Reservation ALOHA

ALOHA is an access method of the data link layer (DLL, OSI layer 2 ) in the field of computer networks. The ALOHA protocol was developed in 1970 by Norman Abramson and 1971, first used for the ALOHANET, which joined the many islands around Hawaii with the University of Honolulu, Hawaii. It was originally conceived as a radio network and later formed the basis for the Ethernet protocol. It is a TDMA multiplexing.

ALOHA is a stochastic process in networks without access channel scanning and the CSMA method similar but which have channel scan. It can be time -independent ( as a pure, pure or non-synchronized ALOHA ) or by using a time-dependent process ( divided, slotted ALOHA or synchronized ) are used.

Non-synchronized ALOHA

In unlock ALOHA, each participant may at any time be ( always the same length ) data packet (frame, engl. Frame ) can be obtained. Sending several participants at the same time a package, so collide and mutilate it and need to be transferred later. The participants recognize a corruption of the data by the lack of confirmation of the arrival of the data packet by the receiving computer. Each participant must wait a certain amount of random time period for retransmission. ALOHA is therefore like most wireless protocols is not real-time capable, as it can not be guaranteed when a packet to be sent is actually transferred successfully.

This method allows a throughput of 18 % of the channel capacity achieved. The throughput can be calculated with ( = throughput per frame time = attempts per packet time ). When you get the max. Flow rate of 18 % or more, with the Euler number.

Synchronized ALOHA or slotted ALOHA

Unlike the pure ALOHA, a participant may, in this method does not send at any time, but must keep the length of a packet at fixed predetermined time slices ( engl. slots). Each user can always write in one of these slots. When concurrent use a slot, a collision occurs. Thus, the mangled packets can only fully overlap with the occurrence of a collision, respectively. This allows a throughput of 37% or the maximum channel capacity achieved, which corresponds to a doubling of the pure ALOHA.

Synchronized ALOHA is however more complex to implement than the unsynchronized method, since all the participants need a unified, centralized time source, so that the "slots" are clocked in sync. The method to synchronized ALOHA was developed by LG Roberts published in 1975.

Calculation follows with maximum and the relationship of the value of about 36.788 %, similar to the 37 % rule.