Topics ! Introduction ! Theory ! Transmission Media Computer Networks Physical Layer Purpose of Physical Layer Theoretical Basis ! Transport bits between machines ! 19th century: Fourier Analysis (eq 2-1) – How do we send 0's and 1's across a medium? ! Any periodic function can be represented by a series of sines and cosines – Ans: vary physical property like voltage or current ! Representing the property as a function of ! Treat bit pattern as periodic function time ex - 01100010 – analyze it mathematically ! co-efficients to summation terms are called ! Does the receiver see the same signal harmonics generated by the sender? – Why or why not? Transmit Bits over Analog Phone Line ! Harmonics – attenuate (weaken) – distortion unevenly – spectrum (cutoff) ! Time depends upon changes/second – baud ! Signal can have more than 1 bit – several volt levels 1
Review Maximum Data Rate of Channel ! How many layers are in the OSI reference ! Nyquist’s Theorem: model? How many in the TCP/IP reference max data rate = 2 H log 2 V bits/sec model? – H is filter bandwidth ! What are the layer differences? – V discrete levels ! What is the purpose of the Physical Layer? ! example: noiseless 3000 Hz line (phone) – 6000 bps max, with 2 levels ! only need to sample at 2 H , to get all ! noise on channel? Noise on Channel Max Data Rate with Noise ! Every channel has background noise – Thermal noise from agitation of electrons in a ! signal-to-noise ratio (S/N) conductor. Uniform. “White noise.” – use 10 log 10 S/N ( decibels, dB ) – Intermodulation noise different frequencies share the – ex: S/N = 100 then 20 dB same medium ! Shannon’s theorem: – Crosstalk noise results from coupling signal paths N Ex: Other conversation (faintly) on a telephone max data rate = H log 2 (1+S/N) bits/sec – Impulse noise from sharp, short-lived disturbances – ex: 3000 Hz, 30 dB noise (typical phone) N Ex: from lightning – max is 30 Kbps! ! Measure (or quantify) background noise? ! Modems use compression Summary Transmission Media ! Nyquist gives upper bound on sampling ! Two types: ! Nyquist gives max data rate for noiseless – Guided (a physical path) channel – Unguided (waves propagated, but not in a directed manner) – can always increase by increasing signal levels ! Shannon gives max data rate for channels with noise – independent of signal levels! 2
Magnetic Media Twisted Pair ! Put files on tape, floppy disks, … ! Two copper wires are strung between sites ! Physically carry (“Sneaker Net”) ! “Twisted'' to reduce interference ! Example ! Can carry analog or digital signals – 8mm video tape holds 7gigabytes ! Distances of several kilometers – box 20”x 20”x 20” holds 1000 tapes ! Data rates of several Mbps common – 24 hour delivery via FedEx – wire thickness and length – = 1000 x 7GB * 8 / (24 * 3600) = 648 Gbps – shielding to eliminate noise (impacts S/N) – = 1000 times faster than high-speed ATM ! Good, low-cost communication Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway – existing phone lines! ! High delay in accessing data Baseband Coaxial Broadband Coax ! Broadband means analog over coax – telephone folks mean wider than 4 kHz ! Typically 300 MHz, data rate 150 Mbps ! Copper core, insulating material (“coax”) ! Up to 100 km (metropolitan area!) ! Baseband indicates digital transmission ! Inexpensive technology used in cable TV – as opposed to broadband analog ! Divide into MHz channels ! To connect, need to touch core: ! Amplifiers to boost, data only one-way! – vampire taps or T junction – Dual cable systems (still, root must transmit) ! 10 Mbps is typical – Midsplit systems divide into two Evaluation of Broadband vs. Fiber Optics Baseband ! Hair-width silicon or glass ! Signals are pulses of light (digital) ! Which is better, broadband or baseband? – Ex: pulse means “1”, no pulse means “0” ! Baseband: ! Glass “leaks” light? – simple to install – interfaces are inexpensive – short range ! Broadband: – more complicated – more expensive – more services (can carry audio and video) 3
Fiber Optics Fiber Optics ! Advantages – Huge data rate (1 Gbps), low error rate ! Three components required: – Hard to tap (leak light), so secure (hard w/coax) – Fiber medium: 100s miles, no signal loss – Thinner (per logical phone line) than coax – Light source: Light Emitting Diode (LED), laser diode – No electrical noise (lightning) or corrosion (rust) N current generates a pulse of light ! Disadvantages – Photo diode light detector: converts light to – Difficult to tap, really point-to-point technology electrical signals N training or expensive tools or parts are required – One way channel N Two fibers needed for full duplex communication Wireless Transmission Fiber Uses ! 1870’s: moving electrons produce waves ! long-haul trunks--increasingly common in – frequency and wavelength telephone network (Sprint ads) ! Attach antenna to electrical circuit to send ! metropolitan trunks--without repeaters (have 8 miles in length) ! rural exchange trunks--link towns and villages ! local loops--direct from central exchange to a subscriber (business or home) ! local area networks--100Mbps ring networks Radio Waves Microwave Transmission ! Easy to generate, travel far, through walls ! Low bandwidth ! Tight beam, (dish plus transmitter) ! Restricted use by regulation ! Blocked by walls, absorbed by water (rain) ! Need repeaters ! Inexpensive (buy land and voila! MCI) ! Used extensively: phones, TV … – shortage of spectrum! ! Industrial/Scientific/Medical bands – not govt regulated – cordless phones, garage doors, … 4
Infrared Transmission Lightwave Transmission ! Short range ! not good in rain or fog ! Cheap ! need very tight ! Not through objects focus ! Used for remote controls (VCR …) ! Maybe indoor LANS, but not outdoors Satellites Comparison of Satellite and Fiber ! Satellite typically in geosynchronous orbit ! Propagation delay very high – 36,000 km above earth; satellite never “moves” – antenna doesn’t need to track ! One of few alternatives to phone companies – only about 90 are possible for long distances ! Satellite typically a repeater ! Uses broadcast technology over a wide area ! Satellite broadcasts to area of earth – everyone on earth could receive a message! ! Easy to place unauthorized taps into signal ! International agreements on use ! Weather effects certain frequencies ! Fiber tough to building, but anyone with a roof can lease a satellite channel. ! One-way delay of 250ms ! Analog vs. Digital Transmission Shift towards digital transmission ! Compare at three levels: ! improving digital technology – Data--continuous (audio) vs. discrete (text) ! data integrity. – Signaling--continuously varying ! easier to multiplex electromagnetic wave vs. sequence of voltage ! easy to apply encryption to digital data pulses. ! better integration :voice, video and digital – Transmission--transmit without regard to signal data. content vs. being concerned with signal content. Difference in how attenuation is handled. 5
Analog Transmission Digital Data/Analog Signals ! Phone System ! Must convert digital data to analog signal before be transmitted – see fig 2-15 – Local phones are connected to a central office ! Modem(Modulator & Demodulator) (Fig 2- over a 2-wire circuit, called local-loop 17) – Today analog signal is transmitted in local-loop Modulation Modes An example of modulation ! amplitude-shift ! 30 degree phase shifts ! frequency-shift ! eight of frequencies have one amplitude ! phase-shift modulation ! four of frequencies – shift by 45, 135, have two amplitudes 225, 315 degree(2 bits/interval). ! Result: 8 + 4 * 2 = 16 values = 4 bits ! When 2400 baud : 2400*4=9600bps Analog Data/Analog Signals A physical layer example ! RS-232-C ! Can actually transmit analog data in a – Pins, signals, and protocols for the interaction similar manner with amplitude-, phase- and between DTE and DCE. frequency-modulated waves. – DTE:Data Terminal Equipment, computers or ! Frequency-division multiplexing can be terminals used. – DCE:Data Circuit Terminating Equipment, modems – Specifies a 25-pin DB-25 connector 6
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