ALOHAnet: The World’s First Wireless LAN

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Believe it or not, there was a time when communication between computers was done point-to-point, necessarily though a wired connection, by manual control. True, this would have been back around the time Richard Nixon appeared on “Laugh-In,” but it did happen.

 

 

What changed this — or, at least, one of the developments that facilitated its change — occurred in the southernmost state in the United States: Hawaii.

 

 

In 1970, faculty at the University of Hawaii wanted to create a network to link computers at campuses that were geographically far removed from one another. So they developed ALOHAnet, a computer networking system that used radio transmitters as ports passing data from machine to machine.

 

 

As it was originally designed, ALOHAnet used two different radio frequencies to transmit data, with a hub machine sending packets of data on the “outbound” channel and client machines sending data back to the hub on the “inbound” channel. Data received was immediately sent back, allowing client machines to know whether the data had been received. A machine receiving back corrupted data would wait and resend the packet.

 

 

Data would be corrupted when it “collided” with other data, that is to say, two client machines had attempted to send data at the same time. ALOHAnet’s main challenge was managing these collisions. Under its original configuration, the network had a throughput rate of just 18 percent, with a vast majority of the available bandwidth wasted.

 

 

The first attempt at a solution to this problem was to create time slots assigned to all the computers in the network, during which time they were allowed to send data packets, and others were not.

 

 

The flaw here was that if a certain machine on the network had nothing to send during its slot, that time was wasted. Nevertheless, this did double the throughput rate.

 

 

With the second attempt at a solution to this problem, the developers of ALOHAnet really hit on something. The idea was to have client machines on the network “listen” in on the channel to determine whether it was in use, and if it wasn’t, begin sending data packets. To avoid one client machine getting on the frequency and staying on it for too long, thereby blocking other client machines trying to send packets, the data was broken into small packets, so all machines on the network could share the channel continually.

 

 

The idea became known as carrier sense multiple access (CSMA), an innovation the developers of ALOHAnet turned around and improved by having client machines also listen to see whether their send packet made it back to the central hub machine on the network.

 

 

This addressed problems that would arise if two client machines attempted to send a data packet at the same time, which could happen even with the machines having listened to see whether the channel was open.

 

 

This idea became known as collision detection (CD). Put it all together, and you have carrier sense multiple access with collision detection (CSMA-CD).

 

 

The ALOHAnet is not in use anymore, but in fostering the development of CSMA-CD, it provided a sophisticated network control protocol that proved to be a huge step forward in the development of the Ethernet. And it prefigured many of the design characteristics of modern wireless LANs.

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