cs 453 computer networks lecture 4 layer 1 – physical layer
TRANSCRIPT
CS 453CS 453Computer NetworksComputer Networks
Lecture 4Lecture 4
Layer 1 – Physical LayerLayer 1 – Physical Layer
Data Communications GrowthData Communications Growth
A little more that 25 years ago A little more that 25 years ago The IBM PC had a clock speed of less than 5 MHzThe IBM PC had a clock speed of less than 5 MHz Networking technology (specifically ARPANET) ran at Networking technology (specifically ARPANET) ran at
56Kbps56Kbps
TodayToday PC clock speeds run up to 4 GHzPC clock speeds run up to 4 GHz High speed networks run at a max of 10 GbpsHigh speed networks run at a max of 10 Gbps
In comparison in about 25 yearsIn comparison in about 25 years CPU clock speed improved by a factor of 800CPU clock speed improved by a factor of 800 Communications speeds improved by a factor of Communications speeds improved by a factor of
178,000178,000
Data Communications GrowthData Communications Growth
During the same timeDuring the same time Communications error rates dropped from Communications error rates dropped from
about 1 error per 10,000 bitsabout 1 error per 10,000 bits To near zeroTo near zero
Due to a large extent to Fiber OpticsDue to a large extent to Fiber Optics
A Brief HistoryA Brief History
The idea of guiding light has been around The idea of guiding light has been around for a whilefor a while Tyndall’s Water FountainTyndall’s Water Fountain
Early 20Early 20thth century – glass tubes for century – glass tubes for projecting images from hard to reach projecting images from hard to reach placesplaces Medical images, equipmentMedical images, equipment
A brief HistoryA brief History
1950s Kapany did early work that lead to 1950s Kapany did early work that lead to optical fibersoptical fibers
Fiberscope – use of fibers for internal Fiberscope – use of fibers for internal medical examinationsmedical examinations
There was a strong interest in using fiber There was a strong interest in using fiber optics for communicationsoptics for communications
Light attenuation to greatLight attenuation to great
A Brief HistoryA Brief History
Many believed that light attenuation was Many believed that light attenuation was due to principles of physicsdue to principles of physics1960s Kao and Hockham theorized that 1960s Kao and Hockham theorized that attenuation was due to impurities in the attenuation was due to impurities in the glassglassKao and Hockham suggested that optical Kao and Hockham suggested that optical fiber could be used for fiber could be used for telecommunications if …telecommunications if …Attenuation could be made less than 20 Attenuation could be made less than 20 dB/kmdB/km
A Brief HistoryA Brief History
1970 Researchers at Corning Glass 1970 Researchers at Corning Glass Works developed an optical fiber …Works developed an optical fiber …
With 17 dB/km light attenuationWith 17 dB/km light attenuation
A few years later they developed fiber with A few years later they developed fiber with 4 dB/km attenuation4 dB/km attenuation
A Brief HistoryA Brief History
For more on the history of fiber opticsFor more on the history of fiber optics http://en.wikipedia.org/wiki/Fiber_optics#Historyhttp://en.wikipedia.org/wiki/Fiber_optics#History http://www.sff.net/people/Jeff.Hecht/history.htmlhttp://www.sff.net/people/Jeff.Hecht/history.html http://www.fiber-optics.info/fiber-history.htmhttp://www.fiber-optics.info/fiber-history.htm
Fiber OpticsFiber Optics
A waveguide for propagating light along its A waveguide for propagating light along its lengthlength
Fiber OpticsFiber Optics
Fiber Optics as a data communication medium is Fiber Optics as a data communication medium is based on a principle of physicsbased on a principle of physics
The principle of refractionThe principle of refraction
When light passes the boundary from one When light passes the boundary from one medium to another –medium to another – It is refracted --- i.e. it bendsIt is refracted --- i.e. it bends Recall looking at a coin in the bottom of a clear pool Recall looking at a coin in the bottom of a clear pool
of waterof water Most noticeable with prisms, magnifying lens, etc.Most noticeable with prisms, magnifying lens, etc.
Fiber OpticsFiber Optics
Light passing a boundary between, for example, Light passing a boundary between, for example, glass and air at an angle A will be refracted glass and air at an angle A will be refracted (bent) to angle B. (bent) to angle B.
Beyond a certain angle all of the light will be Beyond a certain angle all of the light will be refracted back into the original media (glass)refracted back into the original media (glass)
That “certain angle” is dependent on That “certain angle” is dependent on characteristics of the media on both sides of the characteristics of the media on both sides of the boundary – boundary – Refraction IndexRefraction Index
Fiber OpticsFiber Optics
Refraction of light at the Glass (silica)/Air Refraction of light at the Glass (silica)/Air boundaryboundary
From Tanenbaum (2003) pg. 94
Fiber OpticsFiber OpticsIncredibly high bandwidthIncredibly high bandwidth
Data rates (theoretical) greater that 50,000 Data rates (theoretical) greater that 50,000 GbpsGbps
Very low light attenuationVery low light attenuation
Fiber OpticsFiber Optics
Long distances without attenuationLong distances without attenuation
1 Gbps data rates common1 Gbps data rates common
10 Gbps available and economically 10 Gbps available and economically feasible – major trunksfeasible – major trunks
40 Gbps – currently possible40 Gbps – currently possible
Fiber optics can achieve much higher data Fiber optics can achieve much higher data ratesrates
Limited by transceiver electronicsLimited by transceiver electronics
Fiber OpticsFiber OpticsFiber Optic cable includesFiber Optic cable includes A core – made of glass – about 50 microns in A core – made of glass – about 50 microns in
diameter for multimode or 10 microns for diameter for multimode or 10 microns for single modesingle mode
Cladding – usually also glass but with a lower Cladding – usually also glass but with a lower refraction indexrefraction index
This keeps the light trapped in the cableThis keeps the light trapped in the cable A sheath – plastic outer jacket of the fiber A sheath – plastic outer jacket of the fiber
cablecable Often “packaged” in multi-fiber cables…Often “packaged” in multi-fiber cables…
But always in pairsBut always in pairs
Fiber OpticsFiber Optics
Multimode FiberMultimode Fiber Multiple wavelengths of lightMultiple wavelengths of light Thicker core (50 microns)Thicker core (50 microns) CheaperCheaper
Single ModeSingle Mode Small diameter coreSmall diameter core Propagates light in a straight linePropagates light in a straight line Longer distancesLonger distances More expensive fiber, end equipmentMore expensive fiber, end equipment
Fiber OpticsFiber OpticsInterconnecting FiberInterconnecting Fiber Termination in connectorsTermination in connectors
Plug into “patch panels”Plug into “patch panels”Connectors up to 20% light attenuationConnectors up to 20% light attenuation
Mechanical SpliceMechanical SpliceCut fibers, polish ends and connect in sleevesCut fibers, polish ends and connect in sleevesRequires skill – with skill about 5 minutes per Requires skill – with skill about 5 minutes per splicesplice
Fusion – welding Fusion – welding Expensive equipmentExpensive equipmentVery little attenuationVery little attenuation
Fiber Optic NetworkFiber Optic Network
A fiber optic link must have – A fiber optic link must have – The medium – fiberThe medium – fiber A light emitter A light emitter
LEDLED
Semiconductor laserSemiconductor laser A receiverA receiver
Fiber is unidirectionalFiber is unidirectional Must use in pairsMust use in pairs Fiber InterfaceFiber Interface
Convert light to electrical signal and electrical signal to lightConvert light to electrical signal and electrical signal to light
Fiber Optic NetworksFiber Optic Networks
Fiber connector informationFiber connector information http://www.fiber-optics.info/articles/connector-care.htmhttp://www.fiber-optics.info/articles/connector-care.htm
Fiber OpticsFiber Optics
Fiber NetworksFiber Networks
Popular for long distance linksPopular for long distance links
Used in LANs and high performance Used in LANs and high performance applicationsapplications
Fiber connections must be point to pointFiber connections must be point to point
Cannot use broadcast technologyCannot use broadcast technology Like Bus topologyLike Bus topology
So, how do we connect many computers with a So, how do we connect many computers with a fiber networkfiber network
Fiber Optic NetworkFiber Optic Network
Long Distance LinkLong Distance Link Router to RouterRouter to Router Routers hand off to individual computersRouters hand off to individual computers ……or to computers on LANor to computers on LAN
LANsLANs Pass TapsPass Taps
Active RepeaterActive RepeaterTakes incoming light converts to electrical signal…Takes incoming light converts to electrical signal…
Converts electrical signal to light and sendsConverts electrical signal to light and sends
Fiber Optic NetworksFiber Optic Networks
Remember that we could squeeze all of the Remember that we could squeeze all of the bandwidth out of fiber opticsbandwidth out of fiber opticsSo, how do we get more of the bandwidthSo, how do we get more of the bandwidthWave Division Multiplexing (WDM)Wave Division Multiplexing (WDM)Remember that emitter diodes can be tunable – Remember that emitter diodes can be tunable – to different wavelengths of lightto different wavelengths of lightSuppose –Suppose – You take multiple input channels You take multiple input channels Tune each to a different wavelength of light on its own Tune each to a different wavelength of light on its own
fiber (fiber ()) Then combine them on one fiber….Then combine them on one fiber….
Fiber Optic NetworksFiber Optic Networks
……each each is split out to a different fiber at the is split out to a different fiber at the receiving endreceiving end
From Tanenbaum (2003) pg. 139From Tanenbaum (2003) pg. 139
Fiber Optic NetworksFiber Optic Networks
……that’s Wave Division Multiplexing that’s Wave Division Multiplexing (WDM)(WDM)
……its Layer 1 – protocol independentits Layer 1 – protocol independent
So, how muchSo, how much 96 10Gbps channels on a fiber pair96 10Gbps channels on a fiber pair
Fiber Optics NetworksFiber Optics Networks
DWDM – Dense Wave Division DWDM – Dense Wave Division MultiplexingMultiplexing Very small channel separationVery small channel separation Large number of channelsLarge number of channels
SeeSee http://www.cisco.com/univercd/cc/td/doc/http://www.cisco.com/univercd/cc/td/doc/
product/mels/dwdm/dwdm_fns.htmproduct/mels/dwdm/dwdm_fns.htm
Fiber Optic NetworksFiber Optic Networks
Optical Carrier Levels - OCOptical Carrier Levels - OC
Used on SONET NetworksUsed on SONET Networks
Units of measure measurement for data Units of measure measurement for data rates on fiber optic linksrates on fiber optic links
One OC roughly corresponds to 52 MbpsOne OC roughly corresponds to 52 Mbps
More on this laterMore on this later
Fiber vs. CopperFiber vs. Copper
Fiber has much higher bandwidthFiber has much higher bandwidth
Very low signal attenuation relative to copperVery low signal attenuation relative to copper Repeaters needed after long distance –Repeaters needed after long distance –
50 km for fiber vs. 5 km for copper*50 km for fiber vs. 5 km for copper*
Light weightLight weight One km of 1000 pair copper twisted pair = more than One km of 1000 pair copper twisted pair = more than
17,000 lbs.17,000 lbs. One km of 1 fiber pair = about 220 lbs.One km of 1 fiber pair = about 220 lbs. 1 fiber pair can carry more data than 1000 copper 1 fiber pair can carry more data than 1000 copper
twisted pair cables twisted pair cables
From Tanenbaum (2003)
Fiber vs. CopperFiber vs. Copper
SecuritySecurity Copper leaksCopper leaks Fiber does not leakFiber does not leak
Fiber deployment requires more advanced Fiber deployment requires more advanced skillskill
Fiber sensitive to damageFiber sensitive to damage