Aloha Network

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ALOHA NETWORK, was a networking system developed at the University of Hawaii in the late 1960s. 

Norman Abramson and his colleagues developed a radio network for communication among the Hawaiian Islands. This system was an early experiment in the development of mechanisms for sharing a common communications channel – in this case, a common radio channel.

Aloha Protocol
Aloha Network

The Aloha protocol was very simple: an Aloha station could send whenever it liked, and then wait for an acknowledgment. If an acknowledgment wasn’t received within a short amount of time, the station would assume that another station had transmitted simultaneously, causing a collision in which the combined transmissions were garbled so that the receiving station did not hear them and did not return an acknowledgment. Upon detecting a collision, both transmitting stations would choose a random backoff time, and then retransmit their packets with a good probability of success. However, as traffic increased on the Aloha channel, the collision rate would rapidly increase as well.

Abramson calculated that this system, known as pure Aloha, could achieve a maximum channel utilization of about 18%, due to the rapidly increasing rate of collisions under increasing load. Another system, called slotted Aloha, was developed that assigned transmission slots and used a master clock to synchronize transmissions; this increased the maximum utilization of the channel to about 37%. In 2007, Abramson received the IEEE’s Alexander Graham Bell Medal for “contributions to the development of modern data networks through fundamental work in random multiple access.”

Bob Metcalfe and the Invention of Ethernet

Bob Metcalfe’s 1973 Ethernet memo describes a networking system inspired by an earlier experiment in networking called the Aloha network.

Metcalfe realized that he could improve on the Aloha system of arbitrating access to a shared communications channel. He developed a new system that included a mechanism that detected when a collision occurred (collision detection). The system also included “listen before talk,” in which stations listened for activity (carrier sense) before transmitting, and supported access to a shared channel by multiple stations (multiple access). Put all these components together, and you can see why the original channel access protocol specified for Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Metcalfe also developed a more sophisticated backoff algorithm, which, in combination with the CSMA/CD protocol, allowed the Ethernet system to function at up to 100% load.

In late 1972, Metcalfe and his Xerox PARC colleagues developed the first experimental “Ethernet” network system to interconnect Xerox Altos to one another, and to servers and laser printers. The signal clock for the experimental interface was derived from Alto’s system clock, resulting in a data transmission rate on the experimental Ethernet of 2.94 Mb/s.

Metcalfe’s first experimental network was called the Alto Aloha Network. In 1973, Metcalfe changed the name to “Ethernet,” to make it clear that the system could support any computer‚ not just Altos‚ and to point out that his new network mechanisms had evolved well beyond the Aloha system. He chose to base the name on the word “ether” as a way of describing an essential feature of the system: the physical medium (i.e., a cable) carries bits to all stations, much the same way that the old “luminiferous ether” was once thought to propagate electromagnetic waves through space.

Thus, Ethernet was born.

Drawing of the original Ethernet system

In 1976, Metcalfe drew the diagram shown in the above Figure, and it was used in his presentation at the National Computer Conference in June of that year. The drawing uses the original terms for describing Ethernet components.