cs 453 computer networks

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


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


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


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 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.


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


Refraction of light at the Glass (silica)/Air Refraction of light at the Glass (silica)/Air boundaryboundary

From Tanenbaum (2003) pg. 94


From Wikipedia (http://en.wikipedia.org/wiki/Fiber_optics)

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


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


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 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


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….


……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


……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


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

cs 453 computer networks

Documents