The Physics of the

Document Sample
The Physics of the Powered By Docstoc
					The Physics of the Internet

 Computer Science 01i
 Introduction to the Internet

 Neal Sample

Today’s Outline

• Atoms vs. Bits
• Measuring Information
• Bandwidth
• Latency

About this Lecture

• This is the most abstract theoretical
  lecture of the quarter. It gets easier. :)
• The purpose is to give you enough theory
  to deepen your appreciation of the
  Internet and its power.
• Don’t sweat the details! This is just to give
  you a feel for what we’ll talk about later.

Atoms vs. Bits

• We are used to thinking of them as the
  same, but they’re very different!
• Atom:
   The “smallest recognizable” piece of matter
• Bit:
   Smallest piece of information (“1s and 0s”)

Think about a book

• It has physical form:   atoms
• It has information:     bits

• How are these two things different?

Bits are cheap

• Computers make manipulating bits cheap
• Electronic networks make communicating
  bits cheap
   postal system vs. phone system
   phone system vs. a computer network
    • email, faxes, are they different?

Our ability to manipulate each is
growing at vastly different rates

• Power over bits: exponential growth!
• Power over atoms: linear at best


Moore’s Law

• One version:
  “The power of the microprocessor doubles
  approximately every eighteen months.”
        Gordon Moore, founder, Intel Corporation

• Moore’s law may someday hit a wall,
  but for now, bank on it.
• Nothing similar for atoms
Aside: `Net is growing fast

• The “size” of the internet doubles about
  every 20 months (number of users)
• No telling how much information is added
• Electronic commerce
   up 230% from 1998 to 1999
   about the same for 1999 to 2000
   2000 to 2001? “only” 150%?

Growth Downsides: 1999

• Sites get overloaded
• Commerce sites made big mistakes last
  Christmas season*

• “If hard data were the filtering criterion
  you could fit the entire Internet on a
  floppy disk.”
         Cecil Adams, “The Straight Dope Tells All”

Digital vs Analog

• People often use “bits” and “digital”
  interchangeably, but there is a subtle
• Bits can come in digital or analog versions
• The digital version is less accurate (!),
  but much more easily manipulated

Atoms vs Bits: Conclusion

• The growth in our ability to manipulate
  bits in the next few decades will affect
  society as much as the growth in our
  ability to manipulate atoms did in the
  years of the industrial revolution.
• This class is about a major part of that
               The Internet
People of the ‘Net:
Nicholas Negroponte

• Bits vs Atoms is his idea
• Founder of the MIT Media Lab
• More of his ideas can be found in his
  book, Being Digital.
• “In the past shoes could stink,
   In the present, shoes can blink,
   In the future, shoes will think.”

Measuring Information

• A single bit is an awfully small piece of
     Byte       Eight bits
     Kilobyte   About one-thousand bytes. One “K”.
     Megabyte   About one-million bytes. One “Meg”.
     Gigabyte   About one-billion bytes. One “Gig”.
     Terabyte   About one-trillion bytes.
• Don’t worry too much about these words

A better way to think about it
• A single spaced page of text:
   about 2 kilobytes
• A 3” by 5” full color picture:
   about 10 kilobytes
• A four minute song on a CD:
   about 35 megabytes
• A full length movie
   about 6 gigabytes

The information in CDs and
movies is different from
the information in text and


• Measures information per a unit of time
• Plumbing analogy:
   bandwidth measures pipe capacity or
    pipe requirements
   Also measures processing power

• Bytes or kilobytes or megabytes
 per second

Understanding bandwidth

• Typical phone connection to the Internet
  today is 56 kilobits per second
     (actual is about 35-45 kilobits per second)
• Ethernet (offices and campus housing) is
  10 megabits per second
     (actual is about 7-8 megabits per second)
• Higher speed connections are available

What does that mean?

• Remember the size of a page of text?
  About 2 kilobytes
   phone lines take about 1/2 seconds
   Ethernet appears instantaneous
• A small picture is about 10 kilobytes
   phone lines take about 2 seconds
   Ethernet still appears instantaneous

What about sound and movies?

• CD quality audio:
  megabytes per second
   Four minute song
    • phone line requires almost 2 hours
    • Ethernet requires about 35 seconds
• Television quality video:
  hundreds of megabytes per second
   A full length movie? Not yet =)

Bandwidth Conclusion

• Internet speed and capacity are about
  internet bandwidth

• Well, not completely...

What else is there?

• A semi-trailer loaded with DVD discs
  driving from Palo Alto to San Francisco
  has a greater bandwidth that the fastest
  portion of the internet
   Terabytes per second!!

• So what is missing?


• The overhead required to send any
  information whatsoever (like shipping)
• The amount of time it takes to send no
  information at all!
• The latency of the semi-truck is about
  an hour

More on Latency

• Latency is a big problem for modems
• Certain parts of the internet magnify this
  problem considerably

• Fixing latency problems is much trickier
  than fixing bandwidth problems

Airplane Analogy

• Is a 747 three times as fast as a 737?
• A 747’s bandwidth is much greater than
  a 737’s
• But their latency is about the same
• What plane could you use to decrease
  travel latency?

Fun facts about human bandwidth

•   Brain: over 10 gigabits per second
•   Eyes: 10 megabits per second
•   Skin: 1 megabit per second
•   Ears and nose: 100 kilobits per second (each)
•   Taste: 1 kilobit per second
•   Consciousness: less that 40 bits per second
    -- The User Illusion, Tor Norretranders

Sources and further reading:

• On bits vs Atoms:
   Being Digital
    Nicholas Negroponte
   “Bits and Atoms”
    David Micko, Fingerhut Corporation Webmaster

• On Bandwidth and Latency
   “Understanding Bandwidth”
    Computers@Home electronic magazine
   “It’s the latency stupid!”
    Stuart Cheshire, Stanford Graduate Student