MEDIUM ACCESS CONTROL PROTOCOL : PURE ALOHA MAC PROTOCOL PURE ALOHA Definition Types of ALOHA PURE ALOHA & SLOTTED ALOHA Features Issues Examples with Diagram Flowchart Vulnerable Time Efficiency Throughput

1. PURE ALOHA
SOUMEN SANTRA
MCA, M.Tech, SCJP, MCP

2. ALOHA?
 It is a system for coordinating to shared communication
Networks channel.
 The original system used for ground based radio
broadcasting.
 It has been implemented in satellite communication
systems.

3. Types of ALOHA protocol
ALOHA
PURE ALOHA SLOTTED ALOHA

4. PURE ALOHA ?
It allows the stations to transmit data at any time
whenever they want.
After transmitting the data packet, station waits
for some time for acknowledgement.

5. Issue#1:
Transmitting station receives an acknowledgement from the
receiving station.
Transmitting station assumes that the transmission is successful.
Issue#2:
Transmitting station does not receive any acknowledgement within
specified time from the receiving station.
Transmitting station assumes that the transmission is unsuccessful.
PURE ALOHA : ISSUES

6. PURE ALOHA : EXAMPLE

7. PURE ALOHA : Flowchart

8. PURE ALOHA : Vulnerable Time

9. Efficiency of Pure Aloha (ε) = G x e-2G
where G = Number of stations to which data transmitted.
Maximum Efficiency-
For maximum efficiency,
We put dε / dG = 0
Maximum value of ε occurs at G = 1/2
Substituting G = 1/2 in the above expression, we get-
Maximum efficiency of Pure Aloha
= 1/2 x e-2 x 1/2
= 1 / 2e
= 0.184
= 18.4%
Thus,
Maximum Efficiency of Pure Aloha (ε) = 18.4%
PURE ALOHA : Efficiency

10. The throughput for pure ALOHA is
S= G x e-2G
The maximum throughput is
Smax=0.184 when G=(1/2)

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