Transcript Lecture 4: Layer 1- Physical Layer, Fiber Optics

CS 453
Computer Networks
Lecture 4
Layer 1 – Physical Layer
Data Communications Growth
A little more that 25 years ago
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The IBM PC had a clock speed of less than 5 MHz
Networking technology (specifically ARPANET) ran at
56Kbps
Today
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PC clock speeds run up to 4 GHz
High speed networks run at a max of 10 Gbps
In comparison in about 25 years
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CPU clock speed improved by a factor of 800
Communications speeds improved by a factor of
178,000
Data Communications Growth
During the same time
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Communications error rates dropped from
about 1 error per 10,000 bits
To near zero
Due to a large extent to Fiber Optics
A Brief History
The idea of guiding light has been around
for a while
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Tyndall’s Water Fountain
Early 20th century – glass tubes for
projecting images from hard to reach
places
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Medical images, equipment
A brief History
1950s Kapany did early work that lead to
optical fibers
Fiberscope – use of fibers for internal
medical examinations
There was a strong interest in using fiber
optics for communications
Light attenuation to great
A Brief History
Many believed that light attenuation was
due to principles of physics
1960s Kao and Hockham theorized that
attenuation was due to impurities in the
glass
Kao and Hockham suggested that optical
fiber could be used for
telecommunications if …
Attenuation could be made less than 20
dB/km
A Brief History
1970 Researchers at Corning Glass
Works developed an optical fiber …
With 17 dB/km light attenuation
A few years later they developed fiber with
4 dB/km attenuation
A Brief History
For more on the history of fiber optics
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http://en.wikipedia.org/wiki/Fiber_optics#History
http://www.sff.net/people/Jeff.Hecht/history.html
http://www.fiber-optics.info/fiber-history.htm
Fiber Optics
A waveguide for propagating light along its
length
Fiber Optics
Fiber Optics as a data communication medium is
based on a principle of physics
The principle of refraction
When light passes the boundary from one
medium to another –
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It is refracted --- i.e. it bends
Recall looking at a coin in the bottom of a clear pool
of water
Most noticeable with prisms, magnifying lens, etc.
Fiber Optics
Light passing a boundary between, for example,
glass and air at an angle A will be refracted
(bent) to angle B.
Beyond a certain angle all of the light will be
refracted back into the original media (glass)
That “certain angle” is dependent on
characteristics of the media on both sides of the
boundary –
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Refraction Index
Fiber Optics
Refraction of light at the Glass (silica)/Air
boundary
From Tanenbaum (2003) pg. 94
Fiber Optics
From Wikipedia (http://en.wikipedia.org/wiki/Fiber_optics)
Fiber Optics
Incredibly high bandwidth
Data rates (theoretical) greater that 50,000
Gbps
Very low light attenuation
Fiber Optics
Long distances without attenuation
1 Gbps data rates common
10 Gbps available and economically
feasible – major trunks
40 Gbps – currently possible
Fiber optics can achieve much higher data
rates
Limited by transceiver electronics
Fiber Optics
Fiber Optic cable includes
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A core – made of glass – about 50 microns in
diameter for multimode or 10 microns for
single mode
Cladding – usually also glass but with a lower
refraction index
This keeps the light trapped in the cable
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A sheath – plastic outer jacket of the fiber
cable
Often “packaged” in multi-fiber cables…
But always in pairs
Fiber Optics
Multimode Fiber
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Multiple wavelengths of light
Thicker core (50 microns)
Cheaper
Single Mode
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Small diameter core
Propagates light in a straight line
Longer distances
More expensive fiber, end equipment
Fiber Optics
Interconnecting Fiber
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Termination in connectors
Plug into “patch panels”
Connectors up to 20% light attenuation
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Mechanical Splice
Cut fibers, polish ends and connect in sleeves
Requires skill – with skill about 5 minutes per
splice
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Fusion – welding
Expensive equipment
Very little attenuation
Fiber Optic Network
A fiber optic link must have –
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The medium – fiber
A light emitter
LED
Semiconductor laser
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A receiver
Fiber is unidirectional
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Must use in pairs
Fiber Interface
Convert light to electrical signal and electrical signal to light
Fiber Optic Networks
Fiber connector information
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http://www.fiber-optics.info/articles/connector-care.htm
Fiber Optics
Fiber Networks
Popular for long distance links
Used in LANs and high performance
applications
Fiber connections must be point to point
Cannot use broadcast technology
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Like Bus topology
So, how do we connect many computers with a
fiber network
Fiber Optic Network
Long Distance Link
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Router to Router
Routers hand off to individual computers
…or to computers on LAN
LANs
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Pass Taps
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Active Repeater
Takes incoming light converts to electrical signal…
Converts electrical signal to light and sends
Fiber Optic Networks
Remember that we could squeeze all of the
bandwidth out of fiber optics
So, how do we get more of the bandwidth
Wave Division Multiplexing (WDM)
Remember that emitter diodes can be tunable –
to different wavelengths of light
Suppose –
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You take multiple input channels
Tune each to a different wavelength of light on its own
fiber ()
Then combine them on one fiber….
Fiber Optic Networks
…each  is split out to a different fiber at the
receiving end
From Tanenbaum (2003) pg. 139
Fiber Optic Networks
…that’s Wave Division Multiplexing (WDM)
…its Layer 1 – protocol independent
So, how much
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96 10Gbps channels on a fiber pair
Fiber Optics Networks
DWDM – Dense Wave Division
Multiplexing
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Very small channel separation
Large number of channels
See
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http://www.cisco.com/univercd/cc/td/doc/produ
ct/mels/dwdm/dwdm_fns.htm
Fiber Optic Networks
Optical Carrier Levels - OC
Used on SONET Networks
Units of measure measurement for data
rates on fiber optic links
One OC roughly corresponds to 52 Mbps
More on this later
Fiber vs. Copper
Fiber has much higher bandwidth
Very low signal attenuation relative to copper
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Repeaters needed after long distance –
50 km for fiber vs. 5 km for copper*
Light weight
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One km of 1000 pair copper twisted pair = more than
17,000 lbs.
One km of 1 fiber pair = about 220 lbs.
1 fiber pair can carry more data than 1000 copper
twisted pair cables
From Tanenbaum (2003)
Fiber vs. Copper
Security
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Copper leaks
Fiber does not leak
Fiber deployment requires more advanced
skill
Fiber sensitive to damage