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Optical Networks: WDM & SONET
Chapter 6
Multiple Access Methods • TDMA – Time Division Multiple Access
– Done in the electrical domain
• SCMA – Sub Carrier Multiple Access– FDM done in the electrical domain
• CDMA – Code Division Multiple Access– Not very popular
• WDMA – Wavelength Division Multiple Access (The most promising)
Sub Carrier Multiplexing
Widely used in CATV distribution
Single Mode Fiber
Baseband Data
Baseband-RFModulation
RF-Optical Modulation
Optical - RF Demodulation
Gain
BPF
200 THz 1.8 GHz
RF-Baseband Demodulation
Baseband Data
Receiving End
Transmitting
End
SCM Block diagram
Two different Modulations for each RF Carrier !
Sub Carrier Multiplexing
• Each modulating RF carrier will look like a sub-carrier
• Unmodulated optical signal is the main carrier • Frequency division multiplexed (FDM) multi
channel systems also called as SCM
Frequency
Unmodulated (main) carrier
Sub-carriers
f1
f2
f1
f2
f0
Sub Carrier Multiplexing
• Ability to both analog and digitally modulated sub-carriers
• Each RF carrier may carry voice, data, HD video or digital audio
• They may be modulated on RF carriers using different techniques
• Performance analysis is not straightforward
TDMA• Signals are multiplexed in time
• This could be done in electrical domain (TDMA) or optical domain (OTDMA)
• Highly time synchronized transmitter/receiver
• Stable and precise clocks
• Most widely used (SONET, GPON etc.)
Wavelength Division multiplexing
Each wavelength is like a separate channel (fiber)
TDM Vs WDM
SONET
Wavelength Division Multiplexing
• Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths
Why WDM?• Capacity upgrade of existing fiber
networks (without adding fibers) • Transparency: Each optical channel can
carry any transmission format (different asynchronous bit rates, analog or digital)
• Scalability– Buy and install equipment for additional demand as needed
• Wavelength routing and switching: Wavelength is used as another dimension to time and space
Evolution of the Technology
WDM, CWDM and DWDM• WDM technology uses multiple wavelengths to
transmit information over a single fiber• Coarse WDM (CWDM) has wider channel spacing (20
nm) – low cost• Dense WDM (DWDM) has dense channel spacing (0.8
nm) which allows simultaneous transmission of 16+ wavelengths – high capacity
WDM and DWDM• First WDM networks used just two wavelengths,
1310 nm and 1550 nm• Today's DWDM systems utilize 16, 32,64,128 or
more wavelengths in the 1550 nm window• Each of these wavelength provide an
independent channel (Ex: each may transmit 10 Gb/s digital or SCMA analog)
• The range of standardized channel grids includes 50, 100, 200 and 1000 GHz spacing
• Wavelength spacing practically depends on: – laser linewidth – optical filter bandwidth