ALOHA

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

ALOHA is a stochastic access method in networks without channel scanning and is similar to the CSMA method, but which have channel scanning. It can be used independently of time (as a pure, pure or unsynchronized ALOHA) or using a time-dependent method (subdivided, slotted or synchronized ALOHA).

Unsynchronized ALOHA

Time behavior with unsynchronized ALOHA and four stations. Data packets which collide in gray

In unlock ALOHA each participant can at any time be (always of equal length) data packet ( frame , Eng. Frame ) can be obtained. If several participants send their parcel at the same time, the parcels collide and recipients only receive garbled data; the packets must be retransmitted. The participants recognize a corruption of the data due to the lack of an arrival confirmation from the target computer. Each participant has to wait a random period of time for the retransmission. Like most radio protocols, ALOHA is therefore not real-time capable, since it cannot be guaranteed when a packet to be sent will actually be transmitted successfully.

With this method, a throughput of 18% of the channel capacity can be achieved. The throughput can also be calculated ( = throughput per frame time and = attempts per packet time). With you get the max. Throughput of 18% or , with the Euler number . ${\ displaystyle S = G \ cdot e ^ {- 2G}}$ ${\ displaystyle S}$ ${\ displaystyle G}$ ${\ displaystyle G = 0 {,} 5}$ ${\ displaystyle {\ frac {1} {2e}}}$ ${\ displaystyle e}$

Synchronized ALOHA or slotted ALOHA

Time behavior with synchronized ALOHA and eight stations. In gray: data packets that collide

In contrast to the unsynchronized ALOHA, with this method a participant is not allowed to send at any point in time, but has to adhere to fixed time slices ( slots ) the length of a packet. Any user can send to one of these slots at any time. If several people use a slot at the same time, a collision occurs. If a collision occurs, the mutilated packages can only completely cover each other. As a result, a throughput of 37% or the maximum channel capacity can be achieved, which corresponds to a doubling compared to the pure ALOHA. ${\ displaystyle {\ frac {1} {e}}}$

Synchronized ALOHA is, however, more complex to implement than the unsynchronized method, since all participants need a uniform clock so that the "slots" run simultaneously for all. The procedure for synchronized ALOHA was published by LG Roberts in 1975.

Calculation follows with the maximum and the relationship the value of approx. 36.788%, analogous to the 37% rule . ${\ displaystyle G = 1}$ ${\ displaystyle S = G \ cdot e ^ {- G}}$

Individual evidence

^ Norman Abramson: The ALOHA System - Another Alternative for Computer Communications . Ed .: AFIDS Conference Proceedings. tape 37 . AFIPS Press, 1970, pp. 281-285 .
^ LG Roberts: ALOHA Packet System With and Without Slots and Capture . tape 5 , no. 2 . Computer Communications. Review, April 1975, p. 28-42 .

[abram1-1] Norman Abramson: The ALOHA System - Another Alternative for Computer Communications . Ed .: AFIDS Conference Proceedings. tape 37 . AFIPS Press, 1970, pp. 281-285 .

[robert1-2] LG Roberts: ALOHA Packet System With and Without Slots and Capture . tape 5 , no. 2 . Computer Communications. Review, April 1975, p. 28-42 .