CHAPTER 6B

TRANSMISSION OF SIGNALS

1) Analog Signal

-short range work

E/O and O/E

light signal: a copy of input electrical signal

2) Digital Transmission

accuracy can be reduced by nonlinearity and noise

signal travels along fiber is attenuated

to restore signal amplification,add amplifiers (repeaters),now :optical amplifiers

info signal is represented by ON/OFF levels

signal presence: treshold level

repeaters in digital signal: regenarators-regenarate/rebuild signals rather than amplifying them

increase regenarator spacing: allow signal to become very small before there is any danger of it becoming lost in noise

CONVERT INFO SIGNAL TO DIGITAL FORM: Measure amplitude of input waveform at regular intervals measurements (samples)
-sampling must be much faster than signal frequency
-sampling rate 2x highest frequency

3) Pulse Amplitude Modulation

appears to be digital but it's actually an analog signal in disguise

sample pulses can be any amplitude and digital signal is a series of pulses of all which one of the same size

BIPHASE (MANCHESTER) CODE

4) Pulse Code Modulation

info signal is sampled as in PAM system, amplitude of each sample is then represented by a binary code

quantization noise: misrepresantation

reduced by increasing the no of levels used but never eliminated

send more binary digits, high transmission bandwidth

timeslot: time interval associated with each sample

within time slot, binary code is transmitted as a digital code

level changes: to keep the receiver clock sync to the transmitter

receivingcircuit: to ensure receiver clock remains locked on to transmitter clock

decoder: reconstruct PAM signal and from that original info can be extracted

HOW TO GET MORE DIGITAL SIGNALS ON A SINGLE FIBER?
1)Time-Division Multiplexing (TDM):
-combine info through time,spaces can be filled with another transmission containing data associated with another, quite separate, info signal
-demultiplexer: sorts the samples out into their separate signals from which the info can be recovered
-pulses narrower, bandwidth of signal higher


2) Wave-Division Multiplexing (WDM)
-allows thousands of different signals to be carried along a single fiber at the same time/2D transmission
-by using different wavelengths for each transmission

encoding method: reverse the level of each pulse during 2nd half of each timeslot

HOW TO SEND DATA FURTHER?

different methods of coding data to a PCM transmission to overcome problems of continous level

light detector: convert signal from light to electrical

electrical signal is then amplified and used to power another light source

pump: to inject energy into fluoride fiber

DESIGNING AN OPTICAL SYSTEM AND SELECTING COMPONENTS

1) National and International Standard

1) USA

2) Europe

3) Wold in general

click to edit

2) Picking the right fiber

can select 50/125,62.5/125, single mode fiber

names given by standardizing

LED resulting bandwidth: overfilled bandwidth

laser resulting bandwidth: effective modal bandwidth

3) Maximizing length of optic fiber cable runs

depend upon how fast you want the link to operate

4)Living with copper cables

advantages: familiar, easy connections, a simple metal detector can find a buried cable

disadvantages: susceptible to electrical interference and crosstalk, care has to be taken in high voltage environment, reduce bandwidths

HOW COPPER CABLES WORK

-2 wires, 1 from transmitter to receiver and vice versa

COPPER CABLE DESIGN

1) Twisted pair cables:screening, unscreened, unscreen twisted pair (UTP)

2) Coaxial cable: improve protection against interference, copper conductor, stranded copper braid, outer jacket

FIBER IN AND BETWEEN BULDINGS

Making entrance

Dig a hole and throw it in method

Spend more and do it better mehod

Making a start

Inside the building

cables, copper and fiber go to another equipment room: campus distributor/ main cross-connect

hundreds of cables going between equipment rooms: backbone cables

Inside the room

to connect campus distributor to individual work areas inside the room is the function of a junction box: floor distributor/horizontal cross-connect

cables can be installed horizontal/vertical

Other buildings and other floors

cables are taken from the campus connector to the building distributor and from there to another floor distributor and the on to the work areas