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ready to roll out the Alto. Metcalfe remembered reading a paper about an
ARPA-funded networking project linking computers on the different islands
of Hawaii. This Hawaiian network was called ALOHAnet and was designed by
University of Hawaii professor Norman Abramson. Instead of sending electri-
cal signals down phone lines, ALOHAnet used a radio network that sent digital
signals through the atmosphere. Abramson had come up with a simple way to
manage interference caused by two stations trying to transmit a message to
the same receiving station simultaneously. If the transmitting stations did not
get an acknowledgment of a successful receipt of the message from the receiv-
ing station, they were programmed to resend their messages after waiting
different, random times. This delay ensured that their messages would not col-
lide a second time. Metcalfe realized that this feature could be very useful for a
LAN, in which there could be many computers trying to send messages at the
same time. He also proposed joining the Altos together with a physical cable.
Metcalfe thought of the Altos as sending their digital messages onto a wire
that would merely act as a passive channel, much as the atmosphere served as
an inactive medium through which radio signals traveled for ALOHAnet. He
likened this use of the connecting wire to the way that physicists before Albert
Einstein had assumed the presence of a
luminiferous
(light-bearing)
ether
, a sub-
stance through which light signals were supposed to travel. Metcalfe's first
memo on his idea in May 1973 was titled “The ETHER network” and described
his ideas for linking computers in a LAN.
In collaboration with Stanford graduate student David Boggs, who was
then working part-time at PARC, Metcalfe tested out his ideas using a coaxial
cable to connect the computers. A
coaxial cable
is a cable with an inner cop-
per conductor surrounded by an insulator and a copper conducting sheath,
all encased in plastic. The cable is called
coaxial
because the inner conductor
and the outer conducting sheath have the same
axis
(center). The cable acts as
a transmission line for radio-frequency signals. The cable connecting the com-
puters was usually silent and so resembled the inert “ether.” When a machine
wanted to transmit a message, it sent a “wake up” bit onto the cable to alert
the other machines. It then sent a packet with a destination address, a sending
address, the message, and some checksum bits for error checking. If another
machine sent a packet at the same time, resulting in a collision, both machines
stopped sending and each waited a different random time before resending the
message. Metcalfe and Boggs developed the electronics to make this happen
and produced the first
Ethernet card
for the Alto, a device to attach the computer
to the Ethernet. Adding new machines proved to be very easy and just required
the end of a branch cable to be plugged into the main coaxial cable.
Metcalfe resubmitted his doctoral thesis to Harvard, still about packet-
switching but now including a suitably theoretical analysis of ALOHAnet. The
university accepted his thesis in June 1973. Boggs took a leave of absence from
Stanford and joined PARC full-time. It would be another nine years before he
received his Stanford doctorate. The patent for Ethernet was filed in March 1975
under the names of Metcalfe, Boggs, Thacker, and Butler Lampson. Metcalfe left
Xerox PARC in 1979 to set up a new company called 3Com to produce Ethernet
networking equipment. With the support of the Digital Equipment, Intel, and
Xerox corporations, the DIX standard was published in 1980. It carried Ethernet
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