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