames will be totally destroyed and both will have to be
retransmitted later

7. PURE ALOHA
• Vulnerable time = 2*Tfr
• Throughput = G X e-2G ;where G is no
of stations wish to transmit in the
same time
• Maximum throughput = 0.184 for
G=0.5