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Performance Improves with Time
Simple implementations of each technology initially fall on the basic speed-distance curve.
Over time, the development of additional end-unit circuitry always improves performance.
Just for background, let's take a minute to think about how the world of electronic
communications has progressed. If any of you read that wonderful book about Edison*
that came out recently, you know that the field of telegraphy was not static.
As development progressed, engineers figured out how to work the lines at higher and higher
speeds, eventually surpassing the speed of human comprehension several-fold through the use of
paper-tape based high-speed recording and re-transmission hardware, and fancy multilevel
transmission schemes involving both the polarity and amplitude of the transmitted signal.
That's typical for communications systems. At first, we use only a basic, dumb transmission
scheme that realizes only a fraction of the raw communicating ability (the Shannon
information-theoretic bandwidth) of the transmission channel. Then later, over time,
the transmission and reception circuitry is gradually made more sophisticated to mine
out more and more of the usable bandwidth.
Telephones followed a similar curve. When
first conceived they had to go a few miles, and they had to use cheap, available wiring,
so that established the analog bandwidth of the first systems at about 3 KHz. The
first digital modems to operate over phone lines used a very simple FSK coding with a bit
rate of about 300 baud.
After a few years, engineers found ways to press the operating speed
up towards the limit of analog bandwidth, and how to use multi-level signaling (up to 256 levels)
to make optimal use of the great SNR available on short telephone connections, and the bit rate skyrocketed to its present level of about 56Kbps.
It's been the same way with LAN connections. Operating at shorter distances, the natural bandwidth of the cabling is much greater than miles-long telephone wires, so the first LAN systems came out at about 1 Mbps. Now we're up to 1 Gb/s. The latest Gb/s systems use 5-level PAM, and four wire pairs in parallel, to squeeze everything possible out of a 100-m category-5 link.
*Israel, Paul, Edison: A Life of Invention, John Wiley & Sons, 1998
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