Pure ALOHA is an unslotted, fully-decentralized protocol. It is extremely easy and trivial to execute. The basic form is - "when you want to talk, just talk!" So, a node that wants to transmit will go ahead and send the packet on its communication channel, with no consideration whatsoever as to whether anybody else is transmitting or not.
In the slotted ALOHA, stations are enabled to send frames in slots only. If longer than one station transmits in a similar slot, it will generate a collision. This reduces the appearance of collisions in the network system. Here, every station has to maintain the record of the time slot. The process of transmission will be initiated by any station at the beginning of the time slot only.
The major differences between Pure ALOHA and Slotted ALOHA are as follows−
|Pure ALOHA||Slotted ALOHA|
|Pure ALOHA was introduced by Norman and his associates at the University of Hawaii in 1970.||Slotted ALOHA was introduced by Roberts in 1972.|
|In pure ALOHA, whenever a station has data to send, it transmits it without waiting.||In slotted ALOHA, a user waits till the next time slot begins to transmit the data.|
|The frames are transmitted at arbitrary times.||The time is divided up into discrete slots, and the frame is sent at the start of the slot.|
|The performance is given by S = Ge−2G||The performance is given by S = Ge−G|
|Due to the very low utilization factor, it cannot be used for satellites.||It is used in broadcast satellites.|
|It does not require global time.||It requires global time synchronization, as it is divided up into discrete slots.|
|The maximum utilization is about 18%.||The maximum utilization is about double that of Pure ALOHA, i.e., 37%.|
|The implementation is very simple.||The implementation is complex due to the synchronization of all the stations.|