The pure ALOHA protocol is a random access protocol used for data transfer. The first version of the protocol (now called "Pure ALOHA") was quite simple: If you have data to send, send the data, If the message collides with another transmission, try resending "later”. Pure ALOHA does not check whether the channel is busy before transmitting
- The pure ALOHA protocol is a random access protocol used for data transfer.
- A user accesses a channel as soon as a message is ready to be transmitted. After a transmission, the user waits for an acknowledgment on either the same channel or a separate feedback channel. In case of collisions, (i.e., when a NACK is received), the terminal waits for a random period of time and retransmits the message.
- As the number of users increase, a greater delay occurs because the probability of collision increases
- Thus, the probability of no collision during the interval of 2t is found by evaluating Pr(n) given as
Pr(n)=(2R)ne-2R/(n!) at n=0
- The probability of no collision is The throughput of the ALOHA protocol is found by using Equation as
- The maximum throughput is 0.5/e frames per frame-time (reached when G = 0.5), which is approximately 0.184 frames per frame-time. This means that, in Pure ALOHA, only about 18.4% of the time is used for successful transmissions.
Pure ALOHA protocol in Fig 7.10
Boxes indicate frames.
Shaded boxes indicate frames which have collided
The probability (Probpure) of there being zero transmission-attempts between t-T and t T (fig 7.11)
Prob (pure)= e – 2R