Tài liệu Chapter 4 Transmission Media pdf

Overview — Guided - wire — Unguided - wireless — Characteristics and quality determined by medium and signal W For guided, the medium is more important — For unguided, the bandwidth prod

Overview

— Guided - wire

— Unguided - wireless

— Characteristics and quality determined by

medium and signal

W For guided, the medium is more important

— For unguided, the bandwidth produced by the

antenna is more important

— Key concerns are data rate and distance

Electromagnetic Spectrum

Frequency

| ELF | VE | VLF | LF | MF | HF | VHF] UHF] SHF | EHF |

Power and telephone Radio Microwave Infrared Visible

Rotating generators Radios and televisions Radar Lasers light

Musical instruments Electronic tubes Microwave antennas Guided missiles

Voice microphones Integrated circuits Magnetrons Rangefinders

AM Radio FM Radio| Terrestrial

and TV and Satellite

ELF = Extremely low frequency MF = Medium frequency UHF = Ultrahigh frequency

VF = Voice frequency HF = High frequency SHF = Superhigh frequency

VLF = Very low frequency VHF = Very high frequency EHF = Extremely high frequency

LF = Low frequency

Guided Transmission Media

—E Twisted Pair

— Coaxial cable

E Optical fiber

—Often "bundled" into cables

—LUsually installed in building

during construction (a) Twisted pair

Twisted Pair - Applications

| To private branch exchange (PBX)

— For local area networks (LAN)

| 10Mbps or 100Mbps

Twisted Pair - Pros and Cons

f Cheap

— Easy to work with

E Low data rate

— Short range

Twisted Pair - Transmission

E Limited data rate (100MHz)

E Susceptible to interference and noise

Unshielded and Shielded TP

— Unshielded Twisted Pair (UTP)

| Ordinary telephone wire

| Cheapest

| Easiest to install

| Suffers from external EM interference

š Shielded Twisted Pair (STP)

| Metal braid or sheathing that reduces interference

| More expensive

| Harder to handle (thick, heavy)

Near End Crosstalk

E Coupling of signal from one pair to another

E Coupling takes place when transmit signal

entering the link couples back to receiving pair

i j.e near transmitted signal is picked up by near

receiving pair

—Quter conductor is braided shield

—Inner conductor ts solid metal

—Separated by insulating material

—Covered by padding

Coaxial Cable Applications

— Most versatile medium

— Television distribution

I Ariel to TV

I Cable TV

— Long distance telephone transmission

| Can carry 10,000 voice calls simultaneously

| Being replaced by fiber optic

E Short distance computer systems links

W Local area networks

Coaxial Cable - Transmission

Characteristics

— Analog

| Amplifiers every few km

| Closer if higher frequency

— Laser or light emitting diode Light at less than incidence reflection

—Specially designed jacket ciritical angle ts

Optical Fiber - Benefits

E Greater capacity

| Data rates of hundreds of Gbps

E Smaller size & weight

Hf Lower attenuation

W Electromagnetic isolation

i Greater repeater spacing

| 10s of km at least

Optical Fiber - Applications

BE Long-haul trunks

—f Metropolitan trunks

HE Rural exchange trunks

—E Subscriber loops

f LANs

Optical Fiber - Transmission Characteristics

E Act as wave guide for 1014 to 1015 Hz

| Portions of infrared and visible spectrum

E Light Emitting Diode (LED)

| Greater data rate

— Wavelength Division Multiplexing

Optical Fiber Transmission

(a) Step-index multimode

Output pulse

Wireless Transmission

—E Unguided media

— Transmission and reception via antenna

E Directional

| Focused beam

| Careful alignment required

FE Omnidirectional

| Signal spreads in all directions

| Can be received by many antennae

Terrestrial Microwave

E Parabolic dish

— Focused beam

E Line of sight

R Long haul telecommunications

— Higher frequencies give higher data rates

Satellite Microwave

E Satellite is relay station

E Satellite receives on one frequency, amplifies or repeats signal and transmits on another

frequency

— Requires geo-stationary orbit

| Height of 35,784km

E Television

— Long distance telephone

Ef Private business networks

Infrared

—E Modulate noncoherent infrared light

E Line of sight (or reflection)

E Blocked by walls

— e.g TV remote control, IRD port

Required Reading

E Stallings Chapter 4