CS 453 Computer Networks Lecture 4 Layer 1 – Physical Layer

Data Communications Growth A little more that 25 years ago The IBM PC had a clock speed of less than 5 MHz The IBM PC had a clock speed of less than 5 MHz Networking technology (specifically ARPANET) ran at 56Kbps Networking technology (specifically ARPANET) ran at 56KbpsToday PC clock speeds run up to 4 GHz PC clock speeds run up to 4 GHz High speed networks run at a max of 10 Gbps High speed networks run at a max of 10 Gbps In comparison in about 25 years CPU clock speed improved by a factor of 800 CPU clock speed improved by a factor of 800 Communications speeds improved by a factor of 178,000 Communications speeds improved by a factor of 178,000

Data Communications Growth During the same time Communications error rates dropped from about 1 error per 10,000 bits Communications error rates dropped from about 1 error per 10,000 bits To near zero 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 Tyndall’s Water Fountain Tyndall’s Water Fountain Early 20 th century – glass tubes for projecting images from hard to reach places Medical images, equipment 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

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 – It is refracted --- i.e. it bends It is refracted --- i.e. it bends Recall looking at a coin in the bottom of a clear pool of water Recall looking at a coin in the bottom of a clear pool of water Most noticeable with prisms, magnifying lens, etc. 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 – Refraction Index Refraction Index

Fiber Optics Refraction of light at the Glass (silica)/Air boundary From Tanenbaum (2003) pg. 94

Fiber Optics From Wikipedia (

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 A core – made of glass – about 50 microns in diameter for multimode or 10 microns for single mode 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 Cladding – usually also glass but with a lower refraction index This keeps the light trapped in the cable A sheath – plastic outer jacket of the fiber cable A sheath – plastic outer jacket of the fiber cable Often “packaged” in multi-fiber cables… Often “packaged” in multi-fiber cables… But always in pairs

Fiber Optics Multimode Fiber Multiple wavelengths of light Multiple wavelengths of light Thicker core (50 microns) Thicker core (50 microns) Cheaper Cheaper Single Mode Small diameter core Small diameter core Propagates light in a straight line Propagates light in a straight line Longer distances Longer distances More expensive fiber, end equipment More expensive fiber, end equipment

Fiber Optics Interconnecting Fiber Termination in connectors Termination in connectors Plug into “patch panels” Connectors up to 20% light attenuation Mechanical Splice Mechanical Splice Cut fibers, polish ends and connect in sleeves Requires skill – with skill about 5 minutes per splice Fusion – welding Fusion – welding Expensive equipment Very little attenuation

Fiber Optic Network A fiber optic link must have – The medium – fiber The medium – fiber A light emitter A light emitterLED Semiconductor laser A receiver A receiver Fiber is unidirectional Must use in pairs Must use in pairs Fiber Interface Fiber Interface Convert light to electrical signal and electrical signal to light

Fiber Optic Networks Fiber connector information

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 Like Bus topology Like Bus topology So, how do we connect many computers with a fiber network

Fiber Optic Network Long Distance Link Router to Router Router to Router Routers hand off to individual computers Routers hand off to individual computers …or to computers on LAN …or to computers on LANLANs Pass Taps Pass Taps Active Repeater 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 – You take multiple input channels You take multiple input channels Tune each to a different wavelength of light on its own fiber ( ) Tune each to a different wavelength of light on its own fiber ( ) Then combine them on one 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 96 10Gbps channels on a fiber pair 96 10Gbps channels on a fiber pair

Fiber Optics Networks DWDM – Dense Wave Division Multiplexing Very small channel separation Very small channel separation Large number of channels Large number of channelsSee uct/mels/dwdm/dwdm_fns.htm uct/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 Repeaters needed after long distance – Repeaters needed after long distance – 50 km for fiber vs. 5 km for copper* Light weight One km of 1000 pair copper twisted pair = more than 17,000 lbs. One km of 1000 pair copper twisted pair = more than 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 twisted pair cables 1 fiber pair can carry more data than 1000 copper twisted pair cables From Tanenbaum (2003)

Fiber vs. Copper Security Copper leaks Copper leaks Fiber does not leak Fiber does not leak Fiber deployment requires more advanced skill Fiber sensitive to damage