ALOHAnet

In its simplest form, later known as Pure ALOHA, remote units communicated with a base station (Menehune) over two separate radio frequencies (for inbound and outbound respectively).

In the early 2000s additional ALOHA channels were added to 2.5G and 3G mobile phones with the widespread introduction of General Packet Radio Service (GPRS), using a slotted-ALOHA random-access channel combined with a version of the Reservation ALOHA scheme first analyzed by a group at BBN Technologies.

One of the early computer networking designs, development of the ALOHA network was begun in September 1968 at the University of Hawaii under the leadership of Norman Abramson and Franklin Kuo, along with Thomas Gaarder, Shu Lin, Wesley Peterson and Edward ("Ned") Weldon.

If data was received correctly at the hub, a short acknowledgment packet was sent to the client; if an acknowledgment was not received by a client machine after a short wait time, it would automatically retransmit the data packet after waiting a randomly selected time interval.

This acknowledgment mechanism was used to detect and correct for collisions created when two client machines both attempted to send a packet at the same time.

The ALOHAnet solution was to allow each client to send its data without controlling when it was sent, and implementing an acknowledgment/retransmission scheme to deal with collisions.

This approach radically reduced the complexity of the protocol and the networking hardware, since nodes do not need to negotiate who is allowed to speak.

This solution became known as a pure ALOHA, or random-access channel, and was the basis for subsequent Ethernet development and later Wi-Fi networks.

[8] The probability distribution of this random waiting time for retransmission of a packet that has not been acknowledged as received is critically important for the stability of Aloha-type communication systems.

How long a station waits until it retransmits, and the likelihood a collision occurs are interrelated, and both affect how efficiently the channel can be used.

This means that the concept of retransmit later is a critical aspect; The quality of the backoff scheme chosen significantly influences the efficiency of the protocol, the ultimate channel capacity, and the predictability of its behavior.

The improvements with Reservation ALOHA are markedly shorter delays and ability to efficiently support higher levels of utilization.

DAMA alternates between two phases: During the reservation phase of a frame, DAMA acts like slotted ALOHA for some fixed number of short slots, except instead of ground stations sending complete packets, ground stations only send short requests for later transmission.

For this reason, applications that need highly deterministic load behavior may use master/slave or token-passing schemes (such as Token Ring or ARCNET) instead of contention systems.

Outgoing messages from the 360 were converted into packets by the Menehune, which were queued and broadcast to the remote users at a data rate of 9600 bit/s.

The buffer was designed for a full line length of 80 characters, which allowed handling of both the 40- and 80-character fixed-length packets defined for the system.

Shortly after the original ALOHA network went into operation, the TCU was redesigned with one of the first Intel microprocessors, and the resulting upgrade was called a Programmable Control Unit (PCU).

If an acknowledgment was not received from the Menehune after the prescribed number of automatic retransmissions, a flashing light was used as an indicator to the human user.

Also, since the TCUs and PCUs did not send acknowledgments to the Menehune, a steady warning light was displayed to the human user when an error was detected in a received packet.

Considerable simplification was incorporated into the initial design of the TCU as well as the PCU for interfacing a human user into the network.

In later versions of the system, simple radio relays were placed in operation to connect the main network on the island of Oahu to other islands in Hawaii, and Menehune routing capabilities were expanded to allow user nodes to exchange packets with other user nodes, the ARPANET, and an experimental satellite network.

[4] Two fundamental choices which dictated much of the ALOHAnet design were the two-channel star configuration of the network and the use of random access for user transmissions.

The two-channel configuration was primarily chosen to allow for efficient transmission of the relatively dense total traffic stream being returned to users by the central time-sharing computer.

The random-access channel for communication between users and the Menehune was designed specifically for the traffic characteristics of interactive computing.

To achieve a more efficient use of bandwidth for bursty traffic, ALOHAnet developed the random-access packet switching method that has come to be known as a pure ALOHA channel.

This approach effectively dynamically allocates bandwidth immediately to a user who has data to send, using the acknowledgment and retransmission mechanism described earlier to deal with occasional access collisions.

While the average channel loading must be kept below about 10% to maintain a low collision rate, this still results in better bandwidth efficiency than when fixed allocations are used in a bursty traffic context.

The system was configured as a star network, allowing only the central node to receive transmissions in the random-access channel.

[22] These regulatory developments made it possible to use the ALOHA random-access techniques in both Wi-Fi and in mobile telephone networks.