ALOHA In Random Access Protocols

Aloha was designed for WLAN but is also applicable for shared medium. In Aloha, multiple stations can transmit data at the same time which leads toward collision and loss of data.

Before to start the topic must know about the following points

  • Transmission Time: Time to put entire message from station to medium.
  • Propagation Time: Time spend by message over medium to move toward receiving end.

Versions of Aloha Protocol

Aloha has two version which are explain below with detail

1. Pure Aloha

In pure ALOHA, frames are continuously transmitted over medium without any wait. Whenever a station holds an available frame, then it sends that frame. If there is collision in frames then the frames are lost, the sender waits for a random amount of time before retransmission of frame.

If during the transmission time of one station, any other station also transmits the data on same channel then A collision occurs. Following diagram explain all

Major Features of Pure Aloha are

I. Random Access Transmission: Any device can transmit data at any Time.

II. Acknowledgement: Sender waits for acknowledgement signal, for a random amount of time; if acknowledgment is not received then retransmission comes into picture. Back-off time method is used to retransmission, so that retransmission should be safe.

III. WLAN: It is used in Wireless Local Area Network but also applicable for shared medium.

Note: As it is used in WLAN so, it consider only transmission time not a propagation time

IV. Vulnerable Time: It is the time after which the other device is capable to transmit the data on the same shared medium. It is the double time of transmission time.

Suppose the transmission time of a Station-A is 2ms. In Aloha for successful transmission, it is necessary, when Station-A starts its transmission then no other station transmit data 2ms before. So,

 Vulnerable Time = 2* Transmission Time

Following figure shows Vulnerable Time

If there is violation in Vulnerable Time, then collision occur as shows in the following figure

V. Throughput/Efficiency Formula: Efficiency = G * e – 2 G

G is the total number of stations wants to transfer data at transmission time. 2 is the derived from vulnerable time.

Maximum throughput/ Efficiency in Aloha, when G = ½. It means when half of the stations wants to transmit data in transmission time then efficiency will be the maximum.

When G=1/2 then Efficiency = 18.4%

2. Slotted Aloha 

In slotted aloha is similar to pure aloha, except the use of time slots. Slotted aloha holds time slots, the sending of data are allowed only at the beginning of time-slots. If any station misses out the allowed time, it must wait for the next time slot. This reduces the probability of collision.

Keep in mind: Time Slot is always equal to transmission time, so that message can deliver.

But still there is a chance of collision because when two or more stations appears at a time at the end of previous slot, to transfer data.

Throughput/Efficiency Formula:  Efficiency = G * e – 1 G

G is the total number of stations wants to transfer data at transmission time. 1 is the derived from vulnerable time of slotted aloha.

Maximum throughput/ Efficiency in Aloha, when G = 1. It means when all of the stations wants to transmit data in their transmission time slot  then efficiency will be the maximum.

When G=1 then Efficiency = 36.8%

Note: Double the efficiency as compare to pure Aloha

Comparisons of Pure Vs. Slotted Aloha

Pure Aloha Slotted Aloha
Any device can transmit data at any time: Every device send a data in the beginning of time slot.
Single efficiency (18.4%) Double the efficiency (36.8%)
Vulnerable Time = transmission time Vulnerable Time = 2 x transmission time

 

 

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