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Metcalf's Law and Legacy
magic of ethernets stems from the law of the microcosm, favoring distributed
terminals over centralized hierarchies, peer networks of PCs over mainframe
pyramids. The microcosms relentless price/performance gains on chips
have endowed Metcalfes peer-to-peer scheme with ever more powerful
peers, at ever lower prices. Medium-independent from the outset, the Metcalfe
systems do not require central switching. In an ethernet system the intelligence
is entirely in the terminals, not in the network itself, and most of the
bandwidth is local (where some 80 percent of traffic resides).
Although this ATM is expected to gush jackpots of cash for gaggles of
network companies and investors, it is unrelated to its acronymic twin,
automatic teller machines. Think of ATM rather as an automated postal
center that takes messages (of any size or addressing scheme), chops them
up, puts them into standardized little envelopes and figures the best
routes to their destinations in billionths of a second. The magic of ATM
comes from restricting its services to those uniform envelopes (called
cells) of 53 bytes apiece (including a five-byte address) and creating
for each envelope what is called a virtual circuit through the network.
These features make it unnecessary for intermediate switches in the network
to check the address; the cell flashes through the system on a precomputed
A compromise defined by phone companies as the longest packet size that
can handle voice in real time, 53-byte cells are also short enough to
be entirely routed and switched in cheap hardware; i.e., microchips. This
means that the ATM postal center can function at speeds of up to 155 megabits
per second or even higher. Perhaps most attractive of all, ATM can handle
multimedia data, such as digital movies or teleconferences, with voice,
text and video that must arrive together at the same time in perfect sync.
As the world moves toward multimedia, the industry is flocking toward
ATM, the innovation that can make it possible.
Ethernet: A Legacy LAN?
By contrast, Ethernet seems old and slow: the vacuum tube of computer
communications. Think of it, crudely, as a system where all the messages
are cast into the ocean and picked up by terminals on the beach which
scan the tides for letters addressed to them. Obviously, this system would
work only if the beach terminals could suck up and filter tremendous quantities
of sea water. The magic of ethernet comes from the ever growing power
of computer terminals. The microcosm supplies sufficiently powerful filtering
chipschiefly digital signal processors improving their powers some
tenfold every two yearsto sort mail and messages in the vasty deep.
This is quite a trick. To the experts, it seems unlikely to prevail for
long against the fabulously swift switching of ATM.
True, there is some confusion about just how, where and when this miracle
cure will arrive. The industrys leading intellectual, Robert Lucky
of Bellcorea paragon of long- distance networkspredicts that
ATM will come first in local area networks, while Metcalfe, of local area
network fame, thinks it will come first in wide area networks. James Chiddix
of Time- Warner Cable is probably right in predicting digital cable pay-
per-view as the first big ATM customer, using it for broadcasting films
in his 500-channel digital cable TV project in Orlando. But most experts
agree that one way or another ATM will blow away Ethernet during the next
decade or so.
Nonetheless, as usual, conventional wisdom is wrong. Ethernet is quietly
preparing for a new era of hegemony in the marketplace for computer connections.
The reason Ethernet prevailed in the first place is that, in the words
of Ronald Schmidt, it was incredibly simple and elegant and robust.
In other words, it is cheap and simple for the user. Customers can preserve
their installed base of equipment while the network companies innovate
with new transmission media. When the network moves to new kinds of copper
wires or from one mode of fiber optics to another, Ethernet still looks
essentially the same to the computers attached to it. Most of the processingconnecting
the user to the network, sensing a carrier frequency on the wire and detecting
collisionscan be done on one Ethernet controller chip that costs
a few dollars.
As Metcalfe described the conception of this technology in 1981, I
explored the advantages of moving the transceiver down out of the ceiling
onto the adapter board in the host computer. I had seen many actual Ethernet
installations in which our brick transceivers were not up in the ceiling
tapping into the ether cable, as they were supposed to be...but instead
were on floors behind computers, dropped in the centers of neatly coiled
transceiver cables.... We were discovering that the people buying personal
computers and workstations in those days were not generally the same kind
of people who were allowed to remove ceiling tiles and string cables through
conduits.... The personal computer revolution was taking place in organizations
from the bottom up.... It was time for Ethernet to be re- invented for
bottom-up proliferation among the personal computer work group revolutionaries.
Using silicon compiler design tools to radically reduce the
time to market, Seeq Technology created an Ethernet chip for PCs in time
for a single-board version of the interface unit. Putting the transceiver
on the adapter board eliminated a special transceiver cable and drastically
simplified the system. There is no bulky connection between the coding
device preparing information for the network and the transceiver sending
or receiving the signals on the net. All this processing is done in the
computer, on one printed circuit board, now reduced to the size of a credit
card. While its rival from IBMToken Ringrequires a mostly
proprietary array of token-passing managers, clocking assignments and
other complexities, Ethernet is an open system. Relative to the alternatives,
it offers the possibility of something near plug-and-play. So advantaged,
Ethernet has overcome IBMs Token Ring, 20 million nodes to 8 million
in installed base.