Understanding Data Transmission: Physical Layer and Maximum Data Rates, Slides of Social Work

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Introduction to LAN/WAN

Physical Layer

Topics

)

Introduction

)

Theory

)

Transmission Media

Theoretical Basis

)

19th century: Fourier Analysis (eq 2-1)

)

Any periodic function can be represented bya series of

sines

and

cosines

)

Treat bit pattern as periodic function

ex -

)

co-efficients to summation terms are called^ harmonics

)

More harmonics mean closer representation

Transmit

)

Harmonics

attenuate

(weaken)

distortion

unevenly

spectrum

(cutoff)

)

Signal can havemore than 1 bit

• several volt levels

)

Can calculate max.data rates based onchannel parameters

Noise on Channel

)

Every channel has background noise

Thermal noise

from agitation of electrons in a

conductor. Uniform. “White noise.”

Intermodulation noise

different frequencies share the

same medium

Crosstalk noise

results from coupling signal paths

‹

Ex: Other conversation (faintly) on a telephone

Impulse noise

from sharp, short-lived disturbances

‹

Ex: from lightning

)

Measure (or quantify) background noise?

Max Data Rate with Noise

)

signal-to-noise

ratio (S/N)

• use 10 log

10

S/N (

decibels, dB

• ex: S/N = 100 then 20 dB

)

Shannon’s theorem:max data rate =

H

log

(1+S/N) bits/sec 2

• ex: 3000 Hz, 30 dB noise (typical phone)– max is 30 Kbps!

)

Modems use compression

Transmission Media

)

Two types:

• Guided (a physical path)– Unguided (waves propagated, but not in a

directed manner)

Magnetic Media

)

Put files on tape, floppy disks, …

)

Physically carry (“Sneaker Net”)

)

Example

• Ultrium tape holds 200 gigabytes (Gb)– 1 byte = 8 bits– Assuming a box holds 1000 tapes– 24 hour delivery via FedEx– = 1000 x 200 Gb * 8 / (24 * 3600) = 19 Gbps– If delivered in hour, bandwidth = 400 Gbps Never underestimate the bandwidth of a station wagon full of

tapes hurtling down the highway

)

High delay in accessing data

Coaxial Cable

)

Copper core, insulating material (“coax”) )

Baseband

means in the voice range

)

Broadband

means move to much higher frequencies

by introducing a carrier– telephone folks mean wider than 4 kHz )

To connect, need to touch core:

vampire taps

or

T junction

)

10 Mbps is typical

Evaluation of Broadband vs.

Baseband

)

Which is better, broadband or baseband?

)

Baseband:

• simple to install– interfaces are inexpensive– short range

)

Broadband:

• more complicated– more expensive– more services (can carry audio and video)

Fiber Optics

)

Three components required:

• Fiber medium: 100s miles, no signal loss– Light source: Light Emitting Diode (LED), laser diode

‹

current generates a pulse of light

• Photo diode light detector: converts light to electrical

signals

)

Wide fiber = many diff. Wavelengths of light(multimode fiber) )

Narrow fiber = only 1 wavelength (single mode,better)

Fiber Optics

)

Advantages

• Huge data rate (1 Gbps), low error rate– Hard to tap (leak light), so secure (hard w/coax)– Thinner (per logical phone line) than coax– No electrical noise (lightning) or corrosion (rust)

)

Disadvantages

• Difficult to tap, really point-to-point technology

‹

training or expensive tools or parts are required

• One way channel

‹

Two fibers needed for

full duplex

communication

Wireless Transmission

)

1870’s: moving electrons produce waves

frequency

and

wavelength

)

Attach antenna to electrical circuit to send

Radio Waves

)

Easy to generate, travel far, through walls )

Low bandwidth )

Low radio freqs follow earth )

High freqs travel in straight lines, bounce off obstacles )

Restricted use by regulation