ALOHAnet

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. 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 transmissions 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 (${\displaystyle Pro{b}_{pure}}$) of there being zero transmission-attempts between t-T and t+T (and thus of a successful transmission for us) is:

${\displaystyle Pro{b}_{pure}=e^{-2G}}$

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:

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

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 time slots and increased the maximum throughput. A station can start a transmission only at the beginning of a time slot, 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 time slot. Thus, the probability of there being zero transmission attempts by other stations in a single time slot is:

${\displaystyle Pro{b}_{slotted}=e^{-G}}$

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

${\displaystyle Pro{b}_{slotted}k=e^{-G}(1-e^{-G}{)}^{k-1}}$

The throughput is:

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

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 satellite communications networks by military forces, in subscriber-based satellite communications networks, mobile telephony call setup, set-top box communications and in the contactless RFID 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.

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

Packet Reservation Multiple Access

Packet reservation multiple access (PRMA) is an implicit reservation scheme. Some fixed number of slots form a frame. After each frame, the satellite broadcasts the status of each slot from the previous frame, which indicates the reservation status of the corresponding slots of the next frame. All ground stations wishing to transmit compete exactly like slotted ALOHA during any "free slot" of that next frame (i.e., either no one transmitted in that slot of the previous frame, or there was a collision when multiple ground stations transmitted in that slot of the previous frame). If exactly one ground station happens to transmit during a "free slot", that ground station succeeds in reserving that slot of a frame -- the corresponding slot is implicitly reserved in all future frames. From then on, the satellite broadcasts that that particular ground station has reserved that slot of the frame, and that ground station can continue transmitting with a guaranteed data rate during that slot of the frame; other ground stations are careful not to transmit during that slot of the frame, so there are no collisions during reserved slots. When a ground station with a reserved slot has nothing to send, it simply stops transmitting, which gives up its reservation; the satellite notices its reserved slot is idle in one frame, and broadcasts that fact, which indicates that that slot will be a "free slot" in the next frame.

Maximum channel efficiency for slotted ALOHA is 36%; PRMA improves maximum channel efficiency to 80%.

Demand Assigned Multiple Access

Demand assigned multiple access (DAMA), also called reservation ALOHA, is an explicit reservation scheme often used in satellite communications. 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. The satellite collects all the successful requests (i.e., the ones not destroyed by collision) and sends them back as a reservation list assigning specific ground stations to specific TDM slots. During the TDM phase of a frame, the ground stations obey the reservation list and each one only transmits during the long TDM slot(s) reserved for it. Collisions may occur during the reservation phase, but not during the TDM phase.

Maximum channel efficiency for slotted ALOHA is 36%; DAMA improves maximum channel efficiency to 80%.

Mobile Slotted Aloha

Other protocols

The use of a random-access channel in ALOHAnet led to the development of carrier-sense multiple access (CSMA), a listen before send random-access protocol that can be used when all nodes send and receive on the same channel. CSMA in radio channels was extensively modeled. The AX.25 packet radio protocol is based on the CSMA approach with collision recovery, based on the experience gained from ALOHAnet. A variation of CSMA, CSMA/CD is used in early versions of Ethernet.

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 master/slave or token-passing schemes (such as Token Ring or ARCNET) instead of contention systems.

Hardware

The central node communications processor was an HP 2100 minicomputer called the Menehune, named for the mythical race of dwarf people in the Hawaiian language , and was named for its similar role to the original ARPANET Interface Message Processor (IMP) which was being deployed at about the same time. In the original system, the Menehune forwarded correctly received user data to the University of Hawaiʻi central computer, an IBM System 360/65 time-sharing system. 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. Unlike the half-duplex radios at the user TCUs, the Menehune was interfaced to the radio channels with full-duplex radio equipment.

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.

Network architecture

Two fundamental choices that 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 were 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 bit/s, 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 and then in the Marisat (now Inmarsat) satellite network.

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. 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 1G mobile phones for signaling and control purposes. In the late 1980s, the European standardization group GSM who worked on the Pan-European Digital mobile communication system GSM greatly expanded the use of ALOHA channels for access to radio channels in mobile telephony. In addition, SMS message texting was implemented in 2G mobile phones. 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.