J.Tiberghien - VUB09-07- K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 1.5 Communications Media

J.Tiberghien - VUB09-07- K.Steenhaut & J.Tiberghien - VUB1

TelecommunicationsConcepts

Chapter 1.5

Communications

Media


Contents

• Optical fibers

• Coaxial cables

• Twisted pairs

• Wireless communications


Contents

• Optical fibers

• Coaxial cables

• Twisted pairs



Snell’s Law

sin 1

sin 2

=n2

n1

2

1

n2

n1

c

n2

n1

>c

n2

n1

n2 < n1


Optical Fibers(step index)

n1n2

n2 < n1

Protective coating


Multimode FiberDiameter : > 50

Low cost but limited bandwidth * distancedue to multimode dispersion

Step index fiber

Graded index fiber


Multimode Dispersion

Step index fiber : < 50 MHz.KmGraded Index Fiber : < 1000 MHz.Km (1990)

< 5000 MHz.Km (2000)

t t


Monomode Fiber

Diameter : < 5 Only one propagation mode possible

Higher cost due to end equipmentbut enormous bandwidth*distance product

10 Gb/s over 500 Km optical sections (1995)


Wave Domain Multiplexing

Each color can carry an independent data flow.

In 2000

40 colors carrying each 10 Gb/s or

80 colors carrying each 2.5 Gb/s

were commercially available


Optical amplifiers

Pumplaser

Erbium doped fiber

Erbium atoms are pumped into a higher energy state by the light of the pump laser,

they fall back in synchronism with the incoming light, amplifying it.


Optical Switching

From IEEE Com.Mag.V39,N1, Jan 2001.


Contents

• Optical fibers

• Coaxial cables

• Twisted pairs



Coaxial Cables

Protective coating

Insulator

Conductor

Conductor

Monomode propagation for all data applicationsTransmission rates up to some Gb/s

Distance limited by electrical attenuation


Contents

• Optical fibers

• Coaxial cables

• Twisted pairs



Twisted Pairs

Performance highly dependant on cable

qualityTransmission speed up

to several 100 Mb/s for distances of up to

100 m.with better cables

(class 5 or 6)


Contents

• Optical fibers

• Coaxial cables

• Twisted pairs



Wireless Communications

Why ?

Mobile terminals

Cost of wiring

Why Not ?

Lower data rates

Lower reliability

Potential Lack of Security


Wireless CommunicationsMain restrictions:

• Limited available bandwidth• Uncontrolled sources of noise

Some solutions:

• Displace some heavy users (TV) • Reuse of frequencies at different locations (Cellular radio, Point to point links, …)

• Sharing of a set of frequencies (spread spectrum radio)


Reuse of Frequencies

Pr/Sa = Pt/r2

r

transmitter Pr = Power at receiverSa = Area of receiver antennaPt = Power at transmitterr = Distance

At some distance, a transmitter can no longer be received and the same frequency can be reused


Cellular Radio

Ideally, 3 different frequency sets are sufficient


The Mobile Access NetworkSecond Generation Handover

When a receiver is between two cells, the receiver has to disconnect from one cell and connect into the next one. Circuit routing has

to be adapted accordingly.


Cellular Radiok = number available frequencies per cell

S = Area of a cell = *r2

n = Number of simultaneous calls per km2

pt = Power of transmitterP0/Sa= Minimal field strength at receiver input

n = k / S

pt = p0/Sa * r2

With smaller cells, - more antenna sites are needed ...- more simultaneous calls are possible- transmitted power can be reduced


Cellular Radioin practice

Flanders Ardennes

Propagation conditions depend heavily on geography


Cellular Radio

In practice, seven or more different sets of frequencies are needed


Digital Cellular Phones

Name

Freq.(MHz)

# rad.ch.

P.max.(W)

r.max.(Km)

Voicerate (Kb/s)

Capacity (E/Km2)

DECT

1880-1890

12

0.25

0.2

32

10 000

GSM

890-915

935-960

124

2

35

13

1000

DCS

1800

1710-1785

1805-1880

248

1

8

13

2000


Cellular Radioand frequency

hoppingProblem :

Propagation conditions are extremely variable

in function of location and frequency,

especially in cities

Solution :

Use a set of different frequencies

and switch at a high rate between them.

e.g. In GSM every 20 mS frequencies change.


Wireless CommunicationsMain restrictions:

• Limited available bandwidth• Uncontrolled sources of noise

Some solutions:

• Displace some heavy users (TV) • Reuse of frequencies at different locations (Cellular radio, Point to point links, …)

• Sharing of a set of frequencies (spread spectrum radio)


Wireless Interference Margins

Cause considerable loss in transmission capacity

• Considerable room for improvements by controlling interferences

– Signal hardening

– Signal recovery

– Signal expansion

= Third generation mobile networks (UMTS)

Frequency

Space

Time


The Mobile Access Network

• Data combined with known higher frequency pseudo random sequence

• Resulting modulated radio signal has high bandwidth

• Shannon : low data rate combined with high bandwidth = excellent noise margins!

Fast hopping Spread Spectrum

n b/sData

Pseudo-random sequence

m * n b/s

xor

HF carrier

Modulated signalLarge bandwidth ≈ m times bandwidthneeded for data


The Mobile Access NetworkSpread Spectrum and CDMA

D1

S1

xor

HF

Tx1

D2

S2

xor

HF

Tx2

D2

S2

HF

Correl-ator

Rx2

D1

S1

HF

Correl-ator

Rx1

For radio link Tx1-Rx1, emission by Tx2 is just another source of noise


The Mobile Access Network Multi-path Interference

GSM : interference = noiseUMTS : correlator adds similar input signals with appropriate delays so that they reinforce each other

Different paths have different lengths and different delays


The Mobile Access NetworkThird Generation Handover

When a receiver is between two cells, both transmitters send the same signal. These two signals reinforce each other, as multipath propagation does.


Multi-path Interference

2nd generation (GSM) : interference = noise3rd generation (UMTS) : add similar input signals with appropriate delays so that they reinforce each other

Different paths have different lengths and different delays


Wireless Local Loop

Radio is sometimes cheaper than digging the streets !

Used for telephonyand forInternet access (WiMax)


Microwave Point to Point Links

• Highly directive antennas limit spatial spreading

• High transmission capacity (several Mb/s)

• transmission impaired by heavy rain

• Cost effective for line of sight communications


Satellite Communications

36000 Km

Geostationary

Round trip Delay = 240 ms

High power ground stations


Satellite CommunicationsLow Orbit

Short round trip delays Low power ground stations

In fact, a cellular system with mobile base- stations


Introduced concepts• Optical communications are becoming dominant.

– Low cost, high throughput fixed communications.

• Wireless communications are growing:

– For replacing local wiring

– For mobile communications

• Geostationary satellite communications:

–One way broadcasting

– low traffic point to point but high delays

• Low Orbit satellites:

–Cellular system for global mobile application.

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J.Tiberghien - VUB09-07- K.Steenhaut & J.Tiberghien - VUB 1 Telecommunications Concepts Chapter 1.5 Communications Media