ALOHAnet

ALOHAnet, also known as the ALOHA System,

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The ALOHAnet used a new method of medium access, called ALOHA random access, and experimental ultra high frequency (UHF) for its operation. 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). Nodes did not wait for the channel to be clear before sending, but instead waited for acknowledgement of successful receipt of a message, and re-sent it if this was not received. Nodes would also stop and re-transmit data if they detected any other messages while transmitting. While simple to implement, this results in an efficiency of only 18.4%. A later advancement, Slotted ALOHA, improved the efficiency of the protocol by reducing the chance of collision, improving throughput to 36.8%.

ALOHAnet became operational in June 1971, providing the first public demonstration of a wireless packet data network.^[4][5]

ALOHA was subsequently employed in the

.

History

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

ALOHA was originally a

The original version of ALOHA used two distinct frequencies in a hub configuration, with the hub machine broadcasting packets to everyone on the outbound channel, and the various client machines sending data packets to the hub on the inbound channel. 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.

ALOHAnet's primary importance was its use of a shared medium for client transmissions. Unlike the ARPANET where each node could only talk to a single node at the other end of a wire or satellite circuit, in ALOHAnet all client nodes communicated with the hub on the same frequency. This meant that some sort of mechanism was needed to control who could talk at what 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

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The Aloha network introduced the mechanism of randomized multiple access, which resolved device transmission collisions by transmitting a package immediately if no acknowledgement is present, and if no acknowledgment was received, the transmission was repeated after a random waiting time.[8] The probability distribution of this random waiting time for retransmission of a package that has not been acknowledged as received is critically important for the stability of Aloha-type communication systems. The average waiting time for retransmission is typically shorter than the average time for generation of a new package from the same client node, but it should not be allowed to be so short as to compromise the stability of the network, causing a collapse in its overall throughput.^[9]

Also important was ALOHAnet's use of the outgoing hub channel to broadcast packets directly to all clients on a second shared frequency and using an address in each packet to allow selective receipt at each client node.^[4] Separate frequencies were used for incoming and outgoing communications to the hub so that devices could receive acknowledgments regardless of transmissions.

Protocol

Pure ALOHA

The original version of the protocol (now called Pure ALOHA, and the one implemented in ALOHAnet) was quite simple:

• If you have data to send, send the data
• If, while you are transmitting data, you receive any data from another station, there has been a message collision. All transmitting stations will need to try resending later.

Pure ALOHA does not check whether the channel is busy before transmitting. Since collisions can occur and data may have to be sent again, ALOHA cannot efficiently use 100% of the capacity of the communications channel. 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.

To assess Pure ALOHA, there is a need to predict its throughput, the rate of (successful) transmission of frames.^[10] First make a few simplifying assumptions:

• All frames have the same length.
• Stations cannot generate a frame while transmitting or trying to transmit. That is, while a station is sending or trying to resend a frame, it cannot be allowed to generate more frames to send.
• The population of stations attempting to transmit (both new transmission and retransmissions) follows a Poisson distribution.

Let T refer to the time needed to transmit one frame on the channel, and define frame-time as a unit of time equal to T. Let G refer to the mean used in the Poisson distribution over transmission-attempt amounts. That is, on average, there are G transmission attempts per frame-time.

Consider what needs to happen for a frame to be transmitted successfully. Let t refer to the time at which it is intended to send a frame. It is preferable to use the channel for one frame-time beginning at t, and all other stations to refrain from transmitting during this time.

For any frame-time, the probability of there being k transmission-attempts during that frame-time is:

${\displaystyle {\frac {G^{k}e^{-G}}{k!}}}$

The average number of transmission-attempts for two consecutive frame-times is 2G. Hence, for any pair of consecutive frame-times, the probability of there being k transmission attempts during those two frame-times is:

${\displaystyle {\frac {(2G)^{k}e^{-2G}}{k!}}}$

Therefore, the probability (${\displaystyle Prob_{pure}}$) of there being zero transmission-attempts between t-T and t+T (and thus of a successful transmission for us) is:

${\displaystyle Prob_{pure}=e^{-2G}}$

The throughput can be calculated as the rate of transmission attempts multiplied by the probability of success, and it can be concluded that the throughput (${\displaystyle S_{pure}}$) is:

${\displaystyle S_{pure}=Ge^{-2G}}$

The maximum throughput is 0.5/e frames per frame-time (reached when ${\displaystyle G=0.5}$), which is approximately 0.184 frames per frame-time. This means that, in Pure ALOHA, only about 18.4% of the time is used for successful transmissions.

Slotted ALOHA

An improvement to the original ALOHA protocol was Slotted ALOHA, which introduced discrete timeslots and increased the maximum throughput.^[11] A station can start a transmission only at the beginning of a timeslot, and thus collisions are reduced. In this case, only transmission-attempts within 1 frame-time and not 2 consecutive frame-times need to be considered, since collisions can only occur during each timeslot. Thus, the probability of there being zero transmission attempts by other stations in a single timeslot is:

${\displaystyle Prob_{slotted}=e^{-G}}$

the probability of a transmission requiring exactly k attempts is (k-1 collisions and 1 success):^[10]

${\displaystyle Prob_{slotted}k=e^{-G}(1-e^{-G})^{k-1}}$

The throughput is:

${\displaystyle S_{slotted}=Ge^{-G}}$

The maximum throughput is 1/e frames per frame-time (reached when G = 1), which is approximately 0.368 frames per frame-time, or 36.8%.

Slotted ALOHA is used in low-data-rate tactical

technologies.

Reservation ALOHA

Reservation ALOHA, or R-ALOHA, is an effort to improve the efficiency of Slotted ALOHA. The improvements with Reservation ALOHA are markedly shorter delays and ability to efficiently support higher levels of utilization. As a contrast of efficiency, simulations have shown that Reservation ALOHA exhibits less delay at 80% utilization than Slotted ALOHA at 20–36% utilization.^[12]

The chief difference between Slotted and Reservation ALOHA is that with Slotted ALOHA, any slot is available for utilization without regards to prior usage. Under Reservation ALOHA's

reservation schema, the slot is temporarily considered "owned" by the station that successfully used it. Additionally, Reservation ALOHA simply stops sending data once the station has completed its transmission. As a rule, idle slots are considered available to all stations that may then implicitly reserve (utilize) the slot on a contention basis.

Other protocols

The use of a random-access channel in ALOHAnet led to the development of

.

ALOHA and the other random-access protocols have an inherent variability in their throughput and delay performance characteristics. For this reason, applications that need highly deterministic load behavior may use

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Hardware

The central node communications processor was an

The original user interface developed for the system was an all-hardware unit called an ALOHAnet Terminal Control Unit (TCU) and was the sole piece of equipment necessary to connect a terminal into the ALOHA channel. The TCU was composed of a UHF antenna, transceiver, modem, buffer and control unit. 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. The typical user terminal in the original system consisted of a Teletype Model 33 or a dumb CRT user terminal connected to the TCU using a standard RS-232 interface. 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).

Additional basic functions performed by the TCUs and PCUs were generation of a cyclic-parity-check code vector and decoding of received packets for packet error detection purposes, and generation of packet retransmissions using a simple random interval generator. 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]

Network architecture

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. An additional reason for the star configuration was the desire to centralize as many communication functions as possible at the central network node (the Menehune) to minimize the cost of the original all-hardware terminal control unit (TCU) at each user node.

The random-access channel for communication between users and the Menehune was designed specifically for the traffic characteristics of interactive computing. In a conventional communication system, a user might be assigned a portion of the channel on either a frequency-division multiple access or time-division multiple access basis. Since it was well known that in time-sharing systems (circa 1970), computer and user data are bursty, such fixed assignments are generally wasteful of bandwidth because of the high peak-to-average data rates that characterize the traffic.

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.

Two 100 kHz channels in the experimental UHF band were used in the implemented system, one for the user-to-computer random-access channel and one for the computer-to-user broadcast channel. The system was configured as a star network, allowing only the central node to receive transmissions in the random-access channel. All user TCUs received each transmission made by the central node in the broadcast channel. All transmissions were made in bursts at 9600 

, with data and control information encapsulated in packets.

Each packet consisted of a 32-bit header and a 16-bit header parity check word, followed by up to 80 bytes of data and a 16-bit parity check word for the data. The header contained address information identifying a particular user so that when the Menehune broadcast a packet, only the intended user's node would accept it.

Legacy

In the 1970s ALOHA random access was employed in the nascent Ethernet cable based network^[17] and then in the Marisat (now Inmarsat) satellite network.^[18]

In the early 1980s frequencies for mobile networks became available, and in 1985 frequencies suitable for what became known as Wi-Fi were allocated in the US.^[19] These regulatory developments made it possible to use the ALOHA random-access techniques in both Wi-Fi and in mobile telephone networks.

ALOHA channels were used in a limited way in the 1980s in

References

Further reading

• ISBN 978-0-02-415451-4
  .
• R. Metcalfe, Xerox PARC memo, from Bob Metcalfe to Alto Aloha Distribution on Ether Acquisition, May 22, 1973.
• R. Binder, ALOHAnet Protocols, ALOHA System Technical Report, College of Engineering, The University of Hawaii, September, 1974.
• R. Binder, W.S. Lai and M. Wilson, The ALOHAnet Menehune – Version II, ALOHA System Technical Report, College of Engineering, The University of Hawaii, September, 1974.
• N. Abramson, The ALOHA System Final Technical Report, Advanced Research Projects Agency, Contract Number NAS2-6700, October 11, 1974.
• N. Abramson "The Throughput of Packet Broadcasting Channels", IEEE Transactions on Communications, Vol 25 No 1, pp117–128, January 1977.
• M. Schwartz, Mobile Wireless Communications, Cambridge Univ. Press, 2005.
• K. J. Negus, and A. Petrick, History of Wireless Local Area Networks (WLANs) in the Unlicensed Bands, George Mason University Law School Conference, Information Economy Project, Arlington, VA., USA, April 4, 2008.
• H. Wu; C. Zhu; R. J. La; X. Liu; Y. Zhang. "FASA: Accelerated S-ALOHA using access history for event-driven M2M communications" (PDF). IEEE/ACM Transactions on Networking, 2013. Archived from the original (PDF) on 2015-01-23. Retrieved 2015-01-22.

External links

• Dynamic Sharing of Radio Spectrum: A Brief History at the Wayback Machine (archived 2003-08-31)
• Funding a Revolution: Government Support for Computing Research
• ALOHA to the Web, Norman Abramson, HICCS Distinguished Lecture at archive.today (archived 2012-12-11)
• Norman Abramson and ALOHAnet